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An Alternate PM Alternator Design
By finnsawyer, Section Diaries Posted on Fri Apr 21st, 2006 at 04:23:07 PM MST
A while back I started thinking about a pulsed alternator design.

This led to the simple design shown below:

All coils are wound the same way. The coils move clockwise as shown over the magnets. Coil one is over a north pole with maximum flux through it. Coil two is just about to move over a south pole. Coil three is nowhere near a magnet. As the coils move the flux through coil one will decrease causing a negative pulse. Coil two will also experience a negative pulse as it moves over the south pole. The voltages across the three coils are shown below. Note that I've ignored the ramp up and ramp down of the voltages.

The fourth curve shows the voltage waveform when the three coils are connected in series. It is a single phase series of pulses. The total resistance will be the sum of the resistances of the coils. This can then be connected to a full wave bridge rectifier. However, it is not likely anyone would build this alternator except for demonstration purposes. Let's step it up to the next level.

We now have six coils spaced every 60 degrees and four magnets spaced every 90 degrees. So, the coil to magnet ratio is always going to be 3:2 for this type of alternator. Note the peculiar geometry. The magnets are wedge shaped with an angle of MA degrees. Coil two ends at 90 - MA/2 degrees. So, coil one ends at 30 - MA/2 degrees, since it's 60 degrees behind coil two. Coil one starts at a negative angle. We define an angle WA for the angle subtended by the width of the wire. Then we can write that coil one begins at -(WA + MA/2). So, the total width of coil one is then 30 + WA. The inside dimension of the coils are then given by 30 - WA. This is the maximum size magnet that can be used given WA. In reality you would pick the size of the magnet, the width of the wire or number of turns and then size the rotor to fit the angular requirement.
As mentioned, the three phases would be connected in series to get the output. If we connect the a and b windings of each phase together first we can do some electronic alchemy. Say phase two is in the middle. We can bring out a wire from between a and b of phase two and make it common (ground or G). This is similar to a center tapped transformer winding. What does this get us? We now need only two diodes and we cut the resistance in half. We do not cut the voltage in half. The waveforms are shown below.

The peak voltage is now three quarters of what it was before. It occurs for only one third the time, however. Still this would probably not a problem, since at cut-in the power in the wind is low anyway. In fact, as the wind speed increases the peak value of the wavform will increase untill the lower pulses cause conduction. If we assume we are charging a battery with no change in voltage, the current and hence power into the battery will be limited by the alternator resistance. The plot below shows a general power curve. A hypothetical wind power curve is also shown.

Breaks in the power curve always occur near 1.5 and 3 times the battery voltage. This means that the alternator can follow the wind's power curve somewhat better than a normal alternator design. Of course, the whole thing is also dependent on the blade assembly's ability to track the wind.
One final thing. Just as you can rectify to a positive voltage, you can also rectify to a negative voltage. This means you can also connect a battery to G in a negative fashion. This allows you to get 12 volts out by using two six volt batteries or 24 volts by using two 12 volt batteries. The alternator ground floats on a battery voltage relative to system ground.

An Alternate PM Alternator Design | 50 comments (50 topical, 0 editorial)

Re: An Alternate PM Alternator Design (3.00 / 0) (#1)
by willib (willibur at comcast dot net) on Fri Apr 21st, 2006 at 11:33:48 AM MST
(User Info) http://www.njwind.com/webcam.html#

a quote form Carl Sagan's book " Cosmos "

"There were also Heron of Alexandria, inventor of gear trains and steam engines and the author of Automata, the first book on robots; Apollonius of Perga, the mathematician who demonstrated the forms of the conic sections; Archimedes, the greatest mechanical genius until Leonardo da Vinci; and the astronomer and geographer Ptolemy, who compiled much of what is the pseudoscience of astrology: his Earth-centered universe held sway for 1,500 years, a reminder that intellectual capacity is no guarantee against being dead wrong."

Carpe Ventum (seize the wind)

Re: An Alternate PM Alternator Design (3.00 / 0) (#6)
by finnsawyer on Sat Apr 22nd, 2006 at 10:24:15 AM MST
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O.K., I guess you're trying to say that I,m "dead wrong". Well, instead of pushing out pointless verbiage why don't you come right out and say why it's wrong. Let's start with Faraday's Law. I don't mind being told that I'm barking up the wrong tree, but I like people to have the intellectual honesty of saying why.

By the way, it's my understanding that Heron of Alexandria was dead right. The only problem was that he never thought of connecting his "steam engine" to a gear train. Of course, that would have messed up an economy based on slave labor, but it sure would have helped in pumping out the mines. That was the first use of the steam engine, by the way.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#7)
by vawtman (vawtman(at)charter(dot)net) on Sat Apr 22nd, 2006 at 10:41:47 AM MST
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Finsawyer you should just build it and see what happens since you have the plan.Theres always something better out there somewhere.Dont know till you try.

[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#8)
by finnsawyer on Sat Apr 22nd, 2006 at 11:16:30 AM MST
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You're right, of course, but current circumstances don't permit me to do so. That said, I've seen some postings lately where people are looking for alternate generating possibilities. This might fit the bill for some of them. I put this out as a diary so people could see the possibility. What's the point of sitting on this for a year? And besides, isn't this a community effort, to advance the use of wind power? I have no doubt the concept will work and if someone wants to try it they are welcome to do so. In any case it will take many attempts to feel out the size and other requirements to make an alternator that performs at either six or twelve volts at a reasonable rpm. So, I would hope anyone making one of these would provide their results even if they seem to be a failure. I realize people would not want to sacrifice their expensive magnets to something that's untried, so I suggest using circular magnets mounted on a disk with a threaded rod mounted to it. This way the magnets could be used with different size rotors and coils until a usable design is found. Anyway, I bet some people have four magnets lying around just waiting for a good use.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#2)
by drdongle (Dr.Dongle1@juno.com) on Fri Apr 21st, 2006 at 06:17:37 PM MST
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So in a nut shell..... what would be the advangage to this system?
Carpe Vigor, Dr.D

Re: An Alternate PM Alternator Design (3.00 / 0) (#5)
by finnsawyer on Sat Apr 22nd, 2006 at 10:08:27 AM MST
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Possibly low cogging (see the other comment). There is no limit on the size of the coils. For instance, a one inch diameter magnet could have a three inch diameter coil around it with a one inch diameter center hole. Just scale the rotor accordingly. Because of the upward opening power curve for the center tapped version, which more closely follows the wind power curve, it may be possible to have better low speed performance as well as reasonable higher speed performance. See Flux's recent comments in that regard. By using two six volt batteries with the center tapped version instead of one twelve volt battery, one can cut the cut-in speed almost in half. And of course, you're using fewer magnets, though likely with more copper, which might reduce the overall cost. I guess this really isn't a 'nutshell' explanation, but then considering the complex issues involved, there probably isn't one.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#3)
by Chagrin on Fri Apr 21st, 2006 at 10:40:35 PM MST
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Wouldn't this cog pretty hard?

