Single-Phase Induction Motors

Andy Wendt
Design & Drafting Services
Prattville, Alabama
andywendt@bellsouth.net

NOTE: These illustrations and descriptions are not intended to replace motor nameplate information. As generic wiring guidelines, these illustrations aid in interpretation of motor nameplate information. No warranty is expressed or implied as to the suitability of this information for any purpose.

Single-phase induction motors lack starting torque. To initialize motor rotation, a "start" winding produces magnetic pulses "out-of-phase" with pulses created by the "run" winding. This simply means that the pulses occur at different times. Since a motor, or the generator that produced the power, rotates to a different position after a small time interval, phase is measured in degrees instead of elapsed time. Also, since motors run at varying speeds, elapsed time values would vary with motor speed. The optimal phase angle to produce starting torque is 90 degrees. Note that in the illustrations, the start winding is depicted at 90 degrees to the run winding.

Think about a ship's wheel and about placing your hand between two of the spokes around the outside of the wheel. If you quickly strike one of the spokes with the palm of your hand to start the wheel rotating and then quickly move your hand back to its original position, the next spoke would strike the back of your hand, and the wheel would be pushed back to its original position. Rapidly moving your hand back and forth between two spokes causes the wheel to mimic the behavior of a single-phase induction motor without the start winding. In the United States, alternating current (AC) reverses direction 60 times per second (60Hz). This cyclic rate is much too fast to cause any oscillation of a motor's shaft, but a motor with start circuit problems will emit an audible hum or buzz.

Now, think about placing both of your hands between spokes. Quickly strike a spoke with one hand and immediately strike the spoke in the same direction with the other hand. When you move your first hand back to its original position, the back of your hand is not struck by the next spoke because the second strike moved the spoke past your first hand. Continue striking with both hands until the wheel is spinning rapidly. Then, use one hand to maintain rotation.

Single-phase induction motors operate similarly. Most commonly, a switch operated by centrifugal force, disconnects the start winding after the motor reaches 75 percent of its rated speed. Usually, a capacitor wired in series with the start winding creates "phase shift" between the run and start windings. However, it is possible to create phase shift between two windings without a capacitor by using more wire for one winding and/or making the two windings from different gauge wire. And, some motors, lacking a centrifugal switch, are designed to operate at full speed with the second winding still connected.

Motors with centrifugal switches usually accelerate to operating speed in under 3-seconds. The start winding in these motors are not designed to be energized for prolonged periods. Operating a motor with its start winding energized for more than 15-seconds is not usually recommended, and keeping the start winding energized for more than 60-seconds will likely result in damage from overheating.

So, single-phase induction motors may or may not have a centrifugal switch or a capacitor, but some have two capacitors. Many single-phase induction motors have two run windings and one or two start windings. As single phase power can be either 120 volts or 240 volts, dual-voltage motors have two windings that are connected in parallel for low voltage or in series for high-voltage. Another variation has two capacitors--one capacitor is disconnected by the centrifugal switch, while the other remains connected at all times. Some motors use two separate capacitors while others use one dual capacitor containing two capacitors in one housing.

I know... I didn't explain how a capacitor creates phase shift... But, this is about motors...

Although single-phase induction motors run on single-phase power, all variations are sometimes referred to as "split-phase" motors due to the phase shift between the run and start windings. If there are any standard motor naming conventions, they are not closely adhered to. But, motors without capacitors are frequently called "split-phase" motors. Motors with capacitors but without centrifugal switches are called "capacitor run" motors. Motors with centrifugal switches and capacitors are called "capacitor start" motors or "capacitor start, induction run" motors. Motors with two capacitors and a centrifugal switch are called "capacitor start, capacitor run" motors.

Just as there are variations in motor naming conventions, there are variations in terminal marking and wire colors. Red and black wires usually connect to the start winding, and yellow wires are frequently used to connect the capacitor. The illustrations show common terminal marking conventions, but there are variations. Always refer to motor nameplate data.

