In my last video I explained how DC motors
like the one in this drill work.
And in this video I'll cover a different kind of motor
Induction motors. Like
This furnace motor
or the motor in this fan.
or a small shaded pull motor.
This is a shaded pull motor
which is the simplest form of induction motors.
And I've already taken the screws out.
So let's take the rotor out.
And you can see here this is a simple thing
and it's not in any way magnetic. It doesn't attract the screwdriver in any way.
But it dose have these lines accross it.
And these lines are essentially a short circuit winding on the rotor
that's been cast in place.
It's a bit like the rotor of a DC motor
but with all the windings shorted out.
The rotor's actually passively magnetized by the stater.
But these short-circuited coils here
they actually prevent the magnetic field from changing too quickly
because as the magnetic field changes
a voltage induced that causes the current to flow
which then counteracts the change in the magnetic field.
So we basically have a rotor
that, even though it's very permeable to magnetism
It doesn't like it's field changing very quickly.
I can demonstrate the same principle with this aluminum disc.
It's also like a short circuit winding.
So if I put this disc between these 2 magnets
that induces a current in the disc as it rotates
and that counteracts it so it slows it down.
With the shaded pull motor if I apply DC to the coils
that creates a magnetic field which also slows down the motor.
With this furnace motor I applied just a few volts DC
to the stater will also slow it right down.
The breaking effect is enough to keep this 10 pound weight from dropping very fast.
But if I cut power it really goes.
So the rotor for and induction motorreally doesn't like to spin in a magnetic field.
But if we spin the magnetic field
then the rotor will try to follow.
Coming back to this aluminum disc
if I now move the magnetic field accross the disc
that actually spins it up a little bit.
So we could spin the magnetic field by pinning the stater around the rotor.
But as a motor that would be very useless.
So in a 3-phase motor we pin the magnetic field electrically.
So, imagine we have current going through the blue wires
so that north is here and south is here.
And next we'll apply current to the green wires
and then to the red wires.
And then we'll apply it in the opposite direction throught the blue
through the green and through the red
so that we have this magnetic field that keeps spinning around.
In a 3-phase motor we actually apply these currents
continuously as Sine waves so that this transition is very smooth.
And you really do get the spinning magnetic field.
This spinning magnetic field will magnetize the rotor
but the short circuit windings in the rotor
resist changes in the magnetic field
so as the magnetic field spins
the rotor will follow.
To get this nice spinnning magnetic field we need 3-phase power.
But the problem is most houses aren't wired up for 3-phase power.
With single phase power all we can make is a magnetic field that goes back and forth.
And on this motor, I cut the starter winding so if I power this motor up
it just sits there and hums a bit. But if I give it a slight spin
that spins it up.
Now because it's only going back and forth, this motor has no preference
in direction. So I can just take this motor and spin it the other way around
and its just as happy going that way.
There are many ways to give a single-phase induction motor it's initial kick.
The simplest of these is a shaded pull motor.
In here we have 2 copper short-circuit windings
around part of the pole and that's what called it shading.
And as you recall, the copper winding the short circuit winding
doesn't like the change in magnetic field very much.
So, as this goes north here
the north side on here is delayed a bit and then as it switches to south
again, this part of the pole is delayed switching to south.
So, as the magnetic field goes back and forth
this part will always be a bit behind.
And that will give the rotor it's initial kick to get it going.
In fact, even if I even if I just give this motor some DC pulses
every time the shaded part is a bit behind
then the rotor tries to follow to change a little bit.
But now, if I switch the current to AC.
And I get up to operating voltage.
The motor runs just fine.
These shaded pole windings are actually short circuit windings
and they're very inefficient.
So this sort of thing is only used on very small motors.
Larger motors, like this furnace motor
use an auxillary winding to start up.
That winding's also very inefficient so its always disengaged
once the motor's up to speed.
And as it slows down
that click you hear is the centrifical switch
turning the auxillary winding back on.
There are many different ways that are used for starting AC motors.
And that could be a whole video unto itself.
I'll explain more of that at the article that I'm linking from the description.