Published: June 7, 2016

  • OVERVIEW
  • GUIDELINES
  • BENEFITS & BARRIERS
  • REFERENCES

Permanent Magnet Alternating Current (PMAC) motors are emerging as an alternative to the commonly used alternating current induction motor, which has for generations been the workhorse of almost any application involving converting electrical work into mechanical work. AC induction motor technology is simple, reliable, and adaptable to a wide variety of applications. However, some inherent limitations in the design are being overcome with new motor technologies. PMAC motors preserve the simplicity and reliability of the AC induction motor while offering higher efficiency, synchronous operation, and the opportunity to use a smaller frame size to deliver a given torque.

Traditional AC induction motors use a magnetic field induced by electric current (hence the term induction) to turn the rotor. PMAC motors replace the conducting metal bars in the rotor with permanent magnets, usually made from alloys of rare-earth metals. They then use the magnetic field created by the permanent magnets to create torque and motion.

LEFT: en.wikipedia.org RIGHT: intechopen.com

PMAC motors save energy when compared to AC induction motors, and also have several other advantages. Since no electric current is induced in the rotor, PMAC motors have much lower electric resistive losses than AC induction motors. As a consequence, they are more efficient (typically 2-4% at full load), run cooler than induction motors, and typically have higher power factors. PMAC motors also do not experience slip, and are considered synchronous motors. Finally, since the magnetic fields generated by rare-earth magnets are up to twice as powerful as other commonly used permanent magnets, PMAC motors can deliver the same torque as AC induction motors with a smaller, lighter motor.

Line start PMAC motors: One drawback of PMAC motors is that most must be paired with a matching drive and cannot be started directly from line power. The drive is usually a VFD with software and power electronics which start and control the motor. However, some “line start” PMAC motors are available. These are a hybrid design which us a traditional aluminum induction rotor to start the motor. Once the motor has started, the permanent magnets take over to produce torque. The hybrid rotor design allows the motor to enjoy the advantages of a PMAC motor with regard to efficiency and synchronous operation, but without the need for a matching drive to start and control the motor.

Sectors

PMAC motors may be implemented in both retrofit and new construction applications in the following sectors:

  • Industrial: delivering high torque at low speed – conveyors, mixers, and grinders
  • Commercial: pumps, fans, and blowers; high power density applications – agriculture, automotive, maritime, and off-grid

Implementation costs

PMAC motor costs depend on motor size and the permanent magnet material. The market for rare-earth metals is highly volatile. Prices fluctuate widely, and as a result the costs of PMAC motors fluctuate widely as well. However, some PMAC motors use ferrite magnets, an alternative material with more stable prices. A source has identified the incremental cost of PMAC motors to be 20%-70% greater than induction motors.

Potential energy savings

The amount of energy savings from PMAC motors depends chiefly upon the number of service hours experienced by the motor, the motor size, and the motor loading profiles. Compared to AC induction motors, PMAC motors tend to better maintain efficiency at part-load conditions, both in terms of torque and speed. This makes PMAC motors an attractive alternative to induction motors in variable-speed applications.

The following table provides two examples and associated savings of PMAC motors. These examples demonstrate the potential range of savings, but project-specific factors may influence the amount of energy saved.

The first example represents a bench test study conducted by a California utility. The study tested a PMAC motor against a NEMA Premium Efficiency and a NEMA Standard Efficiency motor in a 3 hp HVAC fan application. Total savings were $11.30 per motor hp compared to the Premium Efficiency motor and $16.00 per hp compared to the Standard Efficiency motor.

The second example represents a retrofit of a two-speed 40 hp cooling tower fan motor by a PMAC motor with a VFD. Energy cost savings were estimated at $275 per hp. A portion of these savings should be attributed to the replacement of dual speed control with a VFD.

When selecting a non-line start PMAC motor, it is very important to verify from the manufacturer that the VFD used is capable of operating with the motor, as not all VFDs are compatible with all PMAC motors. Also, as with all motor selection, it is important to carefully match the motor to the design torque and speed requirements, taking care to not oversize or undersize the motor.

Care should also be taken to not run the motor over its maximum rated speed, as this increases the risk of failure and motor damage from back electromotive forces (EMF).

During servicing, the strong permanent magnets increase the possibility of back EMF voltages. If the motor shaft is turning, this can result in pinch and shock hazards. Individuals wearing pacemakers or other medically implanted devices should always be careful when working around strong magnetic fields, although if the rotor is within the motor housing the radiated magnetic energy is very small.

Benefits:

  • Significant energy savings
  • Longer bearing and winding life
  • Synchronous operation
  • Higher torque-to-size ratio

Market barriers:

  • Capital cost
  • Need for a matched motor drive
  • Back EMF limits maximum speed
  • Pinch hazard / magnetic field safety concerns during maintenance
  • Possible demagnetization at high currents/temperatures

Murphy, J., “Understanding AC Induction, Permanent Magnet and Servo Motor Technologies”, Leeson Electric Corp., accessed May 2016: Conceptual description of induction and PMAC motor technologies, relative advantages, and market barriers.

Murphy, Jim. “Sizing and Selecting Permanent AC Motors”, Leeson Electric Corp., accessed May 2016: Presents guidelines for selecting and operating PMAC motors

Lin, P., and A. Crapo, “MDM Webinar: Introduction to Permanent Magnet AC Motors”. Webinar, accessed May 2016: Applications and market sectors of PMAC motors

Dooher, B., and J. Bersini, “Permanent Magnet Alternating Current (PMAC) Motor Efficiency Comparison – Phase 1”, Pacific Gas & Electric Company, January 2014: Experimental testing of the efficiency of 3 and 5 hp PMAC motors compared to NEMA Premium Efficiency Induction Motors

Premium Efficiency Motor Selection and Application Guide, U.S. Department of Energy Advanced Manufacturing Office, accessed May 2016: Compares efficiency of PMAC motors to NEMA Standard and Premium Efficiency motors and discusses line-start PMAC concepts

ADM Associates, Inc. “NovaTorque® Brushless Permanent Magnet Motor Field Test Report”, Prepared for Sacramento Municipal Utility District, May 2014: Field study evaluating energy savings of PMAC motor to NEMA premium induction motor

Larsen, W., et.al. “Exploring the Customer Benefits of Permanent Magnet Motors: Test Results and Opportunities for Next Generation Motor Programs”, 2015 ACEEE Summer Study on Energy Efficiency in Industry, accessed May 2016: Review of three case studies comparing efficiency of PMAC motors to induction motors

Petro, John. “Achieving High Electric Motor Efficiency”, NovaTorwue, inc., accessed May 2016: Discusses design of PMAC motors constructed with ferrite magnets and impacts on motor efficiency

McCoy, Gilbert A. “’Super Premium’ Efficiency Motors are Now Available”, Washington State University Extension Energy Program, Accessed May 2016: Compares power density and efficiency of PMAC motors to induction motors and line start PMAC motor concepts

Valmiki, M.M., and Antonio Corradini. “Energy Savings of Permanent Magnet Synchronous Motors in Refrigerated Cases”, Alternative Alternative Energy Systems Consulting, Inc., March 2016: Case study of using permanent magnet motors in refrigerated case applications