This post contains everything you need to know about AC Motors on the internet.
You will learn:
- What is an AC Motor?
- How an AC Motor Works
- The Different Types of AC Motors
- The Many Uses for AC Motors
- And much more …
Chapter One – What is an AC Motor?
An AC motor is an electric motor that uses alternating current to produce mechanical energy using magnetism blended with alternating current. The structure of an AC motor includes coils that produce a rotating magnetic field inside a rotor attached to an output shaft, which produces a second magnetic field.
AC motors, which have been used for years, are intriguing and supremely useful to engineers and designers since they can be applied to several applications. The main benefit of an AC motor is its ability to produce constant torque up to the rated speed.
Chapter Two – How AC Motors Work
The main components of an AC motor are the stator, stationary outer drum, and the rotor, the rotating inner portion attached to the motor shaft. The stator and the rotor produce rotating magnetic fields. The winding of the stator that creates the rotating field is created by alternating current.
In an AC motor the winding serves as the armature and field winding. When the stator is connected to an AC supply flux an air gap is formed rotating the flux at a fixed synchronous speed, which produces voltages in the stator and rotor winding.
How AC Motors Work
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.
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.
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 120o 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.
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.
Chapter Three – Types of AC Motors
The AC motor, invented by Nikola Tesla, is used in dozens of applications in every place in the world. The basics of the motor were discovered by Tesla when he identified the rotating magnetic induction (RMF) field principle, which is used in alternators. He pioneered the use of the rotating field and inducting electromagnetic field force to generate torque in rotating machines.
From its beginnings, over a hundred years ago, the AC motor has evolved into several forms that are specially designed to fit multiple functions. One of the basic differences between AC motors has to do with the rotor, which can be squirrel cage or wound. This primary difference expands into AC motor types.
Types of AC Motors
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 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.
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.
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.
The National Electrical Manufacturers Association (NEMA) Classification
The National Electrical Manufacturers Association (NEMA) dictates the standards for motors, which are found in NEMA Standard Publication No. MG 1. The standards are based on electrical equipment manufacturing and best practices. AC motors designed for specialty applications are not classified by the NEMA and are referred to as above NEMA motors.
Induction motors are classified by their electrical design. The NEMA has five classifications for AC motors, which are A, B, C, D, and E. A description of the characteristics of each of the classifications is:
- high breakdown torque
- designed for specific use
- slip characteristic less than 5%
- general-purpose motor
- slip is 3-5% or less
- high starting torque
- normal starting current
- low slip
- little demand for overload
- high starting torque
- high slip of 5 to 13%
- low full load speed
- speed fluctuations due to changes in load
- high efficiency
- low starting torque
- requirements are low
The table below is a general description of the uses for the difference NEMA classifications.
Chapter Four – Why Use An AC Motor?
There are any number of uses for AC motors from running appliances to supplying power to large pieces of machinery. Their low cost with high efficiency makes them applicable to a variety of applications. Wherever electrical motors are needed, AC motors are generally at the heart of the application.
AC motors are more powerful than other types of motors because they can generate more torque using powerful current. They come in a wide variety of sizes, configurations, and strengths to meet the power needs of any industry.
Reasons to Use AC Motors
AC motors are flexible and adaptable to meet the needs of a variety of functions due to their efficiency and low noise. Uses for AC motors include pumps, water heaters, lawn and garden equipment, ovens, and off-road motorized equipment.
AC induction motors are the most common type and the most widely used.
Why use an AC motor?
- Efficiency – AC motors have a high speed to torque characteristic, which offers excellent performance without overheating, braking, or degeneration. The performance of AC motors is why they are used in high demand applications. About 85% of the incoming energy is used to create the outgoing mechanical energy.
- Lifespan – AC motors last in the most demanding of conditions. The only component that may need to be replaced are bearings, which is a simple, easy, and affordable repair. AC motors have two bearings that have to be periodically lubricated. The durability of AC motors is one of the reasons that they are chosen for off road applications or use in rigorous conditions.
- Quiet – Since AC motors have a very low sound output, they are chosen for commercial environments where food is being served or customer service is essential. The sound that AC motors produce is a low hum.
- Adaptability – There are several factors that make AC motors adaptable and flexible. They are powered on using a simple on and off switch, which can be reversed. An additional factor is their variable speed and power output that makes them adaptable to conditions where there are multiple users.
- Accessibility – Every industrial operation has a variety of conditions that require multiple sources of power and energy. Since AC motors come in several shapes, sizes, and different power outputs, they can easily be fitted to any possible situation or be customized and designed to fit specialized and unique conditions.
