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Torque Up, Size Down

Torque Up, Size Down

Achieving higher torque and better overall performance are never-ending goals for motor engineers, whose customers are becoming increasingly demanding in their requirements.

Danaher Motion ( just jumped on the bandwagon with the recent announcement of its new AKM line of brushless servomotors, which it says packs in about 30% higher torque density. The line is the first joint development from several former competitors (Kollmorgen, Pacific Scientific, and Warner Electric) that now make up Danaher Motion. To be marketed under the Kollmorgen name, the motor line leverages new magnetic materials and improved stator slot fill to deliver high torque density across a variety of package sizes. And by employing proven and efficient manufacturing process, Danaher says that the motors will be at least 10% less expensive than some of its current motor offerings that have fewer features.

A Motor Segue

To achieve the performance improvements, engineers leveraged the experience they gained developing the brushless dc motor used on the Segway. Special winding technology developed for that motor reduced its size (2.6 inch diameter by 3.5 inch length) by a factor of two-pumping out 40% more torque per unit volume than comparable-size motors. Peak torque on the Segway motor, which must keep an extra margin of torque in reserve for transient conditions, is 36.0-inch-lbs (4.1 Nm).

An obvious way to pack more torque into a given volume is to improve the amount of copper, or slot fill, in the windings-more wire carries more torque-producing current. This is often done using cut-core technology, which facilitates the winding of more turns by splitting the stator into sections. But Danaher engineers felt that the winding technology used for the Segway motors would allow laying small-gauge wires accurately and sufficiently close together within an assembled stator to produce a comparable slot fill more quickly and at a lower cost-which they say is over 80%. Previous similar would Danaher motors have 50%-60% slot fill.

Engineers further improved torque density by employing new rare earth magnets that have a greater flux density. Though the material price per pound is higher, Senior Product Manager Don Neumann says that this is offset by the need for less material per motor.

Further performance gains were achieved by improving pole shaping and exploiting more of the effective force of the rare earth magnets. Three different slot/pole configurations, aimed at different motor sizes, also help to mitigate magnetic imbalances and resonances that impose radial loads on bearings-and lead to increased noise and vibration, which can reduce bearing life. Engineers are tight lipped about the details other than noting pole counts range from six for the two smallest motors, eight for the AKM 3, and ten for the four largest.

Torqued Off: This log-log plot shows torque overlap between AKM frame sizes (the same torque from different motors), which offers engineers flexibility in matching motor inertia to load and mounting options. Low inertia (J), high dynamic uses (upper left) could be pick and place operations in semiconductor manufacturing. While torque roughtly goes up cubically with motor volume, which contains more windings, this increase shows up closer to a straight line on a log scale.

Pack it in: With the AKM series' improved electromagnetic efficiencies, Danaher claims that the servomotor family's torque density compares favorably to other offerings in the market. The plots are curved because, with higher stack lengths (higher stall torque), thermal output (heat rejection) of the motor does not go up in proportion to stack length (i.e., area doesn't increase as much as volume). The result is a slight drop in efficiency and thus torque density.

Looking for additional areas on which they could improve performance, engineers also sought to increase smoothness by reducing the cogging effect on motor rotation as the rotor magnets pass the stator coils. A common solution is to skew one slightly to the other, for a more gradual buildup and decline in the torque produced between them. But with an eye toward minimizing manufacturing complexity, Neumann says the design team felt, "Skewing complicates manufacturing and is difficult to optimize." Instead, engineers paid attention to magnet shape (using a D cross section), magnet positioning tolerances, and stator lamination tooth-tip configuration. And in concert with a design philosophy of redundancy wherever possible, the motors have dual-retention provisions for the magnets (epoxy and fiberglass) and stator laminations (slots and Loctite).

To further keep costs in check, the company developed a cell-based manufacturing strategy to make up thousands of different versions from a standard motor platform, including seven frame sizes, four stack lengths, up to five different windings, various mounting configurations (European and North American), various connections, and different feedback alternatives. The strategy, says Danaher, reduces the likelihood that an engineer will need to order a custom product and will drop lead times from a current 4 to 8 weeks down to 2 weeks, if not less down the road.

None of the company's 12 existing motor platforms will be dropped immediately as a result of this launch, but it wouldn't be surprising if some ultimately are phased out if the motor line performs as well as Danaher's engineers believe it can.

Contact Senior Technical Editor Rick DeMeis at[email protected].

AKM Characteristics
Model Typ. Diameter (mm) Stall Torque Nm (min/max) Pilot Circle Dia. IEC* mount (mm) Shortest Length w/o Brake/with Brake (mm)
AKM 1 48 0.16/0.45 30 70/NA
AKM 2 69 0.48/1.42 40 95/129
AKM 3 84 1.2/3.0 60 110/140
AKM 4 102 1.95/6.0 80 119/152
AKM 5 130 4.7/14.4 110 128/173
AKM 6 165 11.9/25.0 130 154/201
AKM 7 216 29.4/53.0 180 193/235
*NEMA available
CREDIT: Danaher Motion
December 2001 January 2002 March 2002 May 2002 October 2002 July 2003
Feasibility 3P session Project go-ahead Preliminary engineering and costing, two 3P sessions 3P session Capital asset request (money) approval Start of production
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