# Does this motor ever run?

I've been studying lap winding in dc motor and the geometry is quite interesting.
I stumbled on below arrangement of coils for a 4 pole 4 slot double layer simplex lap winding.

My thoughts:

1. This looks legitimate lap winding.
2. Brushes are not along magnetic neutral axis, so sparks will be there.
3. All the coils are shorted, so the motor will never start from this state.
4. Even if I forcibly give the armature some initial spin, it will get stuck immediately because in the shown state the torque on coils is 0. Also at magnetic neutral axis, after 45 degrees, the torque is again 0.

So it seems the brush-placement-relative-to-poles not only decides the sparks, but also the torque. Is this true? If so, why does my textbook never mention this? It says sparks issue exclusively decides the brush placement.

Another example with 4 pole 8 slot lap winding that seem to never spin due to brush/poles relative placement (conductors under a pole don't run current in the same direction. One conductor runs current into the page, the other conductor runs current out of the page):

This is just a multiplication of a 2 pole 2 slot motor, which has the same problems (shorts out, won't start).

Even if I forcibly give the armature some initial spin, it will get stuck immediately because in the shown state the torque on coils is 0. Also at magnetic neutral axis, after 45 degrees, the torque is again 0.

Not necessarily. The motor's rotational inertia may be enough to keep it spinning, provided the brushes don't short out the windings long enough to brake it (which can be ensured by making the insulating gap between commutator segments wider than the brush width).

This motor won't start by itself, but after being spun up manually it will keep going until stopped manually. The technique was used by Tamiya for the motors in their 1/48 scale 'Propeller Action' plastic model aircraft. It allowed the tiny 2 pole motor to be connected directly to the battery without an on/off switch.

So it seems the brush-placement-relative-to-poles not only decides the sparks, but also the torque. Is this true?

Yes.

If so, why does my textbook never mention this? It says sparks issue exclusively decides the brush placement.

Because torque is maximized when when sparking is minimized, so there is no need to mention it?

Animation of a DC motor: rotation of the armature – variations in current and torque