Ac compressor Box relay|Capstor starting realy.potential relay connection

otential Relays
Typically, a refrigeration or air conditioning compressor
needs high starting torque. A start capacitor,
wired in series with the motor’s
start, or auxiliary,
winding, provides
additional current to give
the motor more starting
torque when the circuit is
energized. Start capacitors,
unlike run capacitors, are
designed for intermittent duty.
Capacitor start motors require an electrical device to
disconnect the start capacitor as soon as the motor
attains the minimum speed needed. The most
practical, reliable, accurate and economical device for
this purpose is a potential relay. A potential relay
senses voltage across the start winding. When this
voltage reaches a sufficient level, the start capacitor,
which is in series with the start winding, must be
disconnected from the circuit.
When a capacitor start motor runs, a voltage is induced
in the start winding. This voltage varies with the speed
of the motor. Too much induced voltage can cause
such a high current that it can destroy the start winding
in seconds. For this reason, it is important to have a
reliable device to disconnect the start capacitor before
the induced voltage exceeds safety limits, but not
before the motor has attained sufficient speed to
continue to accelerate to its full load rated speed.
Accuracy, repeatability, reliability and long life are
attributes that ensure proper starting of compressors
and pumps used in HVAC/R applications.
When a capacitor start, induction run (or capacitor
start, capacitor run) motor runs, voltage is generated in
the start winding. The generated voltage is the product
of magnetic lines of flux, induced in the motor’s rotor,
cutting through the coils of the start winding. This
voltage is a function of the speed of the motor and the
number of turns in the start winding. Higher the speed
of rotation, the greater the induced voltage.
When the motor is first energized, the voltage across
the start winding is lower than the applied voltage.
This is because the start circuit is composed of the
start winding in series with the start capacitor. Each
component takes a portion of the applied voltage. As
the motor comes up to speed, the induced voltage
plus the line voltage reaches a value called the
“continuous coil voltage”, which is an important rating
for a potential relay. If a potential relay with a
continuous coil rating that is too low is applied to a
compressor’s starting circuit, the relay will fail.
Potential relays are normally closed devices.
This means that the relay contacts between “1” and
“2” are closed when a motor is first energized. The
opening of these contacts is the primary function of a
potential relay.
Operation
At the instant voltage is applied to a capacitor start,
induction run motor with a potential relay, the relay
contacts are closed, and the applied voltage is split
between the start capacitor and the motor’s start
winding. As the motor quickly builds up speed,
voltage is induced in the start winding. This voltage is
sensed by the potential relay. As soon as the
potential relay senses a pre-determined value, the
relay’s contacts open or “pick up”. As soon as the
relay contacts pick up, the start capacitor is
disconnected from the motor’s start circuit. Potential
relays are designed or selected with specific motor
speed and torque requirements as criteria. When a
properly selected potential relay picks up, the motor
to which it is applied no longer needs the extra torque
provided by the start capacitor and has enough
momentum to continue to accelerate to full rated
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Potential Relays
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