The influence of variable speed drives on electric motor opperating temperatures

The implementation of Variable Speed Drives (VSDs) on electric motors forms part of strategies and initiatives that contribute towards energy efficient operations in the industry. The electrical efficiency of a motor can be improved by controlling its speed to match the requirements of the process it is applied to. The use of electric motors can be split in two categories; constant torque - and variable torque applications. Depending on the application, the use of VSDs on electric motors can have a negative effect on the motor´s life. Harmonics, voltage spikes and operating temperature increases are some of the side-effects found when implementing VSDs. This paper focuses on the effect a VSD has on the operating temperature of an electric motor under variable torque conditions. A case study was conducted on an electric motor used in the industry and the effect on the motor´s operating temperature was analysed. It was found that the VSD used in a variable torque application had a negligible effect on the motor operating temperature. Although an increase in temperature did occur it was still within the design limits of the electric motor used in the case study.