PM Alternator Phase Losses

[sho2010]

If I have 10 sets of 3-phase windings on a stator where each of their neutrals are isolated from each other ("Y" types), If I begin to connect each set to a 3-phase rectifier one by one, why would I have to keep raising the RPM of the machine to maintain the "expected" current out? For example if I connect one set to the (loaded) rectifier I measure 10ADC on the rectified output at 3000 RPM, but then if I connect two more sets I would expect 30 ADC at the the same RPM, however in order to provide the 30 AMPS, I need to increase the RPM up to 3300 RPM. Where are these losses coming from?

 

[Skogsgurra]

There is interaction between the stator windings and the flux from the PM rotor so that load current weakens the flux. There may also be some interaction between the stator windings, it depends if they are wound on common core or separate cores. Common core makes interaction worse.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[sho2010]

Is there a way to prove or measure this? When we look at each set independently every one checks out within reason. I have not come accross such a problem like this but then again I have not had so many independent windings before on a single stator. Do you know if this is a common problem with multi-phase sets and maybe the reason why I usually see only 1 or 2 sets of 3-phase windings on the stator?

 

[Skogsgurra]

No, I haven't seen a generator like yours before. But I know that AC tachogenerators with a toothed PM rotor and two stator windings (one for speed control and one for measurement) are sensitive to loading of the other winding.

A common mistake, before we learned this, was to disconnect the measuring circuit and that always led to speed change in the driven paper machine section - which caused a costly web break.

Gunnar Englund

http://www.gke.org

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Half full - Half empty? I don't mind. It's what in it that counts.

 

[waross]

voltage drop with load is common on generators. The voltage regulator compensates by increasing the field current as added load causes the voltage to drop. When we are testing a set with a power supply on the field instead of the Automatic Voltage regulator we we may have to double the power supply voltage to maintain rated voltage from no load to full load.
With PM fields I guess your best option is to increase the speed.

Bill
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"Why not the best?"
Jimmy Carter

 

[sho2010]

Voltage drop does occur with load and this makes sense to me. We do a lot of voltage controlled systems by regulating RPM. The problem I am having is that if I take each phase set "individually", they each will produce 10 Amps DC out of 3-phase diode rectifier into the load at 3000 RPM. However if I parallel all three sets onto the rectifier input, I would expect to be able to supply 30 Amps at 3000RPM. For some reason I must increase the speed by 10% to achieve the 30A. Why do they not sum as I would expect?

 

[Skogsgurra]

sho, I told you. Don't you believe what I said?

Gunnar Englund

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[sho2010]

Skogsqurra I think what you are saying appears to make sense. I would like to understand this interaction a bit more. Are there any links or books that discuss this particular issue?