One of the most common electrical motor used in most applications which are known as an induction motor. This motor is also called as asynchronous motor because it runs at a speed less than its synchronous speed.
An induction motor is the most modest electrical machine from a constructional point of view, in the majority of the cases. Induction motor works on the principle of induction where the electromagnetic field is induced into the rotor when the rotating magnetic field of stator cuts the stationary rotor. Induction machines are by far the most common type of motor used in industrial, commercial or residential settings. It is a three-phase AC motor. Its characteristic features are:
• Simple and rugged construction
• Low cost and minimum maintenance
• High dependability and sufficiently high proficiency
• Needs no additional starting motor and necessity not be synchronized
Principle of Induction Motor
Consider a portion of the 3-phase induction motor as shown in Fig. The operation of the motor can be explained as under:
(i) When 3-phase stator winding is energized from a 3-phase supply, a rotating magnetic field is set up which rotates round the stator at synchronous speed Ns (= 120 f/P).
(ii) The rotating field passes through the air gap and cuts the rotor conductors, which as yet, are stationary. Due to the relative speed between the rotating flux and the stationary rotor, e.m.f.s are induced in the rotor conductors. Since the rotor circuit is short-circuited, currents start flowing in the rotor conductors.
(iii) The current-carrying rotor conductors are placed in the magnetic field produced by the stator. Consequently, a mechanical force acts on the rotor conductors. The sum of the mechanical forces on all the rotor conductors produces a torque which tends to move the rotor in the same direction as the rotating field.
(iv) The fact that rotor is urged to follow the stator field (i.e., rotor moves in the direction of stator field) can be explained by Lenz’s law. According to this law, the direction of rotor currents will be such that they tend to oppose the cause producing them. Now, the cause producing the rotor currents is the relative speed between the rotating field and the stationary rotor conductors. Hence to reduce this relative speed, the rotor starts running in the same direction as that of stator field and tries to catch it.
Construction of induction motor
Stator of induction motor
The stator is made up of various stampings with slots to carry three phase windings. It is wound for a distinct number of poles. The windings are geometrically divided 120 degrees separated. Two sorts of rotors are used in Induction motors: Squirrel cage rotor and Wound rotor. No DC field current is required to run the machine. Rotor voltage is induced in the rotor windings rather than being physically connected by wires.
The stator is made up of various stampings with slots to carry three phase windings. It is wound for a distinct number of poles. The windings are geometrically divided 120 degrees separated. Two sorts of rotors are used in Induction motors: Squirrel cage rotor and Wound rotor. No DC field current is required to run the machine. Rotor voltage is induced in the rotor windings rather than being physically connected by wires.
Rotor of induction motor
The rotor is the rotating part of the electromagnetic circuit. The most common type of rotor is the squirrel cage rotor. The rotor comprises of a cylindrical laminated core with axially placed parallel slots for carrying the conductors. Each slot carries a copper, aluminum, or alloy bar. The rotor of three-phase induction motors frequently is likewise implied as an anchor. The purpose behind this name is the anchor shape of the rotors used within quite early electrical devices. In electrical equipment, the anchor’s winding would be induced by the magnetic field, although the rotor takes this part in three-phase induction motors.
Type of induction motor
Singe phase
The single-phase induction motor is not self-starting. When the motor is connected to a single-phase power supply, the main winding carries an alternating current. It is logical that the least expensive, most reduced upkeep sort engine ought to be utilized most regularly. These are of different types based on their way of starting since these are of not self-starting. Those are split phase, shaded pole and capacitor motors. Again capacitor motors are capacitor start, capacitor run, and permanent capacitor motors. Permanent capacitor motor is shown below.
In these types of motors, the start winding can have a series capacitor and/or a centrifugal switch. When the supply voltage is applied, current in the main winding lags the supply voltage because of the main winding impedance. And current in the start winding leads/lags the supply voltage depending on the starting mechanism impedance. The angle between the two windings is sufficient to phase difference to provide a rotating magnitude field to produce a starting torque. The point when the motor reaches 70% to 80% of synchronous speed, a centrifugal switch on the motor shaft opens and disconnects the starting winding.
Application of single phase induction motor
These are used in low power applications and widely used in domestic applications as well as industrial. And some of those are mentioned below
• Pumps
• Compressors
• Small fans
• Mixers
• Toys
• High-speed vacuum cleaners
• Electric shavers
• Drilling machines
Three Phase
These motors are self-starting and use no capacitor, start winding, centrifugal switch or other starting devices. Three-phase AC induction motors are widely used in industrial and commercial applications. These are of two types, squirrel cage and slip ring motors. Squirrel cage motors are widely used due to their rugged construction and simple design. Slip ring motors require external resistors to have high starting torque.
Induction motors are used in industry and domestic appliances because these are rugged in construction requiring hardly any maintenance, that they are comparatively cheap, and require supply only to the stator.
Application of three phase induction motor
• Lifts
• Cranes
• Hoists
• Large capacity exhaust fans
• Driving lathe machines
• Crushers
• Oil extracting mills
• Textile and etc.