Three Phase Induction Motor: Explained

Three Phase Induction Motor: Explained

An electric motor takes electrical energy as input and transforms it into rotational mechanical energy that is delivered at the output. Motors may either by AC or DC, and can further be split into synchronous, single-phase, three-phase, induction and special purpose motors. Out of all types, induction motors rule both industrial and commercial applications because of its self-starting working.

The 3-phase Induction Motor derives its name from its working, i.e. roto current working on the principle of electromagnetic induction rather than through physical electrical connections.

Rotating Magnetic Field

Any electrical motor consists of two primary parts: the stationary stator and the rotating rotor. The stator of an induction motor is made up of several overlapping windings that are separated by a difference of 120 degrees. When three phase current flows in the stator windings, a rotating magnetic field is established that makes the stator rotate at synchronous speeds.

The direction of rotation of the motor is determined by the phase sequence of the supply lines and how the connections are formed with the stator. This allows the direction of rotation to be reversed through terminal-interconnection. The synchronous speed is determined by the number of poles used in motor construction and the frequency of input voltage. The following expression governs the synchronous speed:

Speed of rotation = 120 x supply frequency / number of poles

Production of Magnetic Flux

Next, torque is produced, and rotation is achieved by the rotor. A current flow through the rotor’s conductors which is cut by the stator’s rotating magnetic field, leading to emf induction.

In an induction motor, the rotor’s windings are cordoned off either through an external resistor or directly shorting it out. As a result, the emf induced in the rotor makes the current flow in a direction that is opposite to that of the revolving magnetic field in the stator, ultimately causing rotational motion.

The rotor speed never reaches the speed of the magnetic field of the stator, with the former always trying to catch-up. This catch-up action is responsible for the motor’s operation. If the speed does catch up to that of the stator then no emf is induced and thus no torque is produced. The difference in the stator and rotor speeds is denoted by a term known as “slip”, which usually holds a numeric value of 5 – 6 percent.

The end-result is a motor that is self-starting, requires less maintenance due to the elimination of additional components specifically slip rings and brushes, robust in construction and inexpensive (almost 20% to that of synchronous motors).


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