Re: An Alternate PM Alternator Design (3.00 / 0) (#4)
by finnsawyer on Sat Apr 22nd, 2006 at 09:50:16 AM MST
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Cogging only occurs if the coils have iron cores. There is no need per se to have iron cores. Even if they had iron cores, one third of the coils will not be over magnets at any one time. Furthermore one set of coils will be moving off of a magnet as another set moves onto a magnet. So, one set is pulled in the direction of motion as the other is pulled opposite. The effects should cancel out. This may, in fact be a low cogging design. I hadn't thought of that. Thanks for the comment.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#9)
by kitno455 on Sat Apr 22nd, 2006 at 03:20:33 PM MST
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i dont get it. you have just increased the resistance of each coil significantly by making it longer, but you did not increase the portion of the coil that actually gets hit by flux. and, if you space everything out so that it acts like single phase, it will just vibrate like single phase?

dual rotor aint broke?

allan

Re: An Alternate PM Alternator Design (3.00 / 0) (#13)
by finnsawyer on Sun Apr 23rd, 2006 at 10:00:13 AM MST
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First off, every coil goes from a state of no flux through it to a maximum flux less leakage just like in any design. If you scale a particular version up by a factor of two by keeping the same number of windings, you get four times the flux with twice the speed for eight times the voltage. You also have increased the coil volume by eight for the same number of turns or four if you keep the same length for the coils. Since you probably do not need an eight times factor increase in voltage you have the option of decreasing the number of turns by using heavier wire (the coil size must still be doubled for this example). So, you can lower the resistance. This is much like any design. Resistance does matter and is part of the design. It should be obvious that a larger machine can provide more power, which means lower resistance. It does help that by going to the center tap the effective resistance is cut in half.

What you see here is the first step in an engineering design, a concept. The next step is to prove it. It is possible that vibration issues may arise. But they are mechanical not electrical. Or actually electromechanical as they involve interactions between the electromagnetic forces and the mechanical vibrational states of the physical structure. If you go back and look at the first set of voltage curves, you will notice that the positive voltage spikes, formed from the addition of pairs of coil outputs are 120 degrees apart in time as are the negative spikes. In other words three phase behavior is embedded in the design. This is a hybrid between single phase and three phase and it will have its own characteristics.

Since this is my diary I'm going to do a bit of ranting here. When scanning the net once for wind power I came across a comment concerning the lack of "anything new in wind power". Back in the 70's when the energy crunch hit everyone was out trying different things. That spirit seems to be gone. Now it's become "cookie cutter" wind power design. I doubt that in twenty years everyone will be doing what you see here. There still are improvements to be made. The dual rotor may not be broke, but then neither was the 36 Ford in 1936. The car you drive today is far different. Things change. You can see why I put this in a diary. I'm not speaking to those who are satisfied with the status quo, but rather those who really want to advance the cause of wind power. Time will tell whether this design has a role to play in that. In any case, it's out there for anyone to try. Just call it the Mattson Alternator, if you will.

I don't want to appear to come down too hard on this site. There is an undercurrent of innovation here, and the discussions are definitely useful. The idea for this design came out of Jerry's "Jerry rigging" of the alternator output. To my mind this is the real power of the Internet.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#14)
by kitno455 on Mon Apr 24th, 2006 at 10:39:11 AM MST
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sorry dude, i was not trying to get you angry or hi-jack your diary. i was just trying to point out that you made your coils have lots of extra dead wire that is never crossed by the mags, and therefore contributes only resistance to your output?

allan

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Re: An Alternate PM Alternator Design (3.00 / 0) (#19)
by finnsawyer on Tue Apr 25th, 2006 at 09:03:18 AM MST
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No offense taken. I like the give and take. It tends to expand the mind. A while back we were speculating here about trading magnets for copper. Well, this may do that. Not necessarily a bad thing. But still, it has to produce enough power at a reasonable cost. The jury will be out on that for a while.

When you speak of dead wire I presume you are referring to the wire along the top and bottom of each coil. I guess I missed that in your comment, although I was aware of the issue. That wire is there to complete the loop. Every design has to put up with it. To minimize its effect you have the magnet be as large as the inside of the coil. Also make the magnets and coils round. The alternator should still work in that case, but don't expect nice rectangular pulses.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#23)
by kitno455 on Tue Apr 25th, 2006 at 05:26:51 PM MST
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as i understand it, having the magnet as large as the inside of the coil will not minimize the effect. the parts being passed over by the magnet are the only parts that generate voltage. in your design, thats the short sides of the coil, while the long sides just contribute resistance.

just my guess, but it looks like you can get the more power from the same mags and less wire, by going with smaller coils.

allan

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Re: An Alternate PM Alternator Design (3.00 / 0) (#25)
by finnsawyer on Wed Apr 26th, 2006 at 10:09:55 AM MST
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There are two things that make or break an alternator design. Firstly, you need to get the proper voltage at the cut-in speed. This depends on the number of turns (note turns, you can't make an alternator with only radial wires) and the strength of the magnetic flux. Larger flux, fewer turns. That applies to this design as well. Once you have established the number of turns the second requirement is current carrying ability or resistance. The solution to lower the resistance is to use heavier wire for the same number of turns. You now made the coil bigger. This requires a larger rotor. That will give a lower cut-in rpm. That is true for this design as well. So, for a proper combination of magnet size, wire size, number of turns, and rotor size it should be possible to satisfy the design criteria. The problem of course is that these things are all interrelated. It will take while to sort them out. The design currently used was obtained by trial and error. I suspect the same will hold true for this one.
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#15)
by elvin1949 (elvin1949@yahoo.com) on Tue Apr 25th, 2006 at 03:56:16 AM MST
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Finsawyer
This is something that i will do a lot of studying
about.Got me to thinking [rust flying everywhere]
outside the box? yes but that is good.
later
elvin

