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

It is common to use universal motors as the driving force behind the fan because they are reliable and low cost. But their disadvantages include high acoustic noise, high electromagnetic interference, and low efficiency.

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

Johnson Electric's Single-Phase BLDC Motor Solution

Johnson Electric studied ways of reducing these costs and discovered it could achieve a reduction if a single-phase motor were used in place of the normal 3-phase motor. The study indicated that using a single-phase, 4-pole permanent magnet BLDC motor could result not only in cost savings, but in noise reduction and increased efficiency as well.

A 3-phase motor produces three sinusoidal waves of torque at intervals of 120 deg, whereas a single-phase motor produces only one wave. The three waves ensure that there is no zero-torque point, but the single-phase motor does, in fact, create zero torque points at the crossover position when the current direction is electrically commutated at 180 deg.

Johnson Electric focused on the construction of a 4-pole permanent magnet BLDC motor to analyze the way in which the symmetry of the flux paths might be distributed to create a small disturbing inherent magnetic torque that could create a rotational torque at the null-point.

A finite element analysis (FEA) was conducted on a conventional lamination design to create a clear image of the distribution of flux from an electromagnetic field. This analysis indicated that the permanent magnet rotor would come to rest, when power was switched off, in a very symmetrical location with respect to the field coil lamination poles and, at this position, zero torque would be developed on the rotor when power was reapplied.

It quickly became apparent that a bias was required to ensure that the rotor would come to rest in an offset position -- one that favored movement in one direction when power was reapplied. The asymmetric magnetic couple that needed to be created had to be strong enough to overcome any damping effect of friction from the bearings so that the rotor would always come to rest at a point where some torque would develop on the motor at startup. (Diagram shows Johnson Electric's offset 4-pole stator lamination design.)

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 reapplied.

A bonded neodymium/iron/boron (NdFeB) magnet ring is made longer than the axial length of the stator so that the overhang can be used to trigger a position sensor that can operate the phase switches at the appropriate times for maximum torque development.

The circuit drawing above illustrates commutation of the single-phase motor.

A motor built to the description above was tested and the performance results are shown below.

Conclusions

By using a single-phase stator versus a 3-phase stator, Johnson Electric accomplished a cost reduction, making BLDC more feasible for air-moving applications. The motor performance met specification, and motor noise was lower, with greater efficiency.