An induction motor (IM) is a type of asynchronous AC motor where power is supplied to the rotating device by means of electromagnetic induction. Other commonly used name is squirrel cage motor due to the fact that the rotor bars with short circuit rings resemble a squirrel cage (hamster wheel).
An electric motor converts electrical power to mechanical power in its rotor (rotating part). There are several ways to supply power to the rotor. In a DC motor this power is supplied to the armature directly from a DC source, while in an AC motor this power is induced in the rotating device. An induction motor is sometimes 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, absence of brushes (which are required in most DC motors) and — thanks to modern power electronics — the ability to control the speed of the motor.
The basic difference between an induction motor and a synchronous AC motor is that in the latter a current is supplied onto the rotor. This then creates a magnetic field which, through magnetic interaction, 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.
By way of contrast, the induction motor does not have any direct supply onto the rotor; instead, a secondary current is induced in the rotor. To achieve this, stator windings are arranged around the rotor so that when energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern can induce currents in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and the rotor will turn.
However, for these currents to be induced, the speed of the physical rotor and the speed of the rotating magnetic field in the stator must be different, or else the magnetic field will not be moving relative to the rotor conductors and no currents will be induced. If by some chance this happens, the rotor typically slows slightly until a current is re-induced and then the rotor continues 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 the ratio between the relative speed of the magnetic field as seen by the rotor to the speed of the rotating field. Due to this an induction motor is sometimes referred to as an asynchronous machine.
The relationship between the supply frequency, f, the number of pole pairs, p, and the synchronous speed (speed of rotating field), ns, is given by:
From this relationship:
The rotor speed is:
where: s is the slip.
Slip is calculated using:
In contrast, a synchronous motor always runs at either a constant speed, or zero.
where is the explanation or link for Slip Ring Induction Motor?
The most common rotor is a squirrel-cage rotor. It is made up of bars of either solid copper (most common) or aluminum that span 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:
There are various techniques to produce a desired speed. The most commonly used technique is PWM (Pulse Width Modulation), in which a DC signal is switched on and off very rapidly, producing a sequence of electrical pulses to the inductor windings. The duty cycle of the pulses, also known as the mark-space ratio, determines the average power input to the motor. For example, a 100 V DC signal that is cut into on- and off- pulses of equal width, has an average voltage of 50 V. If the on- pulses are a third of the duration of the off pulses, the average would be 25 V. The frequency of the pulses determines the motor speed.
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, by using the method described above, converts it into the desired AC waveform.
Because the induction motor has no brushes and is easy to control, many older DC motors are being replaced with induction motors and accompanying inverters in industrial applications.
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