Types of Single Phase Induction Motors

The different types of single phase induction motor are defined by the starting method used. Like Capacitor start Induction run motor (CSIR), Capacitor  start capacitor run induction motor(CSCR), Permanent-split capacitor motor. Single-phase motors work on the same principles as three-phase motors, but have lower starting torques and need help to start.

A standard single-phase stator has two windings placed at an angle of  90° to each other. One of these windings is known as the main winding, while the other is called the auxiliary winding or starting winding.
Single phase induction motor auxiliary and main winding diagram
Single phase induction motor auxiliary and main winding diagram
Depending on the number of poles, each winding may be distributed across several sub coils. Shown here is an example of a two-pole, single-phase winding with four sub coils in the main winding and two sub coils in the auxiliary winding. It should be remembered that designing a single-phase motor will always be a matter of compromise.

The design of any given motor depends on what is most important for operation. This means that all motors are designed according to the relative importance of e .g. efficiency, torque, duty cycle, etc.

Single-phase motors CSIR  and  RSIR can be very noisy due to a pulsating field, compared to  “two-phase”  PSC  and  CSCR motors which have much more silent running characteristics,  because they use a capacitor during normal operation.  The “run-capacitor” balances the motor which results in more smooth operation.
Type of single phase induction motor are often classified by the names of the starting method used.

Main basic types of single phase induction motors:

  1. Capacitor  start Induction run  (CSIR)
  2. Capacitor  start Capacitor  run  induction motor (CSCR)
  3. Permanent split capacitor induction motor  (PSC).

Capacitor start Induction run motors (CSIR) 

Capacitor start/Induction run motor diagram
Capacitor  start/Induction  run  motors
Also known as CSIR (Capacitor start/Induction run single phase) motors, this is the largest group of single-phase motors. CSIR motors are available in sizes ranging from fractional to 1 .1  kW. Capacitor start motors feature a special capacitor in a series with the starting winding. The capacitor causes a slight delay between the current in the starting winding and main winding.This cause a delay of the magnetization of the starting winding, which results in a rotating field effectively in producing torque.  As the motor gains speed and approaches running speed, the starting switch opens.  The motor will then run in the normal induction motor mode.  The starting switch can be a centrifugal or electronic switch.
Capacitor start/Induction run single phase motors have a relatively high starting torque, between 50 to 250 percent of the full-load torque. This makes them a good single-phase motor choice for loads that are difficult to start,  e .g . for conveyors, air compressors, and refrigeration compressors.

Capacitor start and Capacitor run induction motor (CSCR) 

Capacitor-start/Capacitor run single phase induction motor
Capacitor-start/Capacitor  run single phase  motors (CSCR)
Known as a  CSCR motor for short, this type of motor combines the best features of the Capacitor start/Induction run the motor and the permanent-split capacitor motor.  Even though their construction makes them somewhat more expensive than other single-phase motor types, they are the perfect choice for demanding applications.A  Capacitor start capacitor run single phase induction motor has a start-type capacitor in series with the starting winding, just like the capacitor-start motor. This provides high starting torque. Capacitor start capacitor run motors also resemble Permanent-split capacitor  (PSC) motors in so far as they have a run type capacitor which is in series with the starting winding once the start capacitor is switched out of the circuit. This means that the motor can handle high breakdown or overload torque. CSCR motors can be designed for lower full-load currents and higher efficiency. One of the advantages of this feature is that it allows the motor to operate at smaller temperature rises than other, similar single-phase motors. Capacitor start capacitor run motors are the most powerful single-phase motors and can be used for quite demanding applications,  e .g.  high-pressure water pumps and vacuum pumps and other high-torque applications which require 1 .1  to  11  kW.

Permanent split capacitor induction motors (PSC) 

Permanent-split capacitor single phase induction motor diagram
Permanent-split  capacitor  motors
As the name suggests, permanent split capacitor single phase induction motors  (PSC motors) have a run-type capacitor which is left permanently in series with the starting winding during operation. This is to say that they do not have a starting switch or a capacitor which is used only for starting. Thus, the starting winding becomes an auxiliary winding when the motor is up to running speed. The design of  PSC motors means that they cannot provide the same initial boost as motors with separate capacitors. Their starting torques are quite low,  between  30  to  90% of rated load,  so they cannot be used for applications which are hard to start.This is offset by their low starting currents - usually less than  200%  of rated load current  - which makes them the perfect choice for applications with high cycle rates. Permanent-split capacitor motors offer many benefits.  Their running performance and speed can be tailored to meet specific needs, and they can be designed for optimum efficiency and high power factor at rated load.As they need no starting mechanism, they can be reversed easily.  Finally, they are the most reliable single-phase motors available. Permanent-split capacitor motors can be used for many different applications, depending on their design. Low-inertia loads such as fans and pumps would be a common example.

Related terms.

Reference// Grundfos Motor Book

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