Kit / torque motors

A "kit motor" comprises a stator and a rotor which can be integrated directly into the customer's own machine. Read more about this topic at our technology page "Embedded Motion". In many cases, a kit motor is a torque motor. What is a torque or "direct drive" motor ? Technically speaking, a direct drive is the same as a servo-drive. Modern versions are electrically equivalent to 3-phase brushless, synchronous motors with permanent magnet field excitation. The geometry of a torque motor is chosen to generate high torques (or forces) instead of high power output. The geometrical shape of the torque motor tends therefore to be short relative to its diameter. In order that the copper losses and electrical time constants remain low, these motors have a higher number of magnetic poles than a conventional servomotor.

The design of these motors is optimised for torque (or force) output rather than for high efficiency. The most important selection criterion for a torque motor alongside the acceleration capability (Nm/kg.m²) is the motor constant Km (Nm/√W).  This parameter tells the user which energy losses can be expected from a motor at the demanded torque level - independent of speed!

There are a number of advantages to using this modern drive technology - both for rotatory and for linear motion control. It is not only a question of performance. Often secondary issues of mechanical assembly are of equal importance, such as:

  • High acceleration capability
  • High dynamic performance (ratio max./min. speed)
  • Hetter stiffness and damping
  • Hetter position accurarcy and repeatability
  • High reliability
  • High efficiency
  • Good, consistant performance over a wide temperature range
  • No wearing parts, long life
  • Lower operational noise level
  • Hollow shaft
  • Short motor length or smaller cross-section
  • Frame size or form

The main reason to employ a direct drive still remains servo-performance. This is generally better than for a conventional servo-drive with mechanical gear reduction. The explanation lies in the extremely stiff connection between the motor and the load which can be achieved in this mechanical configuration.