Difference between a start capacitor and a run capacitor?
My rooftop unit's fan runs fine but sometimes the compressor won't come on. I hear a faint click but it won't engage and doesn't blow cold. Would this be a start or run capacitor issue?
Forum Discussion
well, could be the start capacitor. if it fails, a capacitor start (as opposed to split phase start) motor usually won't start, or at least has very very low starting torque. the start cap goes out of the circuit once the motor rpm goes above a certain rpm. then the motor may or may not utilize a running capacitor.
so it could be a failed start cap or a failure in the contactor for the start cap. the basic circuit is a starting winding with a capacitor in series with the starting winding and a switch usually centrifical.
If it was a run capacitor issue, normally the motor would start to turn and then either not come up to speed, or would come up to speed, or close to it and then either stall or struggle with no run winding.
the other remote possibility is that motor is designed with a starting cap that is always in the cicuit (PSC). downside is lower starting torque, but higher than split phase, but the motor can also be reversed. In this motor if the cap fails (no switch involved) then motor won't start.
the other possibility is that the motor is trying to start to soon after shutdown. In that case, the high pressure from the compressor is high enough that the motor can't start. Usually an AC system is designed so it won't try to restart for a period of time (few minutes) to let pressure drop. If that circuit fails and allows the motor to try to restart then that may be the issue.
so it could be a failed start cap or a failure in the contactor for the start cap. the basic circuit is a starting winding with a capacitor in series with the starting winding and a switch usually centrifical.
If it was a run capacitor issue, normally the motor would start to turn and then either not come up to speed, or would come up to speed, or close to it and then either stall or struggle with no run winding.
the other remote possibility is that motor is designed with a starting cap that is always in the cicuit (PSC). downside is lower starting torque, but higher than split phase, but the motor can also be reversed. In this motor if the cap fails (no switch involved) then motor won't start.
the other possibility is that the motor is trying to start to soon after shutdown. In that case, the high pressure from the compressor is high enough that the motor can't start. Usually an AC system is designed so it won't try to restart for a period of time (few minutes) to let pressure drop. If that circuit fails and allows the motor to try to restart then that may be the issue.
