Thread: using run caps for start caps

I just had a start cap blow, and was wondering if I could put run caps in the bank of start caps while I order some new starts? Thanks

The only reason not to use run caps in the starting bank is cost. Run caps are far better quality components capable of sustaining far more abuse.

Correct!

Run caps are oil-filled, and are rated for 100-percent duty cycle.

Start caps are filled with an electrolyte, and are rated for a few start cycles within a fifteen-minute period, with a maximum on-time of perhaps 1-2 seconds for each such cycle.

Start caps have a very high capacitance per unit volume (translates to very modest cost).

Run caps have a very low capacitance per unit volume (translates to very high cost, when used for start service).

However, there are cases, such as very frequent start/stop/reverse cycles where using a run cap for a start cap makes very good sense.

In contrast, start caps cannot be used for run applications.

Can they be mixed? in other words can I leave the other start caps in and add the value of the one that blew in run caps? Thanks

"Can they be mixed? in other words can I leave the other start caps in and add the value of the one that blew in run caps?"

You may leave oil-filled run caps in the circuit permanently.

Electrolytic start caps can only be in the circuit intermittently ... a maximum of 1-2 seconds.

Originally Posted by bennersaw

can I leave the other start caps in and add the value of the one that blew in run caps? Thanks

Not wise, for a couple of reasons.

1) If one blew, the others will likely fail in short order, possibly causing damage to the motor;

2) It's not advisable to parallel dissimilar caps in any circuit. Current imbalance can stress some components excessively.

Best to replace the set (2 or more) with a single cap having a value equal to the sum of the capacitances of the original parts, if space permits.

- Leigh

Can they be mixed? in other words can I leave the other start caps in and add the value of the one that blew in run caps? Thanks

The short answer is yes provided the rating of the lowest rated capacitor is not exceeded, it would pay to measure the capacitance of the start capacitors, they dry out over time.

Capacitors intended for a given service can be paralleled at will, it is routine to parallel dissimilar capacitors provided the voltage rating, the duty rating and the current rating are correct for the given service.

Current rating (usually expressed as Kilo Volt Amps Reactive) is not usually given for General purpose capacitors but a mains capacitor will have a low enough Equivalent Series Resistance to handle the reactive current due it's capacitance when placed across a sine wave supply of mains frequency at it's rated RMS voltage without undue heating.

The duty rating for general purpose capacitors is 100% so this will not be an issue, start capacitors have duty ratings much less than 100%

Connecting dissimilar capacitors in series is another matter entirely.