# I have a 2-part question about electromagnetism and motors

• I
So, I'm working with a DC shunt wound motor and I want to calculate the actual voltage production on paper mathematically before building. I have been studying the biot-savart law and understand it to a fair degree. One of my questions is after you get your measurements in Teslas from said law do you convert that number to Webers to find the flux density? And if so what is the easiest unit conversion to understand?

Next question, what formula do you use to find out how many volts are produced after cutting through these lines of flux?

Thanks

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NascentOxygen
Staff Emeritus
So, I'm working with a DC shunt wound motor and I want to calculate the actual voltage production on paper mathematically before building.
Hi James Birt.

You're constructing a motor from scratch?

James Birt
Trying to, I'm a journeyman electrician and my step-dad is a mechanic with 20+ years experience. We are both working on it and have different understandings of how it works... well, fundamentally the same I guess. We are both interested in attempting it though. It is not going to be a large scale project, the motor will be relatively small with a small supply.. I'm just having slight difficulties with the math and would like to have the ability of knowing it would work before investing the time it will take to accomplish the task.

NascentOxygen
Staff Emeritus
You're constructing an armature using laminated steel, with slots, and a commutator, carbon brushes, and all? I'm impressed!

Something slightly larger than a slot car motor?

I hope there is someone here who can advise you further.

James Birt
Yes, that size or slightly larger but you're in the ballpark. I'm just having issues determining if I'll get the right voltage output so I've been studying magnetomotive force and flux density but the math is complex.... hey, speaking of the commutator, you know the gap where the brushes jump? I'm sure you do, what is the best way to tell how much the flux is disrupted during the change in cycle? An oscilloscope maybe?? Thanks, and thanks to anyone else that can shed some light on the subject.

The commutator is retarded with respect to the geometric neutral plane. You manually adjust the offset angle by trial and error until arcing is minimized.

...what formula do you use to find out how many volts are produced after cutting through these lines of flux?
Back EMF = (1/2) * (angular speed) * (number of turns) * (rotor radius) * (average magnetic flux) * (conductor length)

James Birt
What formula do you use to find average magnetic flux?? I can get the other numbers, and thanks for the feedback I'm stuck on this.

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Do you have a flux meter?

James Birt
No, I do not. Did you see my first post about unit conversion? I would really like to know the math.

...after you get your measurements in Teslas from said law do you convert that number to Webers to find the flux density?
Measurements are data obtained from experiments.
When you apply a physical law, however, the quantity you seek drops out the bottom of an equation.

Before you proceed, review which quantity corresponds to which unit
(square brackets enclosing a physical quantity mean the unit with which said quantity is measured):

[magnetic flux] = weber
[magnetic flux density] = tesla

The formula is:
(flux density) * (flux area) = flux

James Birt
Ok, I have been using the biot-savart law and that law is for magnetic fields and comes out in Tesla units. So, if the flux density is measured in Teslas then that should be the number I need. Thank you very much for clarifying this for me! I can now move forward with my calculations and hopefully construction very soon. Thank you again Mr.Lewis, I can't express to you how helpful that information is.

NascentOxygen
Staff Emeritus
Do you know the relative permeability of the steel you'll be using, James?

How many commutator segments are you planning?

James Birt
Using an iron core, (2.5x10^-1). As far as commutators, from what I've seen, 2 is a pretty common number on motors the size I'm talking about putting together with many windings.

If the relative permeability of the iron is 0.25 then it has 4 times less permeability than free space.

NascentOxygen
Staff Emeritus
Using an iron core, (2.5x10^-1). As far as commutators, from what I've seen, 2 is a pretty common number on motors the size I'm talking about putting together with many windings.
I think there are "issues" with the use of two commutator segments, including the shorting out of the supply unless the commutator gaps are wider than the brushes. And with wide gaps the motor will sometimes need a helping hand to start it rotating.

A motor with two-segment commutator would make a great demonstration machine, though. Are you intending your machine to do useful work, or just serve as a functional demo?

