Motoring Effect in DC Motors: Magnetic Field Interaction or Lorentz Force?

[cnh1995]

When current flows through the armature winding of a dc motor, what exactly happens in there that produces the torque? Is there a force due to the interaction between two magnetic fields or Lorentz force acts on the conductor electrons? Basically, why does motoring effect take place? Is it due to the magnetic field interaction or Lorentz force? Or are they one and the same?

 

[Hesch]

what exactly happens in there that produces the torque?

When current flows, a magnetic field in the airgap between rotor and stator is created. This magnetic field has an energy density = ½*B*H [ J/m³ ].

Commutating the motor currents so that a pole pair is kept apart from each other, the magnetic field will cross the airgap skew, thus taking up a lot of volume. Nature ( included the motor ) wants to get rid of magnetic energy, which could be done by rotating the motor, thereby closing up the pole pair and minimizing the volume of the magnetic field. To turn the motor, torque is needed ( nature knows all about that ). When the pole pair is closing up, the motor currents are commutated so that the distance between the poles is kept constant.

So torque is produced because the nature attempts to get rid of magnetic energy by reducing the volume of the magnetic field, containing energy density.

 

[cnh1995]

That means interaction of two fields leads to the torque?In regards to this interaction, I saw a diagram in which it was shown that flux at one end of the pole is enhanced and that at the other end is reduced so that a net force is developed. Is that correct?

 

[Hesch]

That means interaction of two fields leads to the torque?

How do you count two fields? Which field is the other field?

I saw a diagram in which it was shown that flux at one end of the pole is enhanced and that at the other end is reduced so that a net force is developed.

I don't know. I've not seen such a diagram.

 

[cnh1995]

The two fields ard due to field current and armature current.. The torque is proportional to the product of these two currents.. My question is basically about the force experienced by a current carrying conductor placed in magnetic field.. Does that force act on individual electron(s) or is it the result of the interaction of the two fields?

 

[Hesch]

Two magnetic fields can never overlap each other ( cannot interact ). Illustrating some magnetic field, lines/curves are often used. Two lines/curves can never cross each other. This is stated in Amperes law:

∫_circulation H⋅ds = N * I

This means that if you have a 3D compass and from some place follows a curve, you will always end up exactly the same place. If two curves were crossing each other, you could "jump" from one curve to the other and thus not ending up the same place.

Magnetic fields are circulation fields.

My question is basically about the force experienced by a current carrying conductor placed in magnetic field.

You must read the post #2 again. The wires are often "hidden" in grooves, and thus is not exposed to a magnetic field of significance: The motor will yield a torque anyway.

 

[cnh1995]

If the wires are hidden in the slots, what causes back emf?

 

[cnh1995]

Forget about motor..If a wire is held in between two poles and current is passed through it, it will have its own magnetic field which will be in circles around the wire. This will distort the original flux(N to S) in such a way that it will increase above the conductor and decrease below the conductor (assuming proper polarity and current direction). Won't this lead to a reasultant force upside down? If no such interaction happens, where's the force coming from?? Also, is its msgnitude BILsinθ??

 

[Jeff Rosenbury]

In Quantum Physics, electric forces are sometimes modeled as virtual photons. But following that answer leads down a rabbit hole. (What is "virtual"? How do photons interact on a quantum level? Etc.) If you wish to know more, get a good textbook on Quantum Electrodynamics.

Otherwise just accept that fields exist and the EM fields follow Maxwell's equations.

 

[jim hardy]

this question comes up often

try this thread, be sure to follow it to the end.

https://www.physicsforums.com/threads/is-counter-emf-a-good-thing.624785/

 

[cnh1995]

this question comes up often

try this thread, be sure to follow it to the end.

https://www.physicsforums.com/threads/is-counter-emf-a-good-thing.624785/

Thanks jim..That helped a lot.. Especially that cross product mechanism which takes place twice..
Now as I'm told that the magnetic fields do not interact, does this diagram make any sense?

Is this the effect of that cross product or what?? This is what I was talking about in #3..

 

[jim hardy]

Yes. That helped me. Glad you liked it.

If you can contort your right hand thusly

You'll have to rotate your hand from the way this sketch is...

index finger in direction of current
middle finger align with magnetic field , toward blue S pole
you'll find thumb pointing down and that's the force.

In freshman physics we were taught the memory aid

cross product: result has direction of a right handed screw rotated from first term into second.

Now as I'm told that the magnetic fields do not interact, does this diagram make any sense?

yes.
Now let the wire move down, repeat the hand gyration and you'll see counter-emf oppose current.

Magnetic fields don't interact ? Sounds cunterintuitive at first but think about it...
If i hold two magnets close to one another, the force i feel is exerted on the magnets .
Probably semantics - our mind tends to leap past the necessary baby steps of thinking .
Likewise in the motor the force is exerted on the wire, even if only because the charges are constrained to stay in it. .
This hyperphysics link is a bit more academic than my explanation, it introduces the idea of magnetic moment .
(unrelated to my 'senior moments')
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magmom.html

old jim

 

[cnh1995]

Yes. That helped me. Glad you liked it.

