12 lead generator running at 3600rpm ?

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Sangha Energy

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Hi, I'm new to the forum but thought this might need engineering or calculations to represent.

Recently I fixed a medium sized generator - 100kw three phase. The 12 lead generator head had been rewired from 480 three phase to 208 three phase plus some other engine repairs had been done. So I assumed that all 12 lead generators run at 1800rpm. I timed the engine with a timing light to run at 1800rpm. Output voltage was like 95 per leg, so I thrashed around trying to adjust the voltage regulator to bring the voltage up. Finally getting the exciter circuit breaker to start shutting down DC current to the exciter.

Went back to the drawing board and checked frequency - 37hz! So I adjusted the governor up to 60hz, bingo output come up to spec, exciter current dropped - all happy except the customer because his big block chevy was running at 3900rpm, I told him 3600 but he complained that his tach said 3900. Bottom line - customer is not happy his generator while working and making good power runs at 3600rpm. With a big block chevy pushing the generator head the fuel costs would be very high.

So, sorry for the long story - can someone explain how a 12 lead generator head runs at 3600rpm and not 1800? Any chance the rewiring was done incorrectly? Can anyone point me to the engineering/physics of 12 lead generator heads and RPM? I know all the output wiring options what I want to understand is the relationship of winding/leads/rpm.

Thanks,
 
The number of leads affects how the windings can be arranged in series and parallel to get different voltages.
The number of poles (a mechanical and magnetic issue) is what governs the relation between rotation speed and frequency.
1800 rpm generators are wound with twice as many poles (which can be one coil in series across all of the poles) the frequency will be twice as high for the same rotation speed.
You are lucky that nothing was damaged by the 30Hz output.
As a very general rule, the higher pole generator heads are used for the prime power and construction power uses where long running time is demanded from the engine. That makes for a larger, more expensive engine for the same power output.

Sent from my XT1080 using Tapatalk
 
Sangha Energy said:
Hi, I'm new to the forum but thought this might need engineering or calculations to represent.

Recently I fixed a medium sized generator - 100kw three phase. The 12 lead generator head had been rewired from 480 three phase to 208 three phase plus some other engine repairs had been done. So I assumed that all 12 lead generators run at 1800rpm. I timed the engine with a timing light to run at 1800rpm. Output voltage was like 95 per leg, so I thrashed around trying to adjust the voltage regulator to bring the voltage up. Finally getting the exciter circuit breaker to start shutting down DC current to the exciter.

Went back to the drawing board and checked frequency - 37hz! So I adjusted the governor up to 60hz, bingo output come up to spec, exciter current dropped - all happy except the customer because his big block chevy was running at 3900rpm, I told him 3600 but he complained that his tach said 3900. Bottom line - customer is not happy his generator while working and making good power runs at 3600rpm. With a big block chevy pushing the generator head the fuel costs would be very high.

So, sorry for the long story - can someone explain how a 12 lead generator head runs at 3600rpm and not 1800? Any chance the rewiring was done incorrectly? Can anyone point me to the engineering/physics of 12 lead generator heads and RPM? I know all the output wiring options what I want to understand is the relationship of winding/leads/rpm.

Thanks,
Click to expand...

Disclaimer: I can't tell you about how the gen is wound - not my area of expertise.

It's a 2 pole generator - not a 4 pole generator. The number of leads don't have anyting to do with the number of poles (I think this is true). If you wish to have an 1800 rpm machine - has to have a 4 pole rotor

The 12 leads are connections to 6 stator windings - two coils per each phase. Series up each phase windings, Wye connect, set VR to 139V per coil, and the gen output is 480 Wye.

Parallel connect the phase coils, set the VR to 120V/coil, output is 208 Wye.

I suspect the alternator nameplate lists the rpm. If not, surely the spec sheet will.

Yes, I imagine a 454 turning 3600 to power a 150hp load, really irates the customer. If it were turning 1800, a 454 would be about right.

Welcome to the world of DIY engineering.

ice
 
The generator is wound for 2 poles and so I think no matter what winding connection configuration-series or parallel you make, you can not get 480 V, 60 Hz supply at 1800 RPM from the generator.
 
