real electric motors work
Printed circuit motors
called "pancake motors", these are a particular
cunning motor configuration whose operation is in some ways
is easier to visualise than that of a conventional motor.
They fit into confined spaces (say inside a car door, to
make the windows go up and down) and, because the rotor
is light and has little rotational inertia, can accelerate
to full speed and stop again very rapidly. This feature
isn't so important for car windows (unless you’re
into drive-by shootings), but is essential for industrial
robots and other servo mechanisms.
• Efficient – no hysteresis or "iron"
• Very low rotational inertia
• Light weight
• Flat, so fits into confined space.
• Expensive to make
• Armature has little mass, and therefore can overheat
is what a typical printed circuit motor looks like. Don't
worry about the two black wires for the moment.
is what's inside. Glued to each "face” of the
motor are (in this case) eight magnets. Their poles alternate
N-S-N-S etc as you go around.
the other face, with its eight magnets. These line up exactly
(north to south) with the magnets on the other face, creating
a strong magnetic field across the gap where the rotor sits.
(Try not to think about that black wire for the moment.)
So, now we've set up a strong magnetic field running axially
(ie, parallel to the motor shaft). That field threads back
and forth eight times through the small gap that will be
occupied by the rotor when we put it all back together.
(Note that the end faces are made of iron and complete the
if the magnetic field is parallel to the shaft, and we want
a tangential force on the rotor, which way does the current
have to be flowing? Well, it has to be at right angles to
both, and therefore radial.
it should all make sense. The brushes contact the rotor
on that blackened area near the shaft. The current goes
out along the copper wire, and is travelling almost radially
as it goes through the region of highest magnetic field.
(Almost radial but not quite, to reduce cogging
torque.) So, a force is exerted on the wire that is at right
angles to the wire and at right angles to the magnetic field,
causing the rotor to turn. Now, if the wire just turned
around and came back in towards the shaft again, the force
on the bit coming back would be equal and opposite to what
it was going out, cancelling out any useful torque and the
whole thing would just sit there with smoke pouring out
of it. So, once the wire has gone out past the magnet, let's
take it over diagonally to the right and bring it back in
to the shaft past the next magnet which, you'll
recall, has its magnetic field in the opposite orientation.
Now the force on the returning piece of wire will add
to the torque, and away we go.
back on the blackened piece of rotor the current can pass
out through the second brush and back to the battery or
whatever it is that's powering the robot.
not to think about the black wire for a moment. In this
picture you can see the two brushes. They simply rub on
the rotor which, as you saw, consists simply of a flat piece
of insulator with copper lines etched or stamped on it,
like a printed-circuit board. The other side of the brushes
ends up as brass terminals on the outside of the motor,
as in the first photo.
there we have it. The only problem now is that the magnets
themselves cannot retain their strongest permanent field
unless they are always in a completed magnetic circuit.
So, you can't magnetise them and then assemble
the motor. But, once you've assembled the motor, you can't
get at them to magnetise them. So, we can't actually build
this kind of motor. Pity, really, it was looking rather
wait a moment! Suppose we thread a black* wire back and
forth between the magnets, as in the picture above. Let's
bring the wire outside the motor, and once everything is
assembled we zap a gazillion amps through the wire. Think
about what direction the magnetic field created by the current
through that wire will be in. Perfect! Admittedly it's not
a very thick wire to be coping with such a large current
(typically several thousand amps), but it's only for a few
milliseconds and the wire doesn't have time to complain.
Also, it only has to happen once...
any colour would do.