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High voltage leg on PP rotary 7.5hp

Toms Wheels

Titanium
Joined
Dec 30, 2005
Location
Jersey Shore
I have a PP Rotary 7.5hp hooked to 5 machines, but only used one at a time. On an occasional basis it will fault out one of the VFD units. Incoming voltage is 240, output is 237, 243, 261. This is all No Load voltage, The 2 vfds are same model at set the same. The fault is overvoltage on one leg. Sometimes months go by without fault, then 5 times in a hour. Both VFDS have current reactors inline.

I blame the power company, but don't know. Any ideas to lower the voltage on that one leg?

Tom
 
Do you have capacitors line-to-line on the output of this rotary? First guess (if they are there) is that it's overcompensated. I am suspecting the highest leg is the manufactured one.
 
Almost has to be that, as the normal condition (due to how motors work) is for the manufactured phase to be LOW.
 
Get at the VERY least a "light" load onto it. Truly NO load measurements with modern high impedence meters or 'scopes don't reflect reality very well as they can even read the weakest of parasitic capacitance effects or unintended resonances, strong or weak alike.

Could was all the "spend" you need is a small 3-P coolant-pump switched-on upstream of yer Vee Eff Dee input diode arrays?

Diodes are also capacitors, y'see. Kinda tiny ones. But there IS that sensitive meter factor.
 
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I have a PP Rotary 7.5hp hooked to 5 machines, but only used one at a time. On an occasional basis it will fault out one of the VFD units. Incoming voltage is 240, output is 237, 243, 261. This is all No Load voltage, The 2 vfds are same model at set the same. The fault is overvoltage on one leg. Sometimes months go by without fault, then 5 times in a hour. Both VFDS have current reactors inline.

I blame the power company, but don't know. Any ideas to lower the voltage on that one leg?

Tom

what is the converter rated/designed for motor wise, should be within 5% of each other ideally. Too much capacitance will pull a generated leg high.
 
Get at the VERY least a "light" load onto it. Truly NO load measurements with modern high impedence meters or 'scopes don't reflect reality very well as they can even read the weakest of parasitic capacitance effects or unintended resonances, strong or weak alike.
.......

Two different things.

The meter impedance deal is a factor with un-loaded wires picking up stray voltages. With capacitors, idler motors etc on there, the meter is a non-factor, the voltages are real.

The boost is due to capacitive compensation. That may be OK, since it may be needed to get loaded voltages to their correct value.

It is, however true that totally unloaded voltages are not terribly important as long as they are within reason. The capacitive compensation is a resonance phenomenon, and any loading will reduce the "Q" of the resonance (no, no, not THAT "Q":D), and lower the voltage.

Max voltage for a 240V line is 264VAC, so you are already skating the edge. A small variation in line voltage, and you are over. Get a few more volts cushion on it by applying a small load, and you should be good.
 
The fault only happens on the BP, and only when at rest, no fault when in use. Its not all the time, but several times a month, and when it happens like 10 times in a row. I have not tried leaving the grinder running but next time it starts faulting I flip it on.
 








 
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