Author Topic: Automotive alternator to PM alternator  (Read 34806 times)

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rustkolector

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Re: Automotive alternator to PM alternator - another one
« Reply #66 on: March 22, 2017, 03:05:20 PM »
I am building another one of these generators and would like to make improvements where I can. The first rotor was built on a solid steel core hollowed out a bit to reduce weight. The rotor is in direct contact with the tool steel shaft. I believe it has some torque drag in addition to the cogging. The torque I am referring to feels like a shorted coil, but not as severe. I am wondering if this could be Eddy current drag. It is more noticeable than the cogging at this point. Would a different rotor construction be advisable, or is this drag normal?

Jeff

joestue

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Re: Automotive alternator to PM alternator
« Reply #67 on: March 22, 2017, 11:03:27 PM »
i would expect to feel significant eddy current drag from an alternator core, i think that's what you were using.

look for a motor typically used to drive the fan in a heat pump. if you're in the usa i could send you this one http://imgur.com/5iztOOb after pulling just the stator stampings out of the sleeve. i have another motor of the same size so i could give you its stack and that would make about  3 inch deep core. i rewound the one in the photo btw just to see if it was reasonable for me to save the motor or melt the copper down. they pulled some shenannigans to fit an 8 pole motor in a 36 slot stamping, you might be able to tell the coil i fitted in there fits in 2 slots on one side and three on the other.

the motor in question is a 5kcp39kf but the ebay listings for that part number don't match what i have exactly. if you drop the last two letters off that part number you'll find a lot of other motors that are very similar.

anyhow the second problem is if your magnets are oversized you will saturate the core and the hysteresis and eddy current drag will be noticable. but if you're using the thick stator laminations from a car alternator, that may be half your problem.

you don't have near enough rpm to worry about what the magnets are made out of, or what's under them btw.


however if the core is saturated you may be able to measure the difference between pressing the stator core into an aluminum housing, or a mild steel one, or open air. some of those older motors had the stator stampings open to the outside air, primarily for cooling, so you may be able to more closely match the original look by bolting the end bells through the motor lamination stack with mere bolts, rather than a tubular shell that covers the entire assembly.

most car alternators did have exposed laminations, but the newer high output ones press fit the stator stack into the aluminum housing so that the aluminum can conduct the heat away from the stator!

if the stator is saturated due to the neodymium magnets you should get the least drag if the stator is not pressed into anything. a compromise would be a non magnetic stainless sleeve. it won't conduct the flux and its high resistance won't generate as much eddy current as most every other material will.
« Last Edit: March 22, 2017, 11:14:24 PM by joestue »
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rustkolector

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Re: Automotive alternator to PM alternator
« Reply #68 on: March 23, 2017, 01:51:05 PM »
Joestrue,
This is a scale model project so the small size of the stator is necessary. I am testing performance using the original alternator automotive housing as I did in the first project. The stator is 3.66" OD with a .456" stator lamination thickness. No room left for drilling through the laminations for bolts. The rotor gap was increased to .01" to improve cogging without much loss in voltage. The wave winding coil center width is .625" measured at the slots and the N45 magnets are 1.0" long x .50" wide x .25" thick so about an 80% fill. However, judging from the slight voltage loss when increasing the gap, I would say the stator is still saturated. The stator is a loose slip fit into a .25" thick aluminum generator frame.

Changing to 304 SS for the stator shell would add weight and cost, but if it would significantly reduce the eddy current drag I am feeling with the aluminum frame, it might be worth it. Smaller, or weaker magnets would further reduce voltage at the design speed of 500 RPM. I can afford about a 20% drop in voltage, but actually the voltage is ideal as it is now?

It there anything that can be done with the rotor design or material to lessen this drag?

Jeff

joestue

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Re: Automotive alternator to PM alternator
« Reply #69 on: March 24, 2017, 01:09:59 AM »
The stator is 3.66" OD with a .456" stator lamination thickness. No room left for drilling through the laminations for bolts. The rotor gap was increased to .01" to improve cogging without much loss in voltage. The wave winding coil center width is .625" measured at the slots and the N45 magnets are 1.0" long x .50" wide x .25" thick so about an 80% fill. However, judging from the slight voltage loss when increasing the gap, I would say the stator is still saturated. The stator is a loose slip fit into a .25" thick aluminum generator frame.

