Electromechanical energy converters that can run in motor- and in generator-mode, i. e. as driving and braking machines. Both options are used in practical operation.
Since there are on the one hand electrical supply systems with different voltage, frequency, and number of phases (DC, AC, three-phase systems) and, on the other hand, the mechanical energy must be provided in many different parameter forms (speed, torque, power, velocity), there exist many different motor types.
According to the type of output motion, a basic distinction is made between motors for rotational and translational motion. Rotary drives typically have a 'problem-neutral' design. They are manufactured as self-contained structural and functional units in corresponding product lines for specific task and application classes. In contrast, translational drives predominantly feature a problem-specific design. I. e., in general, they are developed for specific applications and designed as a structural unit with the mechanism to be driven.
According to the continuity of the drive motion both, rotational and translational drives can be divided into continuously and discontinuously operating drives.
Sorted by motion characteristics, four groups of drive motors are distinguished:
- Motors for continuous rotary movements
- Motors for discontinuous rotary movements (stepper motors)
- Motors for continuous linear movements (linear motors)
- Motors for discontinuous linear movements (linear stepper motors).
At present, the majority of machines used are motors for continuous rotary motion.
Basic designs: With regard to the basic design of stator and rotor, these machines distinguish between internal-rotor, external-rotor, bell-shaped-rotor, and disc motors. Internal-rotor motors (stator outside, rotor inside) are used as standard type. External-rotor motors (stator inside, rotor outside) feature a very high rotor inertia. This ensures excellent smooth running characteristics particularly in small drives. Bell-shaped-rotor and disc motors feature a very low rotor inertia and, as a result, excellent dynamic characteristics. Therefore, they are preferably used as
servo motors.
Speed-torque characteristics: Depending on the type of current they are designed for, and depending on the stator and rotor design, electric motors can be basically divided into three natural forms of speed-torque behaviour: synchronous behaviour, i. e. constant speed within a permissible load range, shunt behaviour, i. e. the speed slightly decreases with increasing load, and series behaviour, i.e. the speed strongly decreases with increasing load.
With electric motors just as with gas, water or steam turbines, the stationarily generated torque is constant over time. Only a few small machines, e. g.
single-phase asynchronous motors generate a pulsating torque oscillating around an average value over time.
Torque overload: All electric motors are able to withstand a certain torque overload. I.e. they can for a short time show torques higher than the rated motor torque Mrat, but do not exceed the maximum permissible torque Mmax of the machine.
The following values are commonly used:
- Standard motors: Mmax / Mrat = 1.6 to 2.5
- Servo motors: Mmax / Mrat = 5 to 50.
Rated control modes: Since the torque requirements as a function of time and the required mode of operation are very different for driven mechanisms and machines, electric motors are available for eight different control modes (S1 to S8, cf. DIN VDE 0520).