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Re: An Alternate PM Alternator Design (3.00 / 0) (#21)
by finnsawyer on Tue Apr 25th, 2006 at 09:51:22 AM MST
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Good!
GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#46)
by tmmjohnson on Sun Nov 25th, 2007 at 10:52:32 AM MST
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Hi, I am a newbie and just read your posting here. I am interested in HVLV applications but am finding that it is difficult without just breaking down and buying a unit. I looked at your idea and thought that maybe I could pass something on that is similar. It was a college paper done on PMG with toroidial windings. The purpose was to create a working axial flux generator that could be built in production status with varying scales. It might be a help, might not be. I am hoping to get to the point of trying some of the ideas, but magnets are hard to get in Africa. I was trying to get the document copied with its pictures, but I can't seem to paste a picture. If you give me an address I will send you a PDF of this document. My address is tmmjohnson@gmail.com. Jon NREL/CP-500-24996 Ÿ UC Category: 1213 Axial Flux, Modular, Permanent- Magnet Generator with a Toroidal Winding for Wind Turbine Applications E. Muljadi C.P. Butterfield Yih-Huei Wan National Wind Technology Center National Renewable Energy Laboratory Presented at IEEE Industry Applications Conference St. Louis, MO November 5-8, 1998 National Renewable Energy Laboratory 1617 Cole Boulevard Golden, Colorado 80401-3393 A national laboratory of the U.S. Department of Energy Managed by Midwest Research Institute for the U.S. Department of Energy under contract No. DE-AC36-83CH10093 Work performed under task number WE803020 July 1998 NOTICE This report was prepared as an account of work sponsored by an agency of the United States government. Neither the United States government nor any agency thereof, nor any of their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States government or any agency thereof. The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States government or any agency thereof. Available to DOE and DOE contractors from: Office of Scientific and Technical Information (OSTI) P.O. Box 62 Oak Ridge, TN 37831 Prices available by calling (423) 576-8401 Available to the public from: National Technical Information Service (NTIS) U.S. Department of Commerce 5285 Port Royal Road Springfield, VA 22161 (703) 487-4650 Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste Axial Flux, Modular, Permanent-Magnet Generator with a Toroidal Winding for Wind Turbine Applications E. Muljadi, C. P. Butterfield, Yih-Huei Wan National Renewable Energy Laboratory 1617 Cole Boulevard Golden, CO 80401 Tel. (303)384-6900, Fax (303)384-6999 Eduard_muljadi@nrel.gov, http://www.nrel.gov/wind Abstract - Permanent-magnet generators have been used for wind turbines for many years. Many small wind turbine manufacturers use direct-drive permanent-magnet generators. For wind turbine generators, the design philosophy must cover the following characteristics: low cost, light weight, low speed, high torque, and variable speed generation. The generator is easy to manufacture and the design can be scaled up for a larger size without major retooling. A modular permanent-magnet generator with axial flux direction was chosen. The permanent magnet used is NdFeB or ferrite magnet with flux guide to focus flux density in the air gap. Each unit module of the generator may consist of one, two, or more phases. Each generator can be expanded to two or more unit modules. Each unit module is built from simple modular poles. The stator winding is formed like a torus. Thus, the assembly process is simplified and the winding insertion in the slot is less tedious. We built a prototype of one unit module and performed preliminary tests in our laboratory. Follow up tests will be conducted in our lab to improve the design. I. INTRODUCTION Using permanent-magnet (PM) generators for small wind turbines is very common. Usually an AC generator with many poles operates between 10-100 Hz. Because the generator is directly driven by the wind turbine [1,3,5], it is commonly known as a direct drive generator. Many configurations use surface mounted three phase PM synchronous generators with a rectifier connected to the generator terminal. Many types of generator concepts have been used and proposed to convert wind power into electricity. An axial flux generator with a different type of winding and a different magnet arrangement was developed [1,2]. A modular concept was proposed to reduce manufacturing costs [3]. The transverse flux generator has a higher power density than a traditional induction generator [4]. In this paper, a combination of a modular, axial flux, and torroidal stator winding are applied to a permanent-magnet generator. Although the design is intended for wind turbine applications, this PM machine can be used for many other applications. A wind turbine generator must be light to minimize the requirements for the tower structure. Since the wind turbine operates at low rotational speed, the generator is built with many poles. We designed, built, and tested a permanentmagnet generator for wind turbines. Several unique properties are included in this design. It uses a modular concept. Each pole is constructed individually, thus the number of poles is based on the requirements. The winding is concentric, like a torus, making it easy to assemble. The rotor core has a focusing capability with a variable magnet area, so the air gap flux density can be adjusted independent of the rotor radius. A single unit module of this generator can have single or multiple phases. Additional unit modules can be stacked in the axial direction to get more power. With this modular concept, any failure in one unit can be replaced immediately or can be bypassed, thus minimizing turbine downtime. The dimension of the generator and the size of each component should be based on the actual wind turbine for which it is to be used. Because the purpose of the prototype unit is to prove the concept, we designed and built it with readily available components. A steady state analysis was done to determine the initial electric loading and magnetic loading. The initial loss calculation was derived. The next step of the calculation was done using finite element analysis. The flux density in the critical components, and the map of the core losses were found. No-load, rated, and short-circuit conditions can be predicted from this analysis. Any changes made were reiterated by using steady state analysis. Thus the process was repeated until the final design is ready. A test was conducted in the lab to find the parameters of the generator and any unpredicted anomalies. Data were collected for no-load and full-load conditions. The first section of this paper is devoted to introducing the background of the PM generator in wind turbine applications. The second section introduces the generator components. In the third section we present our analysis of the PM generator. In the fourth section we describe testing, and lastly, in the fifth section the conclusions are summarized. II. COMPONENT OF THE GENERATOR In this paper we discuss only one unit module of the generator. The generator consists of an eighteen-pole permanent magnet. The stator and the rotor cores are made of pre-cut transformer lamination silicon steel (gauge 26, M19). The stator and rotor cores can be made on a per pole basis, reducing the cost of complete dies required to stamp a conventional lamination configuration. The geometry of the stator and the rotor core could have been optimized, however, this project focuses on the proof of concept. A. Rotor The cross section of the stator and rotor pole is illustrated in Figure 1. Each pole is constructed from two identical corestacks and the permanent magnet is sandwiched in between. The rotor is constructed to allow an expansion in the axial direction, for example, to increase the magnet surface. The flux directions at the top (outer radius) and the bottom (inner radius) of the rotor pole are the opposite. Around the perimeter of the rotor, the flux direction of one pole is opposite of the flux direction in next pole, as shown by the white arrows in Figure 2. The ratio of the magnetic surface area to the pole surface area determines the focusing factor. The chosen geometry enables the designer to increase the length of the rotor core without affecting the stator geometry and vice versa. The rotor poles are attached to a non-magnetic disk that holds the rotor cores. The shaft is attached to the disk to rotate the rotor core. A non-magnetic stainless steel belt is strapped around the rotor core to keep the rotor poles in place. Since the rotor speed is low, centrifugal force created when the rotor rotates is not very high. There are nine pole pairs on the rotor. Between two rotor poles, there is a small gap to minimize interpolar magnetic leakage. B. Stator The stator consists of two stator sides. There are nine poles attached to each stator side. The poles on each side are attached to a plate (not shown in Figure 2) which holds the stator poles in place. In the prototype, one side of the stator core can be rotated (within a limited angle range) with respect to the other stator side. Thus the position of the stator cores in one side can be shifted with respect to the other sides. The shift can be adjusted to control the phase shift between the first stator side and the second stator side. C. Stator winding The stator winding is wound like a torus or a washer. With a toroidal form, the stator winding can be easily assembled and automated for production. The stator winding between the stator poles is exposed to open air, which improves cooling. One advantage of wind power systems is the location of the generator. It is mounted on a tower above the ground. The cooling mechanism is better up on the nacelle than inside a ground level building because the generator is always exposed to air flow that is proportional to the generator load. During low wind speeds, the heat transfer from the winding is lower, however, the heat