Changing to 304 SS for the stator shell would add weight and cost, but if it would significantly reduce the eddy current drag I am feeling with the aluminum frame, it might be worth it. Smaller, or weaker magnets would further reduce voltage at the design speed of 500 RPM. I can afford about a 20% drop in voltage, but actually the voltage is ideal as it is now?

It there anything that can be done with the rotor design or material to lessen this drag?

Jeff

did you litterally say the stator is .45" thick? if so that's a solid block of iron. usually stators are stamped or laser cut from sheet metal. Even .045" is far too thick.

your observations about the slight voltage reduction from increasing the air gap are correct, its saturated. but its hard for me to estimate what reasonable drag (eddy current or hysteresis or whatever) is without some real numbers.

I have a 1/2 hp motor that is fitted into a 4" diameter brass sleeve. we call these well pump motors, which is what it was.  the stator outside diameter is probably 3.85 inches. the lamination stack is about 5 inches long so the whole motor fits in a 4 inch outside diameter brass cylinder that could be shortened to 8 inches long. the entire motor section of the pump is over a foot long but my point being these small motors are hard to come by.

I also have some 90 watt induction motors that might have less than a 4 inch outer diameter but i'm not sure. they are 4 pole, 1750 rpm, the stator is pressed into an aluminum finned housing that is about 4.5 inch diameter, that is about 6 inches long. i was hoping they would be rated for more than 90 watts when i got them, (that 90 watt rating is for the worm gear drive box they are mated with) but, looking inside the 90 watt rating was appropriate. these dimensions are approximate  btw, i would have to go find them in my stack of stuff and measure them again.


but i have run into a few other people who have had great difficulty sourcing 1/4th hp 3 phase motors. they should be about 4 inch diameter and 6 inches long, but it seems no one makes them.
« Last Edit: March 24, 2017, 01:20:56 AM by joestue »
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rustkolector

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Re: Automotive alternator to PM alternator
« Reply #70 on: January 27, 2018, 01:30:09 PM »
Ok, it has been quite a while since I last posted. I have finished the duplicate PM alternator and photos of the finished model gas engine alternator are below. In my previous post I had mentioned that when I rotated the alternator shaft I could feel drag, like there was a load on the winding. The apparent drag seemed to increase when I tried to spin it faster. However, I am now confused at my findings. When the alternator is powered by the model gas engine it responds accordingly as small increments of resistive load are loaded. If I remove the electrical load, the engine runs at an unloaded idle RPM. I then disconnected the generator coupling between the engine and generator and started the engine and found it still ran at the same idle RPM as when connected to the unloaded alternator. If this is true, the drag I am feeling when I spin (accelerate) the shaft by hand is not felt by the engine running at a constant speed. The earlier PM alternator and engine exhibited the same thing. Can anyone explain this?

Also, I have a previous model that has a PM alternator generating 100-120vac but at a frequency in the range of 120 hz. I would like put the output through a switchmode power supply to give a regulated 12vdc output. These power supplies are rated at 120-240v and 50 or 60 hz. Would my higher input frequency still allow the SMPS to work satisfactorily?

The new model is the same size as the last one but with a different engine crankshaft configuration that duplicates the exhaust cadence of the old 2 cylinder John Deere tractor engines. The prototype engine design was introduced in 1896 by the Natioinal Meter Co.
Jeff

https://www.youtube.com/watch?v=P4krjtV8R18


SparWeb

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Re: Automotive alternator to PM alternator
« Reply #71 on: January 27, 2018, 02:47:46 PM »
Quote
...I then disconnected the generator coupling between the engine and generator and started the engine and found it still ran at the same idle RPM as when connected to the unloaded alternator.

