Electrical Controls, Motors, and Wiring for HVAC Equipment
Questions cover control circuits, motors, transformers, capacitors, and safe diagnosis. Know Ohm's law, series and parallel circuits, and the difference between line and low voltage.
Controls and motors
A 24 V control circuit typically includes a transformer, thermostat, contactor, relays, and safety switches. Series opens the circuit on a safety; parallel paths can backfeed and confuse diagnosis. PSC motors use a run capacitor; ECM motors are electronically commutated and often maintain constant airflow or torque. Always check capacitor microfarads against the nameplate.
Worked example
A 24 V transformer supplies a gas valve drawing 0.5 A, a contactor at 0.3 A, and a control board at 0.2 A. Total = 1.0 A. VA = 24 × 1.0 = 24 VA. Add 20% margin = 28.8 VA; select a 40 VA transformer. A 25 VA transformer would run hot and drop out.
Diagnosis
Measure voltage under load, not open-circuit only. A reading of 24 V at the transformer but 18 V at the contactor indicates voltage drop or a bad connection. Check common wires, safety switches, and thermostat wiring before replacing parts. Lock out and tag out line voltage; discharge capacitors before handling.
What costs marks
- Confusing control voltage with line voltage.
- Replacing a capacitor without checking microfarads and tolerance.
- Assuming an ECM motor is bad because it has no simple voltage reading.
- Missing a broken common wire or safety switch.
- Not checking for backfed voltage in parallel circuits.