# So the brushes in a commutator are fixed and assume a constant polarity, so how does the commutator reverse the direction of current?

I know this is probably one of the most common questions ever for someone who just started studying DC machines and I would probably get told by someone to just read somewhere and force myself to understand how the reversal of current mechanism thing works as there are lots of sources.

But I honestly can't get a good grasp on how the commutator mechanism works. I got the idea that for slip rings, the direction of the current is periodically reversed such as from positive to negative it becomes negative to positive. I associate these polarities with the brushes.

So how does a commutator fit in? Just because we have two different materials at all times against the brushes, how does that aid in reversing the direction? Is it really vague to just associate the polarities with the brushes because their interaction with the material should mean something and become responsible for the change in direction? How does this exactly work?

• Look at Andy's answer below, and consider a commutator as a sort of cross-over switch, switching one way then another, in lock step with the position of the coil. It has nothing to do with materials. The material in the brush is chosen so that it wears and the slip ring does not, making the brush the replacement part, not the ring. Sep 19, 2020 at 7:21
• @AndroidV11,Welcome and nice to meet you. Ah, let me see. So your question is the following: "How does the commutator reverse the direction of current?" (1) I must first point out that your question is wrong and misleading from the start, because (2) the commutator does NOT reverse the current, and (3) To explain why I am saying this, we need to first agree on what is the meaning of "reversing the current, and also before that, (4) why we need to reverse the current, whatever we agree its meaning. / to continue, ... Sep 19, 2020 at 8:21
• (5) And to understand (4) (why we need to reverse the current) we need to understand Fleming's Left Rule for motors (Never mind his right hand rule for generators, which in irrelevant in our case. / to continue, ... Sep 19, 2020 at 8:30
• What I mean is how does it convert to DC the half cycle which is located in the negative portion of a voltage graph or how it makes the parabola go upward. Sep 19, 2020 at 8:38
• Okay but I want to understand how it works in relation with DC current, because how does it cause a different effect than just putting slip rings instead? How does putting two different segments with an insulation in between to replace the slip rings make a difference? Sep 19, 2020 at 8:53

So how does a commutator fit in? Just because we have two different materials at all times against the brushes, how does that aid in reversing the direction?

Commutator system (simplified): -

Link to picture. Link to lots of pictures in the search entitled: "commutator brush current reversal".

Because the winding rotates and reverses its position relative to the magnetic field, which causes the current to move in the opposite direction.

Check out Fleming’s right hand rule for generators and Fleming’s left hand rule for motors.

• So how does a commutator make the current moving in opposite direction into a continuous one instead? I'm sorry, I just understand that it is the switch between the two segments that cause it but I can't exactly understand why is that so. What does a segmented connector like a commutator do that a continuous connector cannot? I mean both have brushes. Sep 19, 2020 at 8:57

Question

How does the DC motor commutator reverse the direction of current?

This is a long story. So let us begin with the picture below.

(1) The picture shows an over simplified motor with a commutator of two insulated split half rings surrounding the shaft.

(2) Suppose initially current flows from positive terminal of battery to brush contact B1, to commutation half ring C1, through the coil, to C2, to B2, then back to negative terminal of battery.

(3) Now suppose motor has turned 180 degrees (never mind why it turns, because is irrelevant to the explanation), C1 will be in contact with B2, and C2 in contact with B1. To summarize:

(a) Initial condition current flow = Pos -> B1 -> C1 -> Coil -> C2 -> B2 -> Neg

(b) After 180 degree turn = Pos -> B1 -> C2 -> Coil -> C1 -> B2 -> Neg

So the Coil current changes from (C1 -> Coil -> C2), now (C2 -> Coil -> C1). In other words, current flowing through coil changes direction.

References