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Induction Vs Permanent Magnet Alternators

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riverman

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Hi,

I'm toying with the idea of making/adapting an automotive-type (but smaller) alternator for use on a bicycle to charge a small SLA battery or NIMH battery bank for running lighting and other accessories which may include voltage sensitive circuitry. The idea is based on the assumption that it would provide superior voltage regulation through the ability to vary the excitation, but I am interested in member's opinions on other considerations and the pros & cons of this system Vs the more conventional permanent magnet style generator.

Thanks in advance,

Tony.
 
In a permanent magnet alternator, the voltage is basically proportional to the RPM. That's a problem because you need a mostly fixed voltage unless you also plan to build a DC/DC converter into it. If you have an alternator with a field winding to create the field (like every car alternator does) then it's a much simpler matter to regulate the output voltage through modulation of the field current.
 
Oznog said:
In a permanent magnet alternator, the voltage is basically proportional to the RPM. That's a problem because you need a mostly fixed voltage unless you also plan to build a DC/DC converter into it. If you have an alternator with a field winding to create the field (like every car alternator does) then it's a much simpler matter to regulate the output voltage through modulation of the field current.
Yes, that's pretty much what I figured, except rather than some complicated DC/DC converter for the permanent magnet type, I figured to just use a series or shunt regulator. The problem I see with the shunt regulator is that the generator would be working all the time and thus requiring the extra peddling effort for no gain. I'm sure there is also a problem with using a series regulator, but I can't remember what it is. Any comments on this would be much appreciated.

I also suspect there must be a downside to using an induction type alternator - any suggestions? Would it be any less efficient than a permanent magnet type? Can anyone see any problems with it other than having a set of brushes to wear out? Can anyone suggest where I could source one, preferably through salvage?

Thanks,

Tony.
 
Well either type has brushes. Actually I don't think either of these count as an induction motor though. Induction motor-generator is something different still.

The separate field coil will require you to find or build a PWM regulator circuit to put current into the coil so it will generate an output. It's a low resistance inductor and it will only require a brief pulse of current to maintain the current in the coil.

The current for the field coil will come off the battery and/or the alt output. There is a minor issue that if there is no power in the battery, you may not be able to get the alt to put anything out. Typically I think residual magnetism prevents that problem from coming up though.
 
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Oznog said:
Well either type has brushes. Actually I don't think either of these count as an induction motor though. Induction motor-generator is something different still.
Maybe induction is not the correct term to use, but I came across it elsewhere on the net used to describe an automotive-type alternator and (perhaps foolishly) blindly accepted it. I must admit, I had reservations using it at the time knowing as I do that an induction MOTOR is brushless, unlike the automotive-type alternators, but that still leaves me at a loss as to the correct term to use.

Semantics aside, is PWM regulation the best way to go? Does it have any advantages over the analogue (I consider PWM essentially digital) type of regulator used in the automotive application?

Thanks,

Tony.
 
PWM has much higher efficiency that any other method, most of which have to loss off power as heat (waste and extra energy). The PM alternator sounds like a better way to go for a bike simply because if using rare-earth magnets, you should have a better weight-to-power-out ratio vs. an electromagnetically-excited alternator. And you'd have to add the battery and regulator to the EM version to boot.

The major down-side to a PM alternator is that power is varied by speed and would be a problem if you were trying to mimic 50/60 Hz mains power. So that doesn't figure in here.

Dean
 
riverman said:
Hi,

I'm toying with the idea of making/adapting an automotive-type (but smaller) alternator for use on a bicycle to charge a small SLA battery or NIMH battery bank for running lighting and other accessories which may include voltage sensitive circuitry. The idea is based on the assumption that it would provide superior voltage regulation through the ability to vary the excitation, but I am interested in member's opinions on other considerations and the pros & cons of this system Vs the more conventional permanent magnet style generator.

Thanks in advance,

Tony.

Forget it!!!!!!!!!!!!!
It takes a lot of power just to generate enough power for a bicycle head lamp or a flashlight. As a kid I rode bicycles with a generator head light and used hand generator flashlights, so I should know. Last Christmas the big thing was the "Just Shake it LED flashlight", shake it for a minute and have light for X minutes. No way. Just expensive junk!
 
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Just use a normal bicycle dynamo. Switching regulators are quite easy to build nowadays given the large range of ICs available. You can easilly convert them to constant current sources or just by a charger IC.
 
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