Motors, Controls, and Industrial Equipment
What this domain tests
Motors, Controls, and Industrial Equipment tests motor branch circuits, starters, overloads, disconnects, control transformers, pilot devices, VFDs, and basic PLC interfacing. Expect questions that separate overload protection from short-circuit and ground-fault protection, and that require reading a ladder diagram.
Core rules
- Motor branch-circuit conductors are sized from the motor full-load current tables, not the nameplate, and are generally at least 125% of the motor FLA.
- Short-circuit and ground-fault protection protects the circuit; overload protection protects the motor from sustained overcurrent and is sized from the nameplate service factor or temperature rise.
- A motor disconnect must be within sight and readily accessible, or capable of being locked open where the code permits a remote location.
- A starter's holding circuit keeps the coil energized after the start button is released; a stop button or overload contact opens that circuit.
- VFDs require shielded motor cable, proper grounding, and care with long leads; never use a megger on the drive output.
- Control transformers and Class 2 circuits must be protected and grounded according to their ratings.
Worked example
A 10 hp, 480 V, three-phase motor has a table FLA of 14 A. The branch-circuit conductor minimum is 14 × 1.25 = 17.5 A. A 12 AWG copper conductor with a 75°C termination rating has an ampacity of 20 A, so it is acceptable at the minimum. The overload is selected from the nameplate current and service factor, not from the 17.5 A result, and the short-circuit protective device is selected from the motor protection table. The reasoning: conductor sizing, overload sizing, and short-circuit protection use different inputs; mixing them is a common exam error.
Traps
Using nameplate current for conductor sizing where tables are required; using the overload as the only protection; setting the overload to the branch-circuit breaker size; omitting a disconnect; forgetting that a VFD can retain DC-bus voltage; assuming a PLC input is dry contact without checking the card.
Key points
- Motor conductors use table FLA and 125% sizing.
- Overload protection is separate from short-circuit protection.
- Disconnects must be within sight or lockable as permitted.
- A stop circuit opens the starter holding circuit.
- VFDs need shielding, grounding, and discharge time.