IPM motors get bigger role in appliance energy efficiency

Jan. 26, 2013
Interior permanent magnet motors are starting to become mainstream power sources for appliances that have to be designed with an eye toward energy efficiency.

There is increasing evidence that the tried-and-true induction motor is giving way to more exotic types in the effort to boost the efficiency of home appliances. The problem with boosting the efficiency of appliance-grade induction motors is that it's tough to dynamically slow them to save energy both because their rotor currents can't be measured and because the rotor circuit has a large time constant. These aren't problems for synchronous permanent magnet-type motors, however. PMSMs also benefit from greater torque-per-amp with smaller losses, as well as higher continuous torque compared to similar-sized induction motors.

One of the latest indications of the trend is the introduction of the Flux Multiplier drive motors by Johnson Electric. The motors target washing machines. Johnson says the Flux Multiplier motor output torque can handle 26-lb wash loads and 6-ft3 drum sizes. The systems works with interior permanent magnet (IPM) brushless motors. IPM motors typically employ less-expensive rectangular magnets rather than magnets curved to fit the circular rotor surface. The magnetic blocks slide into slots in the laminated rotor core. This scheme holds the magnets securely and simplifies the manufacturing process. In Johnson's system, an intelligent motor controller gets the drive to deliver a 30:1 spin-to-wash speed ratio for multiple wash modes. Johnson claims its Flux Multiplier motor has a long life, high efficiency and operates quietly. The company also claims its IPM drive scheme reduces energy consumption by 40% compared to traditional induction motors, so appliances so equipped can demonstrate energy savings exceeding A+++ and Energy Star ratings.

One reason IPM motors are seeing use in washers is that they produce more torque than surface-type permanent magnet (SPM) motors, because they exhibit an additional reluctance-torque component in addition to permanent-magnet torque. Use of reluctance-torque control with an IPM motor lets machine designers get a high torque for a given operating current. It also comes  in handy for the high-speed spin cycles that characterize today's efficient washers. The reason why pertains to the pronounced back or counter-EMF that rises with motor speed in all PM motors. Motor speed can only rise to the point where the back-EMF matches the drive’s available bus voltage. The electronic commutation for ordinary PM motors can be altered to attain higher speeds, but current demand increases dramatically  in this mode. Thus IPM motors use both the permanent-magnet torque and reluctance torque as a way to reduce or eliminate the limitation of motor speed imposed by back-EMF and drive-bus voltage.

 A cross section of an IPM motor from Infolytica.com. The magnets (red and blue) are visible inside the rotor (green).

With regard to the Johnson Electric IPM motors, each of the three models released so far exhibit efficiencies on the order of 75% at 550 rpm, which is washing speed. The listed spin speeds are between 15,000 to 16,200 rpm and the motors are about 70% efficient when spinning.


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