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Understanding Permanent Split Capacitor (PSC) Motors and Their Variants

This article explores Permanent Split Capacitor (PSC) motors, which maintain an in-series capacitor during operation to enhance starting torque without requiring relays or switches. Commonly used in residential applications such as condenser fans and furnace blowers, PSC motors exhibit moderate starting torque and efficiency. The piece also covers Capacitor Start Induction Run (CSIR) motors, which use a start capacitor and relay for higher torque, and Capacitor Start Capacitor Run (CSCR) motors, which incorporate a switch to disconnect the start capacitor after motor initiation.

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Understanding Permanent Split Capacitor (PSC) Motors and Their Variants

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  1. 214 Motors Part II

  2. Permanent Split Capacitor Motors • In order to increase the starting torque of a motor you can insert a capacitor in series with the start winding circuit. • In a PSC motor this capacitor remains in series while the motor is operating. There is no switch or relay that removes this capacitor from the circuit.

  3. PSC Motor • Residential Condenser fan motors and residential furnace blower motors are the primary use of PSC motors.

  4. PSC Motor • PSC motors have moderate starting torque and intermediate running efficiency. • These motors are used primarily for small compressors and direct-drive blower and fan applications.

  5. PSC Motor Example

  6. CSIR Motors • CSIR are capacitor start and induction run motors. • They use a start cap and a start relay. • Once the motor has reached 75% of its rated speed the start capacitor is removed from the circuit.

  7. CSIR motors • CSIR motors have a high starting torque and are used in residential and small commercial compressors. • May see them in beverage coolers or small reach ins.

  8. CSCR Motors • CSCR (Capacitor Start Capacitor Run) motors are the same as Permanent Split Capacitor motors except they have a start capacitor wired to them. • A start capacitor is a high capacitance capacitor that will provide the motor with additional torque when starting.

  9. CSCR Motors • Once the motor has been started it, the start capacitor must be taken out of the circuit using a switch or a relay. • These devices are called start components and this is covered in the next section. • This motor can be abbreviated to CSR. (Capacitor Start and Run).

  10. CSCR Motor Example

  11. Motors • If the windings are not marked use your ohm meter and check resistance. • Check between all three windings. • The winding measurement with the highest resistance is between start and Run (S and R). The remaining terminal is Common ( or C).

  12. Motors • Once you know C check between C and the other two terminals the one with the highest resistance is S (or Start). • The one with the lower resistance is Run (or R).

  13. Motors • Best way to do this is to write down C, S, R in a triangle on a piece of paper and make sure you write down each measurement. • If you have any doubt - go back and do it again.

  14. Motor Speeds • Motor speeds are shown in RPM’s • RPM is revolutions per minute. This is a measure of how many times per minute the shaft rotates. • Sometimes you will need to figure out RPM’s for a motor.

  15. Motor Speeds • To figure motor speed first use the motor labels • If no label is found use the formula: • Speed (rpm) = (frequency x 120) / Poles

  16. Shaded Pole Diagram C R

  17. CSCR Diagram R C S

  18. Split Phase Diagram R C S

  19. PSC Diagram R C S

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