If you are here reading this, then you have probably already heard that I seem to like induction motors! I first learned about them when studying physics in 6th form, and followed up a lot in university and in industry. They are not always the favoured method of creating motion, but they do have some really handy features.
What Are They?
Induction motors are fed by AC power. Whatever the voltage, the rotation will be a product of the frequency of the power, the number of poles and the number of phases.
Induction motors are made up of 2 parts, the stator and the rotor:
The stator is the "static" bit the doesn't move. It is a winding that forms an electromagnet. In a 2 pole motor (single phase), they will generally be opposite, and wound to be a north and south facing each other. As the AC power goes positive to negative, the poles of the magnets flip too.
The rotor is a freely rotating axis with either windings or laminations to allow magnetic fields to be induced - it is important to note that there is no electrical connection to the rotor, no brushes to wear down like a DC or universal motor!
How Do They Work?
As the power is applied, it (in one instant) creates a magnetic field between the poles of the stator. This field passed through the rotor. Any magnet geeks out there will know that a wire in a magnetic field will induce a current - in this case the movement in the field is created by the action of the AC power, not movement in the stator.
So now we have an electric current induced (See induction motor!) in the coils or laminations of the rotor, which makes it's own magnetic field. What is interesting is the direction of this magnetic self, it's the opposite direction to the field of the stator, so it moves!
This is a pretty simple explanation, there are more things to this, like additional poles, inrush current, slip, additional phases and so on. Most of which I'm avoiding here because I'll probably do a terrible job of explaining them well!
2 Induction motors with no load on them, designed to the same spec will always spin at the same speed (when fed by the same frequency power). With a load, they will spring at an incredibly similar speed, with only really high torque application producing different levels of slip. This is how record players produced accurate pitch, and why they almost always used belts to reduce the speed, relying on the sychinos nature of induction motors to be consistent later direct drive turn tables needed more advanced electronics and normally feedback).
Induction motors can also be designed to provide higher torque than "more efficient" types of motor. Electronically commutated motors (think the motors originally used in PC fans) use rare earth magnets in their rotors, and switching electronics to create a "stator". But they are limited up to about 5 kW in size due to the limited magnetic flux available in the permanent magnet. Induction motors can be enourmous (I have personally worked on motors up to 200kW, but they go bigger). With a variable frequency drive (VSD or invertor) you can control the speed of an induction motor while providing almost all torque at all speeds.
Finally, I know what you are thinking - induction motors sound electrical, not electronic, so why should we care? Well, it's the combination of an induction motor with a variable speed drive (or invertor) that gets really good! If you characterise the voltage, current and harmonics the motor create for any given frequency, you can calculate the torque, slip, shaft power speed of the motor. These are really useful things to know for things like pumps and fans! especially when you have a 200 kW pump running at 3600 RPM!