Torque motors：Powerful && Efficient
These special motors eliminate power transmission elements to deliver very high levels of torque directly to the load. That’s why they’re called ‘torque’ motors—and now they have even better designs.
Electric motors come in a rich variety of configurations to suit different purposes. One specialty motor type—known as a directdrive, permanent-magnet (PM) torque motor—is characterised by a large diameter-to length ratio and large number of magnetic poles to optimise torque production. These relatively low speed motors, usually operating under 1,000 rpm, come in housed and frameless (or ‘built in’) versions.
There are other advantages of torque motors such as better load inertia matching, ease of control, low noise emission, and streamlined machine design (see ‘Simplifies design’ diagram).
More pole pairs and a larger rotor diameter result in higher torque output. Torque is proportional to rotor diameter squared and directly proportional to rotor length, so manufacturers try to make them as wide as they can, with relatively short lengths.
Torque motors are controlled much like other brushless motors, but require certain special provisions.
Control loops (current, velocity, and position) must be closed as fast as possible to deliver high static / dynamic stiffness. Intelligent servo drives close all loops internally at high rates (typically every 0.25 ms). Since the ‘drive + torque motor’ combination provides torque directly to the workpiece, it also directly impacts accuracy and smooth operation.