New motor designs improve efficiency, reliability, and control

Oct. 7, 2008
Contributing Editor Sheila Kennedy reports that construction changes are improving energy efficiency, reliability and process control, and for good reason.

The electric motors you purchase today are better than those of a decade past, and in 10 years, they’ll be better yet. “The evolution of motor technology often is a product of application changes,” says Noah Bethel, PdMA’s vice president of product development. "We’ve seen changes relating to temperature management, handling voltage transients in nonlinear loads and modifying insulation quality to operate in inverter-duty environments."

Lately, the emphasis is on maintaining motor efficiency. “We’re currently seeing more in the area of efficiency than robustness. The good fight is delivering efficiency with reliability — not either/or,” Bethel says.

Minimum aging: A new approach to coil design has been developed by WEG Electric Motors’ R&D in collaboration with the Federal University of Santa Catarina. It addresses inverter overvoltages as a means to reduce premature aging of the coil insulation and improve motor life and efficiency.

The new design, demonstrated in prototypes but not yet commercially available, reduces the nominal coil-winding supply voltage to eliminate partial discharges at the terminals and initial windings. This is achieved by reducing the voltage pulse amplitudes of the pulse width modulation. In addition, replacing pole-group coils with a larger number of parallel-path coils and applying reduced voltage to the parallel-path coils maintains the same magnetomotive force. The self-regulating mechanism tolerates minor voltage fluctuations and ensures loss increases are insignificant.

Maximum efficiency: The finned-frame, internal permanent magnet (IPM) rotor motors in the Baldor-Reliance RPM AC family deliver the small package size, light weight and high power OEMs seek. “The IPM provides a smaller package with better motor efficiency — typically above NEMA premium,” says John Malinowski, Baldor’s motor product manager.

The frame, designed with a stack of thin, finned laminations, resembles a heavier cast frame. Magnets imbedded in the rotor lamination improve power density and efficiency by eliminating rotor I2R losses, while the IPM salient-pole rotor design helps increase the torque output.

“Compared to a NEMA 250 frame with 20 hp, our IPM motor with the same diameter delivers 100 hp and three points higher efficiency,” says Malinowski. “At 96.5% efficiency, the IPM is even more efficient than a NEMA Premium efficient motor at 95.4%.”

The IPM motor operates as an open-loop vector drive with adjustable-speed control, or in closed-loop mode with an encoder. It’s also capable of being a servomotor. It’s suited to high-performance, quick-response start/stop applications and the more conventional applications in which particularly high energy efficiency might help reduce life cycle costs.

Comprehensive redesign: Frame, rotor and fan redesigns by GE Motors form the basis of the Quantum LVM. This line of medium-voltage, TEFC induction motors has a patent-pending duct system design that allows for compact size, increased output per frame size, cooler operation and improved energy efficiency. To improve temperature distribution throughout the motor, the frame’s seven “D-Ducts” increase airflow to operate cooler at a lower shaft height, and multiple oversized rotor vents yield a low temperature rise and maximize cooling.

The Quantum LVM’s new internal fan design runs at higher power ratings in a smaller frame. The external fan’s improved airflow supports improved heat transfer. Vibration is reduced through a large separation between the frame natural frequency and line frequencies, and the motor’s stiff frame, rigid class 30 cast iron feet and coil brace construction further enhance uptime and reliability.

Ongoing challenges: Regardless of technical advances, proper maintenance remains necessary to ensure your motor lasts as long as it is designed to last. “A lot of PdMA’s technology is used at the quality-control level because ounces of QC deliver pounds of reliability,” says Bethel. “If you can identify conditions conducive to a fault, you can clear the conditions and extend motor life. If you can get to the root cause quickly, you can reduce the mean time to repair and improve your overall equipment effectiveness.

Maintaining design efficiency is another ongoing effort. "The Energy Policy Acts provided incentives to increase efficiency, and the industry is increasingly in a better position to support the standards. More repair shops are capable of performing motor performance and efficiency verification on repaired motors. Larger manufacturers and utilities with strict QC departments also are investing in their own test stands and equipment to perform motor efficiency testing," Bethel explains.

E-mail Contributing Editor Sheila Kennedy, managing director of Additive Communications, at [email protected].

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