Compressor Motor Starting Relays - Tecumseh AEA4440YXA Service Data Book

A LOOK AT SERVICE SAFETY
IV. Compressor Motor Starting
Relays
A hermetic motor starting relay is an automatic
switching device to disconnect the motor start
capacitor and/or start winding after the motor has
reached running speed.
Never select a replacement relay solely by horse-
power or other generalized rating. Select the correct
relay from the Tecumseh Electrical Service Parts
Guide Book.
There are two types of motor starting relays used in
refrigeration and air conditioning applications: the
current responsive type and the potential (voltage)
responsive type.
A. Current Type Relay
When power is applied to a compressor motor, the
relay solenoid coil attracts the relay armature
upward causing bridging contact and stationary
contact to engage. This energizes the motor start
winding. When the compressor motor attains run-
ning speed, the motor main winding current is such
that the relay solenoid coil de-energizes allowing the
relay contacts to drop open thereby disconnecting
motor start winding.
The relay must be mounted in true vertical position
so armature and bridging contact will drop free
when relay solenoid is de-energized.
Figure 3-17. Current type relay.
B. PTC Type Relay
Solid state technology has made available another
type of current sensitive relay—a PTC starting
switch. Certain ceramic materials have the unique
property of greatly increasing their resistance as they
heat up from current passing through them. A PTC
solid state starting device is placed in series with the
start winding and normally has a very low resistance.
Upon startup, as current starts to flow to the start
winding, the resistance rapidly rises to a very high
value thus reducing the start winding current to a
trickle and effectively taking that winding out of
operation.
Usage is generally limited to domestic refrigeration
and freezers. Because it takes 3 to 10 minutes to cool
down between operating cycles, it is not feasible for
short cycling commercial applications.
Figure 3-18. PTC type relay.
C. Potential Type Relay
Generally used with large commercial and air condi-
tioning compressors (capacitor start, capacitor run)
to 5 HP. Relay contacts are normally closed. The
relay coil is wired across the start winding and senses
voltage change. Starting winding voltage increases
with motor speed. As the voltage increases to the
specific pickup value, the armature pulls up, open-
ing the relay contacts, de-energizing the start wind-
ing capacitor. After switching, there is still sufficient
voltage induced in the start winding to keep the
Compressor Motor and Component Information
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