ECN Forum Forums General Topics of the Trade General Discussion Area Three-phase boost transformer

Thanks again for all your comments and information. I think I'm beginning to understand. My earlier post that said:

"The motor has 6 leads. Label says it can be connected Delta or Wye. I thought this meant:
-If connected as Delta, 380 volts phase-to-phase should be used as power source.
-If connected as Wye, 220 volts Phase-to-neutral should be used as the power source."

Bbecause of my misconceptions, I now think the above statement was incorrect. What I now think 380/220 means is:

-if you have a 380 volt three-phase Delta feed, you connected the motor leads for a WYE.
-If you have a 220 volt three-phase delta feed, you connect the motor as a delta.

In both cases, the actual voltage that appears across one of the windings is 220 volts. If this is not correct, please tell me.

The next part of my problem is determining exactly what feed I have. The panel box has three hot leads and a neutral. Using a DVM (true RMS), I read these actual voltages:

ØA to ØB: 223 VAC
ØB to ØC: 220 VAC
ØA to ØB: 219 VAC
ØA to Neutral: 129 VAC
ØB to Neutral: 130 VAC
ØC to Neutral: 124 VAC

I cannot be sure it is true three-phase because I did not measure phase angle. My best guess is this is nominally 220 volts phase-to-phase which should give 127 volts from each phase-to-neutral if the supply is a true Wye (neutral not from a tapped Delta). Due to various other load imbalances connected to the system, I am not getting exactly 127 volts

The final problem is what to do about 50/60 Hz. Assuming the speed increase I get by operating a 50 Hz motor on 60 Hz is not a problem for the application, do I need to increase voltage to the motor by (60-50)/50 = 20% to compensate? 220 VAC * 20% = 44 volts. 220 + 44 = 264 VAC?

Obviously I'm new to three-phase motors, so all suggestions/comments are welcome. HaitiJim

Yes!


Where are you? Haiti? It would be an unusual system in the US, but not outside it. (That's why they design the motors for it. )


If you need the full torque. If the motor is oversize for the application, it might not be necessary. If you increase the voltage you will get more power, which means more heat dissapated in the motor. This doesn't have to be a problem as the fan inside the motor will also rotate 20% faster, improving the cooling.

{Edited after rereading Jim's post to remove irrelevant info}

[This message has been edited by C-H (edited 03-16-2003).]

Thanks for everybody's help. I do volunteer work in Haiti, but live in the US. This project is part of an effort to eradicate a disease called elephantiasis caused by a mosquito borne parasite, filariasis. (http://www.nd.edu/~ndmag/sm2001/Haiti.html}Table salt is treated with both iodine and a chemical called diethyle carbamazine and distributed to affected areas. The machine for treating the salt uses two motors to run converyer belts and a motor driving a chemical sprayer pump. The voltage from the powerline is not well regulated so unless we can regulate input voltage to the machine, it will be difficult to get consistant dosage. With everybody's help, I think we have determined that if I wire the motors in Delta configuration, I can use the existing 220 volt, 60 Hz, three-phase. I may try the suggestion to boost the 220 volts to compensate for the 50 to 60 HZ frequency change. Does anybody have recommendations for how to stabilize voltage? I am considering a Lenco three-phase ferro-resonant transformer; 220 volts, three phase in to 220 volts, (or 220 x 1.2 = 264 volts), three phase out. HaitiJim

The speed of the motors will be determined by the frequency and load rather than the voltage. Luckily, the frequency is usually rather stable, but in a poor country like Haiti I would expect it to vary up to +/-5%, maybe more.

An unstable voltage indicates some additional problems: The motor is only designed to handle a certain undervoltage (10% or similar). Even if you boost the voltage 20% to accomodate the higher frequency, the motor could still be damaged if the supply voltage falls. The impedance of the distribution network is often high in poor countries, meaning that a large load (like the motors) will cause the voltage to drop considerably below what you measured.

To make matters worse, if the phase-to-phase voltages aren't within 2% of each other the motor will run hotter than normal.

All of these potential problems can be addressed with transformers, variable frequency drives and other tricks. But before you run out and buy these things: Check how much they cost. There might be a simpler solution: Putting in one size larger motors. A 4 KVA motor is not very expensive.

The cheapest solution is to see if the load on the conveyer belts can be reduced and run the existing motors off the existing 220V.