are usually of larger tonnage than those used for
Motor Overload Protectors
When the compressor is powered by an electric
motor, either belt driven or as an integral part of the
compressor assembly, the motor is usually protected
by a heat-actuated overload device. This is in addition
to the line power fuses. The heat to actuate the
overload device is supplied by the electrical energy to
the motor, as well as the heat generated by the motor
Figure 7-31.Thermal overload relay.
itself. Either source of heat or a combination of the
two, if too much, causes the overload device to open
and remove the motor from the line.
Figure 7-31 shows a thermal-element type of
overload cutout relay. It is housed in the magnetic
starter box. On current overload, the relay contacts
open, allowing the holding coil to release the starting
mechanism, thereby stopping the motor.
An oil failure cutout switch is provided on many
systems to protect the compressor against oil failure.
The switch is connected to register pressure
differential between the oil pump and the suction line.
Figure 7-32 shows a typical oil failure cutout switch.
The switch contains two bellows, which work against
each other, and springs for adjusting. Tubing from the
oil pump is connected to the bottom bellows of the
switch. Tubing from the suction line is connected to
the upper bellows. When a predetermined pressure
differential is not maintained, a pair of contacts in the
switch is opened and breaks the circuit to the
compressor motor. A heating element with a built-in
delay is in the switch to provide for starting the
compressor when oil pressure is low.
The water-regulating valve used with a water-
cooled condenser responds to a predetermined
condensing pressure. A connection from the discharge
side of the compressor to the valve transmits
condensing pressure directly to a bellows inside the
Figure 7-32.Oil failure cutout switch.