Brushless PM Torque Motors
Electric motors come in a rich variety of configurations to suit different purposes. One specialty motor type—known as a direct-drive, permanent-magnet (PM) torque motor—is characterized by a large diameter-to-length ratio and large number of magnetic poles to optimize torque production. These relatively low speed motors, usually operating under 1,000 rpm, come in housed and framele...
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 direct-drive, permanent-magnet (PM) torque motor—is characterized by a large diameter-to-length ratio and large number of magnetic poles to optimize torque production. These relatively low speed motors, usually operating under 1,000 rpm, come in housed and frameless 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. DDR torque motors tend to be physically large (well over 1 m diameter for some models), but smaller units are on the market as well. At the top-end, more than 20,000 Nm (14,750 lb-ft) peak torque output is not unusual.
Large diameter, many poles
Bosch Rexroth Corp. notes other advantages of DDR torque motors such as better load inertia matching, ease of control, low noise emission, and streamlined machine design (see “Simplifies design” diagram). Karl Rapp, machine tool industry branch manager at the company's Electric Drives & Controls Div. verifies that more pole pairs and a larger rotor diameter result in higher torque output. In addition, refinements in magnet orientation, stator-slot configuration and winding method, and air gap design can minimize torque ripple. “Low torque ripple is required for high-end grinding/honing applications and is desired in other applications,” says Rapp.
Danaher Motion likewise considers large diameter and high pole count distinguishing features of direct-drive torque motors. Torque is proportional to rotor diameter squared and directly proportional to rotor length, explains Tom England, Danaher Motion's director of product management. “High magnetic pole count, which provides a higher percentage of torque-producing copper in the windings, yields higher magnetic efficiency,” states England.
DDR torque motors are available in two classic formats. The “frameless” version consists of a ring-shaped rotor and stator parts set, which a customer must incorporate into the machine structure. Feedback, connectors, and cooling means also need to be provided, requiring significant design and assembly effort, according to England. The frameless motor's thin-ring structure offers a large hollow shaft input. A “housed” DDR motor has a frame, bearings, and either a regular shaft or hollow shaft. “However, if the machine already has bearings in place, a housed motor will not work since direct coupling of three (or more) bearings in a row will result in bearing failure,” he says.
Recently, Danaher Motion has taken a different approach, developing a third, enhanced DDR motor format said to focus on advantages of prior formats while eliminating their disadvantages. Called Cartridge DDR (or CDDR), these torque 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,” comments England.
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,” concludes England. Today, CDDR technology motors find application in packaging, press feed, converting, printing, and medical equipment.
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Direct-drive torque motor technology enables simpler machine design and higher motion accuracy as shown in this rotary index table example. Lost motion in gearbox and drive belt is eliminated.
Torque density, powerful magnets
Siemens views high torque density design as an integral part of its torque motor products. Two key parameters for torque production—rotor diameter and motor length—physically shape these motors. Ralph Baran, product manager for servo motors and mechatronic products at Siemens Energy & Automation (E&A)–Automation and Motion Control Div., puts it this way: Increasing active rotor diameter produces a squared effect on torque output, while increasing length only raises torque output in direct proportion. Therefore, torque motors typically have a large diameter and relatively short length.
Strength of the permanent magnets contributes to torque density of synchronous motors, explains Baran. Siemens uses neodymium-iron-boron (Nd-Fe-B) magnets (regarded as the most powerful and affordable type of rare-earth magnets) in its housed and frameless (built-in) torque motors.
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 realized 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, according to Baran. “As rule of thumb, increasing poles up to 30 is a good measurement for torque density increase,” he states. (Even so frameless torque motors with pole counts well above 100 are on the market.)
Baumüller Nürnberg GmbH likewise pays close attention to establishing an optimal ratio between diameter and length in the design of its multi-pole, PM synchronous torque motors—the DST Series. “As a result, constant high torque over a wide speed range has been achieved,” says Marcel Möller, product manager, motors.
Swiss-based ETEL S.A. notes that availability of more powerful modeling and analytical tools have simplified motor design and optimization. “Optimum motor design will maximize flux density by use of smart lamination tooth design and lamination material selection, while still allowing insertion of the maximum possible amount of winding material to develop torque normal to the flux path,” says Kevin Derabasse, president of ETEL Inc. in the U.S. He mentions a patented ETEL design for improved “filling factor” of copper winding wire into laminations. It achieves about 60% fill versus 30% for prior art, thus doubling fill density compared to other designs. ETEL manufactures a variety of frameless torque motors.
