Servos Optimize Torque per Volume

They're the elite among various electric motor types. Servo motors stand out by incorporating the latest high-force magnet materials, advanced design methods, and precise dimensional tolerances. Combined with appropriate electronics, servo motors offer the most sophisticated motion control available today.

By Frank J. Bartos June 1, 2003
  • Smaller motor footprint

  • High-energy
    magnet materials

  • Built-in gearbox

  • High power density

  • High-resolution feedback


They’re the elite among various electric motor types. Servo motors stand out by incorporating the latest high-force magnet materials, advanced design methods, and precise dimensional tolerances. Combined with appropriate electronics, servo motors offer the most sophisticated motion control available today. Brushless servomotors are steadily gaining market share against older brush dc technology by eliminating the drawbacks of mechanical phase switching (commutation) inherent in the latter motor design.

To gauge trends in this product sector, Control Engineering and Reed Research Group—both part of Reed Business Information —asked subscribers about their preferences on servo motors via an e-mail/Web-based survey. Selected results presented here are based on full responses received in April 2003 from 152 subscribers who evaluate, specify, recommend, install, and/or purchase electric servo motors for in-plant or OEM (resale) requirements.

Servo motor users consistently rate these performance and functional features as “very important”.

Packaging machines and machine tools tied for the top spot as primary applications for servo motors with one-in-five respondents indicating usage in each category. Material handling equipment (15%) and assembly lines (12%) rounded out the next two places. On average, over half (53%) of these motors go into new applications, while replacements and retrofits make up 27% and 20%, respectively.

DC servos fade

Brushless servo motors are the dominant choice over the brush-dc type (87% vs. 13% of respondents). This is a marked reversal from our 2000 survey, where brush dc servos were preferred by a majority of users. What hasn’t changed since 2000 is that users clearly prefer to buy servo motors matched with controllers (64%) rather than separate units (12%); still, 24% of respondents say they have no preference.

When asked about their selection criteria for servo motors, survey participants rated software for setup and tuning, high-torque capability at low speed, and high continuous torque output as the three most important performance features (see “Features Desired” bar graphs). As for functional features, pricing, standard frame sizes, and EMI/RFI protection were deemed most important.

Servo motors are most frequently applied in power ratings up to 1 kW (0.75 hp). Current and planned use in the next 12 months was reported by 77% of respondents in each case. Numbers of servo motors in use decrease steadily as size/power ratings grow. Nearly three-of-ten respondents indicate current use of linear servo motors and 32% non-current users expect to do so in the next 12 months.

Networking gains

Distributed control architectures are on the rise with over one-third (36%) of users applying their servo motors in a networked environment—up from 19% in the 2000 survey. Among networked users, DeviceNet is currently the most popular communications means (with 71% using it now; 64% planning to use it). Results suggest that Ethernet, now at a 59% usage level, will emerge as the most popular network with 74% of respondents planning to use it.

SERCOS (49%), Profibus (34%), and Modbus (32%) rounded out users’ communications choices. However, respondents indicated substantially higher planned usage in the next year for Profibus and Modbus. FireWire (IEEE 1394) and Interbus had a low showing in this survey, but also drew growing interest for near-future usage.

Survey results indicated that for the past 12 months, each user purchased 218 servo motors on average, representing $210,600 in value. In the year ahead, purchases will remain steady overall. Only 11% of respondents say their purchasing outlook will decrease.

Smaller units, better magnets

Servo motor manufacturers are applying several approaches in the quest for higher power density.

One trend at Baldor Electric Co. is to squeeze more torque output from a given size motor by designing the rotor assembly with the most up-to-date magnetic materials. The result in Baldor’s BSM C-Series servo motors is a footprint up to two inches shorter (depending on the model), but providing 50% more torque than previous generation motors.

In another approach, Rockwell Automation, Allen-Bradley Industrial Motion Control, integrated a gearbox and servo motor into one housing to reduce package size by 40% and weight by 25% in its MP-Series Integrated Gear Motors, compared to a conventional motor and gearbox. Yet MP-Series reportedly delivers higher torque output than comparable size units.

“An integrated gear servo motor provides a lower mass, which equates to less energy expended when the motor is mounted on a moving axis,” says Rod Dorschner, senior product marketing engineer at Rockwell.

John Mazurkiewicz, servo product manager at Baldor, notes customers’ growing interest in washdown-duty servo motors able to work in applications requiring water spray and chemical cleaning. He also mentions developments to incorporate feedback devices into these motors. This involves high-precision devices, such as absolute position encoders, as well as lower cost magnetic encoders for the right application.

Emerson Control Techniques likewise advocates economical design via smaller servo motors offering higher output. “Customers require compact size, outstanding performance, and flexibility in today’s motion control solutions,” states Bradley Nelson, director of marketing for Control Techniques Americas.

