Electrical Controls, Motors, and Troubleshooting
This domain tests the 24-volt control circuit, motor types, capacitor and winding tests, and the order in which a technician should troubleshoot.
Control circuit
- The transformer is sized in VA to carry every load on the circuit; adding an ECM or a zoning panel without upsizing causes nuisance lockouts.
- Thermostat terminals: R power, W heat, Y compressor, G blower, O/B reversing valve, C common. A smart thermostat needs C.
- Devices include contactors, fan relays, time delays, defrost boards, sequencers for electric heat, float switches, and high and low pressure switches.
Motors
- PSC motors need a run capacitor; ECM and constant-torque motors are programmed and respond to a low-voltage signal.
- Discharge a capacitor before testing. Measure microfarads and replace when outside the marked tolerance, commonly 10 percent, or when the top bulges.
- Compressor terminals are C, S, and R. Common-to-start plus common-to-run should equal start-to-run.
Worked example
Readings: C to R is 1.8 ohms, C to S is 2.2 ohms, so S to R should be about 4.0 ohms. A reading of 4.1 ohms means the windings are intact. An open reading means an open winding or an open internal overload. A reading far below 4.0 indicates shorted turns. Any continuity from a terminal to the shell means the compressor is grounded.
Traps that cost marks
- Ohming a capacitor without discharging it.
- Replacing a control board when a safety switch simply opened.
- Ignoring nameplate RLA and LRA when judging compressor amp draw.
- Overlooking the C wire on a new thermostat, then blaming the board.
- Testing a PSC motor without checking its capacitor first.