Some basic questions about a DC motor and a DC generator

  • #1
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Summary:

I'm having trouble applying the Fleming's right hand rule in a static magnetic field of four poles. I just wanted to know the direction of induced current in both sides of coil.

Main Question or Discussion Point

Hi,

The picture below shows a DC generator. The magnetic field is uniform. As the coil rotates clockwise, the right side of coil is moving downward and the left side of coil moves upward. Using Fleming's right hand rule, one can deduce the direction of conventional current in both sides of coil. In the right hand side, the current is coming out of page and in the left hand side of coil the current is running into the page. It would produce pulsating DC voltage.

dc_generator.png


Now suppose that the same coil is being rotated in the same direction, i.e. clockwise, using four magnetic poles as shown below. How is the right hand rule applied in such a case? Is there any possibility that the rule could just be applied without doing the calculation? Will the upper side of coil have current going into the page and in the lower side coming out of the page? Thank you!

dc_machine_2.jpg

Source: ("/watch?v=OpL0joqJmqY" insert "youtube.com" in front )
 

Answers and Replies

  • #2
Baluncore
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The same planar coil could not be used in a 4 pole field.
The coil would need to be wound differently for use with a 4 pole field.
The commutator would also need more bars.
 
  • #3
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Thank you!

The same planar coil could not be used in a 4 pole field.
The coil would need to be wound differently for use with a 4 pole field.
Could you please let me know how it should be wound? I used the search phrase 'four pole dc motor coil shape' but it didn't help.

PS: Does it have anything to do with lap/wave/frog-log windings?

The commutator would also need more bars.
Yes, a four segment commutator should be used.

Thanks.
 
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  • #4
Baluncore
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The impractical two pole example in post #1 shows a single loop that covers 180° of the armature.

A four pole machine would need two or more loops, each loop covering 90°.
 
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  • #5
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Thank you.

Is it just impractical, or a non-functional and wrong example circuit as well? I wouldn't think that it's a theoretically wrong example considering the source.

dc_machine_2.jpg

Source: ("/watch?v=OpL0joqJmqY" insert "youtube.com" in front )
 
  • #6
Baluncore
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Is it just impractical, or a non-functional and wrong example circuit as well?
In the four pole symmetrical example pictured, with a single central loop, the total flux through the loop must always sum to zero. It cannot generate or motor.
 
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  • #7
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In the four pole symmetrical example pictured, with a single central loop, the total flux through the loop must always sum to zero. It cannot generate or motor.
Thank you.

So, no voltage would be generated using the shown arrangement from my previous post. I'm asking you again because I wanted to add a comment on that video to let others know that there is an error.

Will the following arrangement work? It still shows a single coil rotating clockwise.

dc_machine_2a.jpg
 
  • #8
Baluncore
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Yes, that will work as the flux will fully reverse every 90°. You show it at the transition point, where the rate of change of flux is greatest, so the voltage generated is greatest.

Don't forget that the video is from a 1966 training film, so the best you can do is warn others of being distracted by the error. How will you do that without causing an even bigger distraction?
 
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  • #9
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Thank you!

Yes, that will work as the flux will fully reverse every 90°. You show it at the transition point, where the rate of change of flux is greatest, so the voltage generated is greatest.
The lettering below represents the coil position. We start with J-K where the coil is vertical. Then, it's X-Y position, followed by A-B, Y-D, E-F, D-G, H-I, G-X, and finally J-K.

How do we define the angle of the coil? Could you please guide me with this?
At J-K, angle is 0 degrees, and voltage is 0 V.
At X-Y, angle is ???, voltage is maximum.
At A-B, angle is ???, voltage is zero.
And so on.

dc_machine_2a_1.jpg



In a practical DC machine, there should at least be four coils to utilize the four pole magnetic field fully and optimally. One coil could be E-F, second F-G, third G-H, and fourth H-E. Do you agree with my statement? (I understand that in the capture below, only two coils are shown.)

