Conversion of Military Generators from 400 Hz to 60 Hz

LowEnergyParticle · Sep 28, 2008

I'm looking at some military surplus generators that are a pretty good deal. The snag is that their output is at 400 Hz, not 60 Hz. I've got a couple questions, if that's OK?

The prime movers on these generators are diesels. My guess is the system works something like this: the diesels are set to run at 2000 RPM and the generators are wound with 24 poles, creating 400 Hz voltage:

Output Frequency = Shaft RPMs * (Number of Poles / 2) * (1 minute / 60 seconds)

1) Does anyone know if this is how 400 Hz military generators actually work?

2) It seems like opening the generator case and rewiring coil connections to reduce the number of poles to 4, then changing the throttle feedback to run the diesel at 1800 RPM, might be a reasonable solution?

3) Does anyone know why the military uses some 400 Hz power?

I don't know much about this, so any information would be useful. Thanks very much!

Dave

Steve in SoCal · Sep 28, 2008

These are APU's my best guess. Aircraft use 400Hz power for avionics.

Steve

LowEnergyParticle · Sep 28, 2008

Steve in SoCal said:
These are APU's my best guess. Aircraft use 400Hz power for avionics.

Steve

Thanks, Steve - good to know. I think they must use 400 Hz for other services too, because these are hefty 100 KW units which seems a lot for most aircraft purposes.

Thanks very much!
Dave

Forrest Addy · Sep 28, 2008

You could reconnect the generator rotor and stator BOTH to work 4 poles maybe but the sine wave may be distorted to the point that AC equipment connected to it may not function to capacity. Also you have to get the stator turns and the magnetic flux right to produce a standard output voltage. A 12 KVA 3 phase diesel gen set producing 57 or 176 volts may not be too useful.

You'd be better off to rewind the whole schmeer if you have the smarts or have access to them. Better yet scrounge up an alternator that can be made to work by grafting it on the existing engine.

I wouldn't touch modding that 400 Hz alternator on a bet. Might make a good induction heater for mid sized stuff if it has enough power.

LowEnergyParticle · Sep 28, 2008

Forrest Addy said:
... the sine wave may be distorted to the point that AC equipment connected to it may not function to capacity. ... get the stator turns and the magnetic flux right to produce a standard output voltage.

Forrest, I think you just saved me a bunch of time; thanks! I had been thinking that reconnecting the coils into 2 pairs of opposing poles would more-or-less automatically generate a reasonable sine wave, and that I could dial-in the magnitude of the output by adjusting the exciter voltage. Ignorance is bliss. I should have a tee-shirt made with that on it!

I'll hunt a bit more for a gen set that's already got a 60 Hz output. :wall:

Thank you!
Dave

JST · Sep 28, 2008

Depends how they are made.....

If they have fairly normal-looking slotted rotor and stator, like a regular induction motor stator, they can be re-wound to produce good sine wave. As good as any generator, of which most are only "OK" sine waves.

if they have "salient poles"... i.e. coils on poles that stick out and are not just slots in a punching, then you may not be able to.

In any case, the amount of iron is likely much too low to get anything like full power out at 60 vs 400 hz. The 400Hz is used to get more power through much lighter weight magnetics.

As for power, a client of ours has an air force surplus load bank trailer that was used for testing power on a KC-135 (like a Boeing 707). it has a dissipation capability of 180 KW 3 phase, PLUS two load banks for 2000A 28V (an added 116KW). Set up switchable for 60 Hz, 400Hz, or DC.

They use it for testing large motor drives.

jim rozen · Sep 28, 2008

Nobody's mentioned one obvious solution: rectify the 400 cycle output to produce
a DC buss, and then run an inverter to create the 60~ power desired.

400 cycle power is used because it allows transformer-based power supplies to
be built, with much less weight.

Jim

JST · Sep 28, 2008

Inverters are not cheap, depending on power level. Especially sine wave inverters.

However, for low power, that is almost exactly what some cheap home gensets do....... At that power level, parts are cheaper than wire.

jim rozen · Sep 28, 2008

Inverters are less expensive than they used to be....

Another approach would be to use a large VFD with the rectified DC feeding the
dc buss on it.

Problem with inverters could be, the power would be pretty nasty.

