Why single-phase IM is not self starting; methods

A single-phase induction motor is not self-starting because its single winding produces a pulsating field, not a rotating one, so the net starting torque is zero.

Why it is not self-starting

By double field revolving theory, the pulsating flux $\phi_m\cos\omega t$ equals two fields of $\phi_m/2$ rotating in opposite directions at $N_s$. At standstill both have slip 1 and produce equal and opposite torques: $T_f = T_b$, so $T_{net} = 0$. If the rotor is pushed in either direction, the torque in that direction becomes larger and the motor keeps running.

Starting methods

To start, a rotating field must be produced, usually by phase splitting:

1. Resistance split-phase motor
2. Capacitor-start motor
3. Capacitor-start capacitor-run (two-value capacitor) motor
4. Permanent split capacitor (PSC) motor
5. Shaded-pole motor
6. Repulsion-start induction-run motor

1. Resistance split-phase method

 AC o---+--------------+
        |              |
   Main wdg (low R,  Aux wdg (high R,
   high X)           low X)
        |              |
        |          centrifugal S
 o------+--------------+

• An auxiliary winding with high resistance (thin wire) and low reactance is placed $90^\circ$ electrical from the main winding.
• Main current $I_m$ lags $V$ by a large angle; auxiliary current $I_a$ lags by a smaller angle, giving a phase difference of about $25-30^\circ$.
• These two currents produce a rotating field and starting torque $T_{st} \propto I_mI_a\sin\alpha$ (moderate, about 1.5-2 times full load).
• A centrifugal switch disconnects the auxiliary winding at about 75 % of $N_s$.
• Used for fans, blowers, washing machines.

2. Capacitor-start method

 AC o---+--------------+
        |              |
     Main wdg       Aux wdg
        |              |
        |          C (electrolytic)
        |              |
        |          centrifugal S
 o------+--------------+

• A capacitor in series with the auxiliary winding makes $I_a$ lead $V$, so the angle between $I_m$ and $I_a$ is close to $90^\circ$.
• Starting torque is high (3-4.5 times full-load torque) with lower starting current.
• The auxiliary winding and capacitor are cut out by the centrifugal switch at 70-80 % of $N_s$.
• Used for compressors, pumps, refrigerators and air-conditioners.