power factor correction capacitors - Resonant condition ? 3

[edison123]

Has anyone encountered resonant condition where capacitive reactance of the power factor correction capacitor is equal to inductive reactance of the motor ? If so, what were your observations during such resonance ? (excessive current, current oscillation etc.)

 

[jbartos]

Suggestion: It depends where the capacitors are located. Capacitors may be connected on the load side of the starter, and the capacitor and motor are switched as a unit. This is preferred and most advantageous location from an over-all standpoint. The capacitors may also be connected on the line side of contactor, i.e. permanently connected to the system. Its main advantage is elimination of a separate switching device for the capacitors. Also, the power factor correction of the system takes place no matter whether the motor is on or off.

 

[jbartos]

Comment: Limitation of Capacitor and Motor when switched as a unit:
1. Transient torques
Transient electrical torques of twenty times full load motor torque have been obtained in tests when too large a capacitor was switched with the motor.
2. Overvoltage due to self-excitation
As a benchmark, the resulting overvoltage with capacitor values to improve the full-load motor power factor to unity will range from 35 to 75 %. Self-excitation can be measured by connecting a voltmeter across the motor terminals and reading the voltage after the motor-starter switch is disconnected from the power source.

 

[Marke]

Hello jbartos

The best way of applying static correction, is to use two contactors, one for the motor and one for the capacitors. That way, there is not danger of the capacitors being connected across the motor when disconnected from the supply. The only problem is the additional cost.

Best regards,


Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion to the previous posting: It depends. The capacitor connected to the power source permanently can permanently correct the power factor within the upstream power distribution from the motor starter.

 

[Marke]

Jbartos,
Yes that is true, but you can end up with excessive capacitance on the supply resulting in a leadsing power factor rather than an almost unity power factor. The idea with static correction is to apply the leading component and the lagging component at the same time so that the influence on the supply is reduced.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion to the previous posting: If that is the case, the power factor may be monitored and some larger capacitors turned of by the motor-capacitor branch circuit interrupter.

 

[Marke]

That is bulk correction as opposed to static correction.
Bulk correction is usually at the point of supply while static correction is controlled with the motor. Ideally static correction uses a separate contactor, but in practice the same contactor is used.
Best regards,

Mark Empson

http://www.lmphotonics.com

 

[jbartos]

Suggestion: Reference:
1. Donald Beeman "Industrial Power Systems Handbook," 1st Edition, McGraw-Hill, 1955, page 478 Figure 8.21 shows the capacitor locations, one location is close to the motor, another location is on the contactor load side, and last location is on the fuse/breaker fuse side. The latter one is at a question. It is not called "bulk correction."

 

[electricpete]

Good comments Mark.

 

[jbartos]

Suggestion: The third location in my previous posting is on the fuse/breaker load side in the motor branch circuit. It is not referred to as "bulk correction" in the provided reference (Beeman Handbook). Therefore, I found Marke's posting somewhat inappropriate to my posting.