Induction motor

Induction motor

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.

History

The induction motor with a wrapped rotor was invented by Nikola Tesla in 1882 in France but the initial patent was issued in 1888 after Tesla had moved to the United States. In his scientific work, Tesla laid the foundations for understanding the way the motor operates. The induction motor with a cage was invented by Mikhail Dolivo-Dobrovolsky about a year later in Europe. Technological development in the field has improved to where a 100 hp (73.6 kW) motor from 1976 takes the same volume as a 7.5 hp (5.5 kW) motor did in 1897. Currently, the most common induction motor is the cage rotor motor.

Principle of operation and comparison to synchronous motors

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.

Types:

  1. Based on type of phase supply
    1. Three phase induction motor (self starting in nature)
    2. Single phase induction motor (not self starting)
  2. Other
    1. Squirrel-cage induction motor
    2. Slip ring induction motor

Formulae

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:

f = frac{pn_s}{2}.

From this relationship:

mbox{Synchronous speed, }n_s = frac{120f}{p}quad[mbox{rev/min}]

The rotor speed is:

mbox{Rotor speed, }n_r = n_s(1-s),!

where: s is the slip.

Slip is calculated using:

s = frac{n_s-n_r}{n_s}

In contrast, a synchronous motor always runs at either a constant speed, n_s = tfrac{120f}{p} or zero.

Construction

The stator consists of wound 'poles' that carry the supply current to induce a magnetic field that penetrates the rotor. The number of 'poles' can vary between motor types but the poles are always in pairs (i.e. 2, 4, 6, etc.). There are two types of rotor:

  1. Squirrel-cage rotor
  2. Slip ring rotor

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:

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

Speed control

The rotational speed of the rotor is controlled by the number of pole pairs (number of windings in the stator) and by 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 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.

Starting of induction motor

In a three phase induction motor, the induced EMF in the rotor circuit depends on the slip of the induction motor and the magnitude of the rotor current depends upon this induced EMF. When the motor is started, the slip is equal to 1 as the rotor speed is zero, so the induced emf in the rotor is large. As a result, a very high current flows through the rotor. This is similar to a transformer with the secondary coil short circuited, which causes the primary coil to draw a high current from the mains. Similarly, when an induction motor starts, a very high current is drawn by the stator, on the order of 5 to 9 times the full load current. This high current can damage the motor windings and because it causes heavy line voltage drop, other appliances connected to the same line may be affected by the voltage fluctuation. To avoid such effects, the starting current should be limited. A soft start starter is a device which limits the starting current by providing reduced voltage to the motor. Once the rotor speed increases, the full rated voltage is given to it.

Types of starters

  1. Direct on line starter
  2. Autotransformer starter
  3. Star Delta starter
  4. Stator Resistance starter

See also

External links

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