PM Generator and battery resistance

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Summary:

Burned PM generator charging a lithium battery. How do I make them play well together?
I have a burned stator for a PM generator mounted in the accessory case of an internal combustion engine. The stator has 2 sets of 5 coils creating 2 20 amp generators. I am using a lithium battery and the engine builder blames the battery for the burned stator. He says the lithium battery has much lower internal resistance that a lead acid battery and thus will allow the generator to create a level of power that overheats the stator. I would like information on two approaches to the problem.

First: is there a way to mimic the resistance of the lead acid battery and still allow the lithium battery to charge. The goal would be to throttle the current output of the generator.

Second: I'm thinking of trying to rewind the stator myself. I would appreciate any suggestions as to how to reduce the heat damage potential. I believe the builder used hi temp magnet wire, probably 200C. I would use 240C. It doesn't look like any type of overall coating was used.
I will appreciate any help, suggestions.
 

Answers and Replies

  • #2
jrmichler
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I'll let the EE's discuss battery charging and charge controllers.

There is no rocket science to rewinding a stator, but you do need to do some homework first if you want it to work. Good search terms are motor stator insulation and rewinding electric motor.

You need to be very careful about stress concentrations where the wires enter the stator slots. Stator slots normally have a special insulation paper that extends slightly beyond the slot. The temperature rating of that slot insulation is at least as important as the temperature rating of the wire insulation. Stress concentrations between copper and stator are important failure points because the thermal expansion of copper is significantly larger than the thermal expansion of the steel stator, causing the copper to rub against the steel every time it warms up. Those stress concentrations are where the insulation fails.

Read carefully about varnish/epoxy impregnation of windings, and the effect on thermal conductivity getting heat out of the windings.

I once set up a HALT test program for a high performance servo motor. Running the motor at full load at 300 deg F did not fail it, but temperature cycling between 120 F and 250 F failed several motors in a short time. And those failures duplicated the field failures. The effect of stress and voltage concentrations is typically more important than pure temperature resistance.
 
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  • #3
berkeman
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I once set up a HALT test program for a high performance servo motor.
HALT is a great tool. :smile:
 
  • #4
jrmichler
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One of the great things about mechanical engineering is that sometimes we get to blow things up. We blew up over two dozen of those motors. They had a manufacturing cost of $20,000 each. I don't know the cost to the customer.
 
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  • #6
Borek
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Charging lithium batteries without a controller to limit the current sounds like a disaster waiting to happen.

And I don't mean the generator side.
 
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  • #7
Charging lithium batteries without a controller to limit the current sounds like a disaster waiting to happen.

And I don't mean the generator side.
I should have included in my posting that the generator feeds the battery through a voltage regulator. I am told that the voltage regulator moderates the voltage but does not prevent the generator from feeding the battery as much current as it can accept and therefore the overheating of the coils and stator. I would like to keep the lithium battery but incorporate something to mimic the resistance of the lead acid battery which the generator was designed for.
 
  • #8
berkeman
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the generator feeds the battery through a voltage regulator
Exactly what voltage regulator are you using? And is it a charging circuit targeted at Li batteries, or is it just a constant voltage output regulator? Designing charging circuits and profiles is generally battery chemistry specific, I believe.
 
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  • #9
Tom.G
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Designing charging circuits and profiles is generally battery chemistry specific, I believe.
...Along with temperature compensation.

With at least Lead-Acid batteries, their voltage varies with ambient temperature. If that isn't compensated for they will be either over- or under-charged as the temperature varies... and Lithium batteries require over-temp protection during use to avoid violent and/or destructive dis-assembly.

For battery details see:
https://batteryuniversity.com (the site for all things battery.)

Cheers,
Tom
 
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  • #10
sophiecentaur
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but does not prevent the generator from feeding the battery as much current as it can accept
So don't you just need a current limited voltage controller?
 
