You can find out lots and lots about electric motors by visiting
this section of howstuffworks.com,
complete with diagrams and explanations. The very simplest DC electric
motors work by placing an electromagnet in a magnetic field made by a
permanent magnet. The electromagnet is a piece of iron or steel with
wire wound around it in one direction, and it turns the drive shaft
which can spin freely along one axis. To supply electricity to the
wires from outside, where the outside wires do not spin with the
electromagnet, moving contacts have to be made so the wires from
outside the motor do not have to wind around the driveshaft. These
moving contacts are called "brushes" because the stationary electrical
contacts rub or brush against the moving ones.
But why does this all work? The electromagnet feels the magnetic
field of the permanent magnet around it and wants to line up with it,
like a compass needle pointing north. The main gimmick is, at this
point, to reverse the direction of electrical current in the
electromagnet when this happens so that the electromagnet wants to turn
around to line up the other way. This is done by aligning the brushes
so that they rub on one contact when the electromagnet is pointed in
one direction and rub on the opposite contact when the electromagnet is
pointing the other way. It is arranged so that whenever the
electromagnet is pointed towards one of the directions of the
permananet magnet's field, that is just when it suddenly changes
current direction to want to line up in the other direction, and thus
always wants to spin around because it is never quite happy where it
is.
These simple motors can spin in either direction -- they sometimes
need a little push to get them going (more complicated motors have a
starting-up mechanism or multiple poles instead of two to get them
going in only one direction - see the above website on that).
Their speed is determined only by how fast the current can change
direction in the electromagnet, how strongly the electromagnet is
attracted to the opposite pole of the permanent magnet, and how much
load there is on the driveshaft. The applied DC current does not have
any time structure to it, and so does not set the turning rate of the
motor. The rate at which the current can switch directions in the
electromagnet and the force of attraction to the permanent poles are
directly related to the applied DC voltage. The more volts you apply,
the faster the motor will run. You can do an experiment with a motor --
what is the relationship between the speed of the motor and the applied
voltage?
To get more torque, it is good to make a bigger motor or apply
more DC volts to it. A larger electromagnet can have more wires wrapped
around it for a stronger field, and also can apply more torque to the
driveshaft because longer levers have more mechanical advantage. It
will take more electrical power, and some may be wasted. If your motor
is bigger than needed for the job, not only will it cost more to buy or
build, but it will take more electrical energy than is needed for the
job.
It is hard to control the speed of a very simple motor like the
one above, another reason why they aren't very popular for
applications. Many household appliance motors work on the AC power
which comes from the power company. AC power is also easier to
distribute and change the voltage with transformers. The frequency of
the AC power is a natural timing base for these motors' turning speed.
Tom
(published on 10/22/2007)
