How It Works – 3 Phase AC Induction Motor

How It Works – 3 Phase AC Induction Motor


Introduction to three-phase AC
industrial induction motors. Welcome back to KEB. Today we’re going to talk about
three-phase AC induction motors. So today is your industrial induction motor
introduction. What I drew here is an example of three-phase power – the first
concept we need to understand about a three-phase AC induction motor. Now to
take a step back, single-phase power, which would be like just this blue line
here, is when you apply two wires to a load. One which is neutral and one is a
sinusoidal voltage. In three-phase power you use three wires to apply the same
sinusoidal voltage, but each phase is shifted by 120 degrees. So you have your
single-phase blue wave here, but for moving to three-phase, you add
this pink and yellow. So each amplitude is the same but
they’re shifted by this 120 degrees. At any point in time, if you were to add up
the sum of each wave your total would be constant. Single-phase power is great for
more simple applications such as residential or low power. While
three-phase power is typically required for industrial or higher power
applications. Now let’s talk about Faraday’s law. Faraday found that by
applying a current you can induce a magnetic flux. So as a visual of this
imagine this is a wire. You can use the right hand rule to predict which way the
magnetic flux will go. So the current will be flowing from this tip to this
tip. So you would put your thumb in that direction. And then your fingers would
wrap around in the direction of the magnetic flux. Now we’ll see how that is
applied in the induction motor. An induction motor is composed of two main
parts the stator and the rotor. For simplicity, picture them as two cylinders
with the rotor being smaller and inside the stator. The stator is the outside and
is composed of the windings and a stationary. Think stator – stationary. The
rotor is the part that actually move and is on the inside. Think rotor – rotate. The
squirrel cage design is the most common type of AC induction motor. This is
because it’s self-starting, reliable, and economical. Since we make motors here at
KEB, here’s an example of that squirrel cage
rotor. It gets its name because this portion looks kind of like a hamster
wheel or squirrel cage. The rotor is made up of an outside cylinder of
metal bar shorted on each end. The interior is hundreds of steel
laminations stacked to form an inner core. And finally, there’s the shaft which
rotates and provides the actual output of the motor. Now that we’ve talked about
three-phase power, Faraday’s law, and the components of
an induction motor, now let’s see how it all actually comes together. So now bear with
me here, there’s gonna be a lot of induced currents and magnets. So to start
with, you apply the current across the stator. This induces a magnetic field,
which in turn also then induces a current across the rotor, which finally
then results in another magnetic field. These two magnetic fields follow each
other and is what results in the actual output rotation of the motor. So if we
start here at Point “A”, you’ll see the North is here the South is here and
you’ll have this magnetic flux. If we then move to point “B”, this North and
South would rotate. So the arrows would be going across “B” here. And then finally,
from “B” to “C” this would then rotate again. So you’d have North here and South here.
And this arrow going across. As you were to continue apply power you’d have this
continual rotation of the motor shaft. The example I drew here is a two pole
motor. Three-phase motors come with poles in the multiple of 2. So 2, 4, 6, 8, and up.
Motors with a lower number of poles will have a higher speed, while a motor with a
higher number of poles will have a lower speed, but a higher torque. Because of
this high pole motors are sometimes referred to as torque motors, because
they can replace a motor with a gearbox. The ideal relationship between frequency
speed and poles is the following. This equation defines the ideal relationship
between frequency poles and speed of the AC induction motor. If you were to use
the most common 4 pole motor with a 60 Hertz input, you would have a speed of
1800rpm. If you were to keep that same 60 Hertz input, but move to the two pole
motor that I drew here, you would have a speed of 3600rpm. To wrap up everything we talked about
here today, we discussed how three-phase power and Faraday’s law are used to
apply current across the stator of the motor, which eventually leads to the
rotation of the rotor, and the output shaft. This rotational speed is defined
by the frequency and number of poles in the motor. And that’s your introduction
to industrial induction motors. Please like and subscribe, check us out at KEBAmerica.com, or follow along with us on social media.

2 Replies to “How It Works – 3 Phase AC Induction Motor”

  1. The sine wave drawn in this video more closely resembles a 60° Phase shift instead of the intended 120°. We apologize for the confusion. All the spoken and written information is correct. Thank you for your understanding. The graphic in the blog article has the correct wave form if you are interested in learning more: https://kebblog.com/how-a-3-phase-ac-induction-motor-works/

  2. at 2:35 you say "apply the current across the stator" – this is incorrect terminology. voltage is "applied" while current "flows" as a result. but, otherwise it is a good explanation 🙂

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