generated in the winding is lower, too. The opposite is true at high wind; more heat is generated in the winding, but more air flow is available to transfer the heat away. In this paper, one module unit is built for a single phase generator. The stator windings at the two sides are connected in parallel to generate a single phase output. The rotor shaft is attached to the stator sides through the bearings, which are attached to the stator plate. The rotor core has a width of 6.35 cm (2.5 in.) and a diameter of 29.2 cm (11.5 in.). The overall width of the generator is 16.5 cm (6.5 in.), excluding the two stator plates. D. Expansion for multimodule generation system The power from the stator can be actively controlled using power switches (IGBTs) or passively controlled using a diode Figure 1. One pole of the stator and rotor core Figure 2. PM Generator with Toroidal Winding stator core rotor core PM Non magnet disk rotor core South Copper North rectifier. Figure 3 shows a possible configuration of the power converter to process the power generated by the generator. The generator may consist of one or more modules. In this configuration, only three unit modules are shown. Each unit module of the generator is paired with one leg module of power switches on the power converter side. Thus the power converter and the generator can be expanded in a similar fashion. The power generated is converted back to the utility via a three-phase inverter, which can be controlled to produce good power quality. III. DESIGN ANALYSIS The analysis of the generator is based on the wind turbine requirements. The steady state analysis was performed as the first step to get the first cut of design criteria. The finite element analysis was performed to refine the magnetic analysis. Finally, a dynamic analysis was performed in the lab to validate generator performance under dynamic conditions. A. Steady state analysis The prime mover for this generator is a wind turbine. One characteristic of wind turbines is that the rotational speed is lower than most prime movers. To avoid using a gearbox, the generator is direct driven. Multiple poles must be used to allow slow speed operation. From steady state analysis, the following criteria are chosen: Number of poles = 18 Max operating frequency = 100 Hz (at 667 rpm) Number of phases per unit module = 1 (two windings in parallel) The electric loading: Stator current = 11.0 Amp RMS (at per phase voltage 58 Volt RMS) The wire chosen is AWG 12 The current density in the slot J = 3.4x106 Amp/m2 Predicted copper losses at rated current = 42 watts B. Finite element analysis To analyze the magnetic circuit, the finite element method was used to compute the flux density in the generator components. The main purpose of this analysis is to get the overall picture of the saturation levels in different parts of the generator, the iron losses in the components of the generators, and the worst case of demagnetization on the permanent magnet. In the finite element analysis presented here, the generator uses a ferrite permanent magnet. No-load condition. In the no-load condition, the magnetic path is analyzed to see the magnetic flux density in different parts of the magnetic paths. With the stator core in each side shifted by 180o the maximum flux in the core happens when the stator core and the rotor core are aligned. Figure 4 shows the flux lines at the no-load condition. Only one side of the stator core is shown. Some flux leakage is shown such as at both ends of the rotor poles. The rotor core has low flux density with the highest flux density at the parts closest to the air gap. As shown in Figure 4, the maximum flux density Figure 3. Expansion for multimodule generation. Figure 4. Flux density at no-load condition Phase3 Phase2 To 60 Hz utility Phase1 occurs at the corner of the U-shaped stator core. Figure 5 shows the magnitude of the flux density along the horizontal line in the middle of the air gap. The maximum flux density at no load is 1.55 Tesla. The flux density at the air gap is 0.9 Tesla and the flux density at the permanent magnet is 0.24 Tesla. The stator core and the rotor core have a flux density below the saturation point. Inductive load at rated current. In this condition, the magnetic path is analyzed to see flux reduction at the air gap at the least favorable power factor. The generator is loaded to have rated current. Short-circuit condition. In this condition, the magnetic path is analyzed to see the demagnetization effects on the permanent magnet. In order to analyze the worst case scenario, the stator core and the rotor core are perfectly aligned and the short circuit current is applied to the stator core. In this case the short circuit current is about ten times the rated current. The result is tabulated in Table 1. Table 1. Flux Density Comparison at Different Magnetic Paths for Different Conditions B airgap B max B at PM No-load 0.91 T 1.55 T 0.244 T Inductive Load (rated) 0.89 T 1.50 T 0.239 T Short Circuit 0.70 T 1.05 T 0.193 T IV. EXPERIMENTAL RESULTS A. Experimental set up The experiment was conducted to observe the performance of the generator. The generator is driven by a motor via a belt. The motor is a four pole motor, with rated speed of 1800 rpm. The motor is fed by a PWM variable frequency drive. The generator speed is driven to 667 RPM. The output frequency at this rpm is 100 Hz. The experiment is conducted only on a single unit generator. In the finite element analysis, the permanent magnet used is ferrite, however, in this experiment the permanent magnet chosen is rare earth permanent magnet (NdFeB). Figure 6. Open circuit woltage B. Voltage and current waveforms The open circuit voltage is measured at the terminal output of winding 2 (open circuit). The stator cores are shifted toward each other by 180 electrical degrees. The voltage waveform is captured from the scope, digitized, and plotted in Figure 6 and Figure 7. In Figure 7, the generator is loaded with resistive load up to rated load at 100 Hz. The voltage across the terminal output of the generator is a unity power factor load. Thus the current waveform is reflected by this terminal voltage waveform. C. Parameter Determination Test A simple modified test is used to get the parameters of the permanent magnet [6]. The experiment is shown in Figure 8. Figure 5. Flux density at no-load condition Figure 7. Terminal voltage across resistive load One side of the generator (winding 1) is connected to a rated load at unity power factor. The generator is driven to generate a rated frequency. The other side of the winding (winding 2) is an open circuit. The voltage output of winding 1 is called terminal voltage V and the open circuit voltage of winding 2 is called open circuit voltage E. The angle difference between V and E is called d, which is the torque angle of the generator at this load. The power, current, and voltage output of winding 1 is recorded. The parameters can be computed from the test data, and the results are listed in Table 2 below. Table 2. Results from Test Data Parameters Lds Lqs Rs 8.41 mH 4.38 mH 0.22 ohm Vopen circuit 75 volts Vrated load 58 volts Irated/winding 11 Amp Prated/winding 650 watt Rotor Speed 667 rpm 100 Hz V. CONCLUSION The proposed generator is investigated for application in wind power generation. In the first stage of implementation, a proof of concept of the generator is investigated. The magnetic and electric loading are shown to be within the limits of common practice of machine design. The generator has the following advantages for wind turbine generation: - The modular concept is suitable for the commercial production of machines of limited quantities and with different sizes and output requirements. The components are manufactured on a per pole basis. The tooling required is minimized. The design can be readily changed, such as the number of poles in one unit or the number of unit modules in a generator system. - The axial flux design makes it easier to increase the flux density in the air gap. - The toroidal form of the stator winding makes it easy to fabricate. The geometry of the stator winding and stator core make the heat dissipation more effective. - To scale up the output power of the generator, more units can be stacked in the axial directions. The power converter required to process the power is readily compatible with the generator. Each unit module of the generator is matched with each leg of the power switches. VI. ACKNOWLEDMENTS The authors wish to thank Jerry Bianchi for his assistance during the test set up and Jim Adams for his help during the fabrication of this generator. We wish to acknowledge our management at NREL and the U.S. Department of Energy (DOE) for encouraging us and approving the time and tools we needed for this project. DOE supported this work under contract number DE-AC36- 83CH10093. VII. REFERENCES [1] B.J. Chalmers, E.Spooner, "An Axial-flux Permanentmagnet Generator for a Gearless Wind Energy System," PEDES 96, January 1996, New Delhi, India. [2] F. Carrichi, F. Crescimbini, F. Mezzetti, "Multistage Axial-flux PM Machine for Wheel Direct Drive," IEEE Transactions on Industry Applications, Vol 32. No. 4, July/August 1996, pp. 882-887. [3] E. Spooner, A. Williamson, "Modular, Permanent-magnet Wind-turbine Generators," Conference Record of the 1996 IEEE Industry Applications Society, Oct. 6-10, 1996, San Diego, California, Volume 1, pp. 497-502 [4] S. Huang, J. Luo, T.A. Lipo, "Analysis and Evaluation of the Transverse Flux Circumferential Current Machine," Conference Record of the 1997 IEEE Industry Applications Society, Oct. 5-9, 1997, New Orleans, Louisiana, Volume 1, pp. 378-384 [5] E.F. Fuchs, A.A. Fardoun, P.Carlin, R.W. Erikson, "Permanent Magnet Machines with Large Speed Variations," Windpower 92, October 1992, Seattle, Washington. [6] Gieras, J.F., Wing, M., "Permanent Magnet Motor Technology, Design and Applications," Marcel Dekker, Inc. New York, 1997. Figure 8. Experimental set-up Watt-mtr / V,E = d motor Winding 1 Winding 2 (open) V E V Rload A V scope v
Climb singing, die climbing tmmjohnson
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#47)
by finnsawyer on Mon Dec 3rd, 2007 at 08:27:45 AM MST
(User Info)

