Video transcription

Hi, I'm Steve Jones and I'm going to explain how to make an electric motor run backwards. This is a fairly complex problem because normally in modern AC motors we have two coils, and both are electromagnets and therefore it only actually is designed to run one way unless we can change over a magnetic field. It's much easier with a direct current, a DC motor, and if we have a simple DC motor which consists of a pair field magnets here. These field magnets--north, south--I mean I can change over the direction of the motor very simply by making the north this side, and the south the other side. Change over the magnetic field. The other controlling factor is the current going through this coil. This is the motor, and there's the axle of the motor, so this rotates. If I wanted to rotate the other way, what I've got to do is change the direction of the current in this coil. And the easiest way to do that is to make that negative, and that positive. In other words, swap over the battery connections. So in a direct current, DC motor I can either change over the magnets, and these could be electromagnets--of course these also could be electromagnets--so these could have coils to make them into electromagnets, in which case I'll have to change over the current in the field coils, but I mustn't change both over at the same time. So I can't use the same current to drive both. That is why the AC motor--the alternating current motor--basically has problems because if you use the same current to drive the coil as for the field magnet, then you change one, you change both, so you still go the same way. So with a DC motor you change over the magnets, or you change over the current. With an AC motor it's best to fit a gear box.