3 stage brushless alternator.

[BeenzMeenzWind]

This is more or less a reposting of a reply I made to someone regarding biodiesel powered generators and frequency control. I apologise for the cut/paste, it was just for the sake of laziness.

He was asking how you could control frequency on a generator in order to run domestic appliances without the need for batteries and a bloody huge inverter.

There is a battery-less, permanent magnet alternator that is controllable by a regulator. I suspect people don't discuss them because they sound too complicated. They aren't.

What you might need to do, if you can't source a commercial item at anything like an affordable price, is build (buy??) a three stage brushless alternator. There are two reasons for this. Firstly, you can electronically regulate the output current according to load by varying the field current of the alternator instead of the rpm of the motor. Secondly, you can use the same controller to throttle the diesel as it's rpm begins to drop off under a heavier load. If you use a decent actuator on the throttle quadrant, it becomes possible to keep the engine rpms very close to optimal for frequency control.

For those that don't know, a 3-stage brushless alternator is a simple and incredibly reliable piece of kit used on most modern military aircraft. I did a stint in the RAF (as a civilian technician), so I have some experience of these generators.

It works thus: The first stage is a disk of permanent magnets, attached to the rotor shaft. It's field is picked up by a conventional stator and the output is passed to a regulator which electronically varies both the field (stator) current of the second stage and adjusts the degree of mechanical drive to maintain frequency. On aircraft this is done by means of a variable clutch, as you obviously need to throttle your jet engines to power the aircraft as well as drive the generators. A simpler solution where the engine drives only the alternators is to use that control line to alter the throttle slightly to maintain a constant rotational speed and hence frequency.

The second stage consists of a stator which provides field current, as determined by your regulator, and a winding on the rotor shaft which generates power from that field. The output is rectified by diodes built into the rotor itself (the ones from large car alternators are easily upto the job mechanically as they're usually over-engineered. If they're secured properly, they'll effortlessly cope with the rotational forces because they've been designed to cope with under-hood temperatures and vibrational loads.)

The output from these rectifiers is the field (rotor) current for the final stage. The potentially very large, and controllable, field current induces an even larger current in the 3rd stage stator and this is taken off to power your load.

Just to give you a point of reference for their performance. The Harrington 2.7 KVA 240v alternator on my Honda genny is physically bigger than the 22 KVA 3 stage unit on a Tornado fighter. Of course, the Tornado uses a 400 Hz, 3-phase , 115v output but that's more a matter of design requirements than anything else. Building a single phase 60 Hz machine is just as easy, though your size may increase slightly. Single phase is less efficient and generator efficiency also increases with frequency. (Aircraft commonly use 400 Hz because it's easier to rectify and smooth with smaller, lighter components, which is a factor in military aircraft.)

The regulator pack's electronics are fairly simple. I may even have a diagram of it somewhere in my loft. If not I, or certainly someone on the board, could come up with a design pretty easily. All it has to do is monitor the output voltage and adjust 2nd stage field current as voltage changes due to load. We messed about with a Tornado one on the bench during training, attached to a 4.2 litre diesel, and the output was stable within about 1.8v no matter what load we applied.

The frequency control side is easy too. Monitor the rpms and adjust the throttle to keep them steady. I'd get a crank position disc from a Ford EDIS ignition system, along with the sensor, and do it digitally, then use a stepper motor to control throttle opening. That's only cos it'd be more fun to build though. You could the same job in a dozen simpler ways. Ask some of the board's electronics experts for ideas.

As an aside, I'm fairly sure the regulator pack on the Tornado uses the aircraft batteries as an off-load dump for the permanent magnet stage, killing two birds with one stone. You could run all your 12v lights off the batteries then, saving some mains load, or buy a cheapo inverter and run all your low power devices off them.

I don't see any reason why someone couldn't build one of these things by expanding the brake disc axial flux alternator idea commonly used here and elsewhere. If you built a nice little one, with a few hundred watts output, you should have more than enough power available for 2nd stage stator current and probably to charge a few batteries most of the time as well. Use a single phase washing machine stator for the second stage (might not be big enough, have to experiment) and build the second stage rotor windings to match it's dimensions. Wire in and secure your rectifiers and wind a nice chunky 3rd stage to cope with the power. Don't forget, you are using the rotational velocity to generate/ magnify the power 3 different times, so it can really get up there by the time you get to the output terminals.

I'm going to build one of these babies myself, when I have the time, so I'll let everyone know how well it works. Any deficiencies will be in my skills, not in the principle. Only problem might be in getting a diesel with enough 'bant' to run the beast.

