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?



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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.

  • \$\begingroup\$ Added photos. I think you're on to something. \$\endgroup\$ – user148298 Dec 24 '15 at 1:01
  • \$\begingroup\$ Cool! Just curious. Can the relay be replaced with a capacitor and operate correctly? \$\endgroup\$ – user148298 Dec 24 '15 at 1:24
  • \$\begingroup\$ No. The motor has been carefully designed as it is. Some value of capacitor can probably be found to get the motor to operate, but its characteristics would change. I don't know if the aux current leads or lags the main current, but it is likely that it lags and a capacitor would make it lead and reverse the direction of rotation unless the winding connection is reversed. \$\endgroup\$ – Charles Cowie Dec 24 '15 at 1:38

When you bypassed the relay, was the wire a sufficient size to handle the current needed by the motor? The point of the relay is so that the large amount of current needed by the motor doesn't all have to go through the switch. A smaller, "signal" current can go through the switch, which actuates the relay which connects the much bigger wires that power the motor. If you bypassed the relay and used the smaller switch wires, they may be too small to power the motor. This may be especially true if it requires a large spike in power to get started (which is why they use start capacitors). Did you notice the wires getting hot when it was running?

  • \$\begingroup\$ This was a small motor. It's less than 1HP and looks like a washing machine motor. It has three wires of about 14 gauge: red, white, black. \$\endgroup\$ – user148298 Dec 24 '15 at 0:43

You might posted name plate details of this motor. From there you may find what kind of motor is that. From your information i guess its split-phase single phase induction motor.

In split phase induction motor, there are two windings

        1. Starting winding

              Fewer Turns with Lower Inductance and Higher Resistance. 

        2. Main (or) Running Winding 

 More no of turns with low Resistance and High Inductance

Here starting winding takes part while starting only, (up to 75-80% of rated speed) after attained this speed starting winding will be disconnected by centrifugal switch. Here instead of centrifugal switch the relay might be used.


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