Torque motors: Why they’re now even more powerful and efficient

01 November 2007

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.

Their shape
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.

Cartridge DDRs
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.

Torque density
Strength of the permanent magnets contributes to torque density of synchronous motors. Siemens uses
neodymium-iron-boron (Nd-Fe-B) magnets in both its housed and frameless torque motors. This material isregarded as the most powerful and affordable type for rare-earth magnets.

Another measure of high torque density is the number of magnetic poles in the design. A higher number of poles translates into higher torque output, but this rule has greater impact at lower pole numbers. For example, major torque increase can be realised by a design change from four to eight poles—while
keeping motor volume constant—but torque gain has much less impact by changing from, say, 32 to 46 poles.

‘As rule of thumb, increasing poles up to 30 is a good measurement for torque density increase,’ explains Ralph Baran, Siemens’ product manager for servo motors and mechatronic products. Even so, frameless torque motors with pole counts well above 100 are on the market.

Control implications
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.

Higher drive amplifier control bandwidth is needed to obtain high stiffness. ‘High dynamics can excite mechanical harmonics that must be filtered by the amplifier using filter settings that do not limit performance,’ cautions Karl Rapp, machine tool industry branch manager at Bosch Rexroth.

The choice of feedback devices also is crucial. Sinusoidal feedback is recommended as intelligent drives derive velocity change from this signal. Square wave and serial-type feedback should be avoided, as they limit performance.

Electronic commutation (or pole switching) is needed to operate a brushless PM motor. Commutation is not
a simple procedure for torque motors, since hollow shaft feedback systems are often incremental rather than absolute, requiring the drive amplifier to perform automatic commutation offset after each control power up.

‘The procedure becomes more complex with a high pole count motor, because pole distances become very small,’ says Mr. Rapp. Intelligent drives, such as Bosch Rexroth’s IndraDrive, provide various commutation functions. The saturation method is preferred, as it can be run without physically moving the motor.

Siemens’ Mr. Baran says, ‘Physically, torque motors have the same control characteristic as other brushless PM motors. However, by eliminating mechanical elements in the drive line, backlash [lost motion] and mechanical 'weaknesses' also are eliminated.’ The result is a dramatic increase of drive line mechanical stiffness.

For the controller, this means it can act more aggressively without overshooting—leading to applications
with higher acceleration / deceleration and more precise positioning and path control.

‘Experience has shown that about a factor of 10 improvement can be realised in machine dynamics by designing the machine for direct drives compared to conventional motor-coupling-gearbox combinations,’ says Mr. Baran.

Because gearboxes and other mechanical transmission elements are absent, direct-drive Baumüller DST
motors are said to offer zero backlash that enables high control effectiveness. This attribute makes it possible to draw conclusions about the quality of the connected process by monitoring motor torque and speed, explains product manager Marcel Möller. Operating parameter changes, such as changes in
lubricant viscosity, are correlated in the controller based on software programs, resulting in better system control and product quality. ‘As a rule, direct drives also improve overall system efficiency and lead to energy savings,’ he adds.

Crucial cooling
High torque output produces heat in the motor windings that must be removed to avoid motor damage. Cooling also minimises heat-related expansion, primarily of the motor stator. Such expansion can influence process accuracy (thermal growth in the mechanics) but can also cause stress and damage in the motor mounting.

Because the motor is integrated into the machine structure, OEMs must account for thermal expansion
differences of dissimilar materials to avoid damage when mounting the stator. Bosch Rexroth cites an extreme example of an OEM design that allowed only partial insertion of the stator into the machine bore. Without liquid cooling, higher thermal expansion on the stator’s side outside the machine caused cracking of the windings over time.

Cooling method and volume—liquid, forced air, or convection—mainly depends on consumed power or duty cycle, plus thermal growth considerations. Cooling also increases torque density. Most thermal losses occur in the stator windings of brushless PM motors since no magnetising currents flow in the mrotor to cause thermal losses. An efficient way to remove heat generated from these motors is to pass cooling water through pipes close to the stator windings. ‘Tests have shown that torque output of a motor designed for natural air cooling can be increased by 30% if the design is optimised for water cooling,’ says Mr. Baran.

Real power output of torque motors is limited by the ability to remove I2R power losses from the windings and eddycurrent losses from the stator laminations. Eddycurrent losses rise with the number of poles used. If the resulting heat cannot be efficiently removed, winding temperature rise will eventually cause insulation breakdown, leading to heat flow to the rotor. In turn, this heat can demagnetise the high-energy magnets in the rotor.

Baumüller integrates water cooling into its DST torque motors as a recognised need for top torque performance. ‘Only this way is it possible to achieve the high torque density and simultaneous high overload capacity,’ states Mr. Möller. ‘Integrated water cooling furthermore enables a higher protection class IP54, which helps DST motors meet harsh conditions found in an industrial setting.’

Besides more cooling capacity, a further—yet counterintuitive—advantage of water cooling is reduced noise
emission. Baumüller and other manufacturers mention that water-cooled DDR torque motors run quieter than their fancooled counterparts.

Application view Torque motors are at home anywhere traditional gear trains, chains, or timing belts have been used in the past. Manufacturers of directdrive brushless PM torque motors firmly believe that OEM users can gain major productivity and quality benefits if their machine design is optimised to apply these motors. Experience at Siemens has shown these benefits to be realistic. ‘In some cases, machine productivity grew by 50%, while precision improved by around 30%,’ says Mr. Baran.

Other reasons for OEMs to apply these torque motors include less maintenance and spare-part inventory with fewer parts used in the construction, energy savings from a more efficient drive line, and space savings with smaller foot print machines versus a motor-gearbox combination.

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