What is a PSC motor

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14 Sep. 2020

A Permanent Split Capacitor (PSC) Motor is a type of single-phase AC motor; more specifically, a type of split-phase induction motor in which the capacitor is permanently connected (as opposed to only being connected when starting).

AC motors can be divided into single- and three-phase motors depending on whether they are driven by a single*1- or three-phase*2 power supply.
A number of different types exist for single-phase induction motors. One of these involves using a capacitor*3 to generate a magnetic field in such a way that it simulates a second power supply phase, thereby generating the torque needed to start the motor rotating*4. Such motors are called "capacitor start motors" to reflect the use of a capacitor for this purpose. The category also includes motors in which the capacitor remains connected at all times (not just when starting) and these are called "capacitor run motors" or "permanent capacitor motors".

  • *1
    Single-phase: The type of power supply used in residential homes.
  • *2
    Three-phase: The type of power supply generated at power plants and supplied to factories and other industrial loads.
  • *3
    Capacitor: An electronic device that is able to accumulate and discharge electrical energy, also known historically as a condenser. An alternative design of single-phase induction motor that does not use a capacitor is the shaded-pole motor.
  • *4
    In addition to capacitor start motors, two other single-phase induction motor designs that do not require a capacitor to generate a starting torque are the split-phase induction motor and the shaded-pole motor.

How capacitor motors work

To use a single-phase power supply available in residential homes to drive a motor, there is a need for a mechanism to start the motor rotating. A capacitor motor does this by having separate main and secondary windings (as shown in the diagram), with the main winding connected directly to the power supply and the secondary windings connected via a capacitor.

When the power supply is turned on, the current flows first in the main winding and then, with a short delay due to the capacitor, in the secondary winding. This difference in the main and secondary winding currents takes the form of a phase difference (meaning their waveforms are offset from one another on the time axis), causing the peak magnetic field to alternate between the two windings and thereby generating a torque that starts the motor rotation.

Background to development of capacitor motors

One of the principles behind the single-phase induction motor (capacitor motor) is the phenomenon of "Arago's rotations" discovered by François Arago in 1824. His discovery was that, when a magnet is rotated adjacent to a disk of non-magnetic material (a metal such as copper or aluminum that is not attracted by a magnet), the disk also starts to rotate along with the magnet.

At end of the 19th century, Nikola Tesla, recognized as one of the main proponents of the alternating current (AC) electricity system, invented the first practical induction motor and established the associated technologies, leading to the extensive uptake of AC motors in industry. The subsequent emergence of simple and low-cost single-phase induction motors that featured ease of use and compactness led to even more widespread use of these motors to power home appliances and other machinery in a variety of settings, including homes and small/medium scale factories.

Nowadays, however, electronically commutated (EC) motors have become commonplace in a wide range of fields, being both more efficient and easier to use than single-phase induction motors. These EC motors are commonly known as brushless direct current (BLDC) motors.

Comparison of capacitor and EC motors

While capacitor motors are practical and easy to use, EC motors have come to be widely used in a diverse range of applications due to advantages that include superior energy efficiency and easier control of speed and other aspects of motor performance.
The following table lists the advantages and disadvantages of the two types of motor.

Capacitor motors EC motors
Configuration Simple

More complex because of the need for a control module

Speed and torque Difficult to control

Easy to control with excellent operational characteristics in terms of ease and flexibility of speed and torque adjustment

Reliability

As long-term use affects capacitor life, therefore maintenance is time-consuming

Being brushless, can remain in use for a long time without maintenance
Efficiency

Low efficiency due to energy losses from heat generated in capacitor

Not only efficient, but the use of a controller reduces power consumption when off-peak
Response

Speed adjustment is difficult with slow response

Fast speed control response
Noise and vibration High Low
Cost Low High

Applications for capacitor motors and EC motors

While the ability of capacitor motors to run on familiar single-phase power has led to their widespread use in areas like the common household, small scale factories and agriculture, the use of EC motors has increased in recent years.

Applications for EC motors include the following.

  • Air conditioning
  • Home appliances
  • Water heaters and burner units
  • Environmental equipment
  • Bathroom products
  • Vending machines
  • Freezer and refrigerator display cabinets
  • ATMs, bill change machines, foreign exchange machines, ticket vending machines
  • Clean rooms
  • Optical products
  • Printers
  • Copiers
  • Healthcare equipment
  • Commercial equipment

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