When I hit stator winding like a torus to make insertion in the slots easier I quit reading, as neither concept would apply to my proposed design. You really should get a picture. To post pictures on this site they need to be 640 by 480 pixels and in .jpg format. One trick you can use to make that conversion is to get a digital camera and take a picture off of the video monitor screen. If you decide to pursue this any further do an independent posting. As it is, I do not appreciate all the extraneous verbiage added to the diary, as it simply obscures the real issues. Another possibility would have been to simply have put a link to the information if such exists. I would imagine it does. Rather than simply reacting to things you need to spend more time reading on this site. Most everything you need to know to navigate on this site is explained somewhere.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#16)
by SmoggyTurnip on Tue Apr 25th, 2006 at 07:24:23 AM MST
(User Info)

Finsayer,

I can't seem to figure out
how the coils are connected
together from your explaination
(for the second diagram).
Maybe you could do a drawing
showing how the coils are
connected? I would like to
do some calculations.
The sooner you start the longer it takes.

Re: An Alternate PM Alternator Design (3.00 / 0) (#18)
by finnsawyer on Tue Apr 25th, 2006 at 08:35:12 AM MST
(User Info)

The coils are assumed all wound the same way with the voltages as shown. Simply connect the negative side of V1a to the positive side of V1b to form V1. Do the same for the other two phases. Then connect the negative side of phase one to the positive side of phase two and the negative side of phase two to the positive side of phase three. If you wish to bring out the 'center tap' that is taken off from the connection on phase two between V2a and V2b. The important thing is that when V1a is going negative (as shown) then so is V1b. Similarly, when V2a is going negative then so is V2b. When phase three encounters the north pole, as it will do, then both V3a and V3b must both create a positive pulse. At this point it's basically defining the winding of the coils such that these relationships hold. In building this it requires checking the voltages when turning the rotor to make sure the relationships hold. In that sense a diagram would be of no real help since I can not predict how someone will set up his coils. Note that if it happens that V1a actually goes positive while V2b goes negative then you just exchange the plus and minus signs on V1a. That is, you redefine the voltage V1a. These kind of ambiguities are quite common in electronic circuits, but are really of no consequence as long as we are consistent in our definitions. I hope this helps.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#17)
by viron on Tue Apr 25th, 2006 at 08:22:25 AM MST
(User Info)

Finsawyer, I'm a no knowledge person trying to research an optimum easy to build design before I play. Your alternator looks intersting so I emailed a link to the discussion to my technical partner, he's head of the electrical failure analysis lab at Kennedy Space Center. He's onboard for the project and has a few other interested people at work. If it seems reasonably plausible to minds that should know I'll build your alternator.

For a wind mill I've settled on a Savonius design optimized by UC Long Beach. It is a full blown lift motor with a TSR of 1.6. The choice is because I live in a class 2 wind zone in Florida. The design appears to have good torque and decent speed that would be advantageous to gentle coastal breezes.

http://www.energy.ca.gov/2005publications/CEC-500-2005-084/CEC-500-2005-084.PDF

Your theoretical output curve is attractive because it ramps up fairly quickly at the low end and reduced cogging is always a benefit.

I to am flying in the face of popular opinion on windmill design. Hopefully we can be helpful partners in this crime.

Viron

Re: An Alternate PM Alternator Design (3.00 / 0) (#20)
by finnsawyer on Tue Apr 25th, 2006 at 09:49:54 AM MST
(User Info)

It'll be interesting to see what your guys say. If they're really taken with it, maybe I'll patent it! Ha! I'm willing to bet someone has tried this design in the past. It seems too obvious to not have been noticed. But one never knows. The super strong magnets are a recent phenomena.

That change in the power curve is interesting. It's something that has not been seen before, as far as I know. It should be possible to hit the power curve of the mill at two points instead of one. Keep in mind that curve was hypothetical. While the shape is correct so will any curve scaled up or down from it be correct. The ramp up at the low end depends on the resistance of the alternator. In general a lower resistance requires a larger alternator. Probably larger than with the regular design. Keep in mind that in this design the coils are not spaced tightly, so a larger rotor would be expected. In other words go into this with your eyes open. That said I am willing to give any help and insights that I can.

This might be a good place to suggest a plan of attack for a first case design. Use circular magnets. Decide on a coil diameter. This will determine the size rotor you will need. Pick a number of wire diameters. Wind coils of the specified size with all the wire sizes (the number of turns will depend on the wire size). Mount the magnets on the rotor and make stators using the different size coils. Try the different rotor stator combinations and evaluate each one. From this data you can predict what size magnets and coils you need to meet your design specifications. It would also be nice if you presented your results here. Good luck!

GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#31)
by viron on Tue May 2nd, 2006 at 09:08:49 AM MST
(User Info)

Finsawyer,

Here's my technical partner's, Larry Ludwig's, assessment. In general he thinks it would be good for producing power at very low wind speeds as it would reduce EMF drag.

Skimmed the discussion and drawings. The only serious advantage to this is that low wind speeds will still produce pulses (less system EFM drag) which can be rectified so that you can always produce some voltage. Electronically the voltge can be built to a higher level with a chopper circuit. This might be useful in a battery based system where trickel charge (high voltage, very low current) helps to maintain the charge. If you were to look at there diagrams and draw lines to the top of each pulse you would get a sine wave which is what overlapping coils whould produce. In an alternator, these three overlapping coil voltages are rectified to a pure dc. The three coil set allows for less ripple smoothing to have to ocur so that less loss occurs in the rectification and smoothing circuit.
If our desire is to always generate some voltage, even at low wind speeds: we could have our PIC keep a low drag PM pulsed alternator engaged at low RPM and a High drag, High voltage generating alternator swithched in ( and the other swithed out) at higher RPMs. Then we could have retification circuits for each one designed around what were doing with the outputs. The High RPM could be a direct feed to the system under use, with a small bleed to the batteries. The low RPM system could only feed/charge the battery. This is typically the type system I had envisioned for a home power system. A high voltage battery set with an inverter for producing AC at high output periods and a low current, high voltage electronic circuitry for just charging the batterys during low output periods. I could talk to Pete about an electriclly actuated clutch system or a mechanicat RPM activated clutch system the we could read electrically to determin system operation parameters/switching.

[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#32)
by finnsawyer on Fri May 5th, 2006 at 09:25:00 AM MST
(User Info)

I disagree that the only advantage is the low speed performance. The changing power curve also would be an advantage. Flux, in his recent postings pointed out that with the conventional alternator design one has to give up low wind speed performance to get the best out of system at moderate wind speeds. For instance, if you size the alternator to give good matching with the turbine at 10 mph, you give up performance at 15 mph where the wind has about three times the power available. With this design at a little over fifteen mph the alternator power curve increases by 67 percent, so you track the available power better in that region. Catching that low wind power may actually be quite beneficial as the wind blows more of the time at lower velocities. So capturing more of that energy while still getting reasonable performance at the higher velocities may actually yield more usable power on average.

I presented this design for the case of charging a battery where pulsed dc doesn't matter. That is, while this will have more ripple than a three phase system, for charging a battery it doesn't matter.

I don't see any reason why this alternator couldn't be made to produce more power at higher wind speeds by introducing centrifugally operated iron cores for the coils. At low rpms they are retracted. At a certain point they are inserted in a controlled manner. This would be easiest to do if the coil assembly rotated. With the magnet assembly rotating it would be necessary to mount the centrifugal weights on the back of the rotor and transmit their effect through the shaft by the use of a rod. The question is how something like this would compare in cost to adding a second alternator?

GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#22)
by SmoggyTurnip on Tue Apr 25th, 2006 at 11:33:22 AM MST
(User Info)

Your curve is showing peak voltage with respect to rpm.
Since the wave form is is pulses the actual rms voltage
is less than the rms value of a sin wave with the
same peak voltage. The output power for this design
will still go with the square of the RPM so it will
not match the wind power curve any better than the
standard design.

Any magnet passing over any air filled coil at a fixed velocity will produce an RMS voltage in that coil that is directly proportional to the velocity of the magnet, this does not depend on the shape of the magnet or the shape of the coil or the strength of the magnet field, or resistance of the wire. That is to say for a given magnet and coil moving the magnet twice as fast will produce an RMS voltage twice the magnitude.

The maximum power that can be derived from that coil is V^2/(2*R).

These 2 facts taken together imply that any alternator that moves a constant
magnetic field over fixed air filled coils will have a power curve that is proportional
to the square of RMP, regardless of the arrangement of the coils and magnets.
The sooner you start the longer it takes.

Re: An Alternate PM Alternator Design (3.00 / 0) (#24)
by finnsawyer on Wed Apr 26th, 2006 at 09:29:40 AM MST
(User Info)

While it is true that each voltage pulse will have a value proportional to the rpm, they have different values. One is two thirds the peak voltage, the other one third. They will cause conduction in turn as the rpm increases. What the battery sees is an average current that increases at different rates at different rpms, the rates being higher at higher rpms. What you get is a 'piece wise linear curve'. The change in power to the battery at high rpms per unit change in rpm is greater at high rpms than say near cut-in. This curve opens up in the same way the wind speed power curve does. You can never get a perfect match, but you may be able to match the power curve at two points.

