Failing Capacitors

[charz]

Problem:
We have a motor driving a vertical pump. The problem is there is no check valve at the discharge, so whenever the pump is stopped, due to the back flow the shaft rotated for about 40 seconds. The excitation to the motor is provided the power factor capacitor. The Power factor of the motor feeder is almost unity PF 0.98~0.99 when the the PF improvement capacitor is connected. The motor is started through reactor starter which has a bypass latched type VC. I also posted about this earlier in this forum

In order to over come this problem, we had put a separate Vacuum contactor for disconnecting capacitors from the circuit whenever the motor is stopped. Now the capacitors have started to fail. In one starter, all the three capacitors have failed and in another starter one of the capacitors got failed. We feel that this is due to switching transients of the capacitor VC. We have no other non linear loads. Could resonance between the motor and the capacitor still damage the capacitor as it is almost unity pf?

Any comments.

 

[waross]

Do you disconnect the capacitors a short time before stopping the motor?

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[crshears]

Perhaps you could solve an electrical problem by mechanical means and install a check valve at the pump discharge...

CR

"As iron sharpens iron, so one person sharpens another." [Proverbs 27:17, NIV]

 

[edison123]

Do the pf capacitors have discharge resistors?

Muthu

http://www.edison.co.in

 

[charz]

Do you disconnect the capacitors a short time before stopping the motor?

The capacitor disconnecting Vacuum contactor gets disconnected, the moment the motor is stopped. This capacitor Vacuum contactor gets operated from the upstream VCB auxiliary contacts.

Do the pf capacitors have discharge resistors?

No the PF capacitors do not have external discharge resistors. The manufacturer may have provided internal discharge resistor.

 

[charz]

The main transformer is wye- wye type, So there could be third harmonics from the system entering in to the capacitors.

 

[jraef]

Older vacuum bottle designs often suffered from having big "chop current" spikes and required surge suppression when used on things like capacitors. Although mostly solved by more modern designs and materials in vacuum bottles, this has reared its ugly head again recently with the advent of cheap vacuum contactors being mass produced in places where design and quality are irrelevant. In other words you may be the victims of a disease that was thought cured decades ago but has cropped up once again due to lack of concern.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington