An induction motor is a type of AC motorwhere power is supplied to the rotor by means of electromagnetic induction. These motors are widely used in industrial drives, particularly poly phase induction motors, because they are robust and have no brushes. Their speed can be controlled with a variable frequency drive.
Fig: Various parts of an Induction motor.
Operation of a Induction motor: In a synchronous AC motor, the rotating magnetic field of the stator imposes a torque on the magnetic field of the rotor, causing it to rotate steadily. It is called synchronous because at steady state, the speed of the rotor matches the speed of the rotating magnetic field in the stator. By contrast, an induction motor has a current induced in the rotor; to do this, stator windings are arranged so that when energised with apolyphase supply they createa rotating magnetic field that induces current in the rotor conductors. These currents interact with the rotating magnetic field, causing rotational motion of the rotor.For these currents to be induced, the speed of the physical rotor must be lower than that of the stator's rotating magnetic field (ns), or the magnetic field would not be moving relative to the rotor conductors and no currents would be induced. If this happens while the motor is operating, the rotor slightly slows down, and consequently a current is induced again. The ratio between the speed of the magnetic field as seen by the rotor (slip speed) and the speed of the stator's rotating field is unitless and it is called slip. For this reason, induction motors are sometimes referred to as asynchronous motors. An induction motor can be used as induction generator, or it can be unrolled to form thelinear induction motor which can directly generate linear motion.
Synchronous speed : To understand the behaviour of induction motors, it is helpful to understand their distinction from a synchronous motor. A synchronous motor always runs at a shaft rotation frequency that is an integer fraction of the supply frequency; the synchronous speed of an induction motor is the same. It can be shown that ns in rpm is determined by
where f is the frequency of the AC supply in Hz and p is the number of magnetic poles per phase.Some texts refer to the number of pole pairs per phase; a 6 pole motor would have 3 pole pairs. In this case, P, the number of pole pairs, takes the place of p in the equation.
Slip: Typical torque curve as a function of slip.The slip s is a ratio relative to the synchronous speed and is defined aswhere nr is the rotor rotation speed in rpm
Construction : The stator of an induction motor consists of poles carrying supply current to induce a magnetic field that penetrates the rotor. To optimize the distribution of the
magnetic field, the windings are distributed in slots around the stator, with the magnetic field having the same number of north and south poles. Induction motors are most commonly run on single-phase or three-phase power, but two-phase motors exist; in theory, induction motors can have any number of phases. Many single-phase motors having two windings and a capacitor can be viewed as two-phase motors, since the capacitor generates a second power phase 90 degrees from the single-phase supply and feeds it to a separate motor winding. Single-phase power is more widely available in residential buildings, but cannot produce a rotating field in the motor, so they must incorporate some kind of starting mechanism to produce a rotating field. There are three types of rotor: squirrel cage rotors made up of skewed (to reduce noise) bars of copper or aluminum that span the length of the rotor, slip ringrotors with windings connected to slip rings replacing the bars of the squirrel cage, and solid core rotors made from mild steel.
Starting: A single phase induction motor is not self starting. Hence it is necessary to provide a starting circuit to start up a single phase induction motor. A single phase induction motor rotates either way; the starting circuit determines rotational direction.
The four methods of starting an induction motor are direct on-line, reactor, auto-transformer and star-delta. Unlike a wound-rotor motor, the rotor circuit is inacessible and it is not feasible to introduce extra resistance for starting or speed control.
For small single-phase shaded-pole motor of a few watts, starting is done by a shaded pole, with a turn of copper wire around part of the pole. The current induced in this turn lags behind the the supply current, creating a delayed magnetic field around the shaded part of the pole face. This imparts sufficient rotational character to start the motor. These motors are typically used in applications such as desk fans and record players, as the starting torque is very low and efficiency is not a problem.