Among trends noted at Yaskawa Electric America (YEA) are improvements in high-energy product magnet materials and motor winding techniques. “We are confident that as a result of these developments, the size per torque output of our motors will continue to decrease,” says Jim Gegg, YEA’s manager of servo systems.

Other trends at YEA include ways to make motors accelerate faster by pushing peak/rated torque ratios from 2 to 3 in standard motors. This is unusual for servo motors sold in North America, according to Gegg. Yaskawa servo motors range into the higher power levels, with standard products sold through 55 kW (nearly 75 hp); and 90 kW available as a special product.

Besides striking advances in motor controls and magnetic materials, GE Fanuc Automation sees the use of high-resolution serial feedback devices and water-cooling jackets as pushing recent advances in servo motor performance. “Extremely high-resolution feedback devices (up to 16 million pulses per revolution) allow for nanometer interpolation and very smooth rotation,” explains Paul Webster, GE Fanuc Automation servo products manager.

Servo motor products

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Motor integrates gearbox

Allen-Bradley MP Series Integrated Gear Motors combine a servo motor and gearbox, delivering a package 40% smaller and 25% lighter than traditional separate designs. Reduced rotational masses yield 80% motor operating efficiency, much higher than standard units. MP Series gear motors are available for 230/460 V input with peak torque ratings up to 300 Nm (221 lb-ft) in three frame sizes. Four gear ratios apply to each size, with output speeds up to 194 rpm. The two-stage planetary gear reducer holds backlash to 3 arc minutes. A single- or multi-turn feedback device is included.
Rockwell Automation

50% more torque

BSM C-Series servo motors provide 50% more continuous torque than the previous generation because of newly redesigned package sizes and components. These new motors offer a torque range of 20-200 lb-in (2.3-23 Nm) with wide speed capability and medium rotor inertia. Various feedback devices such as resolvers and encoders (including absolute-position type) are available. Several models are ready for immediate delivery from stock; custom designs are also available. BSM C-series motors can be matched with Baldor’s digital servo drives for optimal application performance.
Baldor Electric Co.

Servo motors to up 55 kW

Sigma II Series servo motors have expanded to meet application requirements up to 74 hp (55 kW). These large capacity, 480 V ac input, SGMBH brushless servo motors feature totally enclosed, fan-cooled (TEFC) housings and IP44 protective enclosure. Because of their larger rotor diameters, SGMBH motors use internal permanent magnet construction, where the magnets are embedded in the rotor, rather than mounted on the surface. The result is a more compact, lighter weight motor, which reduces centrifugal forces to allow higher accelerations and decelerations with speeds up to 2,000 rpm required in factory-automation machines. SGMBH servo motors are offered as alternatives to ac induction, brush dc, and hydraulic motors.
Yaskawa Electric America, Inc.

Servo and spindle family

Beta i Series servo motors are part of the latest release from GE Fanuc that includes spindle motors and drive amplifiers for handling both motor types. The new servo motors are said to offer smooth rotation and low rotor inertia. Beta i Series servo motors are available with continuous torque ratings from 0.2 to 22 Nm (1.8-195 lb-in.); spindle motors have power ratings of 3-12 kW.
GE Fanuc Automation

Rotary and linear motors

Compumotor’s servo motor offering now includes the MaxPlus line of brushless rotary and linear units, newly acquired MTS Automation. MaxPlus rotary motors output continuous torques of 0.5-1,092 lb-in., and have a continuous current range of 0.5-123 A. Rotary servo motors are IP65-rated and come with high-performance neodymium magnets. Features include 10 standard feedback options, thermal sensor, optional gear reducers, and custom windings. MaxPlus linear motors are available in single- and dual-row layer magnet configurations. Their air-core (ironless) design provides high peak/continuous force ratios and extremely high accelerations.
Parker Hannifin, Compumotor Div.

Torque motor for 4,500 Nm

Direct-drive technology allows peak power production from newly introduced Rexroth IndraDyn T Series torque (servo) motors to reach up to 4,500 Nm (3,320 lb-ft). Yet these motors reportedly achieve an “extremely low degree of torque ripple.” Speed range is 100-1,200 rpm. IndraDyn T torque motors are synchronous frameless motors that consist of a stator with three-phase winding and a rotor with permanent magnets. Direct-drive eliminates gear reducers and other wear-prone mechanical elements, while significantly increasing the motor’s accuracy and precision.

