There are descriptions on internet of the single phase BLDC motors which are powered by Vdc (such as PC cooling fans 12V). Interestingly pictures shown describing outrunner type with equal number of poles and stator coils (usually 4 both). One detailed application note provided by Microchip :
Sensored Single-Phase BLDC Motor Driver
Here is a picture showing the motor and it's control using a full H-bridge:
Controlling full H-bridge is shown reversing the current flow in sync with Hall sensor reading.
What is confusing to me that there are no means to ensure proper direction of the rotation at startup. Even more confusing is the following text from this app note (page 4):
The toggling from Forward-to-Reverse mode produces a clockwise rotation, while toggling from Reverse-to-Forward mode produces a counter-clockwise rotation.
How is it possible? All the toggling of the phase current does is to create repulsive force between stator coils and magnetic poles (knowing the polarity of the current position from the Hall sensor reading) but does not make the poles rotate in any "known" direction. These motors work similar to "switched reluctance" as the poles are driven to the position where their magnetic flux get fully aligned with stator coils flux. In this type of motors Magnetic flux lines of rotor magnets and stator coils are not perpendicular to define the direction of the rotation.
Do the PC cooling fans have equal number of stator coils and rotor poles? How do such type of motors start rotating in one direction only?
Thanks in advance for any input on this.