- Simplicity – The fact that an AC motor has only one moving part is a major benefit to their use. The stator of an AC motor is the same for asynchronous and synchronous motors. This simplicity of design is the reason that they are quiet running, low cost, and long lasting.
- Brushless – A brush motor uses brushes and a commutator to supply electricity to magnetic coils on the armature. This process creates friction, heat, and a loss of energy. A brushless motor, AC motor, eliminates the brushes and commutator, which creates a cooler and more efficient motor that has less wear.
- Self-Starting – Only AC excitation is necessary to operate an AC motor. The simplicity of the starting mechanism does not require any additional component for an AC motor to start.
- Speed Regulation – The speed of an AC motor can be controlled by changing the frequency that is sent to the motor, which causes it to speed up or slow down.
- Single Phase Input – Part of the adaptability of an AC motor is how it can run using a single phase input for a three phase motor even though the location may not have a three phase input.
Chapter Five – How AC Motors Are Made
Three phase AC motors are used for most industrial applications. The three main parts of an AC motor are the rotor, stator, and enclosure with working parts being the stator and rotor, while the enclosure protects the motor and serves as its housing.
AC motors are used for a wide variety of industrial applications because of their strength, adaptability, endurance, and simplicity of design, which makes for easy maintenance. They can operate an industrial pump or a home mixer and adapt to each function with ease.
The stator is the stationary part of an AC motor and the motor’s electromagnetic circuit. It is made from laminations, which are thin metal sheets, that are stacked on each other to form a hollow cylinder. The use of laminations reduces the loss of energy.
Stator windings refer to the copper wire that is wound around the stator in its slots. The number of slots in the stator depends on the phases of power that is provided to the coils. A three phase motor has six slots with three pairs of coil windings that are offset by 120o. The term winding is used to describe an entire electromagnetic circuit composed of multiple coils. The coils are of the same shape and size. The more coils a motor has, the more smoothly it will run.
The number of electric currents energizing the coils is known as the phase of the motor. A three phase motor can have three, six, or twelve coils.
When the motor is activated, the stator is connected directly to the power source, which transforms the coils and stator into an electromagnet.
The rotor is the part of an AC motor that moves or rotates. The squirrel cage type of rotor construction is the most common type. Much like the stator, a squirrel cage rotor is made by stacking laminations to form a cylinder. The squirrel cage is formed by conductor bars that are evenly spaced inserted into the rotor’s slots. The bars for the squirrel cage are made of aluminum or copper.
Once the laminations have been stacked and the conductor bars inserted, a steel shaft is pressed into the middle of the assembly.
The function of bearings on an AC motor are to support and locate the rotor, to keep the air gap small, and transfer loads to the motor. They are able to operate at a variety of speeds while minimizing friction.
There are several types of bearings that are used in AC motors, which include ball and roller bearings. The life of a bearing in an AC motor is determined by the number of revolutions or operating hours a bearing can endure. Other factors include operating conditions and lubrication.
The air gap is the gap between the rotor and stator, which is a necessary part of the motor and a key to its design. The gap has to be large enough to prevent contact between the surfaces of the rotor and stator accounting for tolerances related to their dimensions, loose bearings, and movement. The air gap has to be as small as possible to enhance the efficiency of the motor since larger air gaps require more power to achieve sufficient magnetization.
In AC motors, heat builds up in the windings. For this reason, AC motors have a built in cooling system. Inside the enclosure, a fan is attached to the shaft of the rotor at the opposite end of the axle that drives the machine that the AC motor is attached to. The fan pulls in cool air and forces it across the windings. Hot air is blown out the rear of the enclosure.
The enclosure protects the internal parts of an AC motor from particles and liquids, provides convective cooling, and ensures electrical safety. The amount of protection depends a great deal on the quality of materials used to produce the enclosure. NEMA and IEC have specifications for enclosure designs. An ingress protection (IP) code is used to classify enclosures, such as IP65. The higher the number of the IP code the better is the protection.
Some enclosures come with heat fins on the side and do not have a fan for cooling. Totally enclosed fan cooled enclosures have a fan on the rotor shaft.
- An AC motor is an electric motor that uses alternating current to produce mechanical energy using magnetism blended with alternating current.
- The main benefit of an AC motor is its ability to produce constant torque up to the rated speed.
- The main components of an AC motor are the stator, stationary outer drum, and the rotor, the rotating inner portion attached to the motor shaft.
- The basics of the motor were discovered by Tesla when he identified the rotating magnetic induction (RMF) field principle, which is used in alternators.
- The National Electrical Manufacturers Association (NEMA) dictates the standards for motors, which are found NEMA Standard Publication No. MG 1.
- AC motors are used for a wide variety of industrial applications because of their strength, adaptability, endurance, and simplicity of design, which makes for easy maintenance.