You underestimate my desire for knowledge. Do it. Talk about refrigeration compressor motors.
Hello hi, sorry for the delay. I had to wait for the brain power to return to me.
So, an electric motor's job is to convert electrostatic potential energy into magnetic potential energy in order to rotate a shaft, thus making one final energy conversion from magnetic PE to kinetic energy. There is a secret fourth energy conversion, though, and that is another form of kinetic energy: heat. The heat is waste, though.
Anyways, the power available in your home is not the power generated at the source powerplant; it is just a single phase of alternating current (AC), instead of three. Electric motors running on single-phase encounter an interesting problem: they need a special winding, called a start winding, to get the motor going. It is phase shifted from the run winding (which keeps the motor running after startup). The easiest way to explain phase shift, in this context, is to think of it like the pedals on a bicycle: if the pedals weren't opposite each other (offset from each other 180 degrees), it would be very difficult to pedal. Unlike a bicycle, the phase shift in the motor isn't 180 degrees. It's less than that. About 45 degrees.
Both the start and run windings are coils of wire. The more times the wire is wound, the stronger the magnetic field it generates. The start coil has more windings than the run coil.
To help smooth power delivery and introduce the aforementioned phase shift, we need capacitors. Capacitors are just small batteries. They can hold a charge and be discharged. They also block direct current (DC), but allow AC to pass through it. It's really neat. On startup, a single-phase motor using capacitors only needs the start coil and the start capacitor briefly. If they remain in the circuit afterwards, they risk overheating and destroying themselves. So how do we automatically take the start coil and capacitor out of the circuit after startup?
One way is to use a centrifugal mechanical switch that will disconnect the start components from the circuit once the motor reaches 75% of its maximum RPM. This switch must be in the motor, though, and will create a spark when it breaks contact. This is okay for motors that are in the air, but what about the motors in a hermetically sealed refrigerant compressor? That spark would degrade the refrigerant, and that's a non-starter.
We need an RPM sensitive switch outside the compressor motor, and one way to do this is with a potential relay.
This is a diagram of a potential relay circuit with a capacitor-start, capacitor-run compressor motor.
On startup, the relay contacts, 2 and 1, are closed. Power can flow through the start winding and through both the run and start capacitors. This gives the motor a good grunt to get it going, with the start winding slightly out of phase with the run winding. Once the motor reaches 75% of its max. RPM, though, the magnets on the rotor/shaft of the motor induce a back electromotive force, or BEMF, in the start winding. The start winding becomes a voltage source in tandem with the incoming power from the outlet.
The induced AC voltage is stronger than the incoming power, and is also out of phase with it. This is demonstrated in the diagram with voltmeters 1 and 2. The induced voltage flows through the coil of the potential relay at terminals 2 and 5, and the coil generates a magnetic field which pulls the relay contacts open, breaking the connection to the start capacitor and preventing the start winding from connecting with the other side of our power supply. So long as the motor is running, the BEMF is present and keeps the relay open.
This is what the voltage looks like on an oscilloscope. CH1 is the smaller wave and it's on the common run terminal. CH2 is the larger wave and is at terminal 2 of the relay.
As we can see, the induced voltage on the start winding lags behind the line voltage by about 45 degrees and it is much stronger.
When the motor shuts off and slows down, the induced voltage disappears and the relay contacts close again. The relay sits outside the motor where sparks are acceptable.
As a cherry on top, I've drawn the current paths through the circuit to help illustrate what's happening:
^ Start condition (red). Current flow through the start winding and start + run capacitors
^ Run condition (blue). BEMF across the start winding powers the relay coil and opens the contacts.
There is a lot more going on I could talk about, but that's just the part I wanted to be autistic about. There is actually a better way to draw out this circuit that looks like this:
Sw = Start winding
Rw = Rung winding
PR = Potential relay
Sc = Start Capacitor
Rc = Run capacitor
And isn't that fascinating?
