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Novel PM motor design delivers rare-earth performance from ferrite magnets

07 March, 2012

A Californian developer of permanent magnet motors called NovaTorque has announced a pair of brushless motors with efficiencies higher than similar-sized Nema Premium induction motors, but using low-cost ferrite magnets instead of costly rare-earth types. The 3hp (2.2kW) and 5hp (3.7kW) PremiumPlus+ motors, when driven by variable speed drives, have motor-only rated point efficiencies of 93% and 92% respectively.

“By comparison, 3hp and 5hp Nema Premium induction motors achieve a motor-only efficiency of between 88.5% and 89%,” points out Scott Johnson, NovaTorque’s vice-president of sales. He adds that the motors also maintain their high efficiencies over a much wider speed and load range.

“For example, in a variable-speed, variable-torque application, such as a fan or pump, a 50% reduction in speed results in a 75% reduction in load,” he says. “At that reduced load, NovaTorque motors will maintain an efficiency of approximately 91%, whereas a Nema Premium induction motor will have declined to 80%. The advantage is even more dramatic when compared to the current installed base of induction motors, where improvements of 20% or more are possible."

The graph above shows the efficiency for a 5hp Novatorque motor in a typical fan application (top curve, in blue) compared to the efficiency of a leading Nema Premium induction motor of a similar size (lower curve, in red).

NovaTorque achieves its high efficiencies using a patented flux-focusing stator and rotor hub geometry (shown in the exploded view below). The stator’s field poles have conical end-surfaces, and the rotor hubs have matching shapes. This maximises the surface area available for magnetic field transmission, while minimising material volumes.

The rotor/stator interface area is about twice that of a conventional motor with perpendicular stator field poles. This concentrates the magnetic flux density, allowing low-cost ferrite magnets to achieve efficiencies at least as high as motors using rare-earth magnets that cost about 25 times as much.

“The ability to use ferrite magnets allows NovaTorque to price its motors comparably to induction motors,” Johnson says. “This means that OEMs and HVAC system-builders can now deliver permanent-magnet motor efficiency economically.”

Another characteristic of the NovaTorque design is that flux flows parallel to the shaft through the stator’s axially-oriented field poles. This allows grain-oriented transformer steel to be used, reducing eddy current losses and raising efficiency. In radial flux designs, the flux travels in a circular path, preventing the use of grain-oriented steel.

The axial orientation of the stator field poles also allows the use of bobbin-wound coils, which are easier to wind and need less conductor material than the coils wound in the slots of radially-oriented induction motors. This arrangement also creates a better thermal path, because one coil face is next to the external casing, instead of being inside the lamination stack, as in an induction motor.

The NovaTorque rotors have a pair of conical hubs at opposite ends of the motor shaft. These use an interior permanent magnet arrangement which provides another source of flux concentration and allows higher operating speeds than rotors with surface-mounted magnets.

Because of their high efficiencies, the NovaTorque motors run cooler than comparable motors, enhancing their reliability. With good internal thermal paths and heat-dissipating aluminium cases, heat is transferred rapidly out of the motor.

The PremiumPlus+ motors (above) are being produced in Nema frame sizes, allowing them to be used as direct replacements for standard machines. Their high power density means that the motors are available both in a typical frame size for induction motor of the same power, as well as one frame size smaller. They can be used with most VSDs.

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