IndraDyn T torque motors are designed for link circuit voltages of up to 750 V. Provision for liquid cooling is included, but these motors can also be operated without cooling at reduced output. Features include IP65 protection, temperature sensors integrated into the motor windings to protect against thermal overload, and standard power and cooling connections.
Bosch Rexroth

Cut cogging torque

NT servo motors output more than 50 lb-in. continuous torque (over 100 lb-in. peak torque). This represents more than 30% torque improvement over conventional 2- and 3-in. servo motors, but they’re 20% smaller, according to Emerson CT. “High-motor-slot-fill” and “flux focusing” techniques enable the performance improvement. Other benefits of the patented design are low cogging torque (&1%), reduced flux leakage, and lower I
Emerson Control Techniques

25% smaller footprint

Newest additions to Siemens’ 1FK family of synchronous servo motors are up to 25% smaller than previous models. The 1FK7 rotary servo motors produce maximum instantaneous torques up to 108 Nm (956 lb-in.) and stall torques up to 36 Nm. This capability combined with “a particularly favorable torque-inertia ratio” allows these motors to achieve very high dynamic response and accuracy for increased throughput. A rugged, maintenance-free design is said to make 1FK7 motors highly reliable and suits a wide range of motion-control applications, particularly robots, small machine tools, and material handling tasks. Various encoders and a holding brake are among available options.
Siemens Energy & Automation

Direct-drive rotary servo motors

Kollmorgen F Series frameless direct-drive rotary (DDR) servo motors are available in four frame sizes with diameters ranging from 160 to 333 mm (6.3-13.1 in.). A proprietary rotor structure, containing neodymium-iron magnets, and a skewed armature assembly help to optimize the motor’s torque/volume ratio, allowing continuous torque output of 5-369 Nm (3.69-272 lb-ft) and peak torque of 17-1,340 Nm. F Series DDR servos are available with feedback devices (and preconfigured drives) as a complete speed/torque solution for frameless motor applications. The motor’s frameless parts set carries UL recognition for easier agency approvals. Input voltages are 115/120 and 400/480 V ac.

Other features of F Series motors include: Encapsulated armatures for high-thermal transfer and mechanical protection; high magnetic pole count for high torque density and short axial length; low-cogging for smooth low-speed operation; and easy bolt-on rotor assemblies for quick installation.
Danaher Motion/Kollmorgen

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Online Extra to the Control Engineering June 2003‘Product Focus’ on Servo Motors

Here are additional results of the readers’ survey:

Although numbers of servo motors in use naturally decrease with increasing sizes (or power ratings), somewhat of a surprise was that even for motors 30 kW and larger, 19% of re-spondents currently use them and another 15% plan to do so in the next 12 months. This com-pares to 50% users who currently apply servo motors in the 6-10 kW range, with another 41% planning to use them over the next year.

Speed capabilities of 3,000 rpm or less satisfy requirements of nearly half (48%) of re-spondents. Another 40% have needs in the 3,000-6,000 rpm range, while 9% require speeds up to 10,000 rpm. Only 3% of users indicated the need for servo motors operating above 10,000 rpm.

In the year ahead, servo motor purchases will remain steady overall. Fifty-nine percent of respondents say their purchases will remain the same. Of the 30% expecting an increase in purchases, average increase forecast was 25%. Only 11% of respondents say their purchasing out-look will decrease in the next 12 months; average decrease will be by 50%.

Readers name leading servo motor manufacturers and suppliers
According to the Control Engineering and Reed Research Group June 2003 “Servo Motors Focus Study,” respondents purchased servo motors from the following vendors in the past 12 months (those in double-digits are included here):

43% Rockwell Automation/Allen-Bradley;

28% Baldor Electric

21% Emerson Control Techniques

19% Yaskawa Electric;

18% GE Fanuc;

17% Parker-Hannifin/Compumotor

12% Bosch Rexroth;

12% Siemens;

12% Danaher Motion.

On average each user purchased 218 servo motors, amount-ing to $210,600 value.
Among those specifying, recommending, and/or buying servo motors, 50% do so for in-plant requirements, 32% for OEM (resale) needs, while 18% buy for both OEM and in-plant needs.

Special design cuts size by 40%

Compact, low-inertia P2 Series brushless servo motors claim a special design that results in about a 40% size reduction over conventional units. They’re intended for high-response control of low-inertia loads. P2 motors are available in four sizes: a 100-mm square frame model in a 1.5 kW size and other model styles up to 5 kW size. All P2 motors operate at speeds up to 4,500 rpm; include a built-in 2,000 line, “wire-saving” optical encoder; and have a 24 V dc spring-set holding brake. Watertight connectors, shaft seals, and IP67 rating for environmental sealing are additional P2 motor features. Automation Intelligence Inc., a Sanyo Denki Co.

High-performance servo gearmotors

Moog’s G Series Servo Gearmotors integrate a low-backlash, single-stage gearbox with a high-performance servo motor to obtain up to 98% nominal efficiency for improved speed and performance. Smaller package size and substantial weight reduction are other benefits of the integral design. Users have a variety of motor model choices with three frame sizes (55, 70, and 100 mm), each having two gearbox ratios and four stack lengths per frame size. The gearbox is permanently lubricated, requiring no maintenance. The integrated design eliminates a separate front flange, thereby providing the best possible alignment between motor and gear reducer, as well as improving heat dissipation from the assembly. G Series minimizes the motor’s rotary inertia, while providing greater torsional stiffness for high-gain servo loops.
Moog Inc., Industrial Control Div.