Single-Phase Induction Motors
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Introduction
The two main types of AC motors are “induction motors” (also called asynchronous motor) and “synchronous motors.” The most common of the two is the induction motor. Induction motors only run on AC voltage because DC voltage does not produce induction. Therefore, it is understood throughout this post that an induction motor is purely an AC motor.
The Induction Motor
An induction motor is a rotating electrical machine that converts electrical energy into mechanical energy while being driven by the principles of induction. In order for electrical induction to take place in a motor, it requires an AC (Alternating Current) voltage applied to its windings. In this post, we will discuss the Single Phase Induction Motor which requires only one single AC power phase to operate. In the US, the most common single-phase AC voltages are 120VAC, 208VAC, and 240VAC. Induction motors are used in a wide variety of domestic and industrial applications such as: fans, pumps, conveyors, etc.
Origin
The induction motor was invented by a group of scientists who’s names are: Michael Faraday, William Sturgeon, Thomas Davenport, and Emily Davenport.
Two Major Components
The two major components of an induction motor are the “stator winding” (which is stationary) and the “rotor” (which is the rotating part attached around the outer part of the shaft). These two components work together to create electromagnetic induction and transfer rotating energy to a mechanical load through a rotating shaft. Electromagnetic induction is a process by which current is created in a conductor by moving it through a magnetic field, or by having the magnetic field move or change around a stationary conductor.
The stator contains two pairs of windings: the “main winding,” and the “auxiliary winding.” These pairs of windings are placed on opposite sides of a circle (180 degrees apart) from each other. The main windings are placed perpendicular to the auxiliary windings. When an AC voltage (which is constantly changing its polarity) is applied to the main windings, two equal and opposite rotating fields are created around the rotor. Since the lines of flux in the stator’s rotating magnetic fields cut through the bars in the rotor cage in opposite directions at the same time, the rotor develops two opposing torques. Thus, the net torque on the shaft is zero. This concept is called “double-revolving field theory.” This causes the rotor to buzz or hum without turning. This dilemma is overcome with the use of a starting capacitor connected to the auxiliary winding in the stator which cancels one of the opposing fields in the rotor and increases the magnitude of the remaining field. The rotor is now able to turn freely in one direction at a specific speed. The rotating rotor is attached around the center of the output shaft, which also causes it to turn and perform many types of rotating mechanical work.
The induction motor relies on a small difference in speed between the stator’s rotating magnetic field and the rotor shaft speed called “slip” to induce rotor current in the rotor bars. This is what keeps the motor turning. Slip is expressed as a % of motor speed in RPM’s (Revolutions Per Minute).
The Synchronous Motor
As the name implies, synchronous motors are capable of maintaining a constant speed under load as long as the load is within the capability of the motor. They are high-efficiency machines and are mainly used in high-precision applications. The constant speed characteristic is accomplished by the interaction between the constant magnetic field and the rotating magnetic field. The rotor produces the constant magnetic field, and the stator produces the rotating magnetic field. The field coil of the stator is excited by the AC supply voltage applied to the stator windings. This will produce a revolving magnetic field which rotates at synchronous speed (Ns). The rotor is excited by a DC supply voltage so it acts like a permanent magnet, or the rotor could be made of a permanent magnet (which would not require the DC supply voltage).
A synchronous motor is not inherently self-starting. To make it self-start, a squirrel cage arrangement is placed between the magnetic pole tips in the rotor. This causes the motor to start just like an induction motor. However, the synchronous motor is able to maintain its constant speed with the field produced by the energized or fixed magnets between each rotor bar. If the external torque load is greater than the torque produced by the motor, it will fall out of synchronization and eventually come to rest. Low supply voltage or low excitation voltage can also cause the motor to fall out of synchronization. Synchronous motors can also help improve the overall power factor of the electrical system. See post on “Power Factor Explained.”
How to Control the Speed of an Induction Motor
The synchronous speed (Ns) of a single-phase induction motor is calculated by multiplying the number of cycles (f), times the number of seconds in a minute (T), times 2 for the positive and negative pulses in the cycle, divided by the number of poles (P)…4, for example:
For 60-Hertz System:
(60 x 60 x 2) / 4 = 1,800 RPM
For 50-Hertz system:
(50 x 60 x 2) / 4 = 1,500 RPM
By using the synchronous speed formula, it can be seen that controlling the speed is accomplished by varying the frequency of the supply voltage or the number of poles. The easiest way is to vary the frequency using a variable frequency drive.
Ns = (f x T x 2) / P
Are AC Motors Reversible?
Yes. Reversing the direction of an AC motor is accomplished by switching the power lead on the auxiliary winding from one side of the starting capacitor to the other.
Are DC Motors Reversible?
Yes. Simply switch the polarity of the power leads to the winding and the direction will reverse.
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