# Thread: 3 phase voltage question

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Three phase motors have the same resistance or ohm reading in all three windings so if you have a voltage unbalance or difference in the applied voltage to the three windings the electrical unbalance will cause the windings to over heat. A voltage unbalance of more than 2% can cause damage. I ran your numbers and your unbalance was about .8% so I don't think that is your problem. The formula for voltage unbalance is voltage deviation / voltage average. In your case you said L1-L2=212, L1-L3=214, and L2-L3=215 just add 212+214+215=641, then divide. 641/3=213.6 this is your voltage average. Your voltage deviation would be 213.6-212=1.6, 214-213.6=0.4, and 215-213.6=1.4 your greatest deviation is 1.6. Voltage deviation 1.6 divided by voltage average 213.6 = 0.00749064 x 100= .8%. Current unbalance is calculated the same way, the max current unbalance is 10%. Hope this helps.

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## Three Phase motors

Im a student currently enrolled at Penco tech and we are currently learning about three phase motors, and you equation gave me a better understanding of finding the resistance. thanks

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## Voltages

Originally Posted by jmcgreevey
had to change out a 5hp blower motor in a 2 year old rtu this past week. one year warranty on parts so, needless to say, customer not happy shelling out the cash to buy a new one of those. feeling his pain, I went through the blower circuit pretty thoroughly looking for any week links that may have caused us to lose one if the windings. the only thing that really jumped out at me was varying voltage across the line legs. I don't remember the exact voltages, but on 208v service I had something like L1-L2 212v, L1-L3 214v, and L2-L3 215v. amp draw across the windings also varied a few tenths of an amp in the new I motor.

this is the first time I've lost such a young 3ph motor. in the past I feel like the voltage across each leg on other three phase service that I've checked voltage on have been pretty much identical. I never really had a reason to think about it before now. it has me wondering if this voltage difference, and subsequent current difference, would cause the windings to fire slightly out of phase. I've been trying to research this as much as I can on the web but can't seem to put all the info together to answer my own question. it also seems that, depending on the winding configuration, if the windings fire out of phase they could create a short. any grounds to this? I'm trying to take thus opportunity to learn more about 3ph power and motors while trying to protect my customer from another costly failure.

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The voltages seem ok. Just out of curiosity, do you know if the power source is coming from a open delta transformer?

4. Check your contactor and all the electrical connections. I have seen motor failures where once the 3ph motor starts one of the phases drops out, the motor will stay running in a single phase condition and eventually burn out the new motor from running hot. A bad (new) motor will either not work out of the box or fail soon after installing it.

5. Put a vfd on it. Great motor protection, reduce stress on motor (belt?) because of soft start. Also can fix free-wheeling issues (if that's an issue) Maybe there are incentives from the power company.
You don't even have to change to an inverter-duty motor just use inductive chokes. They are relatively cheap, easy to install and they will clean the voltage spikes up before they hit the motor.

If you are really concerned about it happening again you should at the very least install a phase monitor.

6. Just realized that the OP was 3 years old.
I wonder if he heeded our advice or if harmonic distortion and burned contacts did him in again.
Or that wicked .8% voltage imbalance....

7. Originally Posted by RezAtDorsia
Just realized that the OP was 3 years old.
I wonder if he heeded our advice or if harmonic distortion and burned contacts did him in again.
Or that wicked .8% voltage imbalance....
Oh, that's funny ****!

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