CSR Motor

CSR Motor Definition and Characteristics

  • CSR means Capacitor Start Capacitor Run (CSCR), typically shortened to CSR
  • High starting torque
  • Can handle high-demand loads 1-10 HP
  • Can be designed for lower full-load currents and higher efficiency
  • Operates at lower temperature than other single-phase motor types of comparable horsepower
  • Only disadvantage to a CSR motor is its higher price - mostly the result of more capacitors, plus a starting switch
  • Able to handle applications too demanding for any other kind of single-phase motor
  • A CSR motor uses a potential relay or start relay (see Discussion below)

Discussion

  • Potential or "voltage" starting relays are used with single-phase capacitor-start/capacitor-run motors, which need relatively high starting torque.
  • Their main function is to assist in starting the motor.
  • These starting relays consist of a high-resistance coil and a set of normally closed contacts.
  • The coil is wired between terminals 2 and 5, with the contacts between terminal 1 and 2.
  • Below shows how a potential starting relay is wired to a capacitor-start/capacitor-run compressor motor.

  • Note that the relay coil is wired in parallel (across) the start winding.
  • The normally closed contacts are wired in series with the start winding and the start capacitor.
  • When the power is applied through the cycling control, both the run and start windings are energized.
  • The start and run capacitors provide the phase shift for starting torque because of their capacitance adding when wired in parallel.
  • In fact, both capacitors are wired in series with the start winding.
  • The run capacitor also provides running torque when the motor is up and running.
  • The operation of the potential starting relay is based on the increase in back electromotive force (back EMF) or a bucking voltage that is generated across the start winding as the motor increases in speed.
  • The large metal mass of the motor's rotor turning at high speeds has a voltage generating effect. This generated back EMF opposes line voltage and can be measured across the start winding or across the coil of the potential relay at terminals 2 and 5.
  • The back EMF is usually a higher voltage than the line voltage and can be in the 400-V range.
  • All motors have different magnitudes of back EMF.
  • The back EMF voltage generated across the start winding causes a small current to flow in the start winding and in the potential relay coil since they are in the same circuit.
  • When the back EMF has built up to a high enough value, referred to as pick-up voltage, the contacts between terminals 1 and 2 will be picked-up opened.
  • This will take the start capacitor out of the circuit.
  • The pickup voltage usually occurs when the motor has reached about 3/4 speed.

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