AC induction motors (1 Viewer)

[sychikmoron]

duuuuuuuuuuuudes

i've got a problem with this question. It was a past half yearly question.

a) Discuss why it is important to have an ELECTROMAGNET as opposed to a permanent magnet creating a magnetic field in an INDUCTION MOTOR

b) How does this magnetic field cause rotation?

c) Explain WHY the motor produces a low power and therefore would be suitable for use in HEAVY MACHINERY

I get b), so i really need help with a) and c)

Pleaseeeeee, i'd really appreciate it.

 

[BlackJack]

The induction motor is designed with electromagnets situated around the sides of the rotor coil. The main idea is that you input AC current into the electromagnets to drive the induction process (b). With permanent magnets this would be impossible; one cannot change the field unless he/she moves the magnets physically to drive the induction motor. This defeats the purpose of converting electric energy to kinetic energy.
If one does put permanent magnets around the side, then the current must logically go into the rotor coil, making this motor work the usual way.

The max speed of the rotor depends directly on the frequency of the input AC, and the motor only works best when the orientation of the coil and the field stays perpendicular. This is both a minus for the induction motor, and the power it can produce. Our AC is fixed at 50Hz, and so this rotor (rotating at a max of 50 rev/sec) does not have lot of momentum.
This low power cannot drive heavy machinery; if the rotor slows down significantly, then it cannot maintain a perpendicular with the changing magnetic field => even less power is produced. Plus it doesn't have the power to keep going at 50 Hz once you put the device to work at whatever the heavy machinery is designed to do..

 

[unsure]

thats for an induction motor right? whats the deal of putting current through a wire that acts in a magnetic field isnt it the same effect?

 

[BlackJack]

When you get down to it a normal motor's uses the same effect as an induction motor, it's the different things that move.
Moving charges around in a magnetic field (current in a wire) == 'moving' magnetic fields around stationary charges (induction). It's the same effect, just in a different frame of reference. However, the term induction is specific for making those charges move by using magnetic fields.

Is that what you'reasking for?

And yes, the above post applies for induction motors. For a normal motor, there's no changing magnetic field to worry about, just the rotating coil position.

 

[zenger69]

thats for an induction motor right? whats the deal of putting current through a wire that acts in a magnetic field isnt it the same effect?

Yes, to create the magnetic field. As we know that current in a carrying wire has a magnetic field. Therefore, the current through a coil creates an electromagnet.

a) It is important for induction motors to have electromagnets because there needs to be phases (like two sets of magnets working at once) not all magnets needs to be on. Otherwise the rotor won't spin.

C) are u sure the question is right. I thought it was unsuitable for heavy workloads because when there is a load the motor slows down a lot.

 

[zenger69]

i've found some more information on (C)

Single-phase AC induction motors tend to be low power motors. There are a couple of reasons for this. Firstly, as with all induction motors, current is not fed directly to the rotor from the mains supply, it is induced by the changing magnetic field of the stator. Less energy reaches the rotor than would be the case if the mains supply was connected directly to the rotor. Secondly, the rotating stator field in a single-phase induction motor is irregular and is not of constant strength as in a polyphase induction motor. Therefore the torque developed by a single-phase induction motor is irregular or pulsating, which results in more energy being wasted as vibration and sound than occurs in a polyphase induction motor.

Due to their inherent low power, AC induction motors are used in applications such as power tools (drills, saws, sanders etc), electric kitchen implements (beaters, food processors etc) and other household appliances (hair dryers, fan heaters etc).

So if you can't be bothered reading the above i'll summarise it:
The ac induction motor is less efficient and has irregular torque, that would make them unsuitable.

Also, that question is no longer part of the syllabus and you should argue that question should be taken out of your half yearly assessment task.