If it's a DC generator, it wouldn't make sense to receive the output of each coil separately. I believe this is where the idea armature winding, such lap, wave, frog-leg, comes in. Do I have it right?

dc_machine_3.jpg



Don't forget that the video is from a 1966 training film, so the best you can do is warn others of being distracted by the error. How will you do that without causing an even bigger distraction?
It's a really good video. You are right that it'd be a distraction so perhaps I shouldn't comment there, or just send a message to the uploader so the viewer could be informed in the discription.

Thanks a lot for your help and time!
 
  • #10
Baluncore
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In a practical DC machine, there should at least be four coils to utilize the four pole magnetic field fully and optimally.
To be optimum there need to be four brushes, cross connected to two terminals.

There will be as many commutator bars as there are coils. Each coil will straddle close to 90°, and share two commutator bars with other coils. The way the bars are shared by coils determines the style of armature winding.
 
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  • #11
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Thank you.

I do have some questions about your last post but first I need to clarify something.

I have read that in a simplex lap winding there are as many number of parallel paths as there are poles, and in a duplex lap winding the number of parallel paths is doubled for the same number of poles, and with triplex lap winding the number of parallel paths gets tripled for the same number of poles.

Is the lap winding shown below duplex? I might be wrong but there are four parallel paths and four number poles so it should be a simplex lap winding. The reason I think it has four parallel paths is as follows. Each brushes labelled "+" is taking currents from two coils and as there are two "+" brushes therefore four parallel paths. If you could comment on this this, it'd really help. Thanks.

lap_winding_confusion.jpg
 
  • #12
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Hi,

I'd really appreciate if you could help me with the queries below. A simple 'yes' or 'no' will do.Thank you.

Question 1:
I believe that I should clarify post #9.

We start with the midpoint of J-K position of coil which is making 0 degrees and the induced voltage is 0 V. In X-Y position the midpoint is making an angle of 45 degrees and the voltage is maximum. When in position A-B, the midpoint angle is 90 degrees and induced voltage is zero, and so on. In the picture below, the dashed lines represent axes and 'O' is origin.

Do I make sense? A simple 'yes' or 'no' will work.


dc_machine_2a_1_x.jpg


Question 2:
It looks like that there is another mistake in the video. Please have a look on the capture below.

In the transition from 9:27 to 9:28, an additional coil 1-3 is added in the horizontal position. One coil is vertical and the other horizontal. Previously, you have said that the coil 2-4 in the shown position shouldn't produce any voltage, and from this I deduce that the other coil 1-3 should also not produce any voltage. Could you confirm if I have it correct?

Question 3:
In post #8, you said, "You show it at the transition point, where the rate of change of flux is greatest, so the voltage generated is greatest."

If it is assumed that 1-2 is one coil and 4-3 is the other. According to what you have said, the coil 1-2 in the shown position would produce maximum voltage. Now the question is that the coil 4-3 is also occupying the similar position, what would be its induced voltage? Also maximum?

4_poles_video.jpg



Note to self:
In post #9 I said, "In a practical DC machine, there should at least be four coils to utilize the four pole magnetic field fully and optimally. One coil could be E-F, second F-G, third G-H, and fourth H-E. Do you agree with my statement? (I understand that in the capture below, only two coils are shown.)".

I actually had double layer winding in mind which has as many coil as the number of slots. For simple and smaller DC machine, single layer winding is used which has half the number of coils than the slots available.

Helpful link: https://www.quora.com/What-are-the-...nding-in-an-induction-motor-and-an-alternator
 
  • #13
Baluncore
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Question 1. YES.
Question 2. YES.
Question 3. YES, also maximum. The two coils could be connected in anti-parallel.

To generate the 8 pulse per turn ripple voltage shown would require a coil at an angle of 45°.
 
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  • #14
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Thanks a lot!

Question 3. YES, also maximum. The two coils could be connected in anti-parallel.

To generate the 8 pulse per turn ripple voltage shown would require a coil at an angle of 45°.
In the picture below, I have assumed that the coil 1-2 is at 45 degrees and the other coil 3-4 is at 225 degrees. The rotation is clockwise and angles are also measured clockwise with 0 degrees at left north pole.

