Three phase induction generator on a single phase supply

[dec335]

Hi,
I’m really stuck on a problem for my final year project in my taught Masters degree and any suggestions what might be going wrong would be greatly appreciated.
I’m trying to run a three phase delta connected induction generator on a single phase supply using a Steinmetz connection. The Steinmetz connection is a single capacitor connected across phase B or C depending on which direction the machine is running with phase A connected to the single phase supply. I’ve been using the book Distributed Generation by Chan and Lai to get the formulas shown below. It says in the book for a balanced machine the following applies.

Yp = 1/Zp =|Yp|∠-ϕp
Where Yp is the positive sequence admittance of the generator and ϕp is the positive sequence impedance angle.

B1 = 2Yp sin⁡(2π/3 – ϕp)
B2 = 2Yp sin⁡(ϕp - π/3)
B = 1/Xc
C1 = B1/(2.π.f)
C2 = B2/(2.π.f)

I ran the machine as an induction motor first and a certain calculated speed it had an impedance of Z = 124.26∠60 (62.2 + j107.6) and this gave a value of Yp as 0.0085∠-60, B1 = 0 and B2 = -0.0139388. A modular value of B2 was used to calculate a positive capacitance value and this was calculated as 0.00004439 which is equivalent to 44μF. This capacitance was placed on the motor across phase B and it worked really well. It was able to run with a load and all the phases were in balance with the voltages and currents very close together. (The figures have been rounded up for these examples as I have been using an excel sheet that has much longer values in each cell)

I tried to run the machine as an induction generator and a certain calculated speed it had an impedance of Z = 104.41∠120 (-52.26 + j90.4)(the real value is negative as it has a negative slip) and this gave a value of Yp as 0.00958∠-120, B1 = -0.016588 and B2 =0. A modular value of B1 was used to calculate a positive capacitance value and this was calculated as 0.00005282 which is equivalent to 52μF. This capacitance was placed on the machine across phase B and it wouldn’t work properly. There was a lot of imbalance in the currents and voltages.
Would anyone have any idea of why it is not working for the induction generator the same as for the induction motor as all the literature I have read has said that an induction generator behaves exactly the same as an induction motor but it is just run faster? I have been doing tests by trial and error but I have to give a reason why it is not working. If any more information is required, just post a question and I will answer it.
Thanks
Declan

 

[Carl Pugh]

A schematic would be helpful.

 

[dec335]

Carl,
I have attached a schematic of the circuit.
Declan

 

[Carl Pugh]

No one on this site seems to be an expert on what you are attempting to do.
Here are a couple of suggestions. You can take them for what they're worth.

Contact Bodine Motors and talk to one of their experts. Permanent split capacitor motor may be a more common description than Steinmetz Connection.

Why do you want to run a motor as a generator?
Could a different approach be used?

Question:
Are you driving the motor/generator shaft to use this device as a generator? if so are you driving the shaft at a higher RPM than when it is used as a motor?

Are you attempting to convert single phase voltage to three phase voltage?

 

[dec335]

Carl,
I am hoping to run the motor as a generator as the aim of the project is to install a three phase induction generator in a small hydro scheme and connect it to the grid on a single phase supply. I am currently using a dc motor in the lab to run the induction machine to a higher speed than synchronous speed which will run the machine as a generator. I hooked it up to a three phase supply in the lab and the machine works well as a three phase generator. There will be no three phase supply available in the location of the hydro scheme which is why I am trying to use capacitors to balance the phases so overheating does not occur in the machine and the grid is supplied with a reasonably good supply from the machine.
Declan

 

[Carl Pugh]

Why do you wish to use a three phase induction generator? Why not use a standard generator?

 

[dec335]

I want to use a three phase induction motor as a company that makes them have given my college one for free to do testing on and that is the project that has been given to me by my lecturer

 

[Carl Pugh]

These suggestions may be totally impractical, or you may have already done them.

You are using capacitors for phase shift. If the power required is small, it may be practical to obtain the phase shift electronically. It would only require one voltage at a 90 degree angle from the present voltages.
If 100 watt or less is required, it would be easy/low cost to do.
If 500 to 1000 watt was required, it would require more work/cost.

Contact the motor manufacture. (and other motor manufactures) It is possible that they have some suggestions on using motors in this application.

These motors were researched in detail many years ago. Go through the IEEE engineering data.

Check the basic theory on how induction generators work.
It is possible that induction generators will not work with single phase voltages, with or without capacitors.

Good Luck
Carl

 

[dec335]

Carl,
Thanks for the suggestions but I have a few other ideas I want to try first.
Declan