Three Phase Induction Motor

  • Aviinash 
  • 5 min read

An Induction motor is an electric motor which works on AC current. The torque is produced on rotor by the electromagnetic induction which is induced on stator due to magnetic field produced by AC current. There are two types of induction motors namely single phase and three phase induction motors. The single phase induction motor works on the single phase supply of current and it doesn’t have the ability to start by itself and it is used on low load applications. On the other hand, the three phase induction motor works on three phase supply of current and they are used on heavy load applications on ships. It was invented by Galileo Ferraris and Nikola Tesla and the working model was demonstrated in the year of 1887.

Principle

It works on the principle of electromagnetic induction. As it is produced on the rotor of motor due to magnetic flux which produced on the stator by three phase power supply. This magnetic flux will get cut by rotor which produced EMF in it.

Why it is called Induction Motor ?

As it depends on the current which induces on the rotor, hence it is called as induction motor. This is also called as asynchronous motor as it always rotates less than the speed of synchronous motor.

Construction

induction motor
Three phase induction motor.

The important parts of three phase induction motors are stator and rotor. The three phase induction motors are using in most of the mechanical applications due to its good operating characteristics.

Stator:

As the name indicates, it is stationary part of the motor. The three phase supply is given to winding which is placed in the stator of motor. It consists of frame which is used to support the stator core and windings of the stator. The frame should be rigid, strong and air gap length should be small. Die cast or Fabricated steel are used to make the stator frame. The stator core is used to carry the alternating flux and it is made of laminated type of structure to reduce the eddy current loss. These structure are made up of stamping which are of thickness 0.4-0.5mm. All the stampings are kept together to form stator core. To reduce hysteresis loss, it is made up of silicon steel. The three phase windings are connected in the slots of the stator core. The windings are made either in the form star or delta format according to starters used in the motor.

Rotor:

As the name indicates, it is rotating part of the engine. The shaft is connected to the rotor which is then connected to the mechanical load. Based upon the construction of rotor, it is divided into two types:

Squirrel cage induction motor:

The construction of the squirrel cage induction motor consists of the laminated core in the form of cylinder. The slots are in the parallel form. The rotor conductors are placed in these parallel slots. Copper, aluminium or alloys are used as conductors in the place of wires. The slots of rotors are slightly skewed to reduce locking tendency and to increase the rotor resistance due to increased length of conductor. They are electrically welded at both the ends so it looks like similar to the structure of squirrel cage. These motors are widely used in ships.

Slip ring induction motor:

They consists of three phase and are made up of double layer. The poles of rotor and stator are kept same. Even if the stator has two phase, the rotor will always have three phase. The windings are connected in star format. It can reduce noise as there slots are arranged at 120 degrees. The rotor of this is of wound type.

Working

The windings of the stator are overlapped and placed at 120 degree phase shift and it produces a rotating magnetic field when three phase power supply is given to the motor. This rotating magnetic field will get cut by the rotor conductor and induces an EMF in it. This induced EMF will make the current to flow in the conductor. Hence the rotor will rotate in same direction as the rotating magnetic field but at a speed less than that.

Advantages

  • They are simple in construction.
  • They can work efficiently in rugged environment so they are used in ships.
  • The maintenance cost is less
  • They don’t have brushes.
  • They don’t need any external source to start the motor initially.

Disadvantages:

  • It draws high current at start of the motor due to heavy load.
  • The efficiency is reduced due to low power factor at low load.
  • Speed control is difficult.

Problems

  • Environmental problems
  • Winding problems
  • Single phase problems
  • Crawling (mix of electrical and mechanical faults)
  • Bearing failure
  • Corrosion
  • Fatigue failure
  • Lubrication faults

Protections:

  • Single phasing: To overcome the damage which occurs due to single phasing, the device that operates at above 500KW are protected by removing the power supply automatically when single phasing occurs.
  • Over-temperature: The windings in stator will be sometimes overheated due to overloading and single phasing. Fuses and relays are used in the circuit to overcome this problem.
  • Soft Starting: In three phase induction motor, high current is required for starting the motor. To overcome this, different starters like soft starter, DOL, Star delta starter, autotransformer and so on are used. When we use a soft starter, mechanical and electrical stresses are reduced at start of the motor. DOL starter is used for overload protection.

Applications:

  • It is used in home appliances like fans where slip is not an issue
  • It is also used in electric train engine and cooling fans which are used to cool large machines
  • They are used in ships to run large appliances
  • They are used in printing machines and rolling mills.
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