Electrical Systems and Motors
What the exam tests
This domain covers three-phase power, induction and synchronous motors, starters, overload and short-circuit protection, grounding, variable-frequency drives, insulation resistance, and lockout/tagout. Expect practical questions about why a motor trips or overheats.
Core rules
- Three-phase real power is P = 1.732 x V x I x power factor for a balanced load. Power factor affects current, not the real power delivered for a given load.
- Overload protection protects the motor from sustained overheating; short-circuit protection protects the circuit from fault current. They are not interchangeable.
- Single-phasing an induction motor causes high current in remaining phases and rapid overheating.
- Lockout/tagout must isolate all energy sources, including control power, before work. Verify absence of voltage.
Worked example
A balanced 480 V, three-phase motor draws 20 A at 0.85 power factor. P = 1.732 x 480 x 20 x 0.85 = 14,125 W, or about 14.1 kW. If the power factor rose to 0.95 at the same real load, current would fall. The reasoning: real power stays tied to the mechanical load, while lower power factor requires more current for the same kW.
Traps
Do not use a fuse to replace a properly sized overload heater; do not assume a blown fuse means the motor is bad; do not work on a drive without allowing DC bus discharge; and do not confuse grounded conductor with equipment grounding conductor.