Motor-Driven Appliances and Small Motors
What the exam tests
This domain tests small motors used in fans, mixers, washing machines, refrigerators and power tools: single-phase induction, capacitor-start, capacitor-run, shaded-pole and universal motors.
Core rules
- A single-phase induction motor has no starting torque by itself. A capacitor or shaded pole creates a phase difference to start it. A centrifugal switch may disconnect the start winding.
- A capacitor-run fan motor keeps the capacitor in circuit for running and starting. A universal motor has brushes and a commutator and runs on AC or DC, giving high speed and high starting torque.
- Motor faults include open winding, shorted winding, worn brushes, weak or open capacitor, stuck centrifugal switch, bad bearing and loose connection.
- Discharge capacitors before testing. Replace with the same capacitance and voltage rating.
Worked example
A ceiling fan hums but does not turn. Supply is present. Switch off and discharge the capacitor. Measure capacitance; a very low or open reading means replacement. Check that the shaft turns freely; a stiff bearing also causes humming. If the capacitor and bearing are good, test the winding for continuity and insulation.
Traps
Do not assume the capacitor is faulty because the fan hums; a jammed bearing gives the same symptom. Do not fit a larger capacitor to increase speed. Do not touch capacitor terminals after isolation without discharging. Do not confuse a universal motor with an induction motor.