It's possible that the engine is generating much more power than the drag in the alternator.  Can you take some specific measurements?  RPM/torque? 
I can only guess, but if the engine idles at 500 RPM then 1 foot-pound torque is just 80 Watts, or 0.1 HP.  If internal losses in the motor require (guessing again) about 1HP to maintain idle speed then adding/removing the alternator may not make a noticeable difference.  If you rig up a tachometer you might be able to measure it.

Also bear in mind that there are always losses in any motor (that's why they generate heat) and a 5HP rated motor may be generating 8 HP at its rated speed but wasting 3 HP overcoming friction, running the cams, etc.
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SparWeb

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Re: Automotive alternator to PM alternator
« Reply #72 on: January 27, 2018, 02:48:36 PM »
rustkolector,
I can't see any rust on it.
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
System spec: 135w BP multicrystalline panels, Xantrex C40, DIY 10ft (3m) diameter wind turbine, Tri-Star TS60, 800AH x 24V AGM Battery, Xantrex SW4024
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joestue

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Re: Automotive alternator to PM alternator
« Reply #73 on: January 27, 2018, 05:25:26 PM »
a shorted turn in a high frequency alternator will feel as drag at low rpms, but at high rpm it will not present much drag. the drag will feel like magnetic cogging, but a 3 phase resistive load will provide very little torque ripple and the drag will feel smooth.

anyhow most power factor corrected switch mode power supplies, and for example, most cellphone chargers as well, if they say something like 100-240vac 50/60hz, then they will probably work on just about anything from 60vac to 270vac, and the frequency doesn't matter, though higher is better, particularly at the lower voltage end of the limit.
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hiker

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Re: Automotive alternator to PM alternator
« Reply #74 on: January 27, 2018, 09:51:53 PM »
It's just the metal in the stator..as the mags on the rotor rotate...
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rustkolector

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Re: Automotive alternator to PM alternator
« Reply #75 on: January 27, 2018, 11:12:42 PM »
SparWeb
This is a tiny engine constant speed engine operating at 600 RPM with 1" dia. pistons. A 2.5 watt load is noticeable on the photo tach and also audibly. There is relatively no change in speed with the generator disconnected or connected. I don't understand it, but it works.

Joestue
I had the two 3 phase alternator stators rewound by a rewind company. Cheaper than me setting up to do it. Both rewinds showed exactly the same resistances and no load voltages at 500 RPM. I think they are good, or good enough. The cogging was reduced significantly by using the off-set method, but not totally eliminated.

It sounds like the SMPS should work well on the other project. That's a big help. How can I tell if a specific SMPS is PF corrected? Thanks.
Jeff

SparWeb

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Re: Automotive alternator to PM alternator
« Reply #76 on: January 28, 2018, 12:42:56 AM »
Oh I missed the "1/8 scale" part, didn't I?
No one believes the theory except the one who developed it. Everyone believes the experiment except the one who ran it.
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joestue

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Re: Automotive alternator to PM alternator
« Reply #77 on: January 28, 2018, 03:40:57 PM »
It sounds like the SMPS should work well on the other project. That's a big help. How can I tell if a specific SMPS is PF corrected?

most of them advertize it. cellphone chargers are not power factor corrected, however they are designed to run from a wide range so they can be sold in different markets. cellphone chargers usually use a flyback topology which means it will work as low as say 60vac, but it won't be able to deliver 5 volts at 1 amp. maybe half an amp instead.  the flyback is similar to the boost dc-dc converter, which is why it can handle the wide range of voltage. often times the switch is rated for 600 volts, which means theoretically you could run a cellphone charger as high as 425 vac. (if you were to replace the 400v capacitor with a 600v capacitor and change the resistor that feeds the brains of the supply, so the resistor doesn't burn up)

In my experience most power factor corrected supplies say 100-240v on the nameplate. some say 100-265vac, or 120-265vac . non power factor corrected supplies will usually say 120/240v and there will be a switch to select 120 or 240vac.

at the low end of the input voltage range, and or at the low end of the frequency range, the supply will not be able to deliver full rated load. higher input frequency may partially compensate for lower input voltage.
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