Using the RTD as a Motor-Starting Switch

Another way RTDs are used is to put them in series with the start winding of a single-phase motor to take the place of the start (centrifugal) switch. The start switch is used to energize the start winding of the single-phase motor for just a few seconds until the motor's armature comes up to approximately 75% speed. When the motor reaches 75% speed, the centrifugal switch is turning fast enough to open and disconnect the start winding until the next time the motor needs to start. When the motor is de-energized, it will slow down sufficiently to allow the contacts of the centrifugal switch to go back to the closed position. The RTD can duplicate the function of the centrifugal switch by being at low resistance when no current is flowing, and the resistance will increase to a point to stop current from flowing. The resistance of the RTD will increase as the amount of starting current increases and warms it up. This type of circuit is used predominantly on refrigerator compressors that only start and stop several times an hour. The RTD can't be used to start motors that must turn on and off frequently because the RTD must have time to cool down before its resistance is low enough to allow full current flow to the start winding. The Figure below shows an example of this circuit.

Above: ill. 1 RTD used as the starting switch for a single-phase refrigeration compressor motor.