A few aspects about the squirrel cage induction motors in variable speed drives

Abstract

The paper deals with standard three phase low voltage squirrel cage motors in variable speed drives. The aim is to investigate the possibility of improving the machines parameters at low speed. The considered industrial motors are the existing ones, having the rated power between 0.55 kW and 37 kW and 4 and 6 pole. The paper is focused on the machine parameters analysis for machine design improvement. Therefore the machine parameters at low speed were estimated for the supply voltage that ensures the optimum Volt per Hertz value and also taking into account saturation influence and the iron losses. The calculus was made using the computing program INDARUPT. The input machine data are the geometrical dimensions, material data, supply conditions and the required motor performances – efficiency, power factor, starting torque, starting current, etc. The computing program INDARUPT estimates the magnetic flux densities on the machine cross-section (stator and rotor tooth, stator and rotor yoke) and the machine parameters. Then one finds the required values for the voltage, U’, providing the optimum flux value, at different frequencies. The proposed correction of the supply voltage, improves the machine control and the drive efficiency. For the 1.1 kW / 1500 rpm motor it was obtained an improved solution. In this case the motor efficiency improvement is better at low frequencies. Because the purpose was to find the possible “no tool cost” or “low tool cost” solutions, the proposed values for the outer diameter of the stator are very close to the initial ones.