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  • #11
Baluncore
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So don't you just need a current limited voltage controller?
Yes.
A PM generator will produce an uncontrolled output voltage that is simply proportional to RPM. We don't know what controls or limits the RPM.

The simplest reasonably energy efficient solution is to use a switching buck converter to limit the voltage, to a couple of volts above charged battery voltage, followed by a series resistor to limit the maximum battery charge current.
 
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  • #13
Exactly what voltage regulator are you using? And is it a charging circuit targeted at Li batteries, or is it just a constant voltage output regulator? Designing charging circuits and profiles is generally battery chemistry specific, I believe.
The voltage regulator is a solid state unit supplied by the engine builder, one for each generator.
...Along with temperature compensation.

With at least Lead-Acid batteries, their voltage varies with ambient temperature. If that isn't compensated for they will be either over- or under-charged as the temperature varies... and Lithium batteries require over-temp protection during use to avoid violent and/or destructive dis-assembly.

For battery details see:
https://batteryuniversity.com (the site for all things battery.)

Cheers,
Tom
There is no temperature compensation. The lithium battery has built in electronics to prevent overcharging or unbalanced cells.

So don't you just need a current limited voltage controller?
I have no idea if a current limited controller would work. The generator blew a 35 amp fuse while on it's way to destruction. I'm not sure what the highest current was. I am wary of installing a limiter that simply converts the blocked current into heat since I feel that this will still allow the generator to cook itself. The goal is to protect the generator from itself not protect the battery. The voltage regulator will control the voltage to the battery and there is an overvoltage module to shut down the generator.
So don't you just need a current limited voltage controller?
Is there such a thing for 35amp AC in and adjustable 20 amp 14 volt DC out.

Thanks for all the comments. I'm also interested in what I should do when rewinding to help the heat situation.
 
  • #14
jrmichler
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I'm also interested in what I should do when rewinding to help the heat situation.
The two primary sources of heat are:

Eddy currents in the stator steel. These are a result of the design - the exact type of electrical lamination steel, the lamination thickness, and the insulation between laminations.

Resistance loss in the winding. The number of turns controls the voltage, the diameter and length of the wire controls the ##I^2R## loss. Thinner slot insulation makes room for larger diameter wire. The length of the wire is proportional to the number of turns, and the amount of wire in the end turns. The original manufacturer almost certainly filled the slot with copper, so you are stuck with the same diameter wire and number of turns. Your only real ability to reduce heat is to shorten the end turns, thus reducing the length of the wire.

If you can find a way to blow more air over the stator, you can do a better job of removing heat.
 
  • #15
Tom.G
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Is there such a thing for 35amp AC in and adjustable 20 amp 14 volt DC out.
Don't know specifically, but probably.

The oldtime automotive DC generators controlled output by varying the Field current duty cycle. The regulators had 3 relays in them, one for overvoltage to limit charging voltage, one for reverse polarity which disconnected the generator when its voltage was below battery voltage, and one current relay that would shut off or reduce the field current if the charging current was too high.

Of course you can't control the Field of a PM alternator. You can however disconnect the output if a current limit is exceeded. I'm a little surprised your existing regulator doesn't have this feature already built in. Perhaps it does but at a higher value than the alternator can handle. A data sheet for the regulator , or a talk with the manufacturer support folks, would be helpful.

The battery may have a current limit feature built in, but higher than the alternator rating. Again a datasheet or a talk with the manufacturer support folks would be helpful.

Another idea is to add a 30A auto-resetting circuit breaker. These are generally thermal types that open when they heat up. When they cool off they will re-close. This would give the alternator a chance to cool.

Or perhaps the simplest (if there is room), replace the alternator with one from a car or truck. 100A units are available for cars, and much higher for trucks. For instance this manufacturers site mentions up to 320A.
http://www.delcoremy.com/alternators/find-by-vehicle-type/heavy-duty-on-highway-truck

Cheers,
Tom
 

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