• Like us on facebook here

AC induction motor + transformer questions (1 Viewer)


New Member
Mar 10, 2015
What are the disadvantages of AC induction motors? The only one I have is that the rotational/spinning speed is limited as it is induced. Just wondering if there are any more?

Also, when talking about an appliance that utilizes transformers (e.g. TV, microwaves), how detailed does it need to be?



New Member
Aug 30, 2014
Okay, so the main problem with AC induction motors is that they require multi-phase electricity in order to produce the rotating magnetic field, which is really only available to commercial and industrial users. This means that household appliances such as a ceiling fan cannot use AC induction motors.

Since the rotation of the magnetic field is determined by the frequency of the AC current, AC induction motors have an optimal rotational speed - as you've pointed out. The other implication of this is that induction motors are not as easy to start up as a conventional DC motor, as the torque is independent of the position of the rotor. This means the magnetic field is oscillating rapidly, and the induction motor will not produce a great deal of torque at slower speeds. This can be minimised however through good engineering.

As well, since AC induction motors are very complicated designs, this adds to both the design and manufacturing costs. As a result they tend to be more expensive than conventional motors. Yet for most businesses who use them, this upfront cost is worth it considering conventional motors have a higher operating costs and are less reliable.

For transformers, I would definitely mention whether it is a step-up or step-down transformer, assuming it is for a specific purpose such as a TV, microwave or phone charger (usually an easy 1 mark). Then, I think that an outline of a transformer might be suitable - insulated primary and secondary coils around a laminated iron core (simple diagram may be a nice inclusion); primary coil induces a current in the secondary when supplied with an AC current due to changing magnetic fields -> Faraday's Law. Of course, the detail you go into is really just dependent on the question and the marks.

For appliances, I think that a question on TVs may be possible since it links in with other content from the Ideas to Implementation topic. You may well see a 5-8 mark question if it integrated CRTs, in which case you'd also discuss the need for a high voltage (several kV's) across the cathode and the anode collimators in the electron gun to cause thermionic emission.

Hopefully that helps
Last edited:

Users Who Are Viewing This Thread (Users: 0, Guests: 1)