How does an AC motor work? What controls the speed of an AC?

How does an AC motor work? Most of the appliances, equipment, and tools we use daily are powered by an AC motor. Anything that can be plugged in is likely to be the kind that is powered by an AC motor. This is why AC motors can be called the heart of many machines we use every day. It is the power source for various applications due to its flexibility, efficiency, and quiet operation.

What is an AC Motor?

An AC motor or alternating current motor is an electric motor that consists of a stator with a coil that is supplied with alternating current to convert electric current into mechanical power.

The stator is the stationary part of the motor while the rotor is the rotating part. AC motors can be single or three phase with three phase motors mainly used for bulk power conversion. Single phase AC motors are used for small power conversions.

There are two types of AC motors, which are synchronous and induction. In a synchronous motor, the rotation of the shaft is at the same pace as the frequency of the applied current with multiphase AC electromagnets on the stator that produce a rotating magnetic field.

An induction motor, or asynchronous motor, is a single excited motor where current is applied to one part of the motor, the stator. Flux from the stator cuts the short circuited coil in the rotor, which feels torque that makes the rotor rotate.

AC motors are a power source for a wide variety of applications due to their flexibility, efficiency, and noiseless operation. They are used on pumps, water heaters, garden equipment, ovens, off road equipment and are commonly found in many appliances, equipment, and tools. They are an intriguing and interesting device since they can fit easily into a wide range of applications.

The design of AC motors is rather straightforward with their magnetically driven copper wound stator that has a rotating magnetic field created over it. AC induction motors meet IE3 and IE4 requirements, the international standards for motor efficiency.

How does an AC motor work?

Unlike toys and flashlights, most homes, offices, factories, and other buildings aren’t powered by little batteries: they’re not supplied with DC current, but with alternating current (AC), which reverses its direction about 50 times per second (with a frequency of 50 Hz).

If you want to run a motor from your household AC electricity supply, instead of from a DC battery, you need a different design of motor.

In an AC motor, there’s a ring of electromagnets arranged around the outside (making up the stator), which are designed to produce a rotating magnetic field. Inside the stator, there’s a solid metal axle, a loop of wire, a coil, a squirrel cage made of metal bars and interconnections (like the rotating cages people sometimes get to amuse pet mice), or some other freely rotating metal part that can conduct electricity.

Unlike in a DC motor, where you send power to the inner rotor, in an AC motor you send power to the outer coils that make up the stator. The coils are energized in pairs, in sequence, producing a magnetic field that rotates around the outside of the motor.

How does this rotating field make the motor move? Remember that the rotor, suspended inside the magnetic field, is an electrical conductor.

The magnetic field is constantly changing (because it’s rotating) so, according to the laws of electromagnetism (Faraday’s law, to be precise), the magnetic field produces (or induces, to use Faraday’s own term) an electric current inside the rotor. If the conductor is a ring or a wire, the current flows around it in a loop.

If the conductor is simply a solid piece of metal, eddy currents swirl around it instead. Either way, the induced current produces its own magnetic field and, according to another law of electromagnetism (Lenz’s law) tries to stop whatever it is that causes it—the rotating magnetic field—by rotating as well.

(You can think of the rotor frantically trying to “catch up” with the rotating magnetic field in an effort to eliminate the difference in motion between them.) Electromagnetic induction is the key to why a motor like this spins—and that’s why it’s called an induction motor.

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.

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.

What controls the speed of an AC motor?

In synchronous AC motors, the rotor turns at exactly the same speed as the rotating magnetic field; in an induction motor, the rotor always turns at a lower speed than the field, making it an example of what’s called an asynchronous AC motor.

The theoretical speed of the rotor in an induction motor depends on the frequency of the AC supply and the number of coils that make up the stator and, with no load on the motor, comes close to the speed of the rotating magnetic field. In practice, the load on the motor (whatever it’s driving) also plays a part—tending to slow the rotor down.

The greater the load, the greater the “slip” between the speed of the rotating magnetic field and the actual speed of the rotor. To control the speed of an AC motor (make it go faster or slower), you have to increase or decrease the frequency of the AC supply using what’s called a variable-frequency drive.

So when you adjust the speed of something like a factory machine, powered by an AC induction motor, you’re really controlling a circuit that’s turning the frequency of the current that drives the motor either up or down.

What’s the “phase” of an AC motor?

We don’t necessarily have to drive the rotor with four coils (two opposing pairs), as illustrated here. It’s possible to build induction motors with all kinds of other arrangements of coils. The more coils you have, the more smoothly the motor will run.

The number of separate electric currents energizing the coils independently, out of step, is known as the phase of the motor, so the design shown above is a two-phase motor (with two currents energizing four coils that operate out of step in two pairs).

In a three-phase motor, we could have three coils arranged around the stator in a triangle, six evenly spaced coils (three pairs), or even 12 coils (three sets of four coils), with either one, two, or four coils switched on and off together by three separate, out-of-phase currents.

Advantages and disadvantages of AC induction motors

Advantages

The biggest advantage of AC induction motors is their sheer simplicity. They have only one moving part, the rotor, which makes them low-cost, quiet, long-lasting, and relatively trouble free.

DC motors, by contrast, have a commutator and carbon brushes that wear out and need replacing from time to time. The friction between the brushes and the commutator also makes DC motors relatively noisy (and sometimes even quite smelly).

Disadvantages

Since the speed of an induction motor depends on the frequency of the alternating current that drives it, it turns at a constant speed unless you use a variable-frequency drive; the speed of DC motors is much easier to control simply by turning the supply voltage up or down.

Though relatively simple, induction motors can be fairly heavy and bulky because of their coil windings. Unlike DC motors, they can’t be driven from batteries or any other source of DC power (solar panels, for example) without using an inverter (a device that turns DC into AC). That’s because they need a changing magnetic field to turn the rotor.

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.

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.

Stator Core

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

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.

Rotor

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.

Bearings

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.

Air Gap

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.

Fan

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.

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.

FAQs

How does AC motor work for dummies?

An AC motor is an electric motor driven by an alternating current (AC). The AC motor commonly consists of two basic parts, an outside stator having coils supplied with alternating current to produce a rotating magnetic field, and an inside rotor attached to the output shaft producing a second rotating magnetic field.

How does an AC motor run?

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.

How does an AC motor work physics?

In common AC motors the magnetic field is produced by an electromagnet powered by the same AC voltage as the motor coil. The coils which produce the magnetic field are sometimes referred to as the “stator”, while the coils and the solid core which rotates is called the “armature”.

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