Torque motors: Why they’re now even more powerful and 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.
Frank J. Bartos, P.E., Control Engineering
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.
Direct-drive rotary (DDR) brushless (synchronous) motors combine several design features to deliver their intended function. Direct drive—meaning no power transmission elements between motor and driven load—brings advantages of high dynamic motion with essentially no backlash and excellent static / dynamic load stiffness that allows precise motion control. Large numbers of magnet pole pairs in the rotor aid high torque generation.
Torque motors tend to be physically large (well over one metre diameter for some models), but smaller units are on the market as well. At the top-end, more than 20,000 Nm peak torque output is not unusual.
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.
They are available in two classic formats. The ‘frameless’ (or ‘built-in’) version consists of a ring-shaped rotor and stator parts set, which a customer must incorporate into the machine structure (see photo of Bosch Rexroth’s IndraDyn frameless motor). Feedback, connectors, and cooling means also need to be provided, requiring significant design and assembly effort. The frameless motor's thin-ring structure offers a large hollow shaft input.
A ‘housed’ DDR motor (see photos of Siemens 1FW3 and Baumüller’s DST Series) has a frame, bearings, and either a regular shaft or hollow shaft.
Danaher Motion has taken a third approach, developing a format to focus on advantages of DDR while eliminating their disadvantages. Called Cartridge DDR (or CDDR), these motors retain high magnetic pole count and large diameter, but have no bearings.
‘The rotor is supported by the customer’s bearings, thus providing a simple mounting with minimal design
effort and ability to remove the motor without disassembling the machine,’ says Tom England, Danaher Motion's director of product management.
Historically the drawback for DDR motors has been application difficulty and cost, in Danaher's view.
‘Cartridge DDR technology has changed that. It makes direct-drive benefits available to simple mechanisms, as well as to classic, higher performance servo applications,’ says Mr. England. Today, CDDR technology motors find application in packaging, press feed, converting, printing, and medical equipment.
Strength of the permanent magnets contributes to torque density of synchronous motors. Siemens uses