features of brushless DC fan motor

The brushless DC fan motor has these features:

Brushless fan motor, 2-φ

• The stator has 2-phases distributed between 4-poles

• There are 4-salient poles with no windings to eliminate zero torque points.

• The rotor has four main drive poles.

• The rotor has 8-poles superimposed to help eliminate zero torque points.

• The Hall effect sensors are spaced at 45o physical.

• The fan housing is placed atop the rotor, which is placed over the stator.

The goal of a brushless fan motor is to minimize the cost of manufacture. This is an incentive to move lower performance products from a 3-φ to a 2-φ configuration. Depending on how it is driven, it may be called a 4-φ motor.

You may recall that conventional DC motors cannot have an even number of armature poles (2, 4, etc.) if they are to be self-starting, 3, 5, 7 being common. Thus, it is possible for a hypothetical 4-pole motor to come to rest at a torque minima, where it cannot be started from rest.

The addition of the four small salient poles with no windings superimposes a ripple torque upon the torque vs position curve. When this ripple torque is added to normal energized-torque curve, the result is that torque minima are partially removed.

This makes it possible to start the motor for all possible stopping positions. The addition of eight permanent magnet poles to the normal 4-pole permanent magnet rotor superimposes a small second harmonic ripple torque upon the normal 4-pole ripple torque.

This further removes the torque minima. As long as the torque minima do not drop to zero, we should be able to start the motor. The more successful we are in removing the torque minima, the easier the motor starting.

The 2-φ stator requires that the Hall sensors be spaced apart by 90°electrical. If the rotor was a 2-pole rotor, the Hall sensors would be placed 90° physical. Since we have a 4-pole permanent magnet rotor, the sensors must be placed 45° physical to achieve the 90° electrical spacing. (Note Hall spacing above.)

The majority of the torque is due to the interaction of the inside stator 2-φ coils with the 4-pole section of the rotor. Moreover, the 4-pole section of the rotor must be on the bottom so that the Hall sensors will sense the proper commutation signals.

The 8-poles rotor section is only for improving motor starting.

Brushless DC motor 2-φ push-pull drive

In the figure above, the 2-φ push-pull drive (also known as 4-φ drive) uses two Hall effect sensors to drive four windings. The sensors are spaced 90° electrical apart, which is 90° physical for a single pole rotor.

Since the Hall sensor has two complementary outputs, one sensor provides commutation for two opposing windings.