PFC discharge time

[ptonsparky]

Customer with a new soft start on a 75 hp well. POCO normally requires PFC on larger motors. From what I have read these should be installed on the line side of the SS, a min distance away and brought on line after it has switched into bypass. Question is how long should I hold the PFC offline to allow for discharge in case of multiple starts for whatever reason. Eatons state theirs discharge to less than 50v in one minute. 67 seconds close enough, or better with 5 minutes?

 

[gar]

110804-1302 EDT

ptonsparky:

Consider this:
1. Contactor to power factor correction capacitor is connected directly across the motor.
2. Control this contactor to close after the soft start is bypassed.
3. Provide a slight delay to opening the contactor after power is lost to the motor. The capacitor will be discharged by the time the motor stops rotation.

.

 

[ptonsparky]

To simple. Thank you

 

[SOG38]

Capacitor Discharge Time

Capacitor Discharge Time

460.6 says Capacitors must discharge to less than 50 volts within 1 minute. Most I have been involved with have discharge resistors internal or external to the cap.

 

[Jraef]

If the cap is on the line side of the soft starter, you don't have to worry about it discharging before the next start, makes no difference. You only need to worry about discharge in terms of personnel safety, hence the 50V or less in 1 minute. 50V is considered "safe".

The potential danger in gar's solution is if the bypass doesn't close for some reason, you lose the soft starter and caps (it's usually a race to see which one dies first).

 

[beanland]

5 Minutes

5 Minutes

In the utility industry, IEEE recommends 5 minutes between opening the feed to a capacitor and closing it again, giving the capacitor adequate time to discharge.

If you have a way to accelerate the discharge, that is OK but don't short the capacitors because that can damage them.