Question

Explain why a single-phase induction motor is not self-starting. What are the methods used to make it self-starting?

Hint:

If you manually spin the shaft of a single-phase induction motor in either direction while the power is ON, it will instantly catch speed and continue running in that direction. This is because the slip for that direction becomes smaller, causing the torque in that direction to exceed the opposing torque.

Answer 1

Step 1: Understanding the Stator Magnetic Field:
When a single-phase alternating current is passed through the stator winding of a single-phase induction motor, it produces a pulsating (alternating) magnetic field. Unlike a three-phase system, a single-phase stator field does not rotate; it simply oscillates along a single space axis, changing in magnitude and reversing in direction periodically.

Step 2: Double-Revolving Field Theory:
According to the Double-Revolving Field Theory, any pulsating magnetic field of sinusoidal form can be mathematically resolved into two rotating magnetic fields of equal constant magnitude (each equal to half the maximum value of the pulsating field, i.e., $\Phi_m/2$), rotating in opposite directions at synchronous speed ($N_s = 120f/P$).

• One field rotates in the clockwise direction, known as the Forward Rotating Field.
• The other field rotates in the counter-clockwise direction, known as the Backward Rotating Field.

Step 3: Torque Analysis at Standstill:
At starting (when the rotor is stationary, speed $N = 0$, and slip $s = 1$ for both fields):

• The forward rotating field induces a forward electromagnetic torque ($T_f$) in the rotor.
• The backward rotating field induces an equal and opposite backward electromagnetic torque ($T_b$) in the rotor.
• The resultant net starting torque ($T_{net}$) is the algebraic sum of these two opposing torques: $$T_{net} = T_f - T_b = 0$$

Because the net starting torque is exactly zero at standstill, a single-phase induction motor is not self-starting.

Step 4: Methods to Make it Self-Starting:
To make the motor self-starting, we must establish a temporary rotating magnetic field at startup. This is achieved by creating a phase displacement (phase split) between two physical winding currents. Common methods include:

• Split-Phase Method (Resistance Start): Uses an auxiliary starting winding with high resistance and low reactance in parallel with the main winding (which has low resistance and high reactance).
• Capacitor-Start Method: Connects a high-capacitance starting capacitor in series with the auxiliary winding to create a large phase shift ($\approx 90^\circ$).
• Capacitor-Start Capacitor-Run Method: Uses two capacitors to optimize starting torque and running performance.
• Shaded-Pole Method: Uses copper shading rings on a portion of each salient pole to produce a weak, shifting magnetic field.