Electric motors – and how to improve their control, protection

Economic reasons

Nowadays, apart from lighting devices, electric motors represent the largest loads in industry and commercial installations. Their function, to convert electrical energy into mechanical energy, means they are particularly significant in economic terms, and hence, they cannot be ignored by installation or machinery designers, installers or users.

There are many types of motor in existence, but 3-phase asynchronous motors, and in particular squirrel cage motors, are the most commonly used in industry and in commercial buildings applications above a certain power level. Moreover, although they are ideal for many applications when controlled by contactor devices, the increasing use of electronic equipment is widening their field of application. This is the case for start/stop control with soft start/soft stop units, and when precise speed adjustment is also necessary with variable speed drives/regulators.

However, slip-ring asynchronous motors are used for certain high power applications in industry, and single phase asynchronous motors remain suitable for limited power applications, mainly for buildings applications. The use of synchronous motors, known as brushless or permanent magnet motors, combined with converters is becoming increasingly common in applications requiring high performance levels, in particular in terms of dynamic torque (on starting or on a change of duty), precision and speed range.

After presenting the various types of electric motor and their operating principles, this “Cahier Technique” describes the technical and operating features of asynchronous motors, covering in particular the main starting devices, speed control and braking methods. It provides a solid grounding for the reader to gain a good understanding of all the problems involved with motor control and protection.

This “Cahier Technique” touches briefly on variable speed control of electric motors. However, this subject is covered more specifically in “Cahier Technique” CT 208 “Electronic starters and variable speed drives“. Motor protection is the subject of another “Cahier Technique” currently in preparation.

AUTHOR: Schneider Electric expert | Etienne GAUCHERON