In most air-moving applications that employ a fan, such as vacuum cleaners, the only resistance experienced by the motor at start-up is the static friction of the bearings and the brush gear. The fan creates a load only when it is running, and the load is in proportion to angular velocity.

The most commonly used motor for this application has been the universal motor as the driving force behind the fan because it is reliable and low cost and can easily provide variable speeds for a selection of air flow rates. But its disadvantages include high acoustic noise, high electromagnetic interference and low efficiency.

In contrast, brushless direct-current motors are maintenance-free, quiet, and more efficient, with low electromagnetic interference and no friction at the brush gear during start-up. But, they require the use of high-cost electronic sensors and drivers.

Most BLDC motors use three-phase, bi-polar windings to generate torque to rotate the motor. In three-phase motors the windings are energized for 180 electrical degrees, creating sinusoidal torque waves, and are phased at 120 electrical degrees apart. The result is that there is always torque developed at any point within the full 360 electrical degrees. By contrast, a single-phase bi-polar winding has its winding switched on for 180 electrical degrees before switching current flow direction. This results in zero-developed torque at the moment of switching and, in the event that the motor came to rest in that particular position, it would not begin to rotate when power was switched on.

In motors that use permanent magnets and laminated steel poles for the windings, there is always the presence of another torque element known by various names such as cogging, pole-coupling, or pole-sensitivity. This torque is caused by the magnetic flux forces that exist in the air gaps between the permanent magnets and the steel poles around which the coils are wound.

The magnetic energy stored in this air gap always seeks the lowest reluctance angular position of rotor to stator so that the stored energy at this rest position will be at its greatest and the magnet at its lowest state of stress.

It is most common that the preferred positions of rest coincide with the very position that the electromagnetic torque is zero, as one sine wave is reduced to zero and the next sine wave starts at zero.

Single-phase solution

Fig. 2. Circuit drawing illustrates commutation of the single-phase motor.

Johnson Electric engineers, recognizing the potential cost savings on electronics if they could use a single-phase, bi-polar winding, decided to examine the possibility of using pole sensitivity to their advantage by artificially moving the preferred position of rest to a position where torque is always developed by the phase winding to ensure that the motor starts when power is applied.

The engineers designed a basic motor having four stator poles and a permanent magnet rotor with four magnetic poles. The study indicated that using a single-phase, four-pole permanent magnet BLDC motor could result not only in a cost savings, but in a reduction in noise and increased efficiency as well.

The engineers theorized that, by designing the stator poles to be offset with each other, it would be possible to move the preferred position of rest from the center of the pole shoe to an angularly offset position at which torque is developed by the winding sufficient to overcome the cogging torque and rotate the rotor to start the motor.

Fig. 3. Performance results chart.

Using specialized software that can predict the distribution of magnetic flux fields within a known geometry in which the magnetic properties are known for each element, a finite element analysis was conducted. The engineers focused on the construction of a four-pole permanent-magnet BLDC motor and analyzed the way in which the symmetry of the flux paths might be disturbed to create a small offset cogging torque that would create a force at the normal zero torque point strong enough to move the rotor in a de-energized motor to an angular position at which torque would be developed when power was switched back on.

In this lamination design, the small center offset can determine that, due to the magnetic coupling of the permanent magnet rotor, the rotor comes to rest off-center to the lamination pole, thus allowing torque to develop when power is re-applied.

The bonded, neodymium-iron-boron magnet ring is made longer than the axial length of the stator so that the overhang can be used to trigger a position sensor, such as a Hall Sensor, to trigger the phase switches at the appropriate times for maximum torque development. (See Fig. 2.)

A motor built to the description above was tested and the performance results are shown in Fig. 3.

By using a single-phase stator versus a three-phase stator, Johnson Electric achieved a cost-effective solution, making BLDC more feasible for air-moving applications. The motor performance met specification, and motor noise was reduced, with greater efficiency.

For more information email: chris_brunone@johnsonelectric.com