I'm not sure whether a similar design could be used for wind turbines or micro-hydro. The regulator pack would need to be differently designed, so as not to stall the rotor in low wind (or light water flow) conditions. For wind you could probably use an anemometer to tell the controller what level of 2nd stage stator current to generate. That would have to be referenced to the demand from the batteries, too. I imagine it would only be worth the time and trouble for a big turbine. I think someone with more experience of wind power would be able to work the practicalities out, though?

I invite discussion....

[picmacmillan]

  i will toss in a couple of questions/ comments...what we do here is utilize the force in the winds to give us angular velocity to charge basicly charge batteries...we increase the blade diameter or wind swept area, and the power the genny can make increases exponentially...

  anything i have ever read states we lose power at every conceivable level of mechanics, gearing, what have you, so we are struggling to maintain reasonable power even at wind swept areas of 17' diameter...i would think that our issue is the absence of too much power in the wind(or what we can collect of it),, for a good part of the charging period, so i wonder how we can utilize more electronics, regulators or whatever that may bring our power down more than it does already with resistance etc?

   so if we can only reasonably get say 250 rpms out of a machine at 15 mile an hour winds, and we need about 140 rpm's just to get it to charging voltage, i cant see how we can curb more of the energy created by the genny, as it is limited in its abilities.this is of course in  moderated to low wind conditions(which most of us have)...just my humble opinion, and i would like to here more of what you think?..never know when a good idea is going to come around the corner..i hope you have something there that  will work...give it a try and let us know how it did?...good luck on your project  pickster

[BeenzMeenzWind]

I was primarily aiming the post at those of us who use backup generators because I agree with your arguments. The only real application I can see for wind power is the ability to use the 1st stage as a standard PM generator in light wind conditions by not supplying any field current to the other stages. I was just inviting comment because I've never really seen a discussion of these devices. You never know, someone might think of something that really makes it worth the time.

I do actually subscribe to the KISS philosophy when it comes to something that I depend on to produce reliable power. From my experience of electronically regulated brushless alternators, they will produce very reliable power but at the expense of efficiency, since each stage is only about 85-90% efficient (and there are effectively FOUR, as you have to factor in the electronics) and you DO need a lot of rotational force to turn them under heavy load. Where they probably win is in very strong wind conditions, because they do have the potential to carry on producing power where other systems have hit their limits. You're right when you say that they are almost uselessly inefficient when the wind is light. I've been to a few places where the wind is almost never light, though, and wondered if these things might be useful there?

The only other plus point I can see is: They would most likely suffer less from gyroscopic forces and effects from peripheral speed due to the relatively small diameter of the rotor, though the mass would still be a factor. I've carried them about and they are still pretty heavy.

I realise that the technology is probably never going to be efficient at the single-home scale but I can see applications for larger wind turbines to help supply small remote communities, due to the reliability and controllability (is that how you spell it?) of the system.

The mechanical requirements of the tower and rotor would most likely be a matter for composite structures, too, as they would have to be bigger than they really need to be in order to develop useful power in light wind conditions. Or at least very heavy over-engineering.

AND NOW FOR THE APOLOGY:

I was just pointing out that electronically controlled brushless alternators offer a way to have reasonably good frequency control at the same time as accurate voltage control when used with IC engines. There was a query about frequency control and they're a way to do it. Not the only way, I know.

I'm not trying to be a smart arse! I'm sorry if my writing comes off that way. I don't know enough about renewable energy at present to BE a smart arse. I've spent a fair percentage of my adult life writing reports to people that don't understand anything you're talking about anyway and attempting to get them to sign budget allocations, so I probably do sound that way. If so, I'm sorry. I wasn't trying to sound like an authority, just clearly explain something I know about and ask if it's any use to anyone.

I was just trying to give an example of something outside the box.

When we build our out-of-towner, I'm obviously going to use the technology that's available and reliable and I'll bow down and praise the Lord for this community. We can run our home with solar, wind and a bit of bio-energy backup. There's nothing wrong with these systems. They work.

Anyway, I'll build one when I get the time and see what happens. It'll be fun to do, even if it's rubbish. lol

[picmacmillan]

i find that every day we learn a little bit on here and your post i find very intreguing ,and has some validity, i am all ears when i get to hear someone elses view, i think that if we do like you suggest it gives us an opprtunity to try some thing a little bit different and i hope we can make something like this be a viable source...shoot, you never know when an idea of someones will spark a breakthrough and elivate our level of thinking...i enjoyed your post and it got me thinking...i dont feel we have reached our potential on here, and evry year it seems as a unit, we are a little more successful....i would never make fun of anyone for having an idea...i only know what others have taught me, and i am grateful for that...i feel like an infant on here sometimes, but as long as i ask, someone always helps me through...good luck on your project  there beenz .you will make a fine genny i am sure..this is a lot of fun to take on, and very challenging... pickster