What Is Induction Motor And Types Of Induction Motors
Three Phase AC Induction Motor
The three-phase AC induction motor is a rotating electric machine that is designed to operate on three phase supply. This 3 phase motor is also called as an asynchronous motor. This AC motors are of two types: squirrel and slip-ring type induction motors. The principle of operation of this motor is based on the production of rotating magnetic field.
Three Phase Induction Motor Construction
These three phase motors consist of a stator and a rotor and between which no electrical connection exists. These stator and rotors are constructed with the use of high-magnetic core materials in order to reduce hysteresis and eddy current losses.
Stator frame can be constructed using cast iron, aluminum or rolled steel. Stator frame provides necessary mechanical protection and support for stator laminated core, windings and other arrangements for ventilation. Stator is wounded with three-phase windings which are overlapped with one another at 120 degree phase shift fitted into slotted laminations. The six ends of the three windings are brought out and connected to the terminal box so that these windings are excited by three-phase main supply.
These windings are of copper wire insulated with varnish fitted into insulated slotted laminations. At all working temperatures, this impregnated varnish remains rigid. These windings have high-insulation resistance and high resistance to saline atmosphere, moisture, alkaline fumes, oil and grease, etc. Whichever suits the voltage level, these windings are connected in either star or delta connections.
The rotor of three phase AC induction motor is different for the slip-ring and squirrel-cage induction motors. Rotor in slip-ring type consists of heavy aluminum or copper bars shorted on both ends of the cylindrical rotor. The shaft of the induction motor is supported on two bearings at each ends to ensure free rotating within the stator and to reduce the friction. It consists of stack of steel laminations evenly spaced slots that are punched around of its circumference into which un-insulated heavy aluminum or copper bars are placed.
A slip-ring-type rotor consists of three-phase windings are internally starred at one end, and the other ends are brought outside and connected to the slip rings mounted on the rotor shaft. And for developing a high-starting torque these windings are connected to rheostat with the help of carbon brushes. This external resistors or rheostat is used at the starting period only. Once the motor attains the normal speed, the brushes are short circuited, and the wound rotor works as squirrel cage rotor.
Working Principle of Three Phase Induction Motor:
- When the motor is excited with three-phase supply,three-phase stator winding produce a rotating magnetic field with 120 displacements at constant magnitude which rotates at synchronous speed. This changing magnetic field cuts the rotor conductors and induces a current in them according to the principle of Faraday’s laws of electromagnetic induction. As these rotor conductors are shorted, the current starts to flow through these conductors.
- In the presence of magnetic field of stator, rotor conductors are placed, and therefore, according to the Lorenz force principle, a mechanical force acts on the rotor conductor. Thus, all the rotor conductors force, i.e., the sum of the mechanical forces produces torque in the rotor which tends to move it in the same direction of rotating magnetic field.
- This rotor conductor’s rotation can also be explained by Lenz’s law which tells that the induced currents in the rotor oppose the cause for its production, here this opposition is rotating magnetic field. This result the rotor starts rotating in the same direction of the stator rotating magnetic field. If the rotor speed more than stator speed,then no current will induce in the rotor because the reason for rotor rotation is the relative speed of the rotor and stator magnetic fields. This stator and the rotor fields difference is called as slip. This how 3-phase motor is called as asynchronous machine due to this relative speed difference between the stator and the rotors.
- As we discussed above, the relative speed between the stator field and the rotor conductors causes to rotate the rotor in a particular direction. Hence, for producing the rotation, the rotor speed Nr must always be less than the stator field speed Ns, and the difference between these two parameters depends on the load on the motor
The difference of speed or the slip of the AC induction motor is given as
- When the stator is stationary, Nr=0; so the slip becomes 1 or 100%.
- When Nr is at synchronous speed, the slip becomes zero; so the motor never runs at synchronous speed.
- The slip in the 3 phase induction motor from no load to full load is about 0.1% to 3%; that’s why the induction motors are called as constant-speed motors.