Permanent magnet motors outperform induction motors in many applications

Permanent magnet synchronous motors offer significant advantages on high-energy-consuming and high-dynamic applications, compared to induction motors. See table, photo gallery.

By Harald Poesch September 24, 2012

Permanent magnetic synchronous motors have been in use on machine tools and other production machinery for many years, owing to their reliable performance, durability, relatively low cost, and electrical stability. These motors have been the standard in the machine tool industry for many decades, used on the spindle, worktable rotation, ways and part articulation, in both rotary and linear applications of controlled motion. With the advent of the machine tool robot for materials handling, workpiece and tooling changes, high-precision load and unload functions, and more, the use of these motors has grown exponentially over the years. Behind the cutting theater, they’re also found on the chip conveyor, hydraulic manifold, oil reservoir, and coolant pumps.

Likewise, in the production machine realm, plastics and rubber molding and extrusion, papermaking, packaging, textiles, ceramic, glass, woodworking, and countless other pieces of equipment have utilized these motors for motion control.

Courtesy: Siemens, Control Engineering

Compact, high torque density

Further, the essential operational principle of a permanent magnet in the rotor assembly, generating a steady magnetic field instead of the short-circuit current found on asynchronous induction motor designs, has yielded many advantages for the machine designer, builder, and aftermarket. These advantages include compact form with high torque density and less weight, higher continuous torque over a wider range of speeds, lower rotor inertia, higher dynamic performance under load, higher operational efficiencies with no magnetizing current, and the corresponding absence of heat due to current in the rotor, low torque ripple effect, more robust performance compared to dc motors, good cos phi (a European term for power factor) and ultimately, better drive utilization.

The challenges have been, of course, escalating costs of raw materials due to certain economic factors in the world market, though recent discoveries of rare earth magnets (REM) in the U.S. and elsewhere may impact that situation greatly in the future. Plus, more technical aspects such as limited speed ranges in field applications and degradation due to counter voltage created by the magnets, where normally an encoder for commutation is deemed necessary, along with the inevitable limit overload condition, are being continuously addressed by the manufacturers.

New PM motor applications

New market areas are emerging, however, where the use of permanent magnet (PM) motors is showing great promise. These applications involve the use of PM motors for increasing machine productivity with better operational efficiencies.

One PM motor example is the servo pump, where a mechatronic analysis concluded substantial energy savings and operational improvements could be realized, with the additional benefit of environmental upsides, from the application of a PM motor on the hydraulic oil reservoir, replacing a variable capacity pump with a servo. Essentially, the pump motor runs only when the conditions of the machine mechanics warrant. No more was the long-standing presumption of continuous motor operation necessary. By use of a PM motor and direct drive technology, up to 50% savings are being realized on new and retrofit machines, with the obvious additional advantage of eliminating mechanical components, such as the gearbox.

Permanent magnet synchronous torque motors typically have 30%-60% higher torque capacity and 30% better torque utilization with faster acceleration and deceleration, compared to asynchronous induction type motors, and this has proven advantageous in the field, particularly with machine tools and other metalworking production equipment where the rapid traverse function is critical to maintaining higher productivity.

The German word “Nebenzeit” describes the concept of downtime and, in today’s fast-paced, often unattended machining applications, elimination of that condition is an absolute must. There is also significantly higher response and the absence of the “slip” phenomenon with PM motors.

In the higher power ranges today, these motors are also showing a significantly longer use life, owing to the greater degree of rotor tension compensation. In other words, the reduction of backlash (hysteresis) and the maintaining of precise position are better achieved, whether under load or not. From the magnetic perspective, this condition derives from the combination of a larger air gap and smaller radial magnetic forces, with lower inertia moment and high short-term overload capacity with maintenance of desired torque.

Drive technology improvements have likewise lobbied for increased use of PM motors in applications, as spindle motion precision and accuracy in a machine tool, for example, are directly dependent on the intelligence in the drive. Likewise, the complex current calculations for higher speed and rotor pole pair position identification are being made in the drive for use with simpler encoders or even encoderless configurations in the machine.

Use of the asynchronous induction motor is far from obsolete, especially with the advancements in motor system elements, such as variable frequency drives and inverters, used on various phase configurations. Even so, advancements in magnetics configuration and the higher levels of intelligence in drives technology are opening new applications for PM magnets daily.

Somewhere, both Farraday and Tesla are smiling.

– Harald Poesch is product manager for motion control motors, USA, Siemens Industry Inc. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering and Plant Engineering,