Good thing my help is paying attention.

[ptonsparky]

When did they start putting the 'MAX KVAR' on the Motor nameplates? IDR seeing it before.

What happens If it is exceeded?

 

[synchro]

If PF correction capacitors are larger than that providing MAX kVAR, then the resonant frequency of the capacitor and stator winding can fall below 60 Hz. Then when the motor contactor opens, the stored energy in the magnetic field induces currents in the rotor windings (e.g., squirrel cage), which then induces currents in the stator windings at a frequency corresponding to the rotation rate of the rotor. As the rotor coasts down and the frequency falls and passes through the resonant frequency of the capacitor/stator LC circuit, then relatively large voltages can be produced on the windings because the contactor is open. This overvoltage can damage the insulation of the windings.
By keeping the PF correction capacitors less than the MAX kVAR, the resonant frequency will be greater than 60 Hz and therefore the current produced as the motor coasts down will not go through resonance and cause an overvoltage.

 

[Jraef]

If PF correction capacitors are larger than that providing MAX kVAR, then the resonant frequency of the capacitor and stator winding can fall below 60 Hz. Then when the motor contactor opens, the stored energy in the magnetic field induces currents in the rotor windings (e.g., squirrel cage), which then induces currents in the stator windings at a frequency corresponding to the rotation rate of the rotor. As the rotor coasts down and the frequency falls and passes through the resonant frequency of the capacitor/stator LC circuit, then relatively large voltages can be produced on the windings because the contactor is open. This overvoltage can damage the insulation of the windings.
By keeping the PF correction capacitors less than the MAX kVAR, the resonant frequency will be greater than 60 Hz and therefore the current produced as the motor coasts down will not go through resonance and cause an overvoltage.

Very well put.

I'll only add that the only time I have seen this is on motors designed for high inertia loads, like centrifuges/separators. I have always assumed because the long coast down time can result in being in that resonance zone for longer., this adds credence to that.

 

[ptonsparky]

If PF correction capacitors are larger than that providing MAX kVAR, then the resonant frequency of the capacitor and stator winding can fall below 60 Hz. Then when the motor contactor opens, the stored energy in the magnetic field induces currents in the rotor windings (e.g., squirrel cage), which then induces currents in the stator windings at a frequency corresponding to the rotation rate of the rotor. As the rotor coasts down and the frequency falls and passes through the resonant frequency of the capacitor/stator LC circuit, then relatively large voltages can be produced on the windings because the contactor is open. This overvoltage can damage the insulation of the windings.
By keeping the PF correction capacitors less than the MAX kVAR, the resonant frequency will be greater than 60 Hz and therefore the current produced as the motor coasts down will not go through resonance and cause an overvoltage.

Good explanation. I can almost remember most of this, now, from either electronics or physics classes I took in the mid part of last century.

Thank you

 

[mikeames]

If PF correction capacitors are larger than that providing MAX kVAR, then the resonant frequency of the capacitor and stator winding can fall below 60 Hz. Then when the motor contactor opens, the stored energy in the magnetic field induces currents in the rotor windings (e.g., squirrel cage), which then induces currents in the stator windings at a frequency corresponding to the rotation rate of the rotor. As the rotor coasts down and the frequency falls and passes through the resonant frequency of the capacitor/stator LC circuit, then relatively large voltages can be produced on the windings because the contactor is open. This overvoltage can damage the insulation of the windings.
By keeping the PF correction capacitors less than the MAX kVAR, the resonant frequency will be greater than 60 Hz and therefore the current produced as the motor coasts down will not go through resonance and cause an overvoltage.

This deserves an award.