Small electric motors

Small electric motors are everywhere, and have become such commonplace components that their existence is often overlooked. An industrial society would be inconceivable without the massive use of small electric motors and actuators. With the exception of certain ranges of servomotors and actuators, these motors are generally manufactured in very large series. Industrial imperatives (cost, automation) dictate the design of these motors, which also generally have to blend in with the application.

The definition of a small electric motor can be based either on quantitative power or torque criteria, or on application considerations in which positioning and motion functions take precedence over the energy conversion function. It's worth noting, however, that the definition of a small electric motor is sometimes arbitrary, since it complies with all the rules and principles of electrical engineering. In the following, we will consider motors with a power rating of less than 600 W, with reference to the official regulations of the French Electrical Construction Association (NF C 51 –200). It should be noted that this power limit corresponds to fractional horsepower motors, i.e. with a power rating of less than approximately 735 W.

It is useful to clarify the terminology of motor and actuator:

• The motor is an organ that generates movement in the etymological sense of the term;

• the actuator is a device that produces an action.

The term "motor" is broader than "actuator", since it includes not only the production of actions, but also the generation of movements of any kind. Finally, small actuators and motors can produce rotary, linear or more complex actions or movements.

This article applies to centimetric motors and actuators, excluding, in addition to the above-mentioned power limit :

• at the lower limit, electrostatic, magnetic and piezoelectric microactuators, which will only be mentioned here;

• at the upper limit, motors with separate external excitation, both DC and synchronous, and, in general, motors using bushings.

Constant reference will be made to articles in the Techniques de l'Ingénieur collection on different types of electric motors and their control.