---
title: "Single phase IM principle, torque-slip and starting"
subject: "Electric Machines II"
chapter: 3
key: "semester_5/electric_machines_ii:single-phase-principle-torque-slip"
source: https://www.bctengineer.com/q/semester_5/electric_machines_ii/single-phase-principle-torque-slip
site: BCT Engineering
---

# Single phase IM principle, torque-slip and starting

Electric Machines II · Chapter 3: Fractional Kilowatt Motors

- Asked in: 2080 Bhadra, 3+2+2 marks
- Video: [Single Phase Induction Motor  (Construction & Working) Electrical Machines(BEE) 1st year engineering](https://www.youtube.com/watch?v=SdWM_jSu0PY) — Gautam Varde
- Answer checked by Claude Sonnet 5.5 on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_5/electric_machines_ii/single-phase-principle-torque-slip
- Whole chapter: https://www.bctengineer.com/md/semester_5/electric_machines_ii/chapter-3

## Question

Explain the working principle of single phase induction motor and draw its torque-slip characteristics. Explain one of the starting method.

## Short answer

A single-phase induction motor produces a pulsating field, equivalent to two counter-rotating fields, giving zero net starting torque. After being started, forward torque dominates; the torque-slip curve is zero at standstill and symmetrical. A resistance split-phase start uses an auxiliary high-resistance winding and centrifugal switch to provide starting torque.

## Answer

A single-phase induction motor works on electromagnetic induction like a 3-phase induction motor, but its single stator winding produces a **pulsating** field, which by double field revolving theory is two equal fields rotating in opposite directions.

#### Working principle

1. AC supply to the stator winding produces a flux $\phi = \phi_m\cos\omega t$ that alternates along one axis.
2. This flux is equivalent to a forward field and a backward field, each $\phi_m/2$, rotating at $N_s$ in opposite directions.
3. At standstill both fields induce equal rotor currents and produce equal and opposite torques, so the motor has **no starting torque**.
4. If the rotor is started in one direction (by an auxiliary means), the slip for the forward field is $s$ and for the backward field is $2 - s$. The forward torque becomes much larger than the backward torque, and the motor accelerates to a speed slightly below $N_s$ and continues running in that direction.

#### Torque-slip characteristic

```text
        T
        |            _
        |         .-' \   forward run
        |      .-'     |  (Tf > Tb)
 -Ns ---+----.'--------+---> speed
     |  |  .' 0        Ns
      \_|.'
 reverse run      net T = 0 at N = 0
 (mirror image)
```

- Net torque is zero at $s = 1$ (standstill).
- Net torque is positive for forward running ($0 < s < 1$) and becomes zero slightly before $s = 0$ because the backward torque is not zero at synchronous speed.
- The curve is symmetrical for reverse rotation ($1 < s < 2$).

#### One starting method: resistance split-phase

```text
 AC o---+------------+
        |            |
   Main wdg     Aux wdg (high R)
        |            |
        |      centrifugal switch
 o------+------------+
```

- An auxiliary winding of high resistance and low reactance is placed $90^\circ$ electrical from the main winding.
- Its current lags $V$ less than the main winding current, giving a phase difference of $25-30^\circ$, which creates a rotating field and starting torque $T \propto I_mI_a\sin\alpha$.
- At about 75 % of $N_s$ a centrifugal switch disconnects the auxiliary winding.
- Used in fans, blowers, washing machines and small tools.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. A single-phase induction motor stator winding produces:
   A. A pulsating field equivalent to two opposite rotating fields
   B. A rotating field directly
   C. A constant DC field
   D. No field
   Answer: A. By DFRT, phi_m cos(wt) equals forward and backward fields of phi_m/2. (Key point: Working principle)

2. Once the rotor is turning forward, the forward torque is much larger than the backward torque because:
   A. Slips are s and 2 - s for the forward and backward fields
   B. Both slips are equal
   C. The backward field stops
   D. Voltage rises
   Answer: A. The backward slip is near 2, so its torque is small. (Key point: Torque when running)

3. The net torque of the characteristic is zero at:
   A. Standstill (s = 1)
   B. Maximum slip only
   C. Synchronous speed exactly, as torque is maximum
   D. All speeds
   Answer: A. Tf and Tb cancel at s = 1; the curve is symmetric for reverse rotation. (Key point: Torque-slip curve)

4. In resistance split-phase starting, the phase difference of about 25-30 degrees arises because the auxiliary winding has:
   A. High resistance and low reactance
   B. Low resistance and high reactance
   C. A capacitor
   D. No winding
   Answer: A. Its current lags V less than the main winding current. (Key point: Resistance split-phase)

## Related in Electric Machines II

- [Why single-phase IM is not self starting; methods](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-not-self-starting-methods) — Electric Machines II, chapter 3, asked 2×
- [Input current, pf and torque of single phase IM](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asingle-phase-im-input-torque) — Electric Machines II, chapter 3
- [Split-phase motor winding currents at starting](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Asplit-phase-starting-currents) — Electric Machines II, chapter 3
- [Determining single phase IM equivalent circuit parameters](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Aequivalent-circuit-parameter-tests) — Electric Machines II, chapter 3
- [DFRT and equivalent circuit of single phase IM](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Adfrt-equivalent-circuit) — Electric Machines II, chapter 3
- [Shaded pole motor principle and characteristics](https://www.bctengineer.com/question.md?key=semester_5%2Felectric_machines_ii%3Ashaded-pole-characteristics) — Electric Machines II, chapter 3

## Related in other subjects

- [IM slip and starting current from torque ratios](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machine%3Aim-starting-torque-1-25-max-2-5) — Electrical Machine, chapter 5
- [Induction motor torque equation and torque-slip characteristic](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-torque-equation-ts-characteristic) — Electrical Machines I, chapter 5
- [Torque-slip curve: starting, maximum and operating region](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aim-torque-slip-starting-max-operating) — Electrical Machines I, chapter 5
- [Torque equation of 3-phase induction motor](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Atorque-equation-induction-max-condition) — Electrical Machines, chapter 5