Single phase induction motors are traditionally used in residential applications such as ceiling fans, air conditioners, washing machines, and refrigerators. These motors consist of the split phase, shaded pole, and capacitor motors.
An AC (alternating current) motor is an electromechanical device that converts electrical energy into mechanical movement through the use of electromagnetism and the changing of the frequency and voltages produced by the utility company or motor controller.
AC motors are at the heart of the electrical consumption in the world because they do so much and with very little human intervention. The AC motor is by far the easiest and cheapest motor used in industry.
Fig.1: Motor Stator and Rotor
There are very few parts that make up an AC motor, so long as they stay within their operating characteristics they can run as long as 100 years with very little maintenance here and there. The main parts of the AC motor are the rotor and stator, as seen in figure 1.
A rotor is a rotating part of the AC motor that is supported by a set of bearings to allow flawless rotation housed inside the end bells. The bearings are pressed into the set of end bells which are filled with a lubricant to allow fluid motion.
The stator is the fixed or stationary part of the motor in which the end bells are attached and the windings are wrapped around the laminate sheets of iron that creates an electromagnetic rotating field when the coil is energized.
Motors are very versatile electromechanical components because they can be sized, configured, and constructed to fit any situation or perform any duty. A large percentage of the motors using industry are a single-phase and three-phase motors, as seen in figure 2.
Fig.2: Three Phase induction motor (Image Courtesy: Wikipedia)
Single Phase Induction Motors
A single phase induction motor is an electric motor that operates on a single waveform of alternating current. Single-phase induction motors are used in residential applications for AC motor appliances in single, or multiple dwellings. There are three types of single phase induction motors which are the shaded pole, split phased, and capacitor motors.
Shaded pole Motor
Shaded Pole motors, as seen in figure 3, are single-phase induction motors found operating small cooling fans inside refrigerators in computers. They belong to the family of induction squirrel cage motors that are used in limited applications that require less than 3/4 horsepower, usually ranging from 1/20 to 1/6 horsepower.
The heaviest load a shaded pole motor can turn component that is very light in weight and able to spin in low density, Usually when shaded pole motors go bad they are thrown in the recycle bin and a new one is purchased.
Fig.3: Shaded Pole Motor
Fig.4: Shaded Pole Motor Wiring Diagram
The stator poles are equipped with an additional winding in each corner called a shade winding as shown in fig.4. These windings have no electrical connection for starting but uses induced current to make a rotating magnetic field.
The pole structure of the shaded pole motor enables the development of a rotating magnetic field by delaying the buildup of magnetic flux. A copper conductor isolates the shaded portion of the pole forming a complete turn around it. In the shaded portion, magnetic flux increases but is delayed by the current induced in the copper shield. Magnetic flux in the unshaded portion increases with the winding current forming a rotating field.
Split Phase Motor
A split phase induction motor is a single phase induction motor that has two windings called the run winding and a secondary start winding and a centrifugal switch as shown in figure 6. Split phase motors usually operates at 1/20 HP TO 1/3 HP.
These squirrel cage motors are a step above the shaded pole motors, because they can to a little more work with a heavier load attached to the shaft of the rotor.
Fig.5: Split Phase Motor
Fig.6: Split Phase Motor Wiring Diagram
The split phase motor can be found in applications requiring 1/20 HP up to 1/3 HP, meaning it can turn anything from blades on a ceiling fan, washing machines tubs, blower motors for oil furnaces, and small pumps.
The centrifugal switch is a normally close control device that is wired into the start winding. The purpose of this configuration is that the motor start winding would be taken out the circuit once the motor reaches 75 to 80% of its rated speed. Even though it is considered to be a reliable motor this centrifugal switch is a moving part that sometimes fails to reengage when the motor stops spinning.
How Split Phase Motors Operate
- To start a split phase motor the start and the run windings has to be connected in parallel
- At 75% full speed the centrifugal switch opens, disconnecting the start winding.
- Since the start winding is disconnected from the circuit, the motor is operating through the run winding.
- To remove power from a split phase motor at 40% full load speed the centrifugal switch closes. Powering off the motor.
Single phase capacitor motors are the next step in the family of single phase induction motors. Capacitors motors contain the same start and run winding as a split phase motor does with the exception of the capacitor which gives a motor more torque on startup or when it is running. The purpose of the capacitor is to return voltage to the system when there is no voltage being produced and DAC sine wave of a single phase system.
In the AC single phase system there is only one voltage wave form and during one cycle of the sick 60 cps that it takes to produce voltage no voltage is produced at two points. It is the job of the capacitor to fill this void so the motor is always seeing a voltage which means a lot of torque is produced when the motor is running.
The three types of capacitor motors are capacitor start, capacitor run, and capacitor start and run motors.
Capacitor Start Induction Motor
Capacitor start induction run motors, as seen in figure 7, is a single phase induction motor with the capacitor is connected in series with the start winding and the centrifugal switch of the motor. This configuration gives the motor past starting power but the application does not require a lot of power doing the runtime. During the runtime the inertia of the load plays a big part in the motor operation when there is a problem with the motor it is usually due to a bad capacitor. The motor will generally not rotate unless an outside force spins the shaft; once it is started it will continue to operate fine until power is removed from the motor.
Capacitor start motors are generally found in AC units, large blower motors, and condenser fans. The capacitor of these motors are sometimes built onto the motor or located remotely away from the motor primarily making it easier to replace.
Fig.7: Capacitor Start Motor
Capacitor Motor Operation
- Has a start winding, run winding, and centrifugal switch that opens at 60 to 80% full load speed, as seen in figure 8.
- The start winding and the capacitor are no longer in use once the centrifugal switch opens, as seen in figure 9.
- The capacitor is only used for high torque starting.
Fig.8: Start Capacitor
Fig.9: Centrifugal Switch
Capacitor Run Induction Motor
Capacitor run induction motors, as seen in figures 10 and 11, are much like the capacitor start induction run with the exception of the start winding and run winding stay in the circuit at all times. This type of motor requires low starting torque but needs to keep a constant torque while running. This type of motor can sometimes be found in the air-conditioning compressor. The start winding is permanently connected to the capacitor in series.
Fig.10: Capacitor Run Motor
Fig.11: Capacitor Run Motor
Capacitor Run Operation
- Uses a lower rated capacitor because the capacitor is in the circuit at full load speed at all times.
- Used for higher running torque.
Capacitor Start-Capacitor Run Induction Motor
Capacitor start capacitor run induction motors are single phase induction motors that have a capacitor in the start winding and in the run winding as shown in figure 12 and 13 (wiring diagram). This type of motor is designed to provide strong starting torque and strong running for applications such as large water pumps.
Fig.12: Capacitor Start and Capacitor Run Motor
Fig.13: Capacitor Start-Capacitor Run Motor Wiring Diagram
Capacitor Start-Capacitor Run Motor Operation
- Consist of two capacitors
- One capacitor is connected in series with the start winding; the other capacitor is connected in series with the run winding.
- Both capacitors have different values.
- Capacitor start and run motor has the same starting torque and higher running torque because there is more capacitance.
- Larger value capacitor to start, and lesser value capacitor to run.