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Direct-drive torque motors 'have half the cogging torque’

04 October, 2018

The CNC (computer numerical control) specialist NUM has launched a range of high-performance brushless torque motors which, it claims, display almost half of the cogging torque of rivals. The frameless, hollow-shaft TMX motors are said to be ideal for applications that need smooth, accurate rotation, especially at low speeds. Typical uses are expected to include direct-drive machine tools, rotary tables and radial positioning systems.

NUM is well-known for its brushless AC servomotors, having produced more than 5,000 different types over the past 50 years. However, for direct-drive applications, it has usually specified torque motors made by the German manufacturer Idam (Drives & Mechatronics AG & Co. KG) – a member of the Schaeffler Group that specialises in direct-drive technologies.

But, according to Massimiliano Menegotto, general manager of NUM’s main manufacturing plant in Milan, Italy: “A growing number of our customers request customised motors, and prefer to deal with a single supplier for all their CNC machine tool requirements. We have developed the TMX series of torque motors specifically to address these needs. They complement those produced by Idam and provide us with additional in-house configuration flexibility, enabling us to offer customers an even wider range of solutions.”

Initially, there are two TMX frameless torque motors, with stator diameters of 140mm and 291mm. NUM plans to expand the range in future.

The motors’ hollow-shaft rotors use high-flux-density neodymium-iron-boron (NdFeB) magnets and offer enough internal space for through-motor wiring or mechanical parts. The stator windings are protected against over-temperatures and are encapsulated in a thermally optimised enclosure that offers a choice of air or water cooling.

Like other direct-drive motors, the TMX machines can provide continuous torque even when stalled, and are designed to attach directly to a load. This avoids the need for gearboxes and shaft couplings, and results in accurate, backlash-free rotary transmission. The rotor and frameless stator are supplied as component parts and are intended mainly for applications that need the motor to be integrated in a servomechanism to reduce weight or volume.

NUM’s TMX direct-drive motors have extremely low cogging torque.

The new motors can produce a high S1 torque density and are available in a choice of stator and rotor lengths. A 140mm-diameter motor with an active length of 70mm can produce a peak torque of around 82Nm at speeds up to 1,500 rpm, and a continuous (stall) torque of approximately 50Nm.

The motors can be driven by most industry-standard servodrives, including NUM’s MDLU3 and MDLUX models.

NUM’s range of AC servomotors covers torques from 0.318–160Nm and speeds up to 8,000 rpm. The company also manufactures synchronous and asynchronous motor spindles with flux weakening, and power outputs up to 55kW.

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