I was disassembling a belt disc sander today and noticed that there was a rather large relay between the switch and the motor. The relay consists mostly of a large copper coil. The motor doesn't have a capacitor and neither does the switch assembly. There are three wires coming out of the motor: red, white and black. All of the wires are of the same size about 14AWG.
When I tried wiring the motor without the relay, I couldn't get it to start without turning the shaft by hand. This would mean that the relay was acting as a starter. I am perplexed because I was under the impression that the only way to start a motor was either by hand or a start capacitor.
Can a relay be used to start a motor, if so, how?
Best Answer
The relay is likely connected to an auxiliary motor winding that has a different number of turns, a different wire size and resistance compared to the main winding. That would act to start the motor similarly to capacitor starting. There are several possibilities for the motor design details. It is not possible to completely and accurately explain the motor design without close examination of the construction. Photographs might help, but that might still not be enough to determine exactly how the motor works.
Edit Re Photos
Note that the relay has relatively few turns of heavy wire. I believe it is a current relay, wired in series with the main winding. When the motor is first turned on, a high current flows in the main winding through the relay coil causing the relay to connect the auxiliary winding. Since the aux winding has a different inductance and resistance, its current is phase shifted compared to the main current similarly to the phase shift that a capacitor would cause. That defines the direction of rotation and provides starting torque. When the motor approaches full speed, the current drops and the relay disconnects the aux winding.