The motor start relay is used to remove the starting capacitor from the motor starting circuit when the motor reaches about 75 to 80 percent of its operating speed. Start relays are made that can be actuated by either current or voltage. The start relays used with HVAC equipment motors are normally voltage-actuated relays (potential relays); therefore, the remainder of this discussion will cover the operation of the voltage-actuated start relay.
Because all electric motors are inductive loads, they have some electrical voltage-generating capacity. The voltage generated is called back EMF (electromotive force). One important difference between back EMF and the line voltage applied to the motor is that the back EMF can be a much higher voltage. For example, a motor powered by 230 volts can generate back EMF voltages greater than 400 volts. It is this back EMF voltage that is used to energize the starting relay coil. The start relay is selected so that it will not energize at line voltage, but will energize at some higher voltage. When it is necessary to replace a motor start relay, an identical replacement must be used. Substitution of a relay with a different pickup voltage can cause damage to the start capacitor or compressor motor start winding. Also, the replacement relay must be positioned and wired exactly as the original. When troubleshooting a start relay circuit, it is important to remember that the back EMF can be a much higher voltage than the line voltage applied to the motor.
Figure SP-9-11 shows a C SR compressor motor with a voltage-actuated start relay. A t the instant the motor is turned on, line voltage is applied across the motor start and run windings and the coil of the start relay. However, the back EMF voltage built up across the start winding is not high enough to cause the start relay coil to energize. Because the relay is deenergized, its closed contacts connect the start capacitor into the start winding circuit.
As the motor speeds up and reaches about 75 to 80 percent o f its maximum speed, the back EMF reaches the voltage level that causes the start relay coil to energize. When the start relay coil energizes, its contacts open and remove the start capacitor from the motor circuit. The motor start capacitor will remain disconnected until the motor is turned off and the start relay is once again deenergized.
The procedure for testing the start relay is provided in the detailed procedure given at the end of this section. Motor start relays tend to fail with the contacts closed. When this happens, the start capacitor remains in the compressor start circuit, causing the start winding to overheat and fail. When the contacts are stuck closed, it may also result in the failure of the start capacitor.
