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.
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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.
Photo: The stator makes a magnetic field using tightly wound coils of copper wire, which are known as the windings. When an electric motor wears out, or burns out, one option is to replace it with another motor. Sometimes it's easier to replace the motor windings with new wire—a skilled job called rewinding, which is what is happening here. Photo by Seth Scarlett courtesy of US Navy and Wikimedia Commons.
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.
Photo: An efficient AC induction motor. Photo by Al Puente courtesy of NREL.
Here's a typical, everyday AC induction motor with its case and rotor removed, showing the copper windings of the coils that make up the stator (the static, non-moving part of the motor). These coils are designed to produce a rotating magnetic field, which turns the rotor (the moving part of the motor) in the space between them.
Photo by David Parsons courtesy of US DOE/NREL.
Here's a little animation to summarize things and hopefully make it all clear:
Photo: A variable-frequency motor. Photo by Warren Gretz courtesy of NREL.
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.
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.
Animation: A three-phase motor powered by three currents (indicated by the red, green, and blue pairs of coils), 120° out of phase.
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).
Artwork: Electric motors are extremely efficient, typically converting about 85 percent of the incoming electrical energy into useful, outgoing mechanical work. Even so, there is still quite a bit of energy wasted as heat inside the windings—which is why motors can get extremely hot. Most industrial-strength AC motors have built-in cooling systems. There's a fan inside the case attached to the rotor shaft (at the opposite end of the axle that's driving whatever machine the motor is attached to), shown here in red. The fan sucks air into the motor, blowing it around the outside of the case past the heat ventilating fins. If you've ever wondered why electric motors have those ridges on the outside (as you can see in the top photo on this page), that's the reason: they're cooling the motor down.
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.
Artwork: Nikola Tesla's original design for the AC induction motor. It works in exactly the same way as the animation up above, with two blue and two red coils alternately energized by the generator over on the right. This artwork comes from Tesla's original patent deposited at the US Patent and Trademark Office, which you can read for yourself in the references below.
Nikola Tesla (1856–1943) was a physicist and prolific inventor whose many amazing contributions to science and technology have never been fully acknowledged. After he arrived in the United States at the age of 28, he began working for the famous electrical pioneer Thomas Edison. But the two men fell out disastrously and soon became bitter rivals. Tesla firmly believed that alternating current (AC) was far superior to direct current (DC), while Edison thought the opposite. With his partner George Westinghouse, Tesla championed AC, while Edison was determined to run the world on DC and dreamed up all kinds of publicity stunts to prove that AC was too dangerous for widespread use (inventing an electric chair, to prove that AC could be lethal, and even electrocuting animals with AC to show how deadly and cruel it was). The battle between these two very different visions of electric power is sometimes known as the War of the Currents. [1]
Despite Edison's best (or worst) efforts, Tesla won the day and AC electricity now powers much of the world. That's largely why many of the electric motors that drive the appliances in our homes, factories, and offices are AC induction motors, powered by rotating magnetic fields, which Nikola Tesla designed in the 1880s (his patent, illustrated here, was granted in May 1888). An Italian physicist named Galileo Ferraris independently came up with the same idea at around the same time, but history has treated him even more cruelly than Tesla and his name is now all but forgotten.
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Learn the basics of a 3-phase AC induction motor, one of today’s most common types of industrial electric motors.
This overview will explain what 3-phase power is, how Faraday’s law works, understand the main components of an induction motor, and the effect of the number of stator poles on the rated speed and torque of a motor.
Watch the video below for an overview of 3-phase AC induction motors.
The first concept to understand about a 3-phase induction motor is the first part of its name – three-phase power.
A single-phase power supply uses two wires to provide a sinusoidal voltage. In a three-phase system, three wires are used to provide the same sinusoidal voltage, but each phase is shifted by 120°.
If you were to add up the voltage of each phase at any point in time, the sum would be constant. Single-phase power is fine for residential or other low-power applications, but three-phase power is typically required for industrial or higher-power applications.