James Birt
No, I need the motor to function... what would you advise? I'm sure you've worked with them before, I have worked with them enough to understand their function but not enough to easily understand their construction. I know the gap in the commutator can cause interference in the magnetic flux and smaller magnets can be used to offset that particular variable but if I could avoid having to do that all together I would rather. Is there a better way?

NascentOxygen
Staff Emeritus
I think you'll find what you need from further web searches---more details about practical motor design. It's outside my area, I'm afraid.

Are you sure you want a shunt motor, as it seems to me that a permanent magnet motor would, in principle, be simpler. (Though home-building an efficient motor with permanent magnets may not be a realistic proposition.)

jim hardy
Gold Member
2019 Award
Dearly Missed
Using an iron core, (2.5x10^-1).
that's good iron.
0.25/μ0 = almost 200,000 relative permeability

If you want a working, useable motor....
Before making one from scratch
i'd suggest you fellows take apart an old DC generator or motor and take careful physical measurements .
Study how its armature is wound. , and the field coils.
Figure out how they arrived at number of turns on field
and trace the wires around the armature.

Junkyards are full of old cars. Most are too new to have a generator
but a DC motor and generator have no practical difference
and there are fans motors, windshield wiper motors, electric seat and electric window motors

and of course starter motors. Converting a starter from series to shunt field would be a good exercise for you guys
and you'll learn nearly as much , maybe more because you won't get discouraged machining that tedious copper segmented commutator
Old Chrysler starters (early 60's) had two fields - a series field to provide extreme torque for those big B block V8's and a shunt field to limit RPM in case the Bendix drive gear fails to engage.

Here's a video of a guy replacing fields in a generator...

http://www.bing.com/videos/search?q...033C9AECEAA50410C9D0033C9AECEAA50&FORM=VRDGAR

learn what are meaning and units of
magnetomotive force MMF , Ampere-turns
flux Φ , Webers
Reluctance ℝ Amp-turns per Weber

e = blvsinθ, voltage induced in a wire = b(Flux Density) X l(ength of wire) X v(elocity) X sin(angle between Velocity and Flux)
http://ibphysicsstuff.wikidot.com/electromagnetic-induction

old jim

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NascentOxygen
I misunderstood what you were asking, it will have only one commutator.. thought about self-excited but may be unnecisary. I'll take a look at some other motors and see what makes em tick.. that's an excellent idea. I have plans for the one I want to put together though, but getting a better understanding of existing ones does seem like the most logical starting point. I'll check out those videos in the morning and get back to you. Thank you all for your input, it's been very very helpful.

And the iron I want to use is 95% pure, but it also depends on cost if I go with it or not. The magnetic permeability isn't hard to figure out though.

jim hardy
Gold Member
2019 Award
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And the iron I want to use is 95% pure, but it also depends on cost if I go with it or not. The magnetic permeability isn't hard to figure out though.
A motor must have an air gap and that will dominate your magnetic circuit
so don't obsess over exotic ultra-permeable iron, for any reasonable sized motor the bulk of your MMF will be expended in the air gap.

James Birt
I was assuming that's why the distance in the biot-savart law exists as to know the gap factor.. wouldn't making the MMF stronger at the source give you a stronger field in the gap? Or will it dissipate at the same rate regardless of the initial force?

The commutator is retarded with respect to the geometric neutral plane. You manually adjust the offset angle by trial and error until arcing is minimized.

Back EMF = (1/2) * (angular speed) * (number of turns) * (rotor radius) * (average magnetic flux) * (conductor length)
Copper resistance at cold and hot not taken into account and friction losses are practical variants apart from room temp.. which can alter the calculations I think.

James Birt
jim hardy
Gold Member
2019 Award
Dearly Missed
wouldn't making the MMF stronger at the source give you a stronger field in the gap?
yes, i'm glad you see that.....
and the smaller the gap the easier to establish good flux density

the "source" you mention is the field winding (or field magnet in a permanent magnet motor).
Its MMF 'pushes" flux around the magnetic circuit which is mostly iron but with a small air gap for mechanical clearance.
Flux passes easily through iron but not so easily through air.

rotor not shown
source http://www.rfcafe.com/references/El...ty-basic-navy-training-courses-chapter-14.htm