Single Phase Induction Motor

Double revolving field theory explains the zero starting torque: a single stator winding produces a pulsating MMF equivalent to two fields of half amplitude rotating in opposite directions. At standstill their torques cancel, so the rotor will not self-start but will continue if given a push.

Starting methods create a rotating field by adding a second, displaced winding. Split-phase uses a high-resistance auxiliary winding; capacitor-start adds a series capacitor for a near-90° phase split and much higher starting torque.

Relation 1 —

Write this relation with symbols exactly as in Electrical Machines — Nagrath & Kothari before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 2 —

Write this relation with symbols exactly as in Electrical Machines — Nagrath & Kothari before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

Relation 3 —

Write this relation with symbols exactly as in Electrical Machines — Nagrath & Kothari before substituting numbers. Examiners award partial marks for a correct setup even when arithmetic slips.

In the equivalent circuit, the rotor impedance splits into a forward branch (slip s) and a backward branch (slip 2−s). The forward field produces most of the useful torque; the backward field produces a small braking torque and extra loss.

Capacitor-run motors keep the capacitor in circuit for better running power factor and smoother torque; capacitor-start-capacitor-run uses two capacitors for both high starting torque and good running performance.

Shaded-pole motors use a shorted copper ring on part of each pole to create a weak rotating field — cheap, low torque, used in small fans.

State assumptions explicitly before using any relation for single phase induction motor — steady state, uniform properties, linear elastic material, ideal gas, incompressible flow, etc., as applicable.

Wrong assumptions invalidate the entire solution even when the formula is correct. In Electrical Machines II viva and GATE descriptive questions, listing valid assumptions often earns separate marks.

1. Read the question and list given data with SI units (common in Electrical Machines II papers).
2. Draw a neat labelled diagram where applicable — examiners in Indian universities award diagram marks even when arithmetic slips.
3. Identify which relation from this topic applies to single phase induction motor.
4. Use equation 1: .
5. Use equation 2: .
6. Substitute values, compute, and verify units and sign (direction).
7. State conclusion in one line — e.g. safe/unsafe, stable/unstable, feasible/infeasible.

Single Phase Induction Motor appears in industrial motors and generators. In Indian electrical curricula this topic is tested because it connects theory to induction and synchronous machines.

GATE and semester exams often combine single phase induction motor with earlier units — revise prerequisites before attempting mixed problems.

Industry interview panels sometimes ask: "Where did you use single phase induction motor?" — answer with a lab, mini-project, or plant visit example if possible.

• Claiming a single-phase motor has zero running torque (it is only starting torque that is zero)
• Using slip s for the backward field branch instead of (2−s)
• Ignoring the backward-field losses when computing efficiency
• Confusing capacitor-start (start only) with capacitor-run configurations

Before attempting single phase induction motor problems, confirm you can:

1. Capacitor start/run improves starting torque and pf
2. Shaded pole motor: low starting torque, small fans
3. Double revolving field theory explains zero starting torque

Revise the solved examples in Electrical Machines — Nagrath & Kothari and one previous-year GATE or university paper for this unit.