Theory

Working of Induction Motor

Three-phase induction motor is comprised of a stator and a rotor. The purpose of the stator is to develop the rotating magnetic field. The stator field induces a voltage into the rotor cage. The rotor cage is made up of parallel conductors shorted together on each end by a shorting ring. This shorting ring is used to complete the circuit and allow current to flow through the rotor bars.

When stator windings are energised with a polyphase supply they create a rotating magnetic field pattern which sweeps past the rotor. This changing magnetic field pattern induces current in the rotor conductors. These currents interact with the rotating magnetic field created by the stator and in effect cause a rotational motion on the rotor. At startup, the rotor is stationary and the difference in speed between the rotating field and the rotor bars is at its maximum or what is identified as “slip”.

As the difference in speed between the rotor and the stator begins to decrease, the voltage generated in the rotor is reduced. The resultant current flow will be reduced to some steady state value that is required to maintain steady state torque. During steady state operation of the motor, the torque generated by the motor is equal to the torque being demanded by the load. Changes in load will affect the speed.

Problems faced in induction motor

Broken rotor bar: When a broken bar is present within the rotor, current cannot flow through it, and therefore, it can no longer add its share of torque to the rotors load burden. As the broken bar passes under the pole it will effectively reduce the torque of the rotor for the period of time it is under the field pole, in its torque producing position. Since the torque of the rotor is reduced slip is again introduced. As the rotor slows and the slip is increased, the voltage induced into the rotor is also increased, and therefore, the current in the normal rotor bars will have to increase to produce the needed torque. This continues until the torque of the motor is restored to equal the torque of the load.

Eccentricity related faults: Machine eccentricity is the condition of unequal air gap that exists between the stator and rotor. If the air gap between the stator and rotor is not uniform, the forces on the rotor are not balanced, resulting in high magnetically induced vibration at 120 Hz. The magnetic attraction is inversely proportional to the square of the distance between the rotor and stator, so a small eccentricity causes a relatively large vibration.

There are two types of air gap eccentricity:

(1) Static air gap eccentricity.

(2) Dynamic air gap eccentricity.

Static air gap eccentricity

In case of static air gap eccentricity, the centre of the rotor is not at the centre of stator but the position of the minimal radial air gap length is fixed in space

Dynamic air gap eccentricity

In case of dynamic eccentricity, the centre of the rotor is not at centre of the rotation and the position of the minimum air gap rotates with the rotor.

It follows that dynamic eccentricity is space and time dependent (static eccentricity is only space dependent).

Motor Current Signature Analysis (MCSA) is a technique used to determine the operating condition of AC induction motors without interrupting production. In order to diagnose motor faults, MCSA uses the current spectra, which contains potential information of motor faults. MCSA techniques can be used in conjunction with vibration and thermal analysis to confirm key machinery diagnostic decisions. MCSA operates on the principle that induction motor circuits can be viewed as a transducer. By clamping a hall effect current sensor on either the primary or secondary circuit, fluctuations in motor current can be observed.

The experiment is performed in 4 different types of motors: normal AC motor, AC motor with built-in broken rotor, AC motor with built-in bowed rotor, AC motor with rotor misalignment.