If you can contort your right hand thusly

You'll have to rotate your hand from the way this sketch is...

index finger in direction of current
middle finger align with magnetic field , toward blue S pole
you'll find thumb pointing down and that's the force.

In freshman physics we were taught the memory aid

cross product: result has direction of a right handed screw rotated from first term into second.yes.
Now let the wire move down, repeat the hand gyration and you'll see counter-emf oppose current.

Magnetic fields don't interact ? Sounds cunterintuitive at first but think about it...
If i hold two magnets close to one another, the force i feel is exerted on the magnets .
Probably semantics - our mind tends to leap past the necessary baby steps of thinking .
Likewise in the motor the force is exerted on the wire, even if only because the charges are constrained to stay in it. .
This hyperphysics link is a bit more academic than my explanation, it introduces the idea of magnetic moment .
(unrelated to my 'senior moments')
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/magmom.html

old jim

Well, if the field due to the current in the above diagram is somehow nullified, will it still push the wire down? Because there will still be the charges flowing in a magnetic field, leading to the cross product. What really pushes the wire? Is it the distorted magnetic field or the motion of charges in the magnetic field?

 

[jim hardy]

Well, if the field due to the current in the above diagram is somehow nullified, will it still push the wire down?

Nullified ? No field no force.
Realize that magnetic field in the figure will be affected noticeably when current in the wire produces its own field that's noticeably intense in comparison..

In rotating machines that's called "Armature Reaction" and searching on that will open a new world.

What really pushes the wire? Is it the distorted magnetic field or the motion of charges in the magnetic field?

I'm old school and think of it as the Lorenz force pushing the charges across the wire as in Hall effect.
They can't get past the insulation so that force is transferred to the wire.
Indeed the conductors in huge machines have to be securely wedged against magnetic forces.

Astute folks here on PF have corrected me, and rightly so for there do exist field equations more modern than my simplistic 19th century mechanical analogy.. Jeff hinted at that in post 9.
I leave Poynting vectors et al to people who are better versed than i in higher math.

old jim

 

[cnh1995]

Nullified ? No field no force.
Realize that magnetic field in the figure will be affected noticeably when current in the wire produces its own field that's noticeably intense in comparison..

In rotating machines that's called "Armature Reaction" and searching on that will open a new world.
I'm old school and think of it as the Lorenz force pushing the charges across the wire as in Hall effect.
They can't get past the insulation so that force is transferred to the wire.
Indeed the conductors in huge machines have to be securely wedged against magnetic forces.

Astute folks here on PF have corrected me, and rightly so for there do exist field equations more modern than my simplistic 19th century mechanical analogy.. Jeff hinted at that in post 9.
I leave Poynting vectors et al to people who are better versed than i in higher math.

old jim

Here is why my original question came up in the first place..I was studying indirect loading test on induction motor coupled to a dc generator. When the electrical load (lamp bank) was switched on, the motor on the other end slowed down. How does the motor know that it's been loaded?
The motor (rotor) feels an opposing mechanical torque in order to slow down. How's it generated? Then I thought, it might be due to the armature reaction in the generator but it is nullified by the interpoles. That leaves only the Lorentz force mechanism.. That's what I was talking about in #13. Field due to the current is nullified but still there are charges flowing in a magnetic field. Also, is it technically correct to say that armature reaction is desired in a motor but not in a generator?

 

[jim hardy]

Also, is it technically correct to say that armature reaction is desired in a motor but not in a generator?

Armature reaction in a machine seems at first glance a necessary evil.
It complicates an analysis that we would prefer remained simple.
But the clever giants on whose shoulders we stand figured out how to use it to advantage.

Realize that a motor and a generator are identical, the only difference being direction of power flow.
They added a third brush to generators in early automobiles to power the field. Armature reaction distorted the flux hence the current to the field which made them self regulating. Model A Ford(1932) is the last one of those I've personally encountered.
https://en.wikipedia.org/wiki/Third-brush_dynamo

Should you encounter one here's a practical page to know about
http://www.yesterdaystractors.com/articles/artint4.htm

of course the same applies to motor operation of the three brush machine.
Ford windshield wiper motors as late as the 90's had three brushes for better speed control.
If you peruse junkyards grab one and take it apart .

Automobile alternators are built with a lot of armature reaction which makes them current limiting.
I guess i was lucky to grow up before things got sophisticated -
my first alternator equipped automobile puzzled me because the voltage regulator had no current control coil.
Only when i took AC machinery course did i understand why. In fact it was Prof Gross, mentioned in another thread, who helped me understand the Lundell automotive alternator. It's just a three phase synchronous machine with a rectifier added.

Synchronous machines are inherently current limited by armature reaction.
The terms to search would be "Synchronous Impedance" and "Short Circuit Ratio".

But you're studying DC machines, right ?

Field due to the current is nullified

The field isn't nullified.
Draw your vectors and you'll see the armature's MMF is in direction perpendicular to that of field.
At low current there's hardly any effect, observe that in your right triangle a small opposite side doesn;t affect the hypotenuse very much.
As current increases the hypotenuse and adjacent side separate changing the direction of the field.
So the field is shifted and it's the job of interpoles to put it back where it belongs.
Three brush machines omit the interpoles and use that shift for current control. It affects voltage at commutator over where the third brush is located , changing excitation to the field coil..
It's beautifully simple. The old principle of designing so Mother Nature helps you out.

