Relays are a
necessary part of many control and pilot-light circuits. They are similar in
design to contactors, but are generally lighter in construction so they carry
smaller currents.
Compressors
used for household refrigerators, freezers, dehumidifiers, vending machines,
and water coolers have the capacitor-start, induction-run type of motor. This
type of compressor may have a circuit that resembles below.
When the
compressor is turned on by the thermostat demanding action, the relay is closed
and the start winding is in the circuit.
Once the
motor comes up to about 75% of rated speed, there is enough current flow
through the relay coil to cause it to energize, and it pulls the contacts of
the relay open, thereby taking the start capacitor and start winding out of the
circuit.
This allows
the motor to run with one winding as designed.
Below show
the current type of relay. This is generally used with small refrigeration
compressors up to 3/4 horsepower.
This is
generally used with large commercial and air-conditioning compressors up to 5
horsepower.
Protection
of the motor against prolonged overload is accomplished by time limit overload
relays. They are operative during the starting period and running period.
Relay action
is delayed long enough to take care of the heavy starting currents and
momentary overloads without tripping.
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