Start and Run capacitors...
Hi Louis; it can get confusing with all the relays and capacitors and those kind of things! Here's a run-down on the differences.
Original type relay. This is a "current relay" which engages the start winding when the compressor is drawing a high level of current because it's not running yet. It only engages the start winding when necessary, disengaging it at the moment the compressor starts. It will work again immediately if the compressor is stopped and re-started. There is no time waiting required. This has to be matched to the compressor so it kicks in and out at the correct current level, based on the compressor motor requirement.
The Supco RCO and RO devices. This is the 3-in-1 or the smaller one you linked above without the capacitor. This is not really a relay at all. It's a timing device. I won't go into how it works, but will explain what it does. This device provides power to the compressor's start winding for a short period of time, once, each time the compressor is powered on. The device does not know if the compressor started or not - it just sends a timed start signal. The timing device also requires a reset time of up to 5 minutes. If the power is removed then re-applied before this time has elapsed, there will be no start winding activation and the compressor will not start.
Capacitors alter the relationship between the AC waveforms going to the run and start winding. This is called "phase shift." It generates a rotating magnetic field to get the motor going.
Start capacitor has a relatively large capacitance value. It generates a strong phase shifted signal with the motor at stalled or very low speeds. However, it will generate an excess voltage and high current if it continues to operate while the motor is at full speed; resulting in damage. It is used with a start relay or timed start device such as the Supco. The 3-in-1 has a start capacitor in it.
Run capacitor (or permanent capacitor) provides the correct level of voltage and phase shift while the motor is under normal running conditions. It has a smaller capacitance value than the start capacitor; and can operate continuously. This capacitor will not provide much starting torque while the motor is at a standstill. While running the effect is an increase in efficiency (less amps needed to do the same work).
Often systems will have both start and run caps. The start cap has a switching system so it only works during starting. The run cap is permanently connected and stays in circuit at all times.
Hope this is useful info!
