Overvoltage - motor/capacitor combination

[Skogsgurra]

Dear all,

I have been loking for data on the possible overvoltage that can occur when an induction motor with local compensating capacitors is disconnected from the mains, but my search phrases do not lead anywhere.

My thinking is that some combinations of motor inductance and capacitor size can cause resonant overvoltage when the motor slows down from nominal speed.

Has anyone done actual measurements or simulations on this phenomenon? Or is there a site where I can read about it?

Gunnar Englund

 

[electricpete]

If we model the motor as two parallel branches..
inductive magnetizing branch (Lm) and resistive load branch:

We see that the capacitor required to establish resonance at 60hz is the one which corrects the motor pf to 1.0.

If we do not fully correct to 1.0, then the resonant frequency will be slightly higher. This assures that upon disconnection and during coasts down (frequency decreases), the resonant frequency will not be excited.

In summary, the overvoltage upon switching out motor with caps directly connected is avoided by not fully correcting to ph=1.0.... or by connecting capacitors in a configuration where they will not be switched with the motor.

 

[jbartos]

Suggestion: Conservative rules may be applied:
1. The capacitor current should not exceed the induction motor no-load magnetizing current
2. Some small distinction should be made between:
2a. Normal starting torque, normal starting current, and NEMA Classification Design B motors, versus
2b. High starting torque, low starting current, and NEMA Classification Design C motors.
Motors in 2b may have somewhat higher capacitor kVAr rating, e.g. 10% or so depending on the motor HP size, i.e. from 10HP and up.

 

[busbar]

From Longland, et al, Power Capacitor Handbook: …Capacitors used for individual motor correction should not exceed the no-load KVA of the motor. The reason for this is to ensure that overexcitation of the motor cannot occur. (I)f the rating of the capacitor is more than necessary to provide air-gap magnetization of the motors, …on open circuit the motor will excite to the point where its voltampere magnetization curve is intersected by the straight-line voltampere characteristic of the capacitor…

If the connection between the most and supply is restored while the motor is overexcited, there is risk of a violent electrical transient producing a mechanical shock. This has been known to fracture shaft couplings, damage gearing or cause distortion of cores or motor windings.