Background: A few days ago, the air conditioning unit at work began failing to start. Fans would turn, lights would be on, but no compressor start. I called a technician and he said the start capacitor was damaged (open).

So, I said, please replace said start capacitor (not expensive, excluding workmanship), and he replied that the compressor should be checked because if the compressor is being overloaded the capacitor would blow again.

Knowing that start/run capacitor, when used in monophasic applications, have only the purpose of creating a phase shift allowing for increased starting torque, and after started it effectively is disconnected from the circuit I suspect there is no sense in that.

That said, I ask you if the load profile of a motor, for example, if overloaded, have any influence in the longevity or failure rate of start/run capacitors.

Do not take into consideration ambient temperature, line voltage fluctuations and other factors not related to the motor and the capacitor.



1 Answer 1


The start cap being overloaded can be a symptom of the centrifugal switch not switching the start cap off when the motor is up to speed. Or the motor never getting up to the speed that would trigger the disconnect in the first place.

A start/run capacitor remains connected in the circuit (the "run" part of the name). So the motor being overloaded would mean more current going through the capacitor, which would then lead to overheating and overloading.

  • \$\begingroup\$ Ok, from your answer I understood that in the case of a start capacitor (with centrifugal switch) the motor being overloaded would prevent it from reaching enough speed to actuate the switch, leading to the capacitor remaining connected. And in the case of a start/run capacitor (no switch), the motor being overloaded would prevent it from reaching enough speed so the phase difference would remain high (and power dissipated at the capacitor high as well). Is my understanding correct? \$\endgroup\$ Commented Nov 28, 2018 at 16:07
  • \$\begingroup\$ It's more that an overloaded motor induction motor will be drawing more current in general. This would in turn lead to more current through the capacitor. \$\endgroup\$ Commented Nov 28, 2018 at 16:17

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