Rick Armstrong
Danaher Motion
Wood Dale, Ill.

Edited by Robert Repas

This cutaway view of the Danaher AKM Series motor illustrates the overall compact size, high-density torque/volume stator and rotor, bearings, brake, and encoder. All versions of the AKM series are almost identical to keep manufacturing costs low and quality high.

Brushed and brushless servo-motors are found in many diverse areas including medical, industrial, home, aerospace, defense, and robotic fields. And each application potentially requires a different motor. It can be tough to pick the right motor given the virtually endless choices in types, styles, and configurations.

Fortunately, manufacturers work hard to ensure that certain motor parameters meet some standards that simplify the task of picking a motor. Torque, speed, and voltage ratings have always been the first critical parameters to consider. Motor torque and speed are based on machine load and motion profile as well as load acceleration and inertia. Motor sizing software from many manufacturers lets designers enter the parameters they need. The software then tests each motor against the parameters to determine its suitability to the application. Some software, such as Motioneering from Danaher Motion, automatically takes into account possible performance deratings like those caused from thermal limits of optional equipment or the added inertia of a fail-safe brake.

Factory equipment and machinery continues to shrink. Motors keep pace by boosting torque densities — the amount of torque a motor develops for its size. But as torque densities climb, so do motor temperatures. Motor makers try to deliver that power while keeping motor temperatures below maximum ratings — typically 100°C above the specified ambient. Lower weight and inertia also benefits the dynamic response of the system. Higher torque density motors yield higher torque-to-inertia ratios to create faster accelerating machinery. The net result produces more products per hour.

Stator windings are wound around a single pole tooth with each phase insulated for 480 V. None of the coils overlap to prevent coil-to-coil shorts. High-energy neodymium-iron-boron magnets produce optimum torque in a low-cogging electromechanical design. Redundant magnet retention assures mechanical integrity.

Higher-voltage motors typically produce less torque for the same package size, thus have lower torque densities. This is because higher voltages need thicker insulation compelling the use of small-diameter, high-resistance wire. The high resistance wire boosts I ² R losses generating more heat. Lower-voltage motors need less insulation, so they can use a larger diameter wire that handles greater currents without overheating.

So as not to burden lower voltage motors with high-voltage insulation that leads to lower torque density, some manufacturers create both low and high-voltage versions of the same motor. In contrast, Dana-her makes servomotors 58 mm and larger with high-voltage insulation that still has the same torque density of lower voltage motors. The motors run on 75, 120, or 230 Vac but contain 480-Vac Class F insulation across the line. Consider a case where a machine calls for a 120 or 230-Vac motor on one axis and a 480-Vac motor in another. For some manufacturers, this would require different motors to meet identical torque specifications. Dana-her, however, can generate the same torque for both voltages with the same motor.

A torque versus speed operating envelope describes the performance qualities of a brushless motor and servodrive system. The two shaded areas under the curve indicate the continuous and intermittent duty areas of the system.

Danaher abandoned the typical shuttle winding system used by most manufacturers in favor of a servocontrolled laminar scheme. With the laminar design, the turns of each coil lie much closer to one another. This packs more copper into the winding and, thus, boosts torque density. The technique produces a smaller motor for a given power rating or a higher-power motor in an equivalent package size.

A common cause of motor failure is insulation breakdown from excessive voltage or temperature. High-power-density motors use higher-grade insulation to better withstand the elevated temperatures of operation. In addition, the entire stator is enclosed in potting material that lowers its thermal resistance. Heat conducts quickly to the surface of the stator and dissipates into the atmosphere.