PM Servo Motor Size Perspective

By Frank J. Bartos, CONTROL ENGINEERING September 1, 1999

S upply usually meets demand. It’s not too different in the world of high-performance electric motors.

Demand for permanent magnet (PM) synchronous servo motors has traditionally been in the lower power ranges. This went hand in hand with the majority of applications that had limited power needs. Readily available ‘large’ servo motors were generally limited to the 15-22 kW (20-30 hp) range.

When motor control applications required substantially more power, users had to opt for induction-motor-based, closed-loop flux vector control (VC). This approach is sometimes referred to as an ‘induction servo’ system. However, this scenario is slowly changing, as more heavy-duty applications look for a solution with servo-type performance.

More availability
PM brushless synchronous servo motor sizes offered by GE Fanuc Automation (Charlottesville, Va.) now range up to a 40 kW continuous rating. Other examples of motor size growth include Mitsubishi Electric Automation’s (Vernon Hills, Ill.) PM servo motors up to 55 kW available on ‘special quotation.’ Yaskawa Electric America (Waukegan, Ill.) also makes these motors rated up to 55 kW, with even ‘larger sizes in development.’ However, Yaskawa reports no great volume of calls for the larger power ratings.

MTS Automation (New Ulm, Minn.) makes even larger servo motors in production quantities. Over a year ago, its 12-in. brushless servo motors offered continuous ratings up to 90 hp (see CE, Feb. 1998, p. 137). Now that motor line is reported to extend to 100 hp.

Pacific Scientific, Automation Technology Group (Rockford, Ill.) currently produces a brushless servo motor line called PacTorq rated up to 384 hp, running at 1,750 rpm. ‘These motors use neodymium-iron-boron (Nd-Fe-B) rare-earth magnets and are available in NEMA 180, 210, 250, 280, and 320 frame sizes,’ explains Bradley Trago, engineering manager at Pacific Scientific. For a size comparison, the 320 frame unit corresponds to a 16-in. motor.

‘Torque density and efficiency are two notable features of these servo motors,’ says Mr. Trago.

Some years ago, Powertec Industrial Corp. of Rock Hill, S.C, produced still larger PM brushless dc motors. At one time, Powertec offered brushless dc motors up through 600 hp (CE, Dec. 1992, p. 79). These machines, measuring about 25-in. on the OD, used ferrite permanent magnets and were offered in air-cooled and liquid-cooled versions. Powertec was subsequently acquired by Pacific Scientific.

Pacific Scientific continues to support Powertec brushless dc motors through 300 hp for existing applications, but for new usage, emphsis is on PacTorq motors with Nd-Fe-B magnets for higher performance.

However, size is not the only limiting factor. Other considerations and design constraints enter the picture. PM servo motors rely on costly permanent magnets, with material costs that escalate as motor size grows. These motors are also more complex to design and manufacture, especially in larger sizes. For example, as rotor size grows, peripheral speeds increase and make retention of magnets attached to the rotor OD a serious design consideration.

Still, increasingly larger PM servo motors can and will be manufactured as demand rises. The only question remains, ‘If they build them, will the users come?’

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