Electric Motors & Generators

Read the article "Electric Motors" at HowStuffWorks.com

A motor takes electrical current as input, and its "output" is motion. A generator does the exact opposite: if you turn the shaft of a generator, the output is electrical current. Their physical construction is identical. In other words, if you have an electric motor, you can decide to use it as a generator instead, without modification.

Both motors and generators have AC and DC versions. A DC motor uses a split-ring commutator to switch the direction of the current flowing in the armature coils, so it doesn't get stuck in one spot, aligned with the external field magnet. In an AC motor, the two ends of the armature coil are connected to two slip rings. The direction of the current in the armature switches because the external current source switches direction. This keeps the armature from getting "stuck" aligned with the external field magnet. If you use an AC motor as a generator, the current produced automatically switches direction once for every rotation of the armature. If you turn the armature 17 times per second, you will produce 17-Hertz AC. By agreement, all the power companies in the US turn their generators 60 times per second, which is why 60-Hz is the frequency of standard US electricity. If a DC motor is used as a generator, the output wires are switched by the split-ring commutator at just the time the current would switch by itself. Just like multipling two negative numbers gives a positive number, switching the wires themselves just when the current in the wires is switching direction, results in the current not changing direction.

An advantage of DC motors is they are variable speed --- put more current in and they spin faster. AC motors operate at fixed speed, determined by the frequency of the AC power source. An AC motor running on standard 60-Hertz AC is forced to turn 60 times per second.

Activities & Practice
to do as you read

1. Play with the generator simulation http://phet.colorado.edu/sims/faraday/faraday_en.jnlp.

2. Play with this DC motor simulations.

http://www.walter-fendt.de/html5/phen/electricmotor_en.htm