A LOOK AT SERVICE SAFETY
Operation
113
II.
Crankcase Heaters
When most air conditioning and commercial sys-
tems are started up for the first time each season, a
large part of the system refrigerant charge is in the
compressor. On startup, the refrigerant boils off tak-
ing the oil charge with it, and the compressor is
forced to run for as long as 3 or 4 minutes until the
oil charge circulates through the system and returns
to the crankcase.
Obviously this is not good for the compressor and
definitely shortens its service life.
The initial solution is to charge the system so that
little or no refrigerant collects in the crankcase and
to operate the crankcase heater at least 12 hours
before startup or after a prolonged shutdown.
Three types of crankcase heaters are in common use
on Tecumseh compressors: wrap-around resistance
heater (belly band), immersion type integral heater,
and the run capacitance off-cycle heat.
The wrap-around heater should be strapped to the
housing below the oil level and in close contact with
the housing. A good heater will maintain the oil at
least 10°F above the temperature of any other sys-
tem component and (when the compressor is
stopped) desirably at or above a minimum tempera-
ture of 80°F.
The immersion type heater is factory assembled and
is presently used with AB, AW, AG, AV, and AN
compressors. It is self-regulating and energy effi-
cient.
In the run capacitance off-cycle heat method used
by some OEM, single phase compressors are
stopped by opening only one leg (L
1
) and thus the
other power supply leg (L
2
) to the run capacitor
remains “hot.” A trickle current through the start
windings results, thereby warming the motor wind-
ings and thus the oil on the “off-cycle.”
WARNING!
Before servicing systems with off-
cycle run capacitor type heaters, be sure to dis-
connect ALL power supplies. Make sure that all
power legs are open. Failure to do so can result in
serious injury or death from electric shock.
Capacitance crankcase heat systems can be recog-
nized by one or more of the following:
•
Contactor or thermostat breaks only one leg to
the compressor (and condenser fan).
•
Equipment should carry a notice indicating
power is on compressor when it is not running
and that main breaker should be opened before
servicing.
•
Run capacitor is sometimes split (3 terminals)
so that only part of the capacitance is used for
off-cycle heating. NOTE: Use exact replace-
ment when changing such dual purpose run
capacitors. Capacitor must be fused and carry a
bleeder resistor across the terminals. See
Figure 6-8 for the basic wiring diagram for a
PSC compressor with run capacitance off-cycle
heat.
