AC Motor

The term AC motor describes several versions of the motor, which include single phase, three phase, brake, synchronous, asynchronous, customized, two speed, and three speed single phase. The difference between the various versions relates to the type of work that is required where some forms of AC motors are simple and used for small jobs while other versions are designed for bigger more demanding work. A key difference is the phase of the electrical feed, which is different for residential use compared to industrial use.
Residential electricity is single or double phased while electricity for industrial use is three phased. This distinction is the reason for the difference between industrial AC motors and residential ones.
AC motors are referred to as induction motors since they use electric current to produce torque, which is created by electromagnetic induction from the magnetic field of the stator.
Start up
An AC motor can be started by a simple on and off switch, which can be a contactor or manual starter. A contactor allows the control of toggle power to an AC motor. Manual starters have a manual switch that allows the operator to switch or change the power. This type of starter is known as across the line meaning the motor is wired directly to the power source. It directly connects the contacts of the motor to the full supply of voltage, which is normally six to eight times the rated current.
Star delta starters are common types of starters, which use a reduced supply of voltage in starting. The stator is connected in a star configuration, which switches to a delta configuration once the motor reaches a certain speed. By doing this, the line current drawn at starting is reduced.
An auto transformer starter uses a similar method as a delta starter. Again, the initial current is limited to reduced voltage being applied to the stator. The advantage of an auto transformer starter is that the torque and current can be adjusted by the correct tapping.
A rotor impedance starter is connected directly to the rotor through the slip rings and brushes. At first, the rotor resistance is set to its maximum but gradually decreases as the motor speed increases. A rotor impedance starter is very bulky and expensive.
Since single phase motors produce a pulsating magnetic field they are unable to be self starting since a pulsating magnetic field torque cannot produce.
Soft starters are a complex version, which allow for the control of acceleration and deceleration for stopping and starting the motor smoothly and evenly, which is not possible with across the line versions. The advantage of soft starters is the reduction of the wear on the motor and the devices to which it is connected.
Stator
The stator produces a rotating magnetic field. It has a solid metal axle, a loop of wire, coils, squirrel cage, and interconnections. Though a squirrel cage is not found in all AC motors, it is the most common type. In AC motors, electricity is sent directly to the outer coils of the stator. The stator has multiple plates that extend out from its center with copper magnetic wire.
For a three phase AC motor, it has three phase windings with a core and housing. The windings are 120° apart, which can be six or twelve windings . The windings are placed on a laminated iron core. The construction of the core can be seen in the diagram below.
Rotor
Unlike a DC motor, the rotor on an AC motor does not have any connection with the external power source. It receives its power from the stator. In a three phase induction motor, the rotor can be a squirrel cage or wound version.
In the squirrel cage version, the rotor consists of rotor bars with end rings at both ends. There are several versions of the squirrel cage rotor, which include split phase, capacitor start, capacitor start and run, permanent split phase capacitor run, and shaded pole with classifications of A, B, C, D, and E. In the majority of cases, the squirrel cage is made of aluminum or copper.
In the operation of a squirrel cage motor, the bars of the rotor interact with the stator‘s electromagnetic field (EMF). As the current fluctuates, the EMF does the same causing the rotor to rotate producing rotational motion. A key factor in the motion is that the rotor does not turn at the same frequency as the AC current and is constantly trying to catch up, which is how the rotation is produced. If it did have the same frequency, the rotor would freeze, and there would not be any motion.
A wound or slip ring AC motor is a special type of AC motor. It contains the exact same parts as all AC motors but is always three phase. The cylindrical laminated core of the rotor is wound exactly like the windings on the stator with wire. The terminal ends of the wires are connected to slip rings on the output shaft. The slip rings connect to brushes and a variable speed resistor. The slip rings provide control of the speed and torque of the motor, which is the main positive feature of a wound rotor.
Wound motors are asynchronous where there is a difference between the stator speed and the output speed. When generating current in the rotor, the motor will have slippage between the rotating field and the rotor. As the motor is powered, the rotor lessens the strength of the stator, which allows the control of the rotation and the ability to choose torque and running characteristics.

Single phase ac motor
Single phase AC motors are used where there is a single phase supply. This type of AC motor is smaller and less expensive. They are constructed using fractional kilowatt capacity. The stator is activated by a single phase AC electrical supply. Unlike a three phase AC motor, a single phase motor has one main winding and one auxiliary winding, which is perpendicular to the main winding.
The rotor rotates according to the sum of two oppositely rotating fields, which is the double revolving field theory. The torque that is produced is equal and opposite.
Polyphase ac motor
Polyphase Motors, or many phase motors, are a type of AC motor that can be two or three phase and are similar to single phase motors in how they operate. The stator poles in a polyphase motor are not aligned with each other, which means that the rotor passes by the stator poles at different times. A polyphase system has a group of equal voltages at the same frequency that are placed to have an equal phase difference between the adjacent electromagnetic fields (EMF). A polyphase system can be two, three, or six phase with the majority being three phase.
A polyphase system is commonly referred to as a three phase system and produces 1.5 times more output than a single phase system. The current from a polyphase system is constant, which is unlike the single phase system that is pulsating.
Synchronous ac motor
A synchronous AC motor is where the rotation of the shaft is at the same frequency as the current supply with the rotation period being equal to the integral number of AC cycles. The synchronous speed is constant and at which the motor generates electromotive force.
The speed of a synchronous motor is independent of the load where variations in the load does not affect the speed of the motor. Synchronous motors are not self-starting, which is unlike self-starting motors where the power supply is connected directly to the stator.
Reluctance motors
Reluctance motors are a single phase motor, which operate with an accurate value of rotating magnetic field without any synchronous speed. The motor uses reluctance of torque to operate, a type of torque in iron devices. The torque for the motor is created by the exterior field generating an inner field on the iron device. For the reluctance torque to be generated, it has to be stretched around the axes at angles to the angle of the contingent poles of the outer field.
Hysteresis ac motor
The unique nature of the rotor of a hysteresis motor is what makes it different from other AC motors. The rotor contains semi-permanent magnetic material. Torque is created by the magnetic flux lagging behind the external magnetizing force. The eddy of the current produces the motor‘s torque. Hysteresis motors provide exact speed with low flutter and operate with little noise.
A hysteresis motor has a core of non-magnetic material with a layer of special magnetic material. The rotor is a smooth cylinder without any windings. The hysteresis ring is made of chrome or steel with a hysteresis loop.
Repulsion motor
A repulsion motor is a type of single phase motor that works by the repulsion of similar poles. Aside from the rotor and stator, a repulsion motor has a commutator brush assembly. The rotor has a distributed DC winding that is connected to the commutator like a DC motor with the carbon brushes short circuited on themselves.
As the rotor circuit shortens, the rotor receives power from the stator by transformer action. The working principle and function of a repulsion motor is the repelling of the similar poles where the north poles repel each other as do the south poles.
Asynchronous motor
An asynchronous motor uses an induced current in its rotor to produce rotatory motion. This is the most common of the AC motors since it relies on AC current that is connected to the stator for its power supply. All of the power for an asynchronous motor is connected to the stator, none of which is connected to the rotor. The power for the rotor comes from induction.
The induction for the rotor is due to its close proximity to the stators electromagnetic field, which causes the rotor to generate its own electromagnetic field that causes it to spin. Since there aren‘t any brushes or slip rings, an asynchronous motor is the most efficient and reliable of all of the AC motors. It is used for heavy duty applications because of its simplicity of design and ruggedness.