If the side labelled "2" of coil 1-2 is positive then so should be side labelled "4" of coil 3-4. If what I'm saying is correct then the induced voltage in both coils should be in phase.

The shown voltage waveform has 45 degrees phase difference between them. For this waveform, one coil should be at 45 degrees and the other at 90 degrees.

Do I have it correct?

4_pole_2_coil.jpg
 
  • #15
Baluncore
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The shown voltage waveform has 45 degrees phase difference between them. For this waveform, one coil should be at 45 degrees and the other at 90 degrees.
Correct. But you only show the sides of two coils. For a balanced symmetrical winding to generate the waveform you would need 4 coils, in two parallel pairs.
 
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  • #16
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Thank you!

For a balanced symmetrical winding to generate the waveform you would need 4 coils, in two parallel pairs.
Both coils, dashed blue lines, would be connected in parallel, and so would be other two coils in grey dashed lines. The waveform of blue dashed coils is shown with 'blue' dot mark.

Do I have it correct? If it's not correct, you can just say 'no'. Thank you!

4_coils_correct.jpg



But I don't understand that how those professionals would have such an important segment wrong. The picture below shows four different waveforms for each coil, and each waveform differ in phase. I don't think this is correct.

4_coils_wrong.jpg
 
  • #17
Baluncore
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You have it correct.
But I don't understand that how those professionals would have such an important segment wrong.
50 years ago, the film was shown as part of the classroom training of non-mathematical technicians in the army. As an introduction it gets the ideas across. There was no time to spool the film back to examine the details. It is discussing quite impractical designs with few windings, something that will not be encountered in the real world.

With 2020 hindsight, it certainly has technical errors.
 
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  • #18
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Thank you!

Please have a look on the picture below. The right picture shows the coils after 90 degrees clockwise rotation. One coil is labelled 1-2 and the other coil is labelled 3-4. The coils are rotating clockwise. The mid point of left north pole represents zero degrees.

Let's assume that the both coils are connected in antiparallel configuration; i.e. "1" is connected to "3" and "2" is connected to "4". We can only focus on coil 1-2. From 45 degrees to 90 degrees, the direction of current remains the same so brushes could keep the contact with same commutator segments. From 90 degrees to 180 degrees, the current flow gets reversed therefore brushes should make contact with different commutator segments. From 180 to 270 the current flow is the same as it was from 0 degrees to 90 degrees therefore same commutator segments should get in contact with the brushes to keep the current flowing in same direction. From 270 degrees to 360 degrees, the current direction reverse in the same fashion as it did from 90 degrees to 180 degrees, so same commutator segments should get in touches with the brushes.

We need 2 brushes and 4 segments commutator to get the shown waveform, and each segment would be 90 degrees apart.

Question:
I have a strong feeling that what I'm saying is wrong but honestly I don't see how to get the shown waveform any other way. If you also read below what I say about the case of four coils, you might be able to understand my confusion better. Could you please guide me where I'm having it wrong? Thank you.

armature_winding_1.jpg


Following what I have said above, in order to get the waveform shown below using four coils, there should be two brushes and eight commutator segments.

4_coils_correct_a.jpg
 
  • #19
Baluncore
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I think you are in transition from the absolute minimum to the simple practical, trying to use the old diagrams with the confusing annotations.

For a 4 pole motor, to produce 4 separate rectified pulses per turn, requires only a single coil, spanning cross connected commutator bars spaced 90°. A practical implementation will have 4 slots, with 4 coils, terminated on 4 commutator bars. Only 2 brushes, separated by 90° are needed to collect the waveform, but to generate full current from all 4 coils, there will need to be 4 brushes, cross-connected to the 2 external terminals.

To reduce the ripple, a duplicate set of 4 coils, in 4 slots, with 4 commutator bars, will be interleaved with the first phase. That second phase will be rotated in position from the first phase by 45°. The existing 4 brushes will be used alternately by both phases, to produce the 8 pulse per turn, lower ripple waveform.
 
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