Capacitor start capacitor run motor characteristics

A capacitor-start capacitor-run (CSCR) motor, also called a two-value capacitor motor, is a single-phase induction motor that uses one large capacitor for starting and a smaller capacitor that stays in circuit while running.

Construction

• Stator with a main winding and an auxiliary winding displaced $90^\circ$ electrical in space.
• Auxiliary branch has two capacitors in parallel:
  • Starting capacitor $C_s$ (large, electrolytic, short-time rated) in series with a centrifugal switch
  • Running capacitor $C_r$ (small, oil-filled paper, continuously rated)
• Squirrel-cage rotor.

  o-------+------------+
          |            |
         Main      Auxiliary
        winding     winding
          |            |
  1-ph    |       +----+----+
  supply  |       |         |
          |      C_s       C_r
          |       |         |
          |      CS         |
          |       +----+----+
  o-------+------------+
          (rotor: squirrel cage)

Operating principle

1. At starting both capacitors are in circuit. The large total capacitance makes the auxiliary current $I_a$ lead $V$, while $I_m$ lags $V$. The angle $\alpha$ between them is close to $90^\circ$.
2. Two currents displaced in time and space produce a rotating magnetic field, so the motor starts. Starting torque $T_{st} = K I_m I_a \sin\alpha$ is high (about 3–4.5 times full-load).
3. At about 70–80% of synchronous speed the centrifugal switch disconnects $C_s$.
4. The running capacitor keeps the two windings working as an almost balanced two-phase motor, so the backward field is small during running.

          I_a (with C_s + C_r)
           ^
           |  alpha ~ 90 deg
  ---------+-------------> V
            \
             \
              v I_m

Speed–torque characteristic

• High starting torque from start to switching speed (upper curve, both capacitors).
• At the switching point the torque drops to the lower curve (only $C_r$).
• Running characteristic is smooth, like a two-phase motor; full-load slip is small.

 Torque
   ^ ___   C_s + C_r
   |    \___
   |        \__  switch opens
   |           |\
   |  C_r only |  \_
   |___________|____\_____> speed
   0          75%    Ns

Advantages and applications

• High starting torque, better power factor and efficiency, quiet running, smooth torque (less 100 Hz pulsation).
• Used in refrigerators, air-conditioners, compressors, pumps and conveyors.