Compressor Motor Starting Relays - Tecumseh AH5540E Service Handbook

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 horsepower or other generalized rating. Select the cor-
rect 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.
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 ener-
gizes the motor start winding. When the compressor motor attains running 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. See Figure 3-11.
PTC Type Relay
PTC type relay is a solid state current sensitive relay. 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. See Figure 3-12.
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 com-
mercial applications.
Potential Type Relay
Potential type relays are generally used with large commercial and air conditioning com-
pressors (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, opening the relay contacts, de-energizing the start winding capaci-
tor. After switching, there is still sufficient voltage induced in the start winding to keep the
relay coil energized and the relay starting contacts open. When power is shut off to the
motor, the voltage drops to zero, the coil is de-energized, and the start contacts reset. See
Figure 3-13.
When changing a compressor relay, care should be taken to install the replacement in the
same position as the original. Table 3-4 lists useful information regarding starting relays.
FIGURE 3-11:
Current Type Relay
FIGURE 3-12:
PTC Type Relay
FIGURE 3-13:
Potential Type Relay
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