Abnormal PSC Motor Current Draw? 1

[jimolson]

We use 3/4 horsepower PSC motors operating at 120VAC. Normal current draw is 8 amps. Locked rotor current is 13 amps. We have a circuit that monitors the motor current and shuts off AC power if it exceeds 12 amps for a fraction of a second. The PSC capacitor is 90uF.

With an average frequency of once every year or two our motor controls are getting fried by motor current. We think a rare and short term spike of motor current is killing our drive triacs. (Our drive electronics use triacs so that we can select motor shaft rotation direction under software control.)

I have tested these our electronic controls extensively. They are bulletproof for 15 amps continuous current and 50+ amps of short term current. The over current detection circuit works reliably, but takes tens of AC line cycles to respond and shut off current. We have protections in software that prevent reversing motor drive from clockwise to counter-clockwise while the shaft is spinning.

Question to readers: Can a PSC motor's stator core get saturated and/or left with residual magnetic field when the motor turns off? Would this residual field in the core cause the motor to draw super-large current during start?

Another observation: Very occasionally the motor's load causes the shaft to stop spinning for about 10 AC line cycles, thereafter releasing it to spin freely with little or no load. When this occurs, it is common for the motor shaft to "rebound" from lock rotor with spin opposite to the applied AC drive current. Helping the spin reversal phenomenon is that the mechanism that causes brief locked rotor has some spring wind up to it. When the lock rotor scenario ends, the load spring has a tendency to spin the motor backwards for a fraction of a second. The dynamics of a PSC motor are able to sustain backward spin if there isn't much load on the motor.

Does this description (possible core saturation or locked rotor plus reversed spin) hint at a reason for brief high motor current that exceeds locked rotor current?

Thanks in advance for any comments you might offer on this.

 

[jraef]

In my experience, solid state control and single phase motors with capacitors are incompatible. To a triac, the charging current of a motor the capacitor looks like a short circuit. In a PSC motor this is dampened by the fact that the cap is in series with the winding, so of the single phase motors with capacitors it is the “least unfriendly” to electronic control, but still not ideal and it’s usually a race to see what fails first; the controller or the motor. Under the right conditions however, such as the phase angle at the moment the triac fires for the first time or in your case maybe a reverse command while the motor is still spinning, the dI/dt seen by the triac as the capacitor charges can be high enough to cause it to short.


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