HARD START KITS THEORY:
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The relay is a potential relay. That is one that senses the potential of the electrical apparatus
to which it is attached. In our case, the relay senses the electromotive force produced by the
movement of the compressor. As it picks up speed, the compressor produces more electromotive
force and it transmits (in a sense) it to the potential relay that is monitoring it. It is always producing
this force when it is moving. It is a generation of an even higher voltage than the one that is being
supplied to it to move it in the first place. All motors do this.
The potential relay contacts will be initially closed, allowing a circuit from the start winding, through (not exactly) the capacitor, and then to the run and start windings. When the supplied voltage (current) is applied to these terminals of the compressor it will "charge" the capacitor and in some nanoseconds it will discharge back to the windings its designed value of voltage. Ah! But wait. It is not of the same movement upon a sine wave spectrum as is the 230-voltage supply that was there. In other words, it is delayed and sent back. Thus, it is a different "phase" of electrical supply current.
Note on alternating current sine wave movement: If a horizontal line indicates a zero voltage level of current and if above the line is positive and below the line in negative, and movement from a point at the left to the right is time, then we have a sine wave of alternating current that we can see. Start at zero to the left. As time passes, assume that the first action is positive. As time occurs and the voltage increases, it would display an arc upward to a given level and start to arc back down toward zero again. It would continue past (below) the zero line and arc downward to a given level opposite but equal to the positive arc and then start arcing back upward to return to zero. This is a phase, a sine wave, of one cycle of alternating current. If it does this 60 time per second, then it is a 60 Hz electicity sine wave form. If a second voltage (current) is displayed on the same sine wave form, but delayed a few nanoseconds, it would be identical, but slightly to the left at all times in each of the same movements. Now we see two phases of current displayed that we call "single phase". One phase is making a force upon something and the second wave is applying the same force, but at a different interval of time. Kind of like shoving something and then immediately shoving it again. Let us assume it is 230 Vac.
You can display a third voltage on the same form but slightly delayed a few more nanoseconds and this is "three phase" current's sine wave form. Now it is like shoving something three times. You can visualize how three phase motors start easier.
Now back up to the original 230 Vac supplied. One leg of it is a phase on the sine curve. The second leg is a slightly delayed identical movement, but since it is delayed, then it will give a "another" force to the windings as it comes in from the other side of this load. Now across the start and run windings introduce the capacitor's third phase intermittently pulsed current. You have a simulated third phase.
You know a three-phase motor requires no start kit and yet it will start better at all times. Now we have made the start capacitor produce a third (however, a simulated) phase for a "single" phase motor (that is really two phases). It works better.
The problem is complex as to the selection of the MFD (microfarad--a measure of its capacitance) rating of the capacitor. It is best to check the manufacturer's specification. If you don't have one, then start low. With a 2 to 5 ton unit use a 135-150 with a 370 or 440 V rating.
Run capacitors are very similar to start capacitors. Run capacitors are designed to stay cool because they remain in the circuit.
The start capacitor will explode if left in the circuit too long. We don't need it to stay very long anyway. Its purpose is to get the compressor moving from that dead stopped position. Once it is moving, then we want it out of the circuit to allow the regular voltage to the windings to do the job they do more efficiently and better. So....
The start relay (potential relay) will sense the electromotive force of the compressor winding's generation as it starts to move and increase in speed. When it senses a pre-determined voltage generation by the compressor, it will drop out of the circuit by opening the contact which will remove the possibility of a circuit between the start windings, capacitor and run windings. Selection of the relay it important and, again, use the recommended one specified by the compressor manufacturer.
If you want to know something about electromotive force, try this. Attach a voltage meter to the leads of a motor and spin the shaft and watch the current reading on the meter. That is generated voltage called electromotive force.
