Efficiency of VFD Coupled Induction Motors Operating in the Scalar Mode with Different Types of Loads

摘要

In this paper, we investigate the behaviour of scalar controlled AC Variable Frequency Drives for induction motors when coupled with different types of loads. It is well known that slip is more pronounced at high speeds rather than low speeds in loads such as in pumps and fans operation because the torque load varies as the square or cube of the speed. Slip compensation in Voltage/Frequency controlled Variable Frequency Drives may work only up to a certain limit since the problem can be exacerbated by overcompensation causing low magnetisation flux of the stator and the Variable Frequency Drive becomes uncontrollable. In this research work, we investigate the behaviour of the motor and Variable Frequency Drive assembly with regards to slowly changing loads from low to high speeds. Results show that at reduced speeds for the Variable Frequency Drive and motor assembly, the efficiency is lowest for square and cubic torques loads compared to linear and constant torque loads. Highest efficiency and torque are achieved only in a restricted interval of speed. This problem therefore compels us to avoid using the Variable Frequency Drives in the Voltage/Frequency mode at low speeds for pumps and fans.