Improving Efficiency in Fractional HP Motors

A new permanent magnet motor structure promises to greatly improve the efficiency of appliance and servo motors without increasing overall manufacturing costs. Utilizing a unique axial motor design and patented dual conical magnet rotor, efficiencies for a 55 mm diameter motor are 85 percent over a significant portion of the operating range and over 90 percent in selected regions.

"A key characteristic of our new motor technology is its efficiency over a broad range of speed and torque," says John Petro, president of NovaTorque. "There are small motors that have peak efficiencies that reach 80-85 percent, or even the higher 80s, but they are usually not efficient over a broad range of torques and speeds," he adds.

Petro says nearly all of the motors manufactured in the world are based on a radial design. This new unit, however, uses an axial design. Most engineers think of axial motors as being pancake motors. NovaTorque has leveraged axial design and, by packaging it with the dual conical magnets, created motors that are closer to the form factor of a traditional motor.

A new configuration of magnets and axial field poles creates inherent advantages. Opposite polarized magnets create a very efficient, low-leakage and mostly straight magnetic flux path containing no wasteful back iron. The conical magnet angle allows modification of the air gap and field pole face area to allow setting optimum flux levels in the field pole core.

Petro says this motor also features a simple, low-cost coil with close-to-perfect packing and no end turns. In addition, the axial coil structure leads to superior thermal performance, since a significant portion of the coil coincides with the outside of the motor and the rest of the coil is in good thermal contact with the outside of the motor.

Superior efficiency results from a magnet flux path that has very low leakage and high air gap surface areas. Since the flux path in the field poles is straight, lower loss, Cold Rolled Grain-Oriented (CRGO) electrical steel laminations have been used.

Petro notes that these factors combine to give the motor design a high value for the torque constant, Kt.

The coil can be lengthened without much change in the magnetic flux circuit, which allows the motor constant, Km, to be directly set by changing coil length.