Experimental Investigation on the Effects of Direct Torque Control Strategy in Eccentricity-related Frequency Components of Line Current of Induction Motors

Abstract

Eccentricity is a major fault in squirrel-cage induction motors. If this fault is not diagnosed and removed in the initial stages, the rotor may rub the stator and seriously damage the motor. The most well-known and probably most applicable fault diagnosis indices are harmonics components with definite frequencies in the stator current. The main objective of this article is to investigate how a direct torque control strategy applied to induction motor affects these harmonics. Due to the non-linear nature of hysteresis controllers within direct torque control, performing an analytic study on the subject is very complicated. Therefore, an experimental approach is adopted here. An experimental setup has been provided to test the induction motor and to sample and store its line currents, while its load can be adjusted and made eccentric with various types and degrees. Also, the motor is supplied directly from mains, from an industrial drive in direct torque control mode, and from the drive in constant volts/frequency mode. The motor was tested in various eccentricity conditions, load levels, supplying manners, and speeds, and the relevant harmonics amplitudes of the line current were calculated using fast Fourier transform. Analysis for three supplying manners indicates considerable changes in the amplitudes of the proposed harmonics.

Keywords:

• cage induction motor
• eccentricity index
• current harmonics
• direct torque control effects