Electrical motors are most commonly used as spindle drives. Depending on the requirements on the machine tool, 3 different types of rotational electrical motors can be employed here: The DC motor, the synchronous motor and the asynchronous motor.
Externally excited DC motors have high torsional stiffness under load when regulated and can vary their speed significantly. For this reason they have long been employed as the main drives in machine tools. The stationary exciter field is generated in the motor stators in separately excited DC motors. The conductor windings are located on the rotor of the motor and must therefore be supplied through brushes as a sliding contacts, which means that motor maintenance is required.
Synchronous and asynchronous motors are three-phase electrical motors. Asynchronous motors are normally used in machine tools in conjunction with stepped-gear transmission systems. There is a trend towards using regulated synchronous motors and regulated asynchronous motors as main drives. Both types of regulated motors exhibit a large speed range and a switching transmission is therefore commonly no longer required. [DUBBEL], [WECK06]
Very high power densities can be achieved with the synchronous motor. Because the roles of the stator and the rotor are swapped in synchronous motors in comparison to DC motors, this prevents the undesirable heat development on the shaft and the lack of brushes means that they are maintenance-free. In addition, there are opportunities for optimisation due to the different rotor designs, which likewise permit a very large shaft diameter if no large field-weakening range is required. [WECK06], [GESTSLS], [DUBBEL]
Asynchronous motors are most often cage rotor designs. Their characteristics are their low-maintenance requirements, their stable behaviour in the nominal load range and their robust construction. Different types of cage construction and rotor materials enable the motors to be adapted to suit the requirements of different machine tools. In addition, they are also distinguished by a less complex control system and parameterisation, as well as the easily realised large field-weakening ranges. [WECK06], [GESTSLS], [DUBBEL]