According to Bosch Rexroth, DDR 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,” says Rapp. As mentioned, reducing torque ripple becomes especially important for accurate machining.
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 Rapp. 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,” says Rapp.
Electronic commutation (or pole switching) is needed to operate a brushless PM motor. Commutation is not a simple procedure for DDR 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 Rapp. Intelligent drives, such as Bosch Rexroth IndraDrive, provide various commutation functions. The saturation method is preferred, as it can be run without physically moving the motor, he explains.
Siemens E&A's Baran states, “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, explains Baran. “Experience has shown that about a factor of 10 improvement can be realized in machine dynamics by designing the machine for direct drives compared to conventional motor-coupling-gearbox combinations,” he says.
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 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,” adds Möller.
ETEL suggests that a highly damped servo loop is vital for torque motor control, along with a drive able to handle regeneration energy during rapid decelerations. It's due to direct-drive design, which can “see” all of the driven load's resonance and directly reflected inertia. In an emergency stop situation, the motor quickly becomes a generator, producing large amounts of regeneration energy that must be dissipated in the drive or returned to the power source under appropriate control, explains Derabasse.
High torque output produces heat in the motor windings that must be removed to avoid motor damage. “Cooling also minimizes heat-related expansion, primarily of the motor stator,” states Rapp. “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,” adds Rapp.
Siemens also notes the vital role of cooling to increase torque density. Most thermal losses occur in the stator windings of brushless PM motors since no magnetizing currents flow in the rotor 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, explains Baran. “Tests have shown that torque output of a motor designed for natural air cooling can be increased by 30% if the design is optimized for water cooling,” he says.
According to ETEL, real power output of torque motors is limited by the ability to remove Ir. In turn, this heat can demagnetize the high-energy magnets in the rotor, explains Derabasse. “Water circulated as closely as possible to the stator windings is an economically and thermally efficient means to maximize heat removal,” he adds. That's why circumferential channels often are found on the stator's outside diameter, allowing close placement of cooling tubes.
Baumüller integrates water cooling into its DST torque motors as a recognized need for top torque performance. “Only this way is it possible to achieve the high torque density and simultaneous high overload capacity,” states Möller. “Integrated water cooling furthermoreenables a higher [sealing] protection class (IP54), which helps DST motors meet harsh conditions found in an industrial setting.” Besides more cooling capacity, a further counter-intuitive advantage of water cooling is reduced noise emission. Baumüller (and other manufacturers) mention thatwater-cooled DDR torque motors run quieter than their fan-cooled counterparts.
Danaher Motion claims its CDDR motors are very efficient and able to replace water-cooled systems with totally enclosed, non-ventilated solutions at substantial cost savings. However, to further increase torque output, water or air-cooling also can be added to CDDR motors.
Although not a high-volume product, DDR torque motors range over wide applications. Machine tools, machining centers, metal forming, rotary transfer systems, printing/converting machines, and plastic processing equipment are prime markets. More exotic applications cited by Bosch Rexroth are wind power generation and wave power harvesting. ETEL also mentions usage in ocean wave power conversion and in new-generation elevators, replacing hydraulic solutions with lower maintenance cost and simpler installation benefits.
Siemens mentions use of its 1FW3 (housed) and 1FW6 (built-in) torque motors in numerous machine tool and production applications. The latter frameless motors are designed to be built into the user's machine, which supplies the bearings. An encoder system also must be provided on the machine for 1FW6 motors. Housed 1FW3 motors include bearings and encoder. They're applied in plastics production (extruders, winders, injection molding machines, etc.) as well as in paper and textile industries.
Baumüller emphasizes wide use of DST motors particularly in the screw and closing drives of plastics extrusion/injection molding machines, and in powering plate and blanket cylinders for the printing industry.
In short, torque motors are at home anywhere traditional gear trains, chains, or timing belts have been used in the past, suggests ETEL.
Manufacturers of direct-drive brushless PM torque motors firmly believe that OEM users can gain major productivity and quality benefits if their machine design is optimized to apply these motors. Experience at Siemens E&A has shown these benefits to be realistic. “In some cases, machine productivity grew by 50%, while precision improved by around 30%,” says Baran.
Other reasons for OEMs to apply these torque motors cited by Siemens E&A 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.
Frank J. Bartos, is Control Engineering consulting editor. Reach him at firstname.lastname@example.org .