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Energy & Electricity in Science : How Does an Electric Motor Work?
Poziom:
Temat: Nauka i technologia
Hi, I'm Steve Jones and I'm going to explain
how an electric motor works. Now I've drawn
this diagram, it looks rather complicated
but it isn't really that complicated. There
are two main elements. The first is this pair
of magnets, one with a north here, the other
with a south here. These are usually permanent
magnets on small motors, but on big motors
they are not. The second main element is this
coil here, although I've shown just a single
piece of wire, actually it starts here, it
goes around, around, around, around, around,
around and then comes out here. So although
it looks like one piece of wire, it's actually
several hundred turns on a coil. And then
maybe several coils. Simply how it works,
we have two things, a magnetic field going
from north to south represented by these arrows
and if I use my left hand, I can say my first
finger is that direction is the field direction.
Now the second thing we have is an electric
supply going from plus to minus so the electric
supply goes up this wire through what is a
carbon brush, this is a graphite carbon brush
into this that we call a commutator. The commutator
is made of two pieces of copper, this copper
disc is connected to this side, this copper
disc to this side. And these are fixed so
as the coil rotates, this rotates. And as
you can imagine, if this rotates half a turn,
the insulating part is going to be against
the brushes and the electricity will not flow.
So what happens when we get a flow of electricity?
We've got our field. C stand for current,
center finger, current, and if I use my left
hand and put it this way, this is called Flemming's
left hand rule and my thumb represents the
direction of a force which is exerted on the
coil. They must be at right angles. So my
first finger is the field going that way.
Now the electric current is coming in here,
it's going around here and it's going down
there. So if I put my center finger in the
direction of the current, I can see that in
fact I will get a force down here and if I
do the same this side, because this is going
this way, the opposite way in fact I'll get
a force going upwards. So that's my force
there. And what will happen is the coil will
rotate in that direction. If I didn't have
this device, it would rotate until it was
upright and then it would stop and it would
stop because as soon as it went over, again
the force would still be going the same way
and it would just hold it upright. So what
happens is when it reaches the vertical in
fact these swap over, the current goes the
opposite way around and in fact it makes sure
that the current in this side of the coil
that is, even when this side reaches here,
the current is always going that way and therefore
the force is always making it continue to
rotate in a circle. Obviously this is going
to be a very uneven device, it's going to
accelerate until it gets vertical, it's going
to slow down and then it's going to sweep
around quickly and slow down again when it's
vertical. So what normally happens, we have
at least three and very often six, nine or
twelve separate coils, each put at a different
angle with separate connections on this side.
This makes a very smooth electric motor where
three or four coils are working at once. So
this is very simply how an electric motor
works.
