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A torque motor is 'either a rolled-up linear motor or a classic servo drive with a large number of poles'. As I understand it they are ironless 3 phase supply motors, though perhaps neither of these attributes are necessary. Also as I understand it both torque and hybrid synchronous stepper motors have a permanent magnet rotor.

Am I correct in saying then the fundamental difference between a torque motor and a hybrid synchronous stepper motor is the torque motor is 3 phase whereas the stepper is only 2? Or am I missing something? And also is a torque motor necessarily 3 phase and ironless, and if not are they and stepper motors not then the same thing?

Please explain how I'm wrong! Thanks

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    \$\begingroup\$ an unloaded stepping motor will stay stopped and hold position when energized ... an unloaded torque motor will not \$\endgroup\$
    – jsotola
    Feb 26 at 21:18
  • \$\begingroup\$ @jsotola ok good point, I suppose the ironless nature of the torque motors is another fundamental difference! Unless one can get ironless steppers? Or torque motors with iron cores? \$\endgroup\$ Feb 26 at 22:58

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Brushless motor torque is limited by the attractive force of the magnets used in the rotor. The "torque motors" to which you are referring are really just like standard brushless motors; they have a wound stator and magnets on the rotor. The also act just as any brushless motor; they can be stepped or driven synchronously as with any standard brushless motor.

A more common, small diameter brushless motor design may have two to eight magnets, or poles, meaning that to generate torque, each of these poles exerts a tangential force to the energized windings. Compared to a torque motor, the magnets are close to the center of rotationand thus do not enjoy the mechanical advantage of the torque motor, whose magnets are much farther away from the center. This provides a lever arm to produce many times the torque of a small diameter motor from each magnet for the same field strength. In addition, a torque motor can have tens of poles. The combination means that although each pole may provide the same tangential force as in a smaller motor, the distance from the center and number of magnets means that the torque motor can produce orders of magneitude more torque than a small motor.

The trade-off is speed. A 60-pole torque motor must have a drive frequency of thirty times the rate of revolution, whereas a 4-pole motor will require only two times the revolution. Put another way, a 60-hz drive would result in 1800 RPM for a four-pole motor, but only 120 RPM for a 60-pole motor.

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