Induction motor

Induction Motor (IM) is one kind of AC motor where power is supplied to the rotating device by induction. An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotating part of the motor. In a DC Motor this power is supplied to the armature directly from a DC source. But in an AC motor this power is induced in the rotating device. An induction motor can be called a rotating transformer because the stator (stationary part) is essentially the primary side of the transformer and the rotor (rotating part) is the secondary side. Induction motors are widely used, especially polyphase induction motors, which are frequently used in industrial drives.

Induction motors are now the preferred choice for industrial motors due to their rugged construction, lack of brushes (See DC Motors) and -- thanks to modern power electronics -- the ability to control the speed of the motor.

Basic Operation

The basic difference between a sychronous AC motor and an Induction motor is that in the former a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic attraction, links to the rotating magnetic field in the stator which in turn causes the rotor to turn. It is called synchronous because at steady state the speed of the rotor is the same as the speed of the rotating magnetic field in the stator.

The Induction motor does not have any supply onto the rotor, instead a secondary current is induced onto the rotor. Conductors in the rotor induce a current as the rotating magnetic field created by the stator windings sweep past them much in the same way as in a transformer. This current in the rotor conductors will therefore induce a magnetic field which will be attracted to the rotating magnetic field in the stator and the rotor will turn. For this to happen though the speed of the rotor and the speed of the rotating magnetic field in the stator must be different, otherwise the rotor conductors won't 'see' a moving magnetic field and no current will be induced. If this happens the rotor slows slightly until a current is reinduced and the rotor will continue as before. This difference between the speed of the rotor and speed of the rotating magnetic field in the stator is called slip. It is unitless and is a ratio of the speed of the rotor over the speed of the rotating field. Due to this an Induction motor is sometimes referred to as an asynchronous machine. The relationship between frequency(f), no of pole pairs(p) and synchronous speed(n) is given by f = p*n.

from this relationship

Speed of rotating field (n) = f/P (revs.s^-1)

Where f is the supply frequency and P is the number of pole pairs.

Speed of Rotor = n(1-S) (rev.s^-1)

Where S is the slip.

Construction

The stator consists of wound 'poles' that carry the supply current and will induce a magnetic field in the conductor. The number of 'poles' can vary between motor types but are always in pairs (ie 2,4,6 etc). There are two types of rotors:

1. Squirrel-cage rotor
2. Slip ring rotor

The most common rotor is a squirrel cage rotor. It is made up of either solid copper (most common) or aluminum bars that run the length of the rotor and are connected through a ring at each end. The rotor bars in squirrel cage induction motors are not straight but have some skew to reduce noise and harmonics.

The motor's phase type is one of two types:

1. Single-phase induction motor
2. 3-phase induction motor

A drawing of an Induction motor can be seen here [1]

Speed Control

The rotating speed of the rotor is controlled by the number of pole pairs (number of windings in the stator) and the frequency of the supply voltage. Before the development of cheap power electronics it was difficult to vary the frequency to the motor and therefore the uses for the Induction Motor were limited.

There are various techniques to produce a desired frequency available today, the most widely used of which is called PWM (Pulse Width Modulation). This takes a DC signal and cuts it into pulses at varying frequencies, when this is supplied to the inductor windings they smooth the signal which therefore gives an average over that time period. For example, if a 100 V DC signal was cut equally on and off then the average seen by the winding would be 50 V. If it was now cut so that it was on for a third of the time that it is off then the average would be 33 V etc. By changing the mark-space ratio, otherwise known as the duty cycle, a sine wave can be produced at whatever frequency (Motor speed) is required.

The general term for a power electronic device that controls the speed as well as other parameters is called an 'Inverter'. A typical unit will take the mains AC supply, rectify and smooth it into a "Link" DC voltage and then using the method described above will convert this into the desired AC waveform.

Because the Induction motor has no brushes and is now easy to control many older DC motors are being replaced with Induction motors and accompanying circuitry in industrial applications.