Primary lost phase

[ptonsparky]

A dairy customer had one of the POCO primary lines break. Customer is served 480v three phase, wye.

They lost one of their 30hp 480v pumps that was on at the time. No phase loss protection and that’s understandable. They also had two 120v contactor coils burn up that are on a 480 to 208 customer owned wye transformer about 500’ from the POCO primary transformer bank. Stuff happens or is it predictable? No other damage that we know.

 

[gar]

190614-1352 EDT

ptonsparky:

Assume the primary was also a wye, then one secondary leg would go to zero, and there would be one lost 120 circuit, but the other two would remain at 120.

If we have a delta to wye transformation, and assume three separate transformers for concept simplicity, then loss of one input line creates a single phase source.

One transformer receives its nominal input voltage, and thus its output line to neutral remains relatively unchanged at about 120.

The other two transformers have their primaries in series. The secondary voltages will vary from 0 to 120 depending upon the relative loading of the two secondaries. The sum of the two secondaries will be about 120. Thus, low voltage on an AC relay coil can cause failure to pull in and thus burn out.

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[ptonsparky]

Ok, makes sense. The power supply for PLC didn’t care about the low voltage but the contactor coils did.

 

[synchro]

Yes, it's counterintuitive that AC solenoids can pull less current at low voltage. But they do, as gar mentions, if the low voltage reduces the magnetic field enough to prevent the armature from being pulled in. Without the magnetic material of the armature in place, the inductance is significantly reduced and it draws excessive current.