Motor Controls and Variable Frequency Drives
What the exam asks
This domain covers the control chain from the disconnect to the shaft: starters and overloads, control-circuit logic, and the VFD as converter–DC link–inverter. Questions mix component selection with drive application math.
Core rules
- Starter anatomy. Disconnect, short-circuit protective device, contactor, overload relay, control transformer. Overload class (10, 20, 30) sets the trip time at six times FLC - Class 20 suits high-inertia loads, Class 10 suits standard motors.
- Control schemes. Three-wire control uses momentary start/stop with a seal-in contact and drops out on power loss; two-wire control uses maintained contacts and restarts after a power blip. Interlocks and permissives belong in the control circuit, not the operator's memory.
- VFD topology. Diode rectifier → DC link capacitor → IGBT inverter producing PWM. The output is not a clean sine wave, which drives cable, grounding and bearing-current requirements.
- Volts-per-hertz. Below base speed the drive holds V/Hz roughly constant, giving constant torque. Above base speed, voltage is capped and the drive enters the constant-power (field-weakening) region.
- Affinity laws. For centrifugal loads, flow ∝ speed, head ∝ speed², power ∝ speed³.
- Installation. Separate equipment-ground conductor from the motor frame to the drive PE, shielded VFD cable with 360° termination, line reactor for harmonics, dv/dt filter or terminator on long motor leads, and skip frequencies to avoid mechanical resonance.
Worked example
A centrifugal pump absorbs 40 kW at 60 Hz. What does it draw at 45 Hz, and what changes for a positive-displacement pump?
Speed ratio = 45 ÷ 60 = 0.75. Power = 40 × 0.75³ = 16.9 kW; flow falls to 75% and head to 56%. Reasoning: the cube law holds only for variable-torque (centrifugal) machines. A positive-displacement pump or conveyor is a constant-torque load, so torque stays constant and power falls only linearly with speed - 30 kW, not 16.9 kW.
Traps that cost marks
- Applying affinity laws to constant-torque loads.
- Expecting nameplate torque above base speed.
- Raising carrier frequency beyond the manufacturer's cable-length limit, causing nuisance trips and bearing damage.
- Insulation-testing a motor with the drive still connected.
- Forgetting that the drive's DC bus holds a charge after isolation.