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HVAC School - For Techs, By Techs

Sep 17, 2020

Kaleb, Joe, and Eric join us again to discuss some myths about single-pole contactors. We also cover some weird crankcase heater wiring configurations.

When you have a single-pole contactor on a unit with no other resistance crankcase heater attached, the contactor energizes the compressor but is NOT a source of crankcase heat. That myth about single-pole contactors likely stems from a misunderstanding of Ohm's law and resistance heat.

We care about crankcase heat because we want to prevent refrigerant from migrating into the compressor during the off cycle. A crankcase heater keeps the compressor shell warm and prevents vapor refrigerant from condensing in the compressor. Overall, crankcase heat helps prevent flooded starts and oil loss. Some crankcase heaters can be wrapped around the outside of the crankcase, and others can be inserted into the compressor.

The crankcase heater and compressor winding can connect across an open contact to form a series circuit. (If you hook across L1 and T1 so that the other side has constant potential when the contact is open, a path can go to the crankcase heater.) The resistance in the compressor winding can contribute to the crankcase heat strategy, but Joe and Eric argue that the resistance is insignificant.

Overall, we need to remember that resistive heat is resistive heat; in a resistive circuit, your wattage is your wattage, and you can convert that directly to BTUs.

Kaleb, Joe, Eric, and Bryan also discuss:

  • Two-pole and three-pole contactors
  • Resistive heat
  • Operating A/C and heat pumps in low-ambient conditions
  • Ohming compressors
  • Jumpering in place of a single-pole contactor
  • Wire sizing
  • Loud thumping when the unit shuts off
  • Trickle current during the compressor off cycle
  • Power factor, reactive power, and actual power
  • Low-resistance circuits
  • Capacitor purposes, wiring, and sizing
  • Small charge and flood back prevention
  • 3/8" lines

Learn more about Refrigeration Technologies HERE.

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