Crankcase Heaters - Tecumseh AH5540E Service Handbook

Crankcase Heaters

When most air conditioning and commercial systems 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 taking 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 shut-
down.
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 system component and (when the compressor is stopped) desirably at or above a
minimum temperature 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 efficient.
In the run capacitance off-cycle heat method used by some OEM, single-phase compressors are
stopped by opening only one leg (L2) and thus the other power supply leg (L1) to the run capaci-
tor remains "hot." A trickle current through the start windings results thereby warming the motor
windings and thus the oil on the "off-cycle."
WARNING! Before servicing systems with off-cycle run capacitor type heaters, be sure to discon-
nect 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 recognized 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 run-
ning 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 replacement 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.
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