PFC

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ptonsparky

ptonsparky

Tom
Location
NE (9.06 miles @5.9 Degrees from Winged Horses)
Occupation
EC - retired
POCO to the South of me has just started implementing customer owned PFC for motors 20 HP and above. I've done it for years in my immediate area. Each motor got its own PFC.

One of the issues coming up is that each irrigation system may have a well motor that has been running into SFA for years. The nameplate total of two motors on location may say 30 & 15, the POCO metering says 56 total connected. (grabbed #s form the air but they are not unreasonable) POCO is suggesting one unit total, with a FPN that few read, is that it shall not exceed the recommended KVAR for the motor. Installation would be a 20 kvar connected to the 30HP Pump Panel. Fine for a total 56 HP connection but too much for the 30 alone. Typical PFC for a 30 would be between 10 & 17.5 KVAR.

What grief, if any, will this arrangement cause?
POCO engineering had not thought of the reduced motor current seen by the overloads. Typical connection would be at the load side lugs of the starter.
 
Having PFC at each motor normally would be best. But you don't want to exceed the recommended PFC kVAR for the rated horsepower of the motor. If you install too large of a capacitor then the resonant frequency of the capacitor and the inductance of the stator may fall below 60 Hz. In that case, when the contactor opens residual magnetism will cause AC to be produced on the stator with the frequency ramping down from 60 Hz as the motor slows down. Then when it passes through the LC resonant frequency, an excess voltage can be generated that may damage the insulation of the stator windings, etc. By not exceeding the recommended kVAR, the LC resonant frequency will be above 60 Hz and an overvoltage will not occur.
 
synchro said:
Having PFC at each motor normally would be best. But you don't want to exceed the recommended PFC kVAR for the rated horsepower of the motor. If you install too large of a capacitor then the resonant frequency of the capacitor and the inductance of the stator may fall below 60 Hz. In that case, when the contactor opens residual magnetism will cause AC to be produced on the stator with the frequency ramping down from 60 Hz as the motor slows down. Then when it passes through the LC resonant frequency, an excess voltage can be generated that may damage the insulation of the stator windings, etc. By not exceeding the recommended kVAR, the LC resonant frequency will be above 60 Hz and an overvoltage will not occur.
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May I quote you?
 
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