How exactly does an AC motor work? It’s helpful to have a working understanding of AC asynchronous motors so that when you’re replacing one or troubleshooting, you have some context on each of the components and how they work together. Check out the explainer video below for the full story on how AC motors work.
You likely come across multiple AC motors in your day-to-day life, finding them in almost every room in your home as the main driver of your appliances. They’re in washing machines, ovens, gardening equipment. They’re also very common in industrial applications.
Use the below link to jump to different sections of this article:
- Parts of an AC Motor
- How Does an AC Motor Work?
- Are AC Motors Better Than DC Motors?
- Where to Buy AC Motors?
Before we discuss how the induction motor works, let’s do a quick run-through of the individual parts.
Starting on the outside of the motor:
- Starting at the large drum, this is the housing that holds most of the parts. We usually refer to it as the enclosure.
- The box attached to the enclosure is the connection box. This is where you’ll hook up power to the motor or wire peripheral controls into the circuit.
- At the back of the motor is a cover that protects the fan. This fan is there for reducing heat. The fan is attached to the shaft, so its cooling ability is directly influenced by the speed of the motor. The shaft is the only moving part of the motor and is the component that drives the equipment it’s attached to.
We’ll take the motor apart now to take a look at the internals:
- The shaft sits on bearings at the front and back of the motor.
- The bearings hold the shaft in position and help it rotate smoothly, reducing friction and wear.
- The rotor is always made of a conductor, whether it’s a coil, or squirrel cage type. This motor’s conductor is squirrel cage.
- And finally, we have the stator. It’s made up of thin steel laminations and copper coils that run through these slots. The copper wires are covered in an enamel insulation to avoid short-circuiting and to help protect from heat damage. The ends of the wires connect to the terminals in the connection box. The coils are pairs that make up a phase of the motor. Each pair sits opposite from the other in the stator and is rotated at 120 degrees from the other two phases.
Before we dive in, let’s cover some basic electrical theory:
- AC motors run on alternating current power. Alternating current, or AC, periodically reverses direction. This means that it alternates between positive and negative multiple times per second.
- Frequency is how fast the polarity changes from positive to negative. For example, if you have a frequency of 60 hertz, that means the current switches direction 60 times per second.
- AC motors take alternating current and convert it into mechanical power through electromagnetic induction, a phenomenon discovered by Michael Faraday in 1831.
Dubbed “Faraday’s Law,” electromagnetic induction is the process of creating a current by passing an electrical conductor through a static magnetic field or by placing a moving magnetic field around an electrical conductor.
It also works in the reverse, when a current is passed through an electrical conductor, a magnetic field is produced.
An AC induction motor works on these principles, here’s how it happens:
- Power’s applied to the coils in the stator. The charged coil becomes an electromagnet, generating a magnetic field.
- The coils are supplied with alternating current, so they’re charged in pairs, alternating in sequence with the frequency of each phase. This causes the magnetic field to rotate around the stator.
- The rotor is mounted inside the stator, suspended in the magnetic field. As the magnetic field rotates around the rotor, an electrical current is induced in the rotor.
- This current then produces its own magnetic field. As the stator’s magnetic field rotates, it repels and attracts the rotor’s magnetic field. The rotor begins to spin as it tries to "catch up” with the rotating magnetic field. And in turn, drives the intended load.
The AC Motors simple design includes only a single moving part, offering you a low-cost, quiet operation, and a long-lasting motor option.
Keep in mind that the speed of an AC motor is based on the frequency of the power supply, so it runs at a constant speed. You can easily adjust the frequency of the power supply by using a VFD.
Check out our article on how DC motors work to compare the differences.
The fundamental difference is the power that AC and DC motors run on. AC motors run on alternating current and DC motor run on direct current. Electricity to homes and businesses is alternating current, and AC motors are much more common.
AC motors are less sensitive to changes to load than DC motors, making AC motors ideal for heavy load, continuous-speed applications. AC motors are generally less efficient than DC motors.
For a full comparison, read this article on the differences between AC and DC motors.
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