Electrical Fundamentals and Controls
What the exam expects
Expect Ohm's law, series and parallel circuits, AC versus DC, single- versus three-phase power, and the control devices that sequence a system: thermostats, contactors, relays, transformers, capacitors, pressure switches, limit and safety switches, defrost boards, and lockout controls. You should read a ladder diagram and identify the power circuit versus the control circuit, and know that 24 V control wiring is Class 2 and separate from line voltage.
Motor knowledge matters: identify common, start, and run terminals; remember the highest resistance reading is across start-to-run (the winding sum); test for a short to ground; recognize PSC versus ECM blower motors and start-assist components.
Traps
- Testing a capacitor without discharging it, or trusting a visual inspection.
- Confusing normally open and normally closed contacts on a schematic.
- Assuming a tripped breaker is the root cause when a grounded compressor is.
- Forgetting that a transformer's secondary must be sized for the VA of all connected loads.
- Reading a switch on a de-energized circuit and calling it good.
Worked example
A 24 V contactor coil measures 12 ohms. Current draw = V / R = 24 / 12 = 2 A. If the transformer secondary is rated 40 VA, that coil alone consumes 48 VA and the transformer is undersized - the circuit will brown out and the contactor will chatter.