Capacitor start motor operating principle

A capacitor-start motor is a split-phase single-phase induction motor in which a capacitor is placed in series with the auxiliary (starting) winding so that the two winding currents are nearly $90^\circ$ apart at starting, giving high starting torque.

Construction

 AC o----+-----------------+
         |                 |
      Main wdg          Aux (start) wdg
      (Rm, Xm)             |
         |               C (electrolytic,
         |                 |  70-300 uF)
         |          centrifugal switch S
 o-------+-----------------+
       rotor: squirrel cage

• Main winding and auxiliary winding are placed $90^\circ$ electrical apart in the stator.
• An electrolytic capacitor $C$ and a centrifugal switch S are in series with the auxiliary winding.
• Rotor is a normal squirrel cage.

Operating principle

1. At start, the main winding current $I_m$ lags $V$ by about $40-60^\circ$ (inductive winding).
2. The capacitor makes the auxiliary current $I_a$ lead $V$. Choosing $C$ properly makes the angle $\alpha$ between $I_m$ and $I_a$ about $80-90^\circ$.

       Ia (leads V)
        \
         \  alpha ~ 90 deg
 ---------+--------------> V
           \
            \ Im (lags V)

3. Two windings displaced in space by $90^\circ$ carrying currents displaced in time by about $90^\circ$ produce an almost uniform rotating magnetic field, as in a two-phase motor.
4. The rotating field induces rotor currents and develops starting torque $T_{st} = K I_m I_a \sin\alpha$, which is large (3-4.5 times full-load torque) since $\sin\alpha \approx 1$.
5. At about 70-80 % of synchronous speed, the centrifugal switch opens and disconnects the capacitor and auxiliary winding. The motor then runs on the main winding alone (by double field revolving theory the forward field dominates).

Characteristics and uses

• High starting torque with lower starting current than resistance split-phase.
• Reversible by reversing the auxiliary winding connections.
• Used for compressors, refrigerators, air-conditioners, pumps, conveyors and machine tools.