Induction Motors (1 Viewer)

[Big Willy]

Can someone break it down for me?
No matter how many people i ask, or how many books i read, i dont actually get the workings of the induction motor.

Also, if someone has a link, or a demonstration of the induction motor, send me a link etc.

 

[wogboy]

OK, imagine an large circular piece of aluminium with a hole in the middle of it, with a rod going through this hole (like in car wheels or dumbells) such that the aluminium disc is free to rotate around the rod.

Now imagine a horseshoe magnet placed so that the edge of the aluminium disc is between the north and south poles of the magnet. If you were now to move this magnet around the circumference of the disc, there would be relative motion between the magnetic field and the metal, and as you would know, eddy currents would be generated in this metal disc.

Due to these eddy currents, Lenz law kicks in and there is a force created that opposes you from moving the magnet relative to the disc. Now this force won't make it any harder to move the magnet at all, however it will make it harder to move the magnet RELATIVE TO THE DISC. This means that as the magnet is moved, the disc rotates around too with it (the resistance force caused by Lenz law opposes RELATIVE motion between the magnet and disc, not ABSOLUTE motion of the magnet). So as we move the horseshoe magnet around the circumference of the disc, we will see the disc moving too with the magnet.

Now imagine that we replace this horseshoe magnet with several electromagnets that are placed equal distances apart on the circumference of the disc. If we were to momentarily pass a pulse of current through each electromagnet sequentially (one after the other) in order, this would simulate moving a magnet around the disc (the magnetic field made by each electromagnet would "move" as one is switched off and the next is switched on). This would cause the aluminium disc to spin just like before with the horseshoe magnet. Note that the disc doesn't have to be made out of aluminium but any metal such as steel would do.

The benefits of the induction motors are that they're simple & cheap to manufacture, and more importantly have no wearing parts (no parts come into ontact with eachother) so there's less friction (making it more energy efficient and last longer).

Induction motors are a bit compilcated to understand, but just read this a couple of times and also read your textbook too and you'll understand how it works. HTH

 

[Big Willy]

thanks for that, i kinda get it, but when u say "aluminium with a hole in the middle of it", it gets a bit confusing.

But dont worry, ur explaination and a diagram from a book should make it all good.

 

[quartic]

That was a very good explaination of the principle behind the induction motor. If you can understand that then you should be alright. Just remember that in actual A.C. induction motors there is a "sqirrel cage" instead of a metal disk this cage isn't anything fancy, just a couple of metal rings at each end and metal bars at regular intervals inbetween these rings. The cage is free to rotate and so when the magnetic coils are sequentially turned from north to south poles the cage experiences a magnetic field as if a permanent magnet was being moved around it. Wogboy explained how current is then induced and the cage rotates with the moving magnetic field.

Look at the diagrams in your textbook untill you understand it. I think however that the syllabus also stresses the advantages/dissadvantages of induction motors so also be prepared for a question on this.

 

[1234567]

hm....i start to feel realy bad about motors and generators now.
so an ac induction motor works like that....

ok....we need a ac source connected to opertate it right?

what a bout anormal ac motor? how is it different from a normal ac motor ?

is the ac motor exactly the same as the dc motor except the slip ring thingy/?

 

[kaseita]

if your talking about the simple ac motor vs the dc motor, like just an armature, powersupply, and blah, then yea, the only different is the slip ring
but the more commonly used ac induction motor vs dc motor, they're very different. the ac induction motor uses all that squirrel cage, rotating magnetic field, and so on, whilst your dc motor is just the ol'spinning armature.

 

[smithy]

you will generally find that the "normal" or most used type of ac motor is an induction motor because of its relative cheapness- the other type of ac motor- a synchronous motor is generally only used in large machinery because it in fact has to be brought up to the speed of ocsillation in the ac current before it can be effective.