What happens to the motor when the wire is perpendicular to the magnetic field?

Question

In a electric motor, we know that the wires that are parallel to the magnetic field wouldn't have any movement. So when the 'rectangular circuit' rotates until it is perpendicular to the magnetic field, the two portions of the wire that are parallel before this, now have directions of force out of the page and into the page therefore they will cancel out the force. But what about the other two portions of the wire that is 90 degrees to the two other portions of the wire?

According to the Fleming's left hand rule, they now have directions of force upwards and downwards, wouldn't this stop the motor? My approach would be the force generated before reaching 90 degrees is great enough to make the motor turns fast enough to avoid this problem. Note that this is not about commutator. I hope the image in the link will make this clear. https://www.tech-faq.com/how-do-electric-motors-work.html

Answer 1

The question is a bit unclear. Now for simplicity assume the armature (rectangular coil) is resting in the position shown in the diagram in link. The long sides have current flowing perpendicular to the field direction. So when you apply Flemming's left hand rule for both the wires, the left wire has a force going into the page and right wire has a force popping out. Try to visualize this. This creates a torque or rotational motion. So the coil turns. At this position, the other shorter sides have a force in the upper and lower directions on the plane of paper by Flemming's left hand rule so they cancel out.

Now when the coil has turned such that the coil's plane is perpendicular to magnetic field plane, the long wires do experience a downward and upward force but they cancel out. At the same position shorter wires experience inwards and inwards force ( But since they are facing each other they turn out opposite and cancel again) - no net force .

But a fan switched off still keeps rotating for a short while. This is due to inertia of motion. For this reason the coil is able to rotate once even when no force acts and comes back to the original position where it receives a force and rotates again and this cycle keeps on going as long as there is current.

But this results in non- uniform motion. So an extra coil is placed horizontally. I think this extra coil is the answer to your question.

This video is worth 1000 times my answer due to the visualizing power it gives : https://www.youtube.com/watch?v=LAtPHANEfQo

Lastly, rather than learning the fleming's left hand rule, I suggest you to learn vector cross product (if u don't know it) and then learn the Lorenz force law. This might take a couple of hours but it is better than memorising that rule for years and you get an amazing insight with it. Make use of Khan academy.