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.
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