7-15 kW 3 phase squirrel cage motor as AC generator using oil caps

[koksic]

Is there a way to calculate the capacitor values according to the output power, rpm and/or some other characteristic of the motor. What would be the (tipical) efficiency?
thx
branko

 

[electricpete]

I remember this subject was been extensively discussed in alt.engineering.electrical

There was a guy named Mark..... I can't remember his last name but it wasn't empson. I think he was a phd.

You can use google group's search.

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[koksic]

thanx electricpete,
you are right, this subject is discussed but I was not able to find the equations. Mark (probably the one you mentioned) only mentions that that's possible... maybe the right posting is gone...

 

[electricpete]

The problem is one of establishing a voltage divider to produce the correct voltages.

Apply single phase voltage V. Select C1,C2,C3 so that V1,V2,V3 are approximately equal and 120 degrees apart. Zmot is the equivalent motor per-phase impedance (varies with load) which forms part of the volt divider. I'll try to draw a diagram. Points labeled Vn are connected together (not shown) and need not be 0.

<====1/jwC1 ==V1 ==Zmot====Vn
V<
<====1/jwC2===V2 ==Zmot====Vn

0 <=====jwC3=====V2===Zmot====Vn


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[electricpete]

Correctio: bottom one should be V3 as shown

<====1/jwC1 ==V1 ==Zmot====Vn
V<
<====1/jwC2===V2 ==Zmot====Vn

0 <=====jwC3=====V3===Zmot====Vn

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[electricpete]

Since you ask about efficiency:
I played around with this excercize before and it was difficult to get anything close to balanced, especially over a range of operating conditions. I remember mark reported experimental results also his voltages were unbalanced. Unbalanced voltages will definitely kill your efficiency. One reason is negative sequence currents which produce reverse rotating field and almost twice rotational speed currents in the rotor. They produce heat but no work.

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[mc5w]

You DO NOT want to try to build a self excited induction generator using capacitors as the sole source of magnetizing current - the voltage will be difficult to control and likely to run away into overvoltage until the iron saturates.

An induction generator needs to get part of its magnetizing current from either a conventional generator to fix voltage and frequency or a variable frequency drive that is rigged to pump excess direct current into something such as a battery bank.

Remember that an induction generator needs extra magnetizing current to develop countervoltage greater than line voltage meaning lower power factor AND lower power capability than when running as a motor.

Mike Cole, mc5w@earthlink.net

 

[koksic]

The idea was to use used cheap 3 phase induction motor and conect it to the 25HP tractor shaft (750-1000 rpm) in order to drive 1 and 3 phase loads (other motors) The solution must be cheap because my friend (the farmer) can't afford something expensive... :-(

mc5w, appart from efficiency problem, would some kind of mechanical manual adjusting of rpm help? Means that somebody must serve the other machine anyway so that person can correct the diesel's rpm...

electricpete, please, by V you ment single phase voltage from a outlet (e.g. 220VAC for Europe)? What are the V1-V3? points to measure voltages against Vn?

thanks again for the discussion!

 

[aolalde]

To operate an Induction Generator, Three Phase Voltage is required in order to produce the rotating magnetic field. Then the rotor shaft is driven above the synchronous speed to convert mechanical power into electricity.

Apparently you have only single phase power supply. Your problem is how to generate a three phase excitation source starting with single phase. That is why Electripete refers to V1, V2, and V3.

In my opinion you need a transformer with a winding leg similar to a Scott connection and capacitors to move the phase angle of that leg. We are trying to buildup an equilateral triangle starting only with the base line.

 

[mc5w]

Adjusting the speed of the prime mover will not help - below the resonance point of the capacitors the voltage will collapse and above resonance the voltage will run away. You need to have an additional synchronous rotating machine or inverter to supply about 1/3 of the magnetizing current of the system to get stable voltage and frequency.

Actually, a 3-phase induction generator can get its initial excitation from a single phase line. This has been done with small hydropower plants where the 3-phase induction generator serves as both a generator and phase converter. There are about 1 to 3 books at my local library on how this has been done. In this kind of configuration the turbine is a centrifugal pump running backwards just like the turbine side of a turbocharger.

In this application you would get about 2/3 of your magnetizing current by connecting you capacitor across only 2 leads so that the capacitor and the inductance of two of the other leads acts as a Steinmetz Balancer Set which is a type of static phase converter. Search for my comments elsewhere in regard to phase converters. You would then get the other third of your magnetizing current off of the utility or a fixed speed single phase generator that has a REAL exciter and voltage regulator, not the permanent magnet kind which uses a switched power factor capacitor.

BTW, if you are using a 3-phase induction motor as a phase converter your overload relays have to be set at no more than 2/3 of nameplate durrent so that the reverse sequence rotating field will not overheat the rotor.

Where you would lose efficiency is in having a speed reduction gearbox and then a speed increasing gearbox to be able to use a standard 4 pole or 6 pole motor. You would be better off to find a way to direct connect your induction generator to the front end of the crankshaft.

You will also lose efficiency because phase converters have extra rotor heating.

 

[mc5w]

See thread 237-103280 for my explanation of how phase conversion works.