Capacitor start and run motor principle

A capacitor-start capacitor-run (two-value capacitor) motor uses two capacitors with the auxiliary winding: a large one for high starting torque and a small one that remains in circuit while running, so the motor works as a nearly balanced two-phase motor at all times.

Construction

 AC o----+------------------------+
         |                        |
      Main wdg                 Aux wdg
         |                        |
         |                 +------+------+
         |                 |             |
         |               Cs (start,    Cr (run,
         |               electrolytic) oil-paper)
         |                 |             |
         |               switch S        |
         |                 |             |
 o-------+-----------------+-------------+

• Main and auxiliary windings $90^\circ$ apart in space; the auxiliary winding is permanently connected.
• Starting capacitor $C_s$ (large, electrolytic, about 10-15 times $C_r$) in series with a centrifugal switch.
• Running capacitor $C_r$ (small, oil-filled/paper, continuous rating) always in circuit.

Operating principle

1. At start: $C_s$ and $C_r$ are in parallel, giving a large capacitance. The auxiliary current leads $V$ so that $I_a$ and $I_m$ are about $90^\circ$ apart; with nearly equal mmfs, a strong rotating field is produced and starting torque $T = KI_mI_a\sin\alpha$ is high.
2. At about 75 % of $N_s$: the centrifugal switch removes $C_s$.
3. Running: only $C_r$ remains. Its value is chosen so that at full load the two winding currents are still nearly $90^\circ$ apart with balanced mmfs. The backward field is almost eliminated, so the field is nearly uniform and rotating, as in a balanced 2-phase motor.

 T  |\  start (Cs + Cr)
    | \___
    |     \    switch opens
    |      \__________ run (Cr only)
    |                 \
    +------------------\---- speed
                       Ns

Advantages

• High starting torque and the best running performance of the single-phase motors.
• Better power factor and efficiency, quiet and smooth running (very little double-frequency pulsating torque at full load).
• Higher overload capacity.

Applications

Refrigerators, air-conditioners, compressors, pumps, and other loads needing high starting torque and continuous quiet operation.