So, just about every handheld power tool
such as this circular saw or this drill
have got a DC or Universal motor in them.
And in this video I'll explain how they work.
So here's my Ryobi hand drill and on the front we've got the chuck and some gears.
And these are driven by the rotor
which fits inside the stater.
The rotor has got a fan on one side for cooling.
And then on here we've got layers of laminated iron
with copper windings in between
which are connected to the individual pieces of copper
on the commutator.
On both ends of the shaft we have a ball bearing.
And I'm just gonna put this in this block of wood here.
And I'll hold that with a clamp.
The windings in here and the iron core
essentially form an electromagnet.
And they're wired up to the commutator in such a way that if I apply electricity from here
to here, the north and south poles will be here and here.
So, let me demonstrate by applying a little bit of current.
Like so. And now I'll hold the compass on here.
And you can see there is one pole here.
And the other side has got the south pole pointing at it.
And now the cool thing is if I move these around
somewhere else
the north pole here is always going to be pointing to whichever side
I'm applying that wire to.
The thing with magnets is that north and south are always attracted to each other.
So this magnet is trying to orient itself
with the south pole facing north 'cause north is this way here.
So now if I bring the south pole of another magnet
nearby, that wants to reoreint itself.
And so if I bring it in from the side
whichever direction.
It's always the north and south that are attracted to each other.
So, if I have the magnet like this and I
bring this magnet in, like so. It wants to rotate
this way. This rotor forms an electromagnet.
And if I apply just a little bit of current in this case
from a AA battery
then I put some leads on with a rubber band
and I apply current here. We've got the north and south poles here and here.
And now if I bring a magnet in from the side
it always tries to orient itself towards that.
And you notice now its spinning.
So, the cool trick is
the north and south poles will always be this way.
And with a magnet here it tries to orient itself this way.
But as soon as it reorients itself we're connecting to different contacts
on the commutator. So north and south will still be this way.
And so the rotation is just continuous.
But as a motor this is actually very ineffective because
magnetic flux travels about a thousand times better
through magnets and through an iron core in here
than it does through the air. And if I hold this near
I'm not really getting that much magnetic flux through here.
And if I get too near it just gets stuck.
To give the flux a better path I devised this thing here where
I've got this bent piece of iron.
And I've got 2 rare earth magnets here and here.
And now if I put the rotor in between
I can have a much smaller air gap although its still pretty large for a motor.
And the flux can then go around here
so I'll get much more flux going through everything.
So, let's apply the AA battery again.
This time I have to go I have to go top and bottom because my magnetic poles are on the sides.
And you can see it spins but not terribly fast.
Part of this is, this rotors actually made for 110 volt motor
and I'm only applying 1.5 volts here.
Partially what's limiting the speed of rotation here
is that as this spins it generates a reverse voltage
just from the changing magnetic field going through the coils.
So, with this homemade volt meter set to 1 volt, full scale.
I now put the contacts here and I spin this.
You can see I get a voltage here.
And I can easily get up to about .6 volts.
And that .6 volts works against the batteries own voltage so
cuts down on how much current I can get through.
So now, back to the power supply and 3 volts on my wires. Let's see how that does.
So that already spins alot faster but let's turn this up
and now we're at about 30 volts on here.
And let's try that again.
Now that seems pretty fast but in an actual drill
this rotor would be spinnning this rotor would be spinning 100 to 200 times per second.
How powerful a motor this actually is, is a function of how strong this magnet is
and how strong the magnets on the other side are.
And this is just a very crude setup.
In an actual drill
the magnet around it is
very close
and much stronger.
And with the flux being able to go through iron, about a thousand times
more easily than through air. There's a very small gap
around the rotor
so that we have a lot of flux going through here.
Now what I was explaining is how this works as a DC motor.
But this is actually a Universal motor and it runs on AC.
And the way that works is the stator
rather than having permanent magnets in it
actually has electomagnets as well.
So, with the AC reversing direction 60 times per second
and if we had permanent magnets it would just wiggle back and forth
60 times per second. But thats no good at all.
But because the field magnets here also reverse in the same way
Whatever the magnetism in here reverses
it reverses in here as well. So it's always in the same relative phase with each other.
And so it continues to drive in the same direction.
Now in a drill
We don't just use a couple of pieces of copper wire
to connect to the commutator here.
We actually use something called brushes.
And we call these brushes even thought they're actually just little pieces of graphite.
And they go into holders here and here.
Now these carbon brushes will slide much better and wear much slower
than a copper wire would. But even these brushes wear out.
So the brushes are always inside of a spring-backed holder
like this thing here. So as the brush wears out you can continue to move forward.
But, of course, eventually there'd be no brush left
and then the motor needs to be serviced. The brushes replaced.
And that's why we never use a motor like that in applications that have to run
for a very long time such as a fridge or an air conditioner
or a furnace. For things that need to run quietly and reliably for
a long time. We use induction motors.
This is just a simple shaded pull motor, much like the one that would be used
to power the fan inside the freezer/refrig.
And if you look here there is a stator with a coil on it
But there's absolutely no electrical connections to the rotor
so no brushes to make a lot of noise and get worn out.
Induction motors vary from the very small to much larger.
For example, this 2 horsepower motor for powering larger tools.
In many ways Induction motors are actually simpler than Universal motors.
But the theory behind them is much harder to explain.
So I'll cover that in my next video.