I am working on a project where power factor correction capacitors are employed on most motor loads down to 25HP. Most are approximately 30 years old. The power factor runs at a .82 monthly average. The facility is being penalized by the utility for being below .88 power factor. Before we install a new bank of capacitors I would like to check the existing capacitors to make sure that they are functioning. What is the best way to evaluate the existing capacitors to determine if they are functioning properly? I know some manufacturers have blown fuse indicators, some have burst protection that will take the capacitor off line if the case becomes over pressurized. Those are clear indicators of a non functioning capacitor. Are there other evaluation techniques anyone could suggest?
Capacitor Evaluation
- Thread starter mcnichol
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petersonra
Senior Member
- Location
- Northern illinois
- Occupation
- Semi-retired engineer
get an ammeter
measure current to capacitor
measure current to capacitor
Twodollar
Member
- Location
- San Jose, CA
I would also be suspect of 30 year old electrolytic capacitors, especially if they are well used.
As a first test, apply a dc voltage (less than what is rated) to it with an ammeter in series and see if there is any current flowing. If so, it is a bad capacitor. Maybe use one of those wall-wart battery chargers for the dc voltage because they have current limiting built in (just in case your capacitor is a dead short) You could also put a 1M ohm resistor in series with the dc voltage supply and see if there is any voltage drop across it (that would serve to limit any inrush current as well)
Find someone who owns an LCR meter to see if the measured capacitance matches the rating. The dielectric may have degraded.
Another concern is increased internal resistance which will cause heating within the capacitor. To measure this you will need an impedance analyzer (I hope you have an EE friend) Probably you won't need to go this far unless these are very big and expensive caps worth testing.
Have you considered other ways of power factor correction like a synchronous condenser.
As a first test, apply a dc voltage (less than what is rated) to it with an ammeter in series and see if there is any current flowing. If so, it is a bad capacitor. Maybe use one of those wall-wart battery chargers for the dc voltage because they have current limiting built in (just in case your capacitor is a dead short) You could also put a 1M ohm resistor in series with the dc voltage supply and see if there is any voltage drop across it (that would serve to limit any inrush current as well)
Find someone who owns an LCR meter to see if the measured capacitance matches the rating. The dielectric may have degraded.
Another concern is increased internal resistance which will cause heating within the capacitor. To measure this you will need an impedance analyzer (I hope you have an EE friend) Probably you won't need to go this far unless these are very big and expensive caps worth testing.
Have you considered other ways of power factor correction like a synchronous condenser.
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