Motor Controls & Drives — Industrial Electrician (442A) Practice Questions

Free Motor Controls & Drives practice questions

1. 1.A NEMA Size 1 starter is rated for motors up to approximately:

   • A.5 HP at 480V
   • B.10 HP at 480V✓
   • C.25 HP at 480V
   • D.50 HP at 480V

   Why: NEMA sizes: 00=2HP, 0=5HP, 1=10HP, 2=25HP, 3=50HP, 4=100HP, 5=200HP (at 480V). Size 1 is the most common industrial starter size.

2. 2.A VFD's DC bus voltage for a 480V input is approximately:

   • A.480V DC
   • B.680V DC (480 × √2)✓
   • C.240V DC
   • D.960V DC

   Why: The VFD rectifier converts AC to DC. Peak DC voltage = Vpeak = Vrms × √2 = 480 × 1.414 = 678V ≈ 680V DC. This bus voltage is then inverted to variable frequency AC.

3. 3.A servo drive differs from a standard VFD in that it provides:

   • A.Only speed control
   • B.Precise position, speed, and torque control with feedback (closed-loop)✓
   • C.AC power only
   • D.No feedback capability

   Why: Servo drives use encoder/resolver feedback for precise closed-loop control of position, speed, and torque. They respond much faster than standard VFDs. Used in CNC, robotics, packaging.

4. 4.What must be installed on a VFD output to protect the motor from voltage spikes (dV/dt)?

   • A.A standard breaker
   • B.Output line reactor or dV/dt filter, especially on long cable runs✓
   • C.A bigger motor
   • D.Nothing is needed

   Why: VFD output waveform contains steep voltage spikes. Long cable runs amplify these (reflected wave effect), damaging motor insulation. Line reactors or dV/dt filters attenuate these spikes.

5. 5.Motor branch circuit protection (fuses/breaker) is sized at what percentage of FLA?

   • A.100%
   • B.150-300% depending on motor type and starter type per CEC Table 29✓
   • C.50%
   • D.80%

   Why: CEC Table 29 specifies maximum branch circuit protection: dual-element fuses typically 175% FLA, standard fuses 300%, inverse-time breakers 250%. This allows starting inrush without tripping.

6. 6.When meggering a VFD output, what precaution must be taken?

   • A.None
   • B.Disconnect the VFD from the motor leads — NEVER megger through a VFD✓
   • C.Increase test voltage
   • D.Test only one phase

   Why: The megger's high DC voltage will destroy the VFD's IGBT transistors. Always disconnect the motor cables from the VFD output terminals before meggering the motor.

7. 7.What is the function of a phase monitor relay?

   • A.Count phases
   • B.Monitor for phase loss, phase reversal, and phase imbalance to protect 3-phase equipment✓
   • C.Generate phases
   • D.Store energy

   Why: Phase monitor relays continuously check all three phases for: loss (single-phasing), reversal (wrong rotation), and voltage imbalance. They trip the control circuit to protect motors and equipment.

8. 8.What does 'V/Hz' ratio mean in VFD operation?

   • A.Voltage divided by time
   • B.Maintaining constant voltage-to-frequency ratio for constant motor flux and torque below base speed✓
   • C.Speed setting
   • D.Power factor

   Why: Constant V/Hz maintains rated magnetic flux in the motor as frequency changes. Below base speed: both V and f change proportionally. Above base speed: V stays at max, f increases (field weakening).

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