Jim

JunkyardJ · Sep 29, 2008

That's what I thought.

JST said:
Depends how they are made.....

If they have fairly normal-looking slotted rotor and stator, like a regular induction motor stator, they can be re-wound to produce good sine wave. As good as any generator, of which most are only "OK" sine waves.

if they have "salient poles"... i.e. coils on poles that stick out and are not just slots in a punching, then you may not be able to.

In any case, the amount of iron is likely much too low to get anything like full power out at 60 vs 400 hz. The 400Hz is used to get more power through much lighter weight magnetics.

As for power, a client of ours has an air force surplus load bank trailer that was used for testing power on a KC-135 (like a Boeing 707). it has a dissipation capability of 180 KW 3 phase, PLUS two load banks for 2000A 28V (an added 116KW). Set up switchable for 60 Hz, 400Hz, or DC.

They use it for testing large motor drives.

The reason for 400 Hz in the military is weight savings. Generators, motors, transformers, all lighter. To power 60 Hz equipment, they couple a 400 Hz motor to a 60 Hz generator. If you look at Fair Radio, they have a TON of military surplus communication equipment, and a LOT of it is 400 Hz. There are some other reasons for 400 Hz power, but they may or may not be classified, so I won't get into that. :cheers:

jim rozen · Sep 29, 2008

Not classified at all! It's physics. Transformers work
better at higher frequencies so you need less inductance,
hence less iron. Also the filter capacitors can be much
smaller as well, on the other side of the power supply.

Jim

Steve in SoCal · Sep 29, 2008

Civil aircraft have used 400Hz for at least 50 years, it is not well known out side aviation but no secret.

Steve

JunkyardJ · Sep 29, 2008

I wasn't saying the 400hz is classified.

It's more the reasons behind it. Can't really get into details, but there are a LOT of advantages to 400 hz power. There's other reasons for it than the function of transformers, and the obvious electrical stuff. :cheers:

jmead · Sep 30, 2008

30 years ago in school it was no secret: higher Hz= less iron= lighter plane.

blackadder · Sep 30, 2008

smoother DC less ripple to filter out

IBM mainframes used to use it to

jim rozen · Sep 30, 2008

Yes they did.

We had a machine room fed from a rotary converter
setup, if you think 60~ noise is bad, the 400 cycle
whine is really incredibly annoying. Summertimes they
used to have to open the back door of the converter
room to keep it cool with fans, what a racket.

Jim

blackadder · Sep 30, 2008

jim rozen said:
Yes they did.

We had a machine room fed from a rotary converter
setup, if you think 60~ noise is bad, the 400 cycle
whine is really incredibly annoying. Summertimes they
used to have to open the back door of the converter
room to keep it cool with fans, what a racket.

Jim

Yep we had six converters running what a din , then we had some Anton Piller converters, that you could parallel up the units ( German made ) a house hold vac made more noise . dare I say we also tried some USA made static converters , but they produced bad harmonics on the supply , then they released the magic smoke.

At a steel works that i worked ( did my apprenticeship there ( electrical )) we had 200hz rotary converters 2 pole motor and 8 pole alternators , what a pig to rewind . We used these for grinders in the fettling dept for the CI castings

Lloyd H · Sep 30, 2008

High cycle gensets

They use to make, maybe still, 180 Hz 3 phase gensets for powering concrete vibrators. Two pole induction motor in the vibrator head run over 10,000 rpm, were very effective as well.

gary350 · Sep 30, 2008

I have a military generator once. Actually it was a dynamotor converter connect it to a 12 VDC battery and it makes 120 volts AC 400 hz. Worked great for lights on my camper. It was way to heavy for it watt rating I sold it on ebay used the money to buy a DC to AC inverter on ebay for $50. Now I have 300 watts instead of 100 watts works great on the camper runs every thing I need. For $75 I bought a 1200 watt DC to AC inverter to run a small refrigerator. It will also run a skill saw at the camp site works great for cutting up firewood.

zeo · Oct 30, 2008

If you saw how much copper wire goes in to a large aircraft you can understand why the 400hz they can use 22ga instead of say 12 or 10 ga. like the man said lighter A/C more payload. G'day zeo

Conversion of Military Generators from 400 Hz to 60 Hz

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