A little elaboration on matching curves may help. The general curve for a cubic is: P = AxV^3 + BxV^2 + CxV + D. In this D = 0, as no voltage no power. With the three coefficients A,B,and C we can easily match the curve of the power into the battery at three points. Unfortunately wind power only has the coefficient A. This severely limits any matching. We can try to improve the matching by modifying the wind mill's characteristic. For instance, we can lower the efficiency of the wind mill at high wind speeds by changing the pitch of the blades. At low wind speeds we might let the mill move toward stall but keep it from stalling.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#26)
by SmoggyTurnip on Wed Apr 26th, 2006 at 11:39:41 AM MST
(User Info)

"While it is true that each voltage pulse will have a value proportional to the rpm, they have different values. One is two thirds the peak voltage, the other one third. They will cause conduction in turn as the rpm increases."

Yes, I agree they will each increase at different rates but they will
each be with the square of RPM.

Coil1 = K1*RPM^2
Coil2 = K2*RPM^2

Coil1+coil2 = (K1+K2)*RPM^2 = K*RPM^2

So in the end the power curve is with the
square of the RPM. This means it can
be reproduced by another dual rotor machine
of standard design.

"A little elaboration on matching curves may help. The general curve for a cubic is: P = AxV^3 + BxV^2 + CxV + D. In this D = 0."

Yes this is true - the power in the wind follows the cube of wind velocity, but B, V, and D are 0.

As far as the power curve of the blades go we really don't
know the formula for it - it could even be a fourth power,
it really depends on the blades, - one thing we can say is that
it is concave down everywhere.

In matching the alternator to the blades the 2 curves that have
to intersect are :
1) the alternator power verses RPM and
2) blade power at the shaft verses RPM.

The blade power curve is concave
down everywhere and the alternator
power curve is K*PRM^2. They
can only intersect at 2 places
- one of them being (0,0).
The sooner you start the longer it takes.
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#27)
by finnsawyer on Thu Apr 27th, 2006 at 08:23:15 AM MST
(User Info)

A short response. Power into the battery is proportional to current. Current is charge per unit time. As the rpm goes up you dump more charge per unit time into the battery after each break point. A regular alternator doesn't do that. By the way, when a regular alternator is charging a battery the current is equal to the alternator voltage minus the battery voltage divided by the alternator resistance. Power into the battery is that current times the battery voltage. It goes up linearly with rpm.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#28)
by SmoggyTurnip on Thu Apr 27th, 2006 at 10:58:47 AM MST
(User Info)

Sorry finsawyer, I guess I just don't understand
what you are saying. I must be missing something
here. I don't see where these "break points"
are. I am not saying you are wrong - I just don't
get it.

The sooner you start the longer it takes.
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#29)
by finnsawyer on Fri Apr 28th, 2006 at 10:18:27 AM MST
(User Info)

Sometimes it's hard to tell where people are having problems. Let's try it this way. When the alternator hits cut-in at let's say 100 rpm, the highest voltage pulse will cause conduction. I've called this voltage Vp, which varies linearly with rpm. I'm going to ignore the diode voltage. So the current that flows from the alternator for the duration of that pulse is: Ib = (Vp - Vb)/Ra, where Vb is the battery voltage and Ra is the alternator resistance. I'm also ignoring the battery resistance to keep this simple. So, the power into the battery is given by VbxIb during the duration of the pulse. The energy into the battery is given by
Eb = PtxVbxIb = PtxVbx(Vp - Vb)/Ra, where Pt is time duration in seconds of the pulse. The energy out of the alternator then becomes Ea = PtxVpxIb = PtxVpx(Vp - Vb)/Ra during the pulse. The percent of energy (or power) going into the battery is given by Eb/Ea = Vb/Vp. This is the type of behavior you would expect from a regular alternator.

When the alternator hits 150 rpm the second highest pulse becomes large enough to cause the diodes to conduct. That speed is the first break point. So now we have a second pulse causing conduction. Its value is 2/3 that of Vp. So, for that pulse we may write Ib2 = (2/3xVp - vb)/Ra. The energy delivered by that pulse is given by Eb2 = PtxVbx(2/3xVp - Vb)/Ra. The energy from the alternator during that pulse is Ea2 = PtxVpx(2/3xVp - Vb)Ra. Since both pulses now contribute, we must add their contributions to get the total energy into the battery, which gives:

Eb = PtxVbx(5/3xVp - 2Vb)/Ra. Ea = PtxVpx(5/3 - 2Vb)/Ra.

This applies from 150 rpm to 300 rpm.

Finally, when the alternator's speed hits 300 rpm, the second break point, the third pulse will be large enough to cause the diodes to conduct. This pulse has an amplitude of 1/3 that of Vp. We now get a current of Ib3 = (1/3xVp - Vb)/Ra, for an energy value of Eb3 = PtxVbx(1/3xVp - Vb)/Ra. Ea3 = PtxVpx(1/3xVp - Vb)/Ra. The other two pulses are also contributing so we need to add this to the contributions from the others. We get: Eb = PtxVbx(2xVp - 3xVb)/Ra. Ea = PtxVpx(2xVp - 3xVb)/Ra. You get the average power by adding up the energy contributions from all the pulses per cycle (there are two of each) and dividing by the time in seconds per cycle. These equations show that the rate at which the alternator puts power into the battery as a function of rpm is increased by 2/3 after the first break point and is twice as great after the second break point as its initial value. This is the basis for the power curve that I gave.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#30)
by SmoggyTurnip on Fri Apr 28th, 2006 at 10:57:58 AM MST
(User Info)

OK now I get it.

I'm not sure why I didn't get it before,
probably because I'm not used to thinking
about batteries - strictly heating for me
so far.

It is not easy to analyze, but interesting
to think about.
The sooner you start the longer it takes.
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#33)
by finnsawyer on Wed Jan 10th, 2007 at 09:22:13 AM MST
(User Info)

There is a relatively simple way to determine rotor diameter assuming you know your magnet size and coil size. Assume we want to make a 12 magnet 18 coil version. The magnets will be spaced every 30 degrees around the rotor and the coils every 20 degrees. Place the zero degree line at 12 0'clock on a sheet of paper or piece of cardboard. A coil will be centered directly over a magnet on this line. The next coil will be at 20 degrees and the next magnet at 30 degrees. Draw radial lines at these two angles. Place a coil or cut out of a coil centered along the 20 degree line and a magnet or cut out of it centered along the thirty degree line. Move them radially along the lines. When the coil just touches the magnet with both the same distance from the center you will have determined the proper size of the rotor. If the rotor size would be too large go to a smaller coil. Note that I assume the center hole of the coil is the same size as the magnet.
GeoM

Re: An Alternate PM Alternator Design (3.00 / 0) (#34)
by Lumberjack on Sun May 20th, 2007 at 11:52:16 AM MST
(User Info)

now that I have digested most of this:

We now have six coils spaced every 60 degrees and four magnets spaced every 90 degrees. So, the coil to magnet ratio is always going to be 3:2 for this type of alternator. Note the peculiar geometry. The magnets are wedge shaped with an angle of MA degrees. Coil two ends at 90 - MA/2 degrees. So, coil one ends at 30 - MA/2 degrees, since it's 60 degrees behind coil two. Coil one starts at a negative angle. We define an angle WA for the angle subtended by the width of the wire. Then we can write that coil one begins at -(WA + MA/2). So, the total width of coil one is then 30 + WA. The inside dimension of the coils are then given by 30 - WA. This is the maximum size magnet that can be used given WA. In reality you would pick the size of the magnet, the width of the wire or number of turns and then size the rotor to fit the angular requirement.

