Electric motors, both a.c. and d.c. motors, come in many shapes and sizes. Some electric motors are standardized for general-purpose applications. Other electric motors are intended for specific tasks. Very different applications and requirements have resulted in the manufacture of many different types and topologies. The customer’s demand for safety, comfort, economy, clean environment and quality is another reason for the explosive growth in the variety of electric motors. The basic technical and economic requirements to electric motors can be classified as follows:

• General requirements: – low cost – simple construction – simple manufacture – high efficiency and power factor – low EMI and RFI level – long service life – high reliability

• Requirements depending on application and operating conditions: – repairability, essential for medium and large power motors – extended speed range and energy efficiency for motors for electric ve-

hicles – low noise, essential for public life and consumer electronics – minimum size and mass at desired performance for airborne apparatus,

handpieces and power hand tools – resistance to vibration and shocks, essential for transport and agricul-

tural drives and also for airborne equipment – resistance to environmental effects and radiation essential for elec-

tromechanical drives operating in nuclear reactors, space vehicles, underwater vehicles and in tropics

– low amount of gas escape, essential for electromechanical drives in-

stalled in vacuum equipment • Additional requirements for motors used in servo drives and automatic

control systems: – fast response – temperature-independent response – high torque at high speed – high overload capacity – stability of performance

Reliability of an electrical machine is the probability that the machine will perform adequately for the length of time intended and under the operating environment encountered. Reliability of machines intended for a long service life is to be ensured on a reasonable level with due regard to economic factors, i.e., the best level of reliability is that obtained at minimum cost. The quantitative estimate of machine reliability is made by using probability and mathematical statistics methods.