Simple question about the DC motor

[scilearner]

Hello everyone,

 

I have a simple questiom about the DC motor. I understand that in the DC motor the commutator reverses the current to get a constant rotation but my question is why do we really need one. For example in this diagram

 

 

If the only thing that rotates is the coil and the commutator wouldn't the current always be in the same direction. I mean the only way the current could change is if the whole circuit turns as well with the coil and commutator. Put it simply I don't understand why the current reverses every half a rotation when only the coil and the commuatator is rotating.

 

I made another different topic about commutator in physicsforums and didn't get a single reply. I hope scienceforums would do better . Your help would be greatly appreciated

[ecoli]

excellent question. It's because the act of the coil rotating causes the change in current direction.

 

When the coil spins in the magnetic field, it briefly disconnects from the carbon brush on one side of the circuit, and finishes its half rotation via momentum, and reconnects with the carbon brush on the other side.

 

At this point, the coil is upside down from its original position, but the voltage is still being applied in the same direction, so from the perspective of the coil, the current is now flowing in the opposite direction, and the coil can continue to turn in the same direction.

 

If the coil never disconnected and replaced in the opposite way (and current reversed) the coil would just keep rotating forward and back.

 

So, to answer your question, the act of the coil rotating is what's changing the direction of the current.

[scilearner]

Thanks a lot for the reply ecoli . It was just what I was after . So the commutator is just like a common sense arrangement to reverse the current. The way our school teaches is that it is some kind of device where some chemical reaction occurs inside to reverse the current . So the commutator doesn't actually reverse the current by some reaction inside. So it is just a common sense arrangement and the rotation of the coil ensures the current is reversed. Am I right?

[Edtharan]

So the commutator doesn't actually reverse the current by some reaction inside. So it is just a common sense arrangement and the rotation of the coil ensures the current is reversed. Am I right?

It is like an automatic switch. When the coil rotates, the arrangement of the commutator allows the current to be switch so that it will always be flowing in the correct direction to allow the coil to keep turning.

 

It really is just a cleverly arranged, automatic switch.

[YT2095]

without it, you`d be left with the working guts of a moving coil meter, they will move roughly up to 180 degrees and stop there depending on the current passing through the coil.

[ecoli]

without it, you`d be left with the working guts of a moving coil meter, they will move roughly up to 180 degrees and stop there depending on the current passing through the coil.

I was told that it would turn 180 degrees, and then rotate back, the reason is that if the direction of current doesn't change direction, the force due to the current flow in a magnetic field will be in the opposite direction.

[swansont]

With no friction it would oscillate, but in a real arrangement it will end up oriented to where the torque is zero, much like a pendulum will end up pointing straight down.

[YT2095]

absolutely, in fact the only thing that makes a meter needle come back to the `0` point is a tiny little spiral spring.