It can transmit three times as much power while only using 1.5 times as much wire, and this makes for a more efficient and economical power supply.
For more information, please visit Asynchronous Induction Motor Price Manufacturers.
Another underlying principle of AC induction motors comes from Faraday’s Law.
The British scientist, Michael Faraday, discovered that a changing magnetic field can induce a current, and conversely, a current can induce a magnetic field.
Using the right-hand rule, you can predict the direction of the magnetic field. To do so, imagine grabbing a straight wire with your thumb pointing toward the current, and your fingers would wrap around in the direction of the magnetic flux lines.
The squirrel cage design is the most common type of induction motor because they are self-starting, reliable, and economical.
In this design, the rotor looks similar to a hamster wheel or ‘squirrel cage,’ thus the name. The rotor consists of an outer cylinder of metal bars shorted on the ends, and the interior consists of the shaft and a solid steel-lined core.
Two main components compose an induction or asynchronous motor: the stator and the rotor. The stator consists of the outer windings or magnets and is stationary. The stator is stationary. For the rotor, the inner core is what actually rotates in the motor, making the rotor rotate.
The squirrel cage design is the most common type of induction motor because they are self-starting, reliable, and economical. In this design, the rotor looks similar to a hamster wheel or ‘squirrel cage’, thus the name. The rotor consists of an outer cylinder of metal bars that are shorted on the ends. The interior consists of the shaft and a solid core built of steel laminations.
A current is applied across the stator to achieve torque at the motor shaft. This creates a rotating magnetic field, inducing a current in the rotor.
Because of this induced current, the rotor also creates a magnetic field and starts to follow the stator due to magnetic attraction. The rotor will turn slower than the stator field, and this is referred to as ‘slip.’
If the rotor were to turn at the same speed as the stator, no current would be induced, and thus no torque. The difference in speed ranges from 0.5 to 5%, depending on the motor winding.
Three-phase motors are available in configurations of 2, 4, 6, 8, and up poles.
The number of poles in the windings defines the motor’s ideal speed. A motor with more poles will have a slower rated speed but a higher-rated torque.
Because of this, high pole motors are sometimes referred to as torque motors and can be used to replace a motor using a gearbox. The ideal relationship between the number of poles, frequency, and speed is defined by the following:
Induction motors offer many advantages, including reduced upfront and maintenance costs.
Because of their basic, economical design, induction machines usually cost less than synchronous and dc motors. This makes them ideal for industrial, fixed-speed applications like wind power and wind turbine generators.
The sheer simplicity of induction motors also makes maintenance easier and less frequent, decreasing operating costs over time. This cost efficiency gives induction machines a significant edge over synchronous and dc motors, which feature many additional components, like slip rings, commutators, and brushes.
Durability is another strength of induction motors. These rugged machines can run for several years with little attention and maintenance, even in demanding environments.
The absence of brushes (and sparks) allows induction motors to operate safely in explosive or other environmental conditions, creating a flexible solution for Oil & Gas, material handling, and more.
3 phase induction motors bear unique advantages as well, including self-starting torque. This feature eliminates the need for starting capacitors, typically required for a single-phase motor.
3 phase machines also deliver exceptional speed regulation and overload capacity, making them viable for various applications.
By converting electrical energy into mechanical energy, 3-phase AC induction motors can drive various components — from pumps and fans to compressors and conveyors — in industrial or higher-power applications.
3 phase AC induction motors represent a premium, low-cost choice for simple, single-speed applications. This includes rotary turntables, material handling conveyors, industrial fans, and other straightforward systems.
3 phase induction motors are also well suited for eMobility applications, including commercial electric and hybrid vehicles.
Induction motors minimize costs and potential points of failure across mining and farming equipment, trucks, and school buses while optimizing motor control performance, providing a comprehensive solution for eMobility machine builders.
The stator and rotor comprise the 3-phase AC induction motors.
During operation, a current is applied through the stator, which induces a magnetic field and leads to the rotation of the rotor. The rotational speed of the shaft and the applied torque depends on the operating frequency and the number of pole pairs in the motor’s windings.
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