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Split-Phase Motor

Although all single-phase induction motors are "split-phase" motors, the term is usually used to describe motors lacking capacitors. This motor has a "run" winding and a "start" winding that remain connected at all times. A motor of this type has very low starting torque.

Split-Phase Motor With Centrifugal Switch

This motor is similar to the "split-phase" motor in the previous illustration--both lack capacitors. Starting torque for this motor is low but more than the starting torque of split-phase motors lacking centrifugal switches as in the previous illustration. A motor of this type is commonly used in washing machines.

Capacitor Run Motor

Often referred to as a "capacitor run" motor, a single-phase induction motor that lacks a centrifugal switch has a capacitor that remains connected at all times. It is so named because the capacitor remains connected while the motor is running. The difference between this motor and the "split-phase" motor in the first illustration is that differences in the run and start windings create phase shift for the first motor, while the capacitor creates phase shift for this motor. Just as for the motor in the first illustration, this motor has very low starting torque.

Capacitor Start Motor

A single-phase induction motor with a start capacitor is known as a "capacitor start" motor. The centrifugal switch disconnects the capacitor and start winding while the motor is running--the capacitor is only used for starting. As it has high starting torque, a motor of this type is commonly used for pumps, compressors, refrigerators and air conditioners. All capacitor start motors possess high starting torque. Note that as depicted, this motor is wired for forward rotation.

Reversing A Capacitor Start Motor

This is the same "capacitor start" motor depicted in the previous illustration, but wired for reverse rotation. Regardless of whether or not it has a capacitor or centrifugal switch, changing rotation of any single-phase induction motor is achieved by swapping places with the two start circuit leads. Most often, these two leads are red and black. Always refer to motor nameplate data.

Dual-Voltage Capacitor Start Motor
(Low Voltage Wiring)

This illustration shows a dual-voltage "capacitor start" single-phase induction motor connected for low voltage (120V). Of motors ranging from 1 horsepower to 5 horsepower, this is the most common motor I've encountered. Motors with horsepower ratings of 1hp to 3hp are often operated from low voltage. Note that the two "run" windings are connected in parallel.

Dual-Voltage Capacitor Start Motor
(High Voltage Wiring)

This illustration shows the same dual-voltage "capacitor start" single-phase induction motor as in the previous illustration but connected for high voltage (240V). Motors over 3-horsepower are usually connected to operate at high voltage. Note that the two "run" windings are connected in series. Acting as a voltage divider, the two series connected windings provide a low voltage connection for the "start" winding. A 3-horsepower motor operating from 120V draws nearly 20 Amps. Changing the internal motor wiring to operate the same motor from 240V reduces the current draw to 10 Amps. With the reduced current draw, smaller supply wires can be used and loses (voltage drop) over distance are reduced.

Dual-Voltage Capacitor Start Motor
With Dual Start Winding (Low Voltage Wiring)

This illustration show a different type of dual-voltage "capacitor start" single-phase induction motor connected for low voltage (120V). Note that it has two "run" windings connected in parallel, but is also has two "start" windings connected in parallel. A previous illustration depicted another type of dual-voltage motor with only one start winding.

Dual-Voltage Capacitor Start Motor
With Dual Start Winding (High Voltage Wiring)

This illustration show the same dual-voltage "capacitor start" single-phase induction motor as in the previous illustration but connected for high voltage (240V). Note that it has two "run" windings connected in series and two "start" windings connected in series. A previous illustration depicted another type of dual-voltage motor with only one start winding using the run windings as a voltage divider for a low voltage connection.

Capacitor Start, Capacitor Run Motor

A single-phase induction motor with two capacitors or one dual-capacitor is called a "capacitor start, capacitor run" motor. The start capacitor provides starting torque exactly as in other capacitor start motors, while the run capacitor improves motor efficiency thru "power factor" correction. As these motors are usually large, 5 horsepower or greater, and used in high-torque applications, they would likely operate at high voltage. They may or may not have dual-voltage capability, but for simplicity in the above illustration, only single run and start windings are shown. Note that the centrifugal switch only disconnects the start capacitor--the start winding remains connected thru the run capacitor.

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