Motors
What the exam asks
Motor questions test the relationship between speed, slip, torque, current and thermal limits, plus the practical detail that separates a technician from a textbook: NEMA design letters, service factor, voltage unbalance and connection practice.
Core rules
- Synchronous speed. n_s = 120 f ÷ P (P = poles). A 4-pole, 60 Hz machine runs at 1800 rpm synchronously.
- Slip. %slip = (n_s − n_rated) ÷ n_s × 100. Slip rises with load and is the reason a motor draws more current as it is loaded.
- Torque. T (lb·ft) = 5252 × hp ÷ rpm; T (N·m) = 9550 × kW ÷ rpm. Use the actual rated speed, not synchronous speed.
- NEMA design letters. Design B is general purpose with moderate starting current and about 150–200% breakdown torque; C has high locked-rotor torque with low starting current; D has high slip and is used for punch presses and hoists.
- Locked-rotor current. Typically 6–8 × FLA for a Design B motor at full voltage - the reason reduced-voltage starting exists.
- Voltage unbalance. Current unbalance runs roughly 6–10 times the voltage unbalance. NEMA MG1 derating curves drop a motor to roughly 95% of rating at 2% unbalance and about 88% at 3%.
- Service factor. A 1.15 SF permits continuous overload only at rated voltage, frequency and ambient - and not automatically on a VFD, which is why MG1 Part 31 motors exist.
Worked example
A 50 hp, 4-pole, 460 V motor runs at 1750 rpm. Line voltages measure 472, 460 and 448 V. Check slip, torque and unbalance.
Slip = (1800 − 1750) ÷ 1800 = 2.8%. Torque = 5252 × 50 ÷ 1750 = 150 lb·ft. Voltage unbalance: average = 460 V, largest deviation = 12 V, so 12 ÷ 460 = 2.6%. Conclusion: the motor must be derated to roughly 93% of nameplate output, and expect current unbalance in the 15–25% range - not a winding fault until a phase-to-phase check proves otherwise.
Traps that cost marks
- Substituting synchronous speed into the torque formula.
- Assuming current unbalance equals voltage unbalance.
- Using service factor to justify running a motor on a drive without the Part 31 marking.
- Treating a NEMA frame size as a design letter.