My gut feeling matches the general consensus above: the generator itself is wound as a 2 pole machine, and is designed to be run at 3600 RPM in order to produce 60Hz. It just wasn't properly matched to the engine from the get go. Getting nameplate information would be ideal.

Questions: how did you test the generator? Was it producing 'good power' under load, or only open circuit with a meter? How did you measure RPM, by output frequency, or with a tach on the prime mover?

For a little bit of background, an AC motor turns through one pole for each 'hump' of the AC cycle. So a 2 pole machine will turn at 60 rotations per second fed with 60Hz; a 4 pole machine will turn at 30 RPS fed with 60 Hz, 6 pole 20 RPS..... Multiply by 60 to get RPM, and this calculation is exact for a 'synchronous' machine, but only an approximation for an induction machine.

When you say you 'fixed' this generator...was it ever working? Or were you hired to make something work that has never worked in the past?

There is a very 'out there' possibility. There is a type of motor winding called a 'consequent pole' winding. In a consequent pole winding you can change double or halve the number of magnetic poles by changing the external lead connections. Doing this requires you to be able to 'get to' six separate phase windings; each phase is split into two pairs and by reversing the polarity of one of each of the three phases you can change the pole count. It is distantly possible that by reversing some of the connections in a 12 lead machine that you could see this sort of consequent pole effect, changing a 4 pole machine into a 2 pole machine.

I say that this is distantly possible simply because your alternator is a _synchronous_ machine with a DC field excitation. This means that even if you could somehow change the stator winding from 4 pole to 2 pole, the _rotor_ magnetic field would not change, and the rotor magnetic field would not have the proper symmetry to drive the (now incorrectly connected) stator. When consequent pole machines are actually used, they are _induction motors_ where the rotor magnetic field will naturally reconfigure to match that of the stator. I could imagine a 'built field' machine that could work with both 2 and 4 pole stator configurations, but I can't see it as being a good design.

If you can't get the generator nameplate information, then I'd chase down the 3900 RPM reading. Where did it come from? Can the actual engine speed be measured with an accurate strobe tach? If the engine is actually spinning at 3900 RPM, but the generator is producing 60 Hz (IMHO _very_ unlikely but again I can see possibilities) then the machine is no longer acting as a synchronous machine, and the 'consequent pole' possibility become more likely.

-Jon
 
Using a 454 big block, IMHO, is a very poor choice. I can understand the owner's concerns of running it, loaded or unloaded, at 3600 rpm. There probably is a reason I have never seen a big block V-8 running a generator.

I have never seen a non-inverter type AC generator driven by a gasoline engine running at anything other than 3600 rpm.

Now, with that being said, the only thing I can think of as a reason why, is the need to have a huge engine for very little power. With that in mind, I checked out some typical power curve charts. An engine that has it's peak hp at 5500 rpm will have less than 1/3 of that at 2000 rpm. I couldn't even find charts that went below 2000 rpm, and the slope of the curve was very steep, meaning 1800 rpm has much less power than 2000.

Torque, on the other hand, is quite high at 1800 rpm. This may be a design, home made perhaps, that uses a purposely overpowered engine so it can run at 1800 rpm. My thoughts would be less wear and tear on the engine and less noise.

As for the 3900 giving 37 Hz, I suspect that is an instrument error.

If you can supply a make and model #, I'll bet the generator guys, or even an search for the specs would indicate if the generator is capable of 60 hz at 1800 rpm.
 
I'm not real familiar with gas engine 100kw generators. The few that I have seen that were big engines. As I recall, they (the two I've seen) were a 500 in^3 GM block (maybe they're 502 in^3) I suspect it is what would be used in a school bus.. They need 150hp at 1800rpm - that's a lot of in^3

Not my area, but I would expect long stroke, little valves, low compression, really mild cam, cast iron everywhere

I just picked a 454 cause it felt like a DIY. This would be where my peer group compadre says, "Worm - you're all wet."

ice
 
iceworm said:
I'm not real familiar with gas engine 100kw generators. The few that I have seen that were big engines. As I recall, they (the two I've seen) were a 500 in^3 GM block (maybe they're 502 in^3) I suspect it is what would be used in a school bus.. They need 150hp at 1800rpm - that's a lot of in^3

Not my area, but I would expect long stroke, little valves, low compression, really mild cam, cast iron everywhere

I just picked a 454 cause it felt like a DIY. This would be where my peer group compadre says, "Worm - you're all wet."

ice
Click to expand...

