Pilot Duty Motor Protector
Some manufacturers will install a pilot duty motor protector (or pilot duty thermostat) on the compressor motor to protect the motor from overcurrent damage. This device is a small temperature- sensitive thermostat mounted inside the compressor on the motor windings. If the motor windings become overheated, the pilot duty thermostat breaks the circuit to the contactor relay and shuts off the compressor motor.
The pilot duty thermostat is usually wired in the 24-volt control circuit. This can be accomplished by interrupting the transformer secondary of the control circuit. A wiring diagram of a pilot duty thermostat connected in a 24-volt circuit is shown in Figure 6-64. Pilot duty thermostat contacts can also interrupt 120-volt and 240- volt circuits.
Capacitors
A capacitor is a device that provides the phase shift in the running and starting windings of an electric motor in order to increase the torque and efficiency of the motor-compressor assembly.
Capacitors are used in the following single-phase induction motors:
• Capacitor-start motors
• Permanent-split capacitor motors
• Capacitor-start, capacitor-run motors
A capacitor-start motor (see Figure 6-65) is used to power fans, blowers, and centrifugal pumps where constant-speed drive is
necessary. This motor develops high starting torque on the frictional horsepower ratings and moderate starting torque in the lower ratings.
In operation, an auxiliary winding is connected in series with a capacitor in the motor circuit. When the motor approaches running speed, a centrifugal switch cuts the capacitor and auxiliary winding out of the circuit.
A capacitor-start, capacitor-run motor (see Figure 6-66) also develops high starting torque. This is accomplished by employing a starting capacitor and a running capacitor. The starting capacitor gives good starting ability but is suited for short-time operation only. The starting capacitor is cut out of the circuit during the running period. The running capacitor provides high efficiency at running speed. Capacitor-start, capacitor-run motors are used to power compressors, reciprocating pumps, and similar types of equipment.
On some capacitor-start, capacitor-run motors, the starting capacitor may be cut out of the circuit with a centrifugal switch (see Figure 6-66). An alternative method of cutting out the starting capacitor is by using a back-emf relay (see Figure 6-67). Back-emf relays are usually used on hermetic (sealed) compressors where it is impractical to install a centrifugal switch. The coil of the back-emf relay is connected in parallel with the auxiliary winding. When the
motor approaches running speed, the relay pulls in, opening its contacts and taking the capacitor out of the circuit.