In one line
Single-phase induction motors are named by how they create starting torque: split-phase, capacitor-start, capacitor-start capacitor-run (and permanent capacitor) and shaded-pole motors.
Explained simply
All single-phase induction motors share the same basic body: a stator with a main winding and a cage rotor. They differ only in the "starting trick" used to create a rotating field. Better tricks give more starting torque but cost more, so each type matches a different job, from a hair dryer to a refrigerator compressor.
Technically, an auxiliary winding placed 90° in space from the main winding is made to carry a current out of phase with the main winding current. The larger this time-phase difference (ideally 90°), the better the starting torque.
How it works
| Type | Starting method | Features | Applications |
|---|---|---|---|
| Split-phase (resistance start) | auxiliary winding of high resistance and low reactance; centrifugal switch cuts it out at about 70–80% of speed | moderate starting torque, phase difference well below 90° | washing machines, small fans, blowers, grinders |
| Capacitor-start | capacitor in series with auxiliary winding; centrifugal switch | phase difference near 90°, high starting torque | refrigerator and air-conditioner compressors, pumps, conveyors |
| Capacitor-start capacitor-run | large capacitor for starting plus small capacitor that stays in circuit | high starting torque, better running pf and efficiency, quieter | compressors, pumps, larger domestic machines |
| Permanent split capacitor | one capacitor permanently in series with auxiliary winding | low-moderate starting torque, quiet, good pf | ceiling fans, coolers, exhaust fans |
| Shaded-pole | copper shading ring on part of each salient pole | very low starting torque, low efficiency, cheap, rotates from unshaded to shaded part | table fans, hair dryers, small exhaust fans, record players |
- Direction of split-phase and capacitor motors is reversed by reversing connections of either the main or the auxiliary winding.
- Shaded-pole motors cannot be reversed electrically because the shading ring is fixed.
Quick example
A domestic refrigerator compressor uses a capacitor-start (or capacitor-start capacitor-run) motor because it must start against refrigerant pressure. A cheap hair dryer uses a shaded-pole motor because it needs very little starting torque and must be cheap.
Common mistakes
- Saying a capacitor gives a lagging auxiliary current; it makes the auxiliary current lead.
- Saying the shaded-pole motor can be reversed by swapping supply leads.
- Saying the centrifugal switch is used in permanent-split-capacitor motors.
How MU asks it
Numericals are not expected in this module (as per the MU syllabus), so questions are theory. Typical: "Explain any one type of single-phase induction motor with diagram" (5 marks) or "Classify single-phase induction motors and give applications" (5 marks). For full marks draw the connection diagram (main winding, auxiliary winding, capacitor, centrifugal switch) and the phasor diagram showing the phase difference between \( I_m \) and \( I_a \), and give applications with reasons.