A 100kw genny at 100% efficiency needs 134 hp. What is the effeciency of a typical generator?
 
The plot thickens -- this is a 1800rpm generator!

The plot thickens -- this is a 1800rpm generator!

Ok, the thread has veered off somewhat - here's what I learned today - the alternator is an 1800! See photo attached.

So how does a 4 pole rotor become a 2 pole rotor - what failure mode or wiring fault would cause this? Why the photo is upside down is also a mystery - but anyway - let's focus...

Generac 100kw Alternator.jpg
 
winnie - thanks for the thoughtful and thorough response. This is a Generac 100kw generator that overheated - we were hired to fix the generator by replacing the engine heads and getting the engine to run. Then we rewired 1st for single phase (zig-zag) and then back to 208 Wye (customer demands) - the original output voltage was 480 wye.

The generator was hooked up to a shop pulling about 100amps per phase at 208 and it ran for a week 24x7 then the cooling fan came apart. The customer decided to go another direction - go figure;) So I got the unit back and now need to repair and resell. She runs good - just at the wrong RPM. Cooling fan replacement not a big deal but getting 1800 power is a big deal.

So your analysis was very interesting - could we as rookies on rewiring actually cause the problem? Very real possibility!

winnie said:
My gut feeling matches the general consensus above: the generator itself is wound as a 2 pole machine, and is designed to be run at 3600 RPM in order to produce 60Hz. It just wasn't properly matched to the engine from the get go. Getting nameplate information would be ideal.

Questions: how did you test the generator? Was it producing 'good power' under load, or only open circuit with a meter? How did you measure RPM, by output frequency, or with a tach on the prime mover?

For a little bit of background, an AC motor turns through one pole for each 'hump' of the AC cycle. So a 2 pole machine will turn at 60 rotations per second fed with 60Hz; a 4 pole machine will turn at 30 RPS fed with 60 Hz, 6 pole 20 RPS..... Multiply by 60 to get RPM, and this calculation is exact for a 'synchronous' machine, but only an approximation for an induction machine.

When you say you 'fixed' this generator...was it ever working? Or were you hired to make something work that has never worked in the past?

There is a very 'out there' possibility. There is a type of motor winding called a 'consequent pole' winding. In a consequent pole winding you can change double or halve the number of magnetic poles by changing the external lead connections. Doing this requires you to be able to 'get to' six separate phase windings; each phase is split into two pairs and by reversing the polarity of one of each of the three phases you can change the pole count. It is distantly possible that by reversing some of the connections in a 12 lead machine that you could see this sort of consequent pole effect, changing a 4 pole machine into a 2 pole machine.

I say that this is distantly possible simply because your alternator is a _synchronous_ machine with a DC field excitation. This means that even if you could somehow change the stator winding from 4 pole to 2 pole, the _rotor_ magnetic field would not change, and the rotor magnetic field would not have the proper symmetry to drive the (now incorrectly connected) stator. When consequent pole machines are actually used, they are _induction motors_ where the rotor magnetic field will naturally reconfigure to match that of the stator. I could imagine a 'built field' machine that could work with both 2 and 4 pole stator configurations, but I can't see it as being a good design.

If you can't get the generator nameplate information, then I'd chase down the 3900 RPM reading. Where did it come from? Can the actual engine speed be measured with an accurate strobe tach? If the engine is actually spinning at 3900 RPM, but the generator is producing 60 Hz (IMHO _very_ unlikely but again I can see possibilities) then the machine is no longer acting as a synchronous machine, and the 'consequent pole' possibility become more likely.

-Jon
Click to expand...
 
Let me verify the symptoms. Here's what I got:
You rewired for 208V Wye.
It ran at 1800 rpm producing 36kw (100A at 208V) for a week.
The cooling fan came apart.
You repaired the fan. (Did nothing to the alternator windings)

After firing back up, the engine speed, and voltage were off. You reset the governor for 3600 rpm and adjusted the VR for 208V. Produces power fine, but the engine is now turning 3600.

