I understand that we use the commutator as a rectifier using two segments that alternatively touch the negative and positive brush. That is when we only have one coil. What happens when we have more than one? enter image description here In the above diagram we have 4 coils and 4 commutator bars. What are the bars? Shouldn't we have 2 segments for each coil ? That means 8 bars. Maybe the bars and the segments are not the same thing. If they are not , what are the bars here? Because the coil has two sides and it has to get to both brushes. The textbook I'm studying also says that the voltage in the first picture below is eb+ec and in the second picture 7+18+20+18+7=70 volts. enter image description here enter image description here Now here is another part I don't understand. In the simple example of one coil each brush is always in touch with one coil. So I figured, if we have 2 coils we have four segments and we take advantage only of the coil which has the highest voltage each moment. However, that is not true. Here we add every coil's voltage. How can we take advantage of all the coils if only two segments touch the brushes each time? And how come the voltage in the first picture is eb+ec? Which coils touch the brushes? (I imagine that two coils could be touching a brush at the same time but only one contributes) I'm very confused and I can't find an answer. I also have this image for the actual physical construction of the first image . enter image description here

  • \$\begingroup\$ Commutator "bar" and "segment" refer to the same thing. Do you realize that these coils are connected in series? \$\endgroup\$
    – user28910
    Feb 21, 2017 at 16:04

2 Answers 2


Commutator segments = Commutator bars. Segments are made of copper bars separated by mica.

Coil B is connected to commutator segment b and c. C to c & d. 4 segments connected to 4 coils. Each segment connects two coils. All coils are connected in series around commutator. Brushes connect commutator in parallel.

As your rotor spins, Faraday's Law applies. Whenever the flux linked or associated with a circuit changes, a voltage is induced in the circuit.

So in Figure 4.7, coil A and C are moving parallel to the flux. No flux lines are cut, so induced voltage is 0. Coils B and D are moving perpendicular to the flux, so maximum voltage is induced. The text says 20V. \$E_B = E_D = 20V\$.

And how come the voltage in the first picture is eb+ec?

This is not correct.

Bottom of p74.

Consequently, the voltage induced in these coils is at it maximum possible value (20V, say). That is also the voltage across the brushes at this particular instant.

So in Figure 4.11b, we have the same size coils, producing 20V at maximum. Coil A and B produce 0V. Coil C and D produce maximum or 20V. The 18V coil is \$ 20\ sin (60°) = 17.3V\$. The coil is cutting the flux lines at roughly 60°. The 7V coil is \$ 20\ sin (30°) = 10V\$. So: $$10V + 17.3V + 20V + 17.3V + 10V = 74.6V$$

The coils are not fully at 30° and 60°, so voltages are less. Or 70V. But this illustrates where we are.

I disagree with the authors Figure 4.8, which covers Figure 4.10, but this has more to do with understanding the theory of how it works. Two coils at 45° will produce more than 20V. No DC Generator commutator has 4 segments.

4 segment DC Generator output

  • \$\begingroup\$ I see you know the textbook. Is it good for a first study on electric machines ? And yes,that diagram confused me more because I thought that one coil stops interacting and the other takes its place because it has a greater voltage . I commented on the previous answer explaining one more little problem I have. Can you check it out ? \$\endgroup\$ Feb 22, 2017 at 10:34
  • \$\begingroup\$ Any textbook has advantages and disadvantages. Read, throw stuff at the wall and see what sticks. There is no easy path to learning. I googled the text. But I'm no expert. I found the other answer was not at your level of understanding, so I tried to clarify your misconceptions. \$\endgroup\$ Feb 22, 2017 at 11:02

The individual physical coils of the armature are connected in series around the ring. When you make contact with two of the segments of the commutator, you are essentially forming two parallel "virtual" coils that are composed of the individual physical coil segments on either of the two paths between the contacts. These virtual coils have the desired alignment with the stator field in order to achieve the desired result. Each physical coil segment experiences an EMF that is related to its actual physical angle with respect to the stator field, which explains the numbers in your Figure 4.11b.

As the armature turns, the virtual coil turns with it, until you get to the point at which a different set of commutator contacts is reached by the brushes. At this, point, you get a different combination of physical coils, and the resulting virtual coils have an alignment that is reset back to the beginning of the desired alignment angle.

In this way, the armature experiences the maximum amount of torque available at all times. More physical coils and commutator segments means that the torque ripple is reduced and the efficiency is increased, at least to a point.

  • \$\begingroup\$ I believe that it is not correct to relate the number of commutator segments to "poles." A 4-pole DC motor has 4 field poles, 4 armature poles and 2 pairs of brush assemblies. At any instant in time, half of a larger number of armature coils would be serving to form each pair of poles. \$\endgroup\$
    – user80875
    Feb 21, 2017 at 21:03
  • \$\begingroup\$ @CharlesCowie: Yes, strictly speaking, you're correct. Edited. \$\endgroup\$
    – Dave Tweed
    Feb 21, 2017 at 21:21
  • \$\begingroup\$ I understand the diagram now but I'm confused as to what happens in the physical construction. Can you help me with the connections there ? I can't understand the voltage here , meaning what is point A and what is point B if Vab is the total voltage induced . Because I try to follow coil B which starts from the bottom segment of the brush on the left and ends up on the bottom segment of the brush on the right. So why isn't the total voltage only coil's B voltage VB ? Similarly on the other side with coil's D voltage VD. The V across the bottom segments is VB and the top ones VD. \$\endgroup\$ Feb 22, 2017 at 10:24
  • \$\begingroup\$ I know I'm wrong . I just explain how I see it so you can find out what's wrong in my thinking . \$\endgroup\$ Feb 22, 2017 at 10:25
  • \$\begingroup\$ @JohnKatsantas All the coils are connected in series and the 2 brushes make two sets of coils in parallel. So each induced voltage adds together to give a total. Segment a to Coil A to segment b to Coil B, etc. Currents are moving in different directions so both voltages are the same. \$\endgroup\$ Feb 22, 2017 at 10:56

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