I am not sure what you really meant to say here. Perhaps you could clarify this for us?

As mentioned, the three phases would be connected in series to get the output. If we connect the a and b windings of each phase together first we can do some electronic alchemy. Say phase two is in the middle. We can bring out a wire from between a and b of phase two and make it common (ground or G). This is similar to a center tapped transformer winding. What does this get us? We now need only two diodes and we cut the resistance in half. We do not cut the voltage in half. The waveforms are shown below.

If I understand you correctly you want the coils wired like this:
- D -1a-1b-2a - G -2b-3a-3b- D -
D= diode , G = ground

Problem, a single normal diode only conducts in one direction so you will only get positive or negative voltage out, the reverse voltage is blocked and the output is zero.

Ignoring the diode problem, this is just a normal multi-coil single phase generator with an odd center tap. The normal output with a full wave bridge would be 6 (4x) pulses. If the system voltage is (1x) then 4 of the pulses are wasted because they would be under system voltage. As an average, only (2x) of usable voltage is generated per pulse. In the normal non tapped the average value would be (3x) per pulse. As your system voltage rises the situation worsens. At (2x) you would only get 2 usable pulses of (1x). There has to be a clearer way to say this but I have had to much coffee.....

You stated elsewhere that you wanted a winding that could be used for 6 or 12 volts. The simple solution is to wind each coil 2 in hand. For 6 volts run them in parallel and for a 12 volt application wire them in series. If you wound 4 in hand the stator could be wired for 6,12 or 48 volts.

I have noticed you tend to use peak voltage when you really mean RMS voltage. Peak voltage is semi-constant whenever the coil reaches saturation while RMS voltage varies with the speed due to frequency and pulse width changes.

The term ground tends to imply a connection to earth or chassis and 0 volts. Neutral implies no connection to a chassis or earth and the voltage is not always zero.

Re: An Alternate PM Alternator Design (3.00 / 0) (#35)
by finnsawyer on Mon May 21st, 2007 at 10:05:01 AM MST
(User Info)

You need not shout. "" will do. I've probably discussed all of this here or there. What I usually do in comparing this with the standard three phase arrangement is to assume one inch diameter magnets and two inch diameter coils with twelve magnets (the three phase is restricted to numbers of magnets divisible by four, this design can use any even number of magnets). The magnets alternate poles and are spaced every 30 degrees. The 18 coils are spaced every 20 degrees. The coils are arranged in three groups of six. When the coils of group one are centered exactly over the magnets having north poles, the coils of group two (and three also) will just be touching magnets having south poles. This determines the geometry as well as the size of the rotor and the stator. I'm going to rotate the coils here instead of the magnets, as I think it's easier to see the action, although the result would be the same. As the coils rotate (in either direction), the coils of group one are moving off of a north pole creating a negative pulse (arbitrary choice). At the same time the next group of coils advancing in the direction of rotation (call it group B) is moving unto a negative pole, also producing a negative pulse. The last group (C) is moving between magnets producing no voltage. Carrying on, group B now moves off of a south pole producing a positive pulse, while group C is moving on to a north pole producing a positive pulse. Group one, (or A if you wish) is now between magnets producing no voltage. At this point we have produced one complete cycle of the output waveform in moving one eighteen of a revolution. So, we get eighteen cycles per revolution.

I'm not going to discuss the center tapped version at this time. It has different characteristics, which may be useful, but I think you should completely understand the basic design first.

The output of this alternator will be single phase, as all the coils are connected in series. As far as winding the coils is concerned, for this exercise the total amount of copper per coil remains constant. If you wind the coils with the same wire as used in the three phase but two in hand, you will end up with the voltage (of my design) cut in half and the resistance reduced by a factor of four (which results in 3/4 of the resistance of the three phase, for instance). Both voltage and resistance are important.

"I have noticed you tend to use peak voltage when you really mean RMS voltage. Peak voltage is semi-constant whenever the coil reaches saturation while RMS voltage varies with the speed due to frequency and pulse width changes."

I don't mean RMS or Root Mean Square voltage. It is not a very good measure for an alternator who's output is not a pure sine wave. Your RMS meters will give an erroneous reading. What you are missing is that for a given alternator the ratio of the peak voltage to the "average voltage" stays constant with RPM. When charging a battery peak voltage not average voltage gets you there first. I explained that in the other thread. Coils never reach "saturation". They can't. They do not contain any iron. Nor are the speeds reached by these alternators high enough for hysteresis or inductive effects to become important. Keep in mind that the iron parts are not in saturation to begin with.

"The term ground tends to imply a connection to earth or chassis and 0 volts. Neutral implies no connection to a chassis or earth and the voltage is not always zero."

Ah, semantics, semantics. The neutral of the 220 volt supply to my house is definitely connected to ground (Earth). On the other hand the center tap of a transformer could be connected to the Datum Node (ever hear of that) or ground of a particular circuit, which you could connect any way you see fit. Many tools today are not grounded. The alternators we are discussing here are by their nature floating and can be "grounded" in any way you see fit. Many circuits will show a circuit ground, the connection of which is optional or up to the user. So, take the "ground" that I mention and connect it to your chassis, or the neutral of your house or the Earth, or leave it floating. I don't care. The sense that I was using "ground" was a common point about which the output voltage is defined. That is, the circuit ground or datum node.

GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#36)
by jimovonz (username at matata dot co dot nz) on Mon May 21st, 2007 at 01:40:27 PM MST
(User Info)

GeoM, you have mentioned a few times recently about RMS voltage measurements and how they relate only to sine waves and not to our typical alternators. This is not my understanding. I believe RMS measurements can apply (and have meaning) to any wave form. Cheaper multimeters expect to see a sine wave and approximate an RMS measurement based on this assumption (0.707 x Vpeak for instance). Meters that are 'True RMS' calculate the RMS value based on the actual waveform measured. I believe that the RMS value gives the 'equivalent DC heating power' i.e. a 10Vrms waveform will dissipate the same heat across a given resistor as 10Vdc. I often see large variations in reading between my cheap meters and those that are 'True RMS' I only ever give any credit to the 'True RMS' meters when such discrepancies arise.

[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#37)
by Lumberjack on Mon May 21st, 2007 at 10:06:14 PM MST
(User Info)

Didnt mean to shout....

I will go through your design again but I dont see anything new in it.Looks fairly close to the normal setup used around here. ( or was that the point?)

On RMS:
As you said RMS really only occurs in true sine waves but since most meters are built to read RMS and most here only have this type of meter it becomes an issue for most of us. This is a differance between theory and the real world. Yes the reading we get is incorrect but it is still refered to as RMS and not peak.

"Coils never reach "saturation". They can't. They do not contain any iron."

Here is a point where you are dead wrong. Any object that is passed through a magnetic field builds up flux to a maximum amount. This is a saturation point. Iron has nothing to do with it. Even a wood block undergoes this process and becomes saturated. All it takes is time spent inside a magnetic field.

"Nor are the speeds reached by these alternators high enough for hysteresis or inductive effects to become important."

These effect are governed by the "rate of change". Two magnets of opposite poles butted against each other create a very steep rate of change. You should also realise that a 60 hertz square wave or sawtooth wave can create harmonics well into the 100 khz range.

The semantics you dont care about killed a co-worker two years ago....All it takes is .1 amp at 5 volts to stop a human heart. A lot of the generators here are 3 phase and the neutrals are not always grounded.

Datum node ... nope never heard of it outside of programming.

[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#38)
by finnsawyer on Wed May 23rd, 2007 at 09:26:16 AM MST
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"Here is a point where you are dead wrong. Any object that is passed through a magnetic field builds up flux to a maximum amount."

