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Single Phase Motors



There are many different types of single phase motors. Most of these motors exist in home appliances such as air conditioners, refrigerators, well pumps, and fans.
The two voltages they primarily operate on are 120V and 240V. Single phase motors aren't as smooth, efficient or quiet as three phase, however, they have the same construction. They range from 1/2 horsepower to 10 horsepower.

Split Phase Motors

Split phase motors are ones that literally split one phase and produce a second phase. See illustration below.

Three Classifications of Split Phase Motors

1) Resistant - Start, Inductive - Run (RSIR)
-most popular due to its low cost.

2) Capacitor -Start, Induction - Run (CSIR)
-used for high starting torque.

3) Capacitor - Start, Capacitor - Run
-used for quiet operation like hospitals.
--Permanent Split Capacitor (PSC)
--Two Value Capacitor Motor (CSCR)


There are two types of windings in a split phase motor:
Start Winding - AUX
-constructed from many turns of thinner gauge wire.
-placed near the top of the stator slots.
-impedance is made up of high resistance, and low inductive reactance, and is primarily resistive.

There are two types of windings in a split phase motor:
Run Winding - MAIN
-constructed of few turns of thicker gauge wire.
-placed deep into the stator slots.
-impedance is made up of low resistance, and high inductive reactance, and is primarily inductive.

The following are the characteristics of split phase motors (induction) Without an additional capacitor (ie RSIR) they are 40 degrees out of phase. Adding a capacitor corrects this poor starting torque.

Resistance - Start, Induction - Run Motor

Basic single voltage drawing - (CW)

- A centrifugal switch disconnects the start winding when the motor attains 75% of the rated speed.
-Reversal method that is preferred is to change the start windings. You can also change the main winding leads, but you CANNOT change both.
-Since there is the absence of a capacitor, the running current and the starting current are 40 degrees out of phase with each other. The reason is because the run winding is more inductive than the start winding - but it still has some resistance which prevents the current from being 90 degrees out of phase with the voltage.
-Poor starting torque (approx -13)

Capacitor - Start, Inductive - Run Motor

Basic single voltage drawing - (CW)

- The capacitor in series with the start winding causes the current in the shaft winding to lead the applied voltage. This corrects the scenario seen in the RSIR motor.
-This leading current produces a 90 degrees phase shift between run winding current and start winding current.
-Although the capacitor start induction motor has a high starting torque, the motor should not be started more than eight times an hour. The capacitors need time to cool down.
- An electrolytic capacitor is always used for start windings.
-High starting torque (approx - 27)

Capacitor - Run, Split - Phase Motor
Permanent Split Capacitors (PSC)

Basic single voltage drawing - (CW)

- No centrifugal switch
-Has low starting torque (50% to 100% of full load)
-Has oil filled capacitor that is permanently connected the run capacitor, in series with the auxiliary windings. This is to create a continuous leading current.
-Quiet motor, as needed in hospitals
-Windings are similar in construction unlike induction motors.

Capacitor - Start, Capacitor - Run Motor

Two Value Capacitor Motor

Basic single voltage drawing - (CW)

- By using two capacitors and disconnecting one with the start switch, the problem of too much phase shift between the aux and main winding (after starting) can be solved.
-Quiet motor as needed in hospitals
-High starting torque (approx - 26)
-High efficiency and high power factor. Nice little machine.
-Windings are similar in construction unlike induction motors.

Motor Connections

The following diagrams are 4 lead, 6 lead, and 8 lead connections. Pay particular attention to the connection charts for clockwise and counter-clockwise.

The preferred method to change (reverse) the motor is to change the AUX 'start' leads.

Terminal Markings identified by color (Nema Standards MG1-2.41)

1 - Blue
2- White
3- Orange
5- Black
6- No color assigned
7- No color assigned
8- Red
P1- No color assigned
P2- Brown

Troubleshooting Single Phase Motors

Cleanliness is probably the most important preventive maintenance measure you can take. This permits the proper flow of cooling air around and through the motor, and the proper operation of the centrifugal switch.

Split Phase
(Failure of the motor to start)
-blown fuses
-opens in main or aux windings
-open in the starting capacitor (short)
-open in the centrifugal switch

Series Motor
-Undesirable films may be either too dull or too shiny but are usually very dark.
No film at all is shown by a light copper color and is often an indication that the brushes are too hard.

Brushes should easily slide in the brush holder. The spring pressure on the brushes should be adjusted to manufacturer`s specifications.
Some causes of excessive sparking are:
-roughness at the commutator edges (levelled and smooth)
-high mica between the commutator bars
-brush position off neutral plane
-incorrect brush pressure
-incorrect brush grade
-brushes worn excessively (less than 1/4 remaining)