Electrical Systems, Motors, Controls, and Instrumentation
Domain 7: Electrical Systems, Motors, Controls, and Instrumentation
This domain tests three-phase motor circuits, starters and protection, variable-frequency drives, instrument loops, calibration, and control interlocks.
Core rules
- For a balanced three-phase motor, input power = sqrt3 x V x I x power factor. Shaft horsepower is less than input power by efficiency.
- Motor overload protection protects the motor from sustained overcurrent; it does not replace branch-circuit short-circuit protection.
- A 4-20 mA loop uses live zero: 4 mA at zero value, 20 mA at full scale. Calibration checks zero, span, and wiring polarity.
- Control valves and dampers should fail safe: usually fail closed for fuel, fail open for cooling water, depending on the hazard analysis.
- Lockout/tagout and arc-flash boundaries apply before opening panels or checking motor terminations.
Worked example
A 50 hp, 460 V, three-phase motor has efficiency 0.90 and power factor 0.85. Input current I = (50 x 746) / (sqrt3 x 460 x 0.90 x 0.85). Numerator = 37,300 W. Denominator = 1.732 x 460 x 0.765 = 609.5. I = 61.2 A. The reasoning: convert horsepower to watts, then divide by the three-phase power expression including efficiency and power factor.
Traps
Confusing kW with kVA; using line-to-neutral voltage in a three-phase formula; calibrating span before zero; and assuming a fail-safe valve always fails closed.