That's my translation so far. How am I doing?

ice
 
OK. The nameplate reads 1800 rpm, 60 Hz. If that is correct, and there is strong evidence from prior operation that it is, what could have changed?
Consider what would happen if the 4 pole behavior was created by a four pole field winding. Essentially two coils at right angles to each other, so that the exciting magnetic field polarity reversed four times per revolution instead of twice.
If one of those two field coils opened up, the output windings would still see a roughly sinusoidal change in field strength, but at 1/2 the frequency.
Now thus would not actually work unless the field excitation normally also was commutated (electronically?) during each rotation. But I am trying to figure out a way that the effective number of poles per rotation could have changed, given that I do not see any way that the 12-lead output winding connection could have done that.

Tapatalk!
 
GoldDigger said:
... If one of those two field coils opened up, the output windings would still see a roughly sinusoidal change in field strength, but at 1/2 the frequency. ...
Click to expand...

No book, and no anecdotal knowledge. I'm definitely out of my area:
I'm wondering is the alternator has an exciter with shaft diodes and one or more diodes are burned out.

ice
 
GoldDigger said:
But I am trying to figure out a way that the effective number of poles per rotation could have changed, given that I do not see any way that the 12-lead output winding connection could have done that.
Click to expand...

Consider a very simplified 2 pole single phase winding. You have 2 coils, each about 90? wide spaced 180? apart on the stator. The coils are wound and connected such that when the coil at 12o-clock is generating an N pole facing the inside of the stator, the coil at 6 o-clock is generating an S pole facing the inside of the stator. Note that each coil has two ends, so with two coils you have 4 leads to deal with if you want full access.

Now reverse the polarity of the 6o-clock coil, so it generates an N pole. You won't have suddenly created a magnetic monopole; you have to get S poles somewhere to balance the N poles, and you do. Roughly hand waving, you get the two S poles in the space between the coils. In other words, the relative polarity between the two coils determines if the winding produces 2 poles or 4 poles.

Take three of the above single phase coil sets, and arrange them 120? apart, and you get a 3 phase, 2 pole winding. Now in a normal 3 phase machine, you would internally connect things to fix the configuration in stone, and only bring out 3 of the leads. But imagine if you bring out all 12 leads. By appropriately connecting these 12 leads, you could change the voltage, connect star or delta, or even _reverse the polarity_ of some of the leads and change the pole count between 2 and 4 poles.

I do not know the exact details of the winding of this alternator. Generac might be able to supply you with a stator schematic, and you could see if my guess is even remotely possible.

The real question is how to deal with the pole count of the _rotor_. It seems totally possible that with a 12 lead generator you can get the _stator_ pole count wrong by changing the connections.

Going out on a limb here with an 'out there' guess, is it possible that the DC rotor excitation is not being used, and that the alternator is somehow acting as an induction generator? This would make teh 3900 RPM fit with the 60Hz output.

IMHO the next step is to get a schematic from Generac before proceeding.

-Jon
 
Singha...

The same 3 winding-pairs used for Zig-Zag, single-phase operation, are used for 208V, 3-phase operation! Pehaps while it operated single-phase one of the six windings was open-circuited and went undetected! Did you compare the phase-resistance of each pair?

Secondly, are rotations of the two diesels the same? Although reverse operation doesn't affect the generatior it could have a deleterious impact on AVR/exciter performance!

Thirdly, for my own edification how was is the diesel started?

Phil Corso
 
Phil Corso said:
Singha...

The same 3 winding-pairs used for Zig-Zag, single-phase operation, are used for 208V, 3-phase operation! Pehaps while it operated single-phase one of the six windings was open-circuited and went undetected! Did you compare the phase-resistance of each pair?

Secondly, are rotations of the two diesels the same? Although reverse operation doesn't affect the generatior it could have a deleterious impact on AVR/exciter performance!

Thirdly, for my own edification how was is the diesel started?

Phil Corso
Click to expand...

I guess I missed the diesel part. I was assuming a '454' would be a gasoline engine. 454 CI is 7.4 liters. I don't know of any 7.4 liter diesels.
 
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