You're showing your lack of understanding of the physics or more precisely Faraday's Law, which states that the voltage induced in a loop of wire is equal to the time rate of change of the magnetic flux. While the flux may reach the same maximum value, which does not constitute saturation, as increasing the magnet's strength will result in more flux, the case where the coil is moving faster will induce a larger voltage or a voltage pulse having a greater peak voltage. If you consider the standard 12 magnet 9 coil three phase design, you find that the a coil will move 30 degrees in creating a single pulse (from magnet center to following opposite polarity magnet center). With my proposed design Two groups of coils act together. When the group one coils are centered over a magnet at zero degrees, the centers of the next group of coils are advanced 20 degrees, since there are eighteen coils. The coils of group two are just touching the next opposite pole group of magnets, the centers of which are advanced 30 degrees from zero. As the group one coils move off of their magnets the second group coils move onto magnets having an opposite pole. Their voltages add. They must move only 10 degrees to produce the pulse. If they move the ten degrees in the same time as the three phase coils move thirty degrees, the combined voltage of a coil from each of the two groups will create an identical pulse of voltage as a single coil of the three phase. If the alternators have the same RPM, then each coil pair of my design will have a peak voltage three time that of the single coil of the three phase as they are moving three times as fast. There will be six such pairs acting at any time, so we could write the peak voltage as 18V. In the case of the three phase alternator there are three coils per phase. So each phase will put out a peak voltage of 3V. In the star configuration two phases are active, but are 120 degrees out of phase, taking into account the resulting 1.73 factor we find the peak voltage will be 5.19V. So, we find that my design will have a peak voltage 3.47 times as great as any lead pair of the star connected three phase. It also will have a resistance tree times as great for identical coils (18 coils in series versus six). When connecting the alternators to matched resistances, you find that my design can output 4 times the power for the same number and size of magnets. That said, this is a theoretical result. Other factors, such as rectification and application will affect the result.

A couple of points about harmonics: Even for a square wave the energy in the harmonics dies out fairly rapidly. The coils do have some inductance, so the harmonics are suppressed fairly quickly with frequency. That is, the waveform will not deviate too much from a sine wave.

Saturation implies a maximum physical limit beyond which one can not go. Iron has a magnetic saturation point. Air, copper, or free space does not. If you can create it, you can ram as much magnetic flux through a coil as you please. There is no saturation point. You are misusing the term to refer to the maximum value of flux that passes through the coil for a given construction, which depends on a number of factors that can be changed.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#39)
by Lumberjack on Wed May 23rd, 2007 at 07:01:10 PM MST
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You are right in that I was mis-using the term saturation however, given that the rotor / stator combination in use cannot be changed while in use my point is still valid. There is a maximum amount of flux that can be created at a given speed and once that it is reached the coil will stop producing. I can not find any term for that point and have always heard the term saturation used. Perhaps you might know a correct term for it?

[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#40)
by finnsawyer on Thu May 24th, 2007 at 08:47:00 AM MST
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Simply refer to it as the value of magnetic flux for that alternator design or produced by the given magnets. Or use the time rate of change of the flux through the coils, which is what really gives the voltage. This is a variable quantity, as it depends for the given design on rate of rotation. Sometime there is no simple all encompassing word. But we do need to try to avoid confusing people.

GeoM
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Re: An Alternate PM Alternator Design (3.00 / 0) (#41)
by bigkahoonaa on Fri Jul 20th, 2007 at 09:45:05 AM MST
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Don't see anything wrong in experimenting with coil design. I had some extra mag wire and tried something like this:

One massive coil with 6 magnets on either side. From my limited understanding of magnetic flux, etc., current is only generated in that part of the coil that is perpendicular to both movement of the magnets and direction of the magnetic field (i.e. those parts of the coil going to the center of your stator). I never tried it with magnets. I think using a big coil like this one may give starting problems and could cog.

Hope you try your idea some day.

Re: An Alternate PM Alternator Design (3.00 / 0) (#43)
by finnsawyer on Tue Oct 30th, 2007 at 08:37:23 AM MST
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Even though I check the diary from time to time I missed this one. Sorry about that. I'm not sure what to make from your statements except to suggest you consider the case of a transformer. One of the coils takes the place of the magnet and it does not move. Nevertheless it does create a time varying magnetic flux through the other coil(s). So, it is the change of flux through the coil with time and not the movement of the magnet per se that is important. Anyway, your scheme is different than what I propose, as my coils still lie above the magnets in the same orientation as the usual three phase. That is, the planes of the turns of the coils are parallel to the faces of the magnets.
GeoM
[ Parent ]

Re: An Alternate PM Alternator Design (3.00 / 0) (#42)
by finnsawyer on Sun Oct 28th, 2007 at 09:17:35 AM MST
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There are different ways to look at a situation. Lately, I've seen a different way of analyzing this alternator design that might help people understand better how Faraday's Law fits it. As usual, I assume 12 magnets 9 coils for the three phase standard, and identical 12 magnets with identical 18 coils for the design presented here. When a magnet, say a north pole is directly centered over a coil the flux through the coil is assumed the same for both cases and given the value of F. In the three phase case as the north pole moves off of the coil and a south pole replaces it the change in flux has a magnitude of 2xF. This occurs in the time it takes the rotor to move 30 degrees, which I will call t. So, from Faraday's Law we can write the average voltage V3 induced in the coil as V3 = 2xF/t.

For the design presented here a magnet moves off of or onto a coil resulting in a flux change of magnitude F. The magnets are spaced every 30 degrees, as before, but the coils are now spaced every 20 degrees. Furthermore, the geometry is specified to be such that a magnet will move off a coil or onto a coil in a rotation of the rotor of 10 degrees. So, it takes one third the time for the flux to change by F. We may then write the average voltage Vm induced in the coil as Vm = F/(t/3) = 3xF/t. Doing a little trivial manipulation we find that Vm/V3 = 3/2 = 1.5. That is, Vm = 1.5xV3.
We also recognize that there is essentially no difference in the shape of the pulse of voltage generated when a magnet moves off of a coils as when it moves onto a coil, although the signs differ. A north pole moving off of a coil will generate a negative pulse, but so will a south moving onto a coil. Consequently, we can write the total average voltage generated per pulse as 12xVm = 18xV3.

In the case of the three phase alternator we have 3 coils per phase, so the voltage per phase will be 3xV3. Since two phases almost add in the star configuration, we can write the voltage out from the three phase alternator as 1.73x3xV3 = 5.19xV3.

So, we find that the average voltage out from this proposed design will be 3.47 times as great as from the standard three phase. What would this mean in practice? Well, let's assume that cut-in for the three phase occurs at exactly 12 volts due to the battery being low. That is, the battery voltage plus the diode forward voltage drops equals 12 volts. Furthermore, the three phase alternator is putting out 18 volts. Finally, for simplicity the individual coil resistances are 1 ohm. So, from the three phase we have I3 = (18-12)/6 = 1 amp. In the case of this design we would have Im = (62.4 - 12)/18 = 2.8 amp, which translates into 2.8 times as much power into the battery. I've been criticized because of the amount of copper needed, but you see that copper or resistance doesn't tell the whole story.

We have other options. We could wind the coils two in hand instead of with a single wire. Doing so cuts the voltage in half, but it also cuts the resistance by a factor of four, with two half as long strands in parallel. We then would find that we get the following current: Im = (31.2-12)/4.5 = 4.3 amp, even more power into the battery! Or we could do three in hand for one third the voltage and one ninth the resistance to get: Im = (20.8-12)/2 = 4.4 amp. Well, there is obviously a limit to this. Be aware also that you are also affecting cut-in for the alternator by making these changes.

GeoM

Re: An Alternate PM Alternator Design (3.00 / 0) (#48)
by 1968jeepman (neilmrogers@yahoo.com) on Sat Mar 29th, 2008 at 10:20:02 AM MST
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I read through all o