Voltage Imbalance Caused by Unbalaced Load

[geolec]

Gentleman,

We are building a permanent magnet frequency converter for one of the major space program. The specification requires us to pass a phase balance test which state,

"Phase Balance: With 1/3 of full load on one phase and zero load on the other two phases, the voltage unbalance will not exceed 4% of the unloaded phase."

We have built the exact same unit before and it passed the phase test smoothly (with 1% difference between loaded and unloaded phases). We are using the same design this time, however, we are having trouble passing the phase test.

We are pretty sure that there is nothing wrong with the stator windings as the voltages on all 3 phases were balanced when running on balanced loads. The voltage regulator might be a problem, however, we have tried 5 different types of voltage regulators and they had no significant affect on the testing result.

We are running out of ideas. Any suggestion you can provide is greatly appreciated.


Thanks,


Jerry

 

[jghrist]

Where is the voltage measured? If measured at the input terminals of the converter, then the voltage unbalance will be a function of the power supply impedance, not a function of the converter design.

 

[geolec]

The measurements were taken from the output terminals. We used a handhold meter and an internal vol/amp/freq meter to take the measurements.