# DC motor working principle help

#### Autobike

Joined Feb 23, 2018
52
hello i found a nice old video which explains the working principle of a DC motor. here you can see the video. i have the basic idea of how a DC motor works. but after watching this video i got a question.

if we refer the below example, when we apply current. the winding becomes an electromagnet. so if we apply the right hand rule, the AB part becomes the North pole and the CD part becomes the South pole. so similar poles meet each other and they repel each other. due to that the rotation happens. when the winding becomes perpendicular to the permanent magnetic field ( just passing 90 degree angle) it begins to attract to the opposite pole. we can continue the rotation by using two commutator segments and two brushes. in that video at 4:53 he explains it.

but if we refer this example, where we change the polarity of the voltage and then apply current to the winding. the AB part becomes the South pole and CD part becomes the North pole. so the opposite poles meet each other and they attract each other. in above video he explains it at 4:41. we can clearly see that the winding doesn't rotate.

the first example rotates that's true. but practically if we change the voltage polarity (as the second example) we know that the DC motor should begins to spin in reverse direction. if we apply Fleming's left hand rule we can see that too. but in that video it doesn't happen.

do DC motors work only by repelling and attracting to each poles (as the 1st example)? like when the winding is parallel to the permanent magnetic field, are the poles supposed to be similar (poles of permanent magnet and winding) to make a rotation? thank you.

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#### profbuxton

Joined Feb 21, 2014
410
While the diagram shown gives a VERY simplified explanation of the workings of a DC motor, please be aware that in a real motor there are MANY coils connected to many commutator segments so that there will be current carrying coils at different angles to the poles which will react to give a motive impulse to the armature(rotating part). About the simplest DC motor you can have a look at is a small model motor which usually has THREE poles and THREE commutator segments so that the armature fields do not simply "lock" into position with the field magnets.

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#### Dodgydave

Joined Jun 22, 2012
8,374
Usually in a DC motor there are 3 comutators in condction, which gives 3 poles of flux, one coming out, one in max flux, and one coming in, to give a smoother rotation.

• Autobike

#### Alec_t

Joined Sep 17, 2013
10,355
if we apply the right hand rule
For motors we use the left-hand rule.

• Autobike

#### ci139

Joined Jul 11, 2016
701
the B vector field around straight wire
+ http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html  ← the B is the external 1 here
↑ your (external) B is from N→ to →S here ↑
so the F direction is different Last edited:
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#### Autobike

Joined Feb 23, 2018
52
While the diagram shown gives a VERY simplified explanation of the workings of a DC motor, please be aware that in a real motor there are MANY coils connected to many commutator segments so that there will be current carrying coils at different angles to the poles which will react to give a motive impulse to the armature(rotating part). About the simplest DC motor you can have a look at is a small model motor which usually has THREE poles and THREE commutator segments so that the armature fields do not simply "lock" into position with the field magnets.
thank you yea the actual motor is more complex. i've disassembled my starter motor in my bike and experienced it Usually in a DC motor there are 3 comutators in condction, which gives 3 poles of flux, one coming out, one in max flux, and one coming in, to give a smoother rotation.
thank you For motors we use the left-hand rule.
sorry i mean i applied the Right hand thumb rule. the rule which shows here. i attached a picture below.
so in my first example AB gets North pole and CD gets South pole. am i correct ? thank you. the B vector field around straight wire
+ http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/elemag.html  ← the B is the external 1 here
↑ your (external) B is from N→ to →S here ↑
so the F direction is different thank you very much. it's helpful #### Autobike

Joined Feb 23, 2018
52
sorry if i confused you by not explaining it clearly. but if i make it simple, the reason for the question lies in what we see here at 4:41
we can clearly see that the winding doesn't rotate. but in theory it should rotate and practically a DC motor rotates either way depending on the current flow.
we can say that there are only two armature segments. but the same thing happens even if there are 4 segments like the image below. doesn't it? in above situation if we consider the "coil 1" the electromagnet S aligns with permanent S. electromagnet N aligns with permanent N. we can see that in the video too and it rotates. but if we reverse the voltage polarity the direction of the current changes. so in this case if we do so, the electromagnet S aligns with permanent N. then electromagnet N aligns with permanent S. in that video this kind of situation doesn't make a rotation. that's what i'm trying to understand. in theory and practically it should make a rotation. thank you.

#### KeithWalker

Joined Jul 10, 2017
413
The two pole motor shown in the example is very simplified but it will actually run in either direction. It does need to be started by twisting the shaft but once started, it will continue to run in the same direction.
I built one like it in my early teens from a book called "The Boy Electrician which I found it on my father's bookshelf. It was very educational. It is a very old book, first printed in 1913. Because of that, the information is in very basic form which makes it very easy to understand.
If you are interested enough I recommend that you try making one for your self. It's the best way to learn. I have attached a url to the book. Chapter 17 is about motors and the one I built is on page 304:
http://rawfire.torche.com/~opcom/tbe/the_boy_electrician.pdf

• Autobike
thank you for your information. i'm downloading the book. appreciate your help 