Induction motors working principle and why Induction motors are self starting while single phase motors not?

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Induction Motor Basics: Working Principle, Types, and Self-Starting Concept

🔹 Introduction

One of the most widely used electric motors in industry and domestic applications is the induction motor (often called an asynchronous motor). It is termed “asynchronous” because it always operates at a speed slightly less than synchronous speed.



To understand this better, let’s first define synchronous speed.

🔹 What is Synchronous Speed?

Synchronous speed (Ns) is the speed at which the rotating magnetic field produced by the stator revolves. It depends on the frequency (f) of the AC supply and the number of poles (P) in the motor:

Ns=120fPN_s = \frac{120f}{P}

  • f = supply frequency (Hz)

  • P = number of poles

For example, a 4-pole motor supplied at 50 Hz has:

Ns=120×504=1500 RPMN_s = \frac{120 \times 50}{4} = 1500 \, \text{RPM}

Since the rotor always lags behind the stator flux, its actual speed (Nr) is slightly less than synchronous speed.

🔹 Why Does an Induction Motor Run Below Synchronous Speed?

When AC supply is given to the stator winding, a rotating magnetic field (RMF) is produced. This field induces an EMF in the short-circuited rotor conductors (Faraday’s Law).

  • The induced current in the rotor produces its own magnetic field.

  • The interaction between stator flux and rotor flux produces torque.

  • Since rotor flux always lags behind the stator flux, the rotor speed (Nr) < synchronous speed (Ns).

This difference is called slip (s):

s=Ns−NrNs×100%s = \frac{N_s - N_r}{N_s} \times 100\%

🔹 Types of Induction Motors

Induction motors are broadly classified based on the supply system:

1. Single-Phase Induction Motor

Not self-starting by nature. Types include:

  • Split-phase induction motor

  • Capacitor-start induction motor

  • Capacitor-start capacitor-run induction motor

  • Shaded-pole induction motor

2. Three-Phase Induction Motor

Self-starting due to the presence of a rotating magnetic field. Types include:

  • Squirrel cage induction motor

  • Slip ring induction motor

🔹 Self-Starting Concept

⚡ Three-Phase Induction Motor (Self-Starting)

  • Three-phase supply creates three currents with 120° phase difference.

  • These currents produce a rotating magnetic field (RMF).

  • The rotor is continuously pulled in sequence by phases A, B, and C, making the motor self-starting.

⚡ Single-Phase Induction Motor (Not Self-Starting)

  • A single-phase supply produces only a pulsating magnetic field, which can be imagined as two equal and opposite rotating fields.

  • These fields cancel out at standstill, producing no starting torque.

  • To overcome this, an auxiliary winding + capacitor is used to create a phase difference, generating a starting torque.

  • Once the motor runs, it continues due to the RMF principle.

👉 Example: Ceiling fans use a capacitor-start single-phase induction motor. If the capacitor is disconnected, the fan won’t start unless rotated manually.

🔹 Direction of Rotation

  • In a three-phase motor, reversing any two supply phases (e.g., RYB → RBY) reverses the direction.

  • In a single-phase motor, reversing the capacitor connection in the auxiliary winding changes the rotation.

✅ Advantages of Induction Motors

  • Simple and rugged construction

  • High efficiency and reliability

  • Low maintenance (no brushes or commutator)

  • Wide range of industrial and domestic applications

⚠️ Limitation

  • Speed decreases slightly with load (slip-dependent)

  • Single-phase versions require auxiliary starting means

📌 Conclusion

Induction motors, both single-phase and three-phase, form the backbone of modern electrical drives. Their simple design, robustness, and efficiency make them indispensable in industries, households, and appliances.


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