I hope this helps you with the concept.
I apologize for bringing a 'junkyard wars' meme to PF . But one learns an awful lot by working on machinery, and that's why lab courses are so vital to a university curriculum.
Glad you were curious.

old jim

 

[cnh1995]

Armature reaction in a machine seems at first glance a necessary evil.
It complicates an analysis that we would prefer remained simple.
But the clever giants on whose shoulders we stand figured out how to use it to advantage.

Realize that a motor and a generator are identical, the only difference being direction of power flow.
They added a third brush to generators in early automobiles to power the field. Armature reaction distorted the flux hence the current to the field which made them self regulating. Model A Ford(1932) is the last one of those I've personally encountered.
https://en.wikipedia.org/wiki/Third-brush_dynamo

Should you encounter one here's a practical page to know about
http://www.yesterdaystractors.com/articles/artint4.htm

of course the same applies to motor operation of the three brush machine.
Ford windshield wiper motors as late as the 90's had three brushes for better speed control.
If you peruse junkyards grab one and take it apart .

Automobile alternators are built with a lot of armature reaction which makes them current limiting.
I guess i was lucky to grow up before things got sophisticated -
my first alternator equipped automobile puzzled me because the voltage regulator had no current control coil.
Only when i took AC machinery course did i understand why. In fact it was Prof Gross, mentioned in another thread, who helped me understand the Lundell automotive alternator. It's just a three phase synchronous machine with a rectifier added.

Synchronous machines are inherently current limited by armature reaction.
The terms to search would be "Synchronous Impedance" and "Short Circuit Ratio".

But you're studying DC machines, right ?The field isn't nullified.
Draw your vectors and you'll see the armature's MMF is in direction perpendicular to that of field.
At low current there's hardly any effect, observe that in your right triangle a small opposite side doesn;t affect the hypotenuse very much.
As current increases the hypotenuse and adjacent side separate changing the direction of the field.
So the field is shifted and it's the job of interpoles to put it back where it belongs.
Three brush machines omit the interpoles and use that shift for current control. It affects voltage at commutator over where the third brush is located , changing excitation to the field coil..
It's beautifully simple. The old principle of designing so Mother Nature helps you out.

I hope this helps you with the concept.
I apologize for bringing a 'junkyard wars' meme to PF . But one learns an awful lot by working on machinery, and that's why lab courses are so vital to a university curriculum.
Glad you were curious.

old jim

Ok..I guess I've got a hang of it..And what's your idea about that induction motor indirect loading? How does that work?

 

[cnh1995]

Indirect loading ? Dont know what you mean by that.

Ever play with two magnets through a wood table? Hold one against bottom of table and drag the top oe around?

A motor is two electromagnets , one dragging the other around a shaft.

Synchronous motor is easy, its rotor is a DC electromagnet, in fact a permanent magnet will work (remember Garrard "Synchro-Lab" turntables?)

http://www.embedded.com/design/indu...ield-oriented-control-of-a-brushless-DC-motor

induction motor is similar, but the magnetic field of the rotor is induced there by its turning a little bit slower than the stator field that's dragging it around. That's called "slip".

The rotor conductors resemble a squirrel cage
http://hsc.csu.edu.au/physics/core/motors/2698/Phy935net.htm
as you doubtless know
http://hsc.csu.edu.au/physics/core/motors/2698/inductionmotor.gif

and the current induced in them makes a magnetic field that's not quite stationary with respect to rotor.
It is dragged around by the stator field.

Here's an animation
http://ffden-2.phys.uaf.edu/212_spring2011.web.dir/patrick_brandon/Tesla's Alternating Current.html

but they got slip the wrong way !
They show rotor moving faster than stator magnetic field, and rotor bars not being cut by motion relative to flux.
They should have locked the black flux lines together and rotated the rotor a little slower than the flux lines.anyhow - rotor bars carry current in presence of a magnetic field.
Does that mental image help?

If you overspeed an induction motor, like they did in that image, the rotor tries to speed up the stator field and it becomes a generator returning power to the grid. Search "Induction generator".

Indirect loading means not applying direct mechanical load. Induction motor is coupled to a dc generator via a shaft. Now if the motor is run, it will generate emf in the generator. When the electrical load at the generator end is switched on, it will draw current from the generator armature. That's how the motor has been loaded indirectly. This will slow down the motor as it feels a load torque. My question was how does it know that its been loaded??

 

[jim hardy]

My question was how does it know that its been loaded??

Force on the rotor bars is in proportion to the current induced in them
by relative motion of rotor bar to rotating field (slip).
To get more torque requires more slip so the rotor slows down.

Since you have a lab setup
shine a florescent light on the shaft and watch very carefully. (old fashioned fluorescent tube with magnetic balast , not a CFL)

You will see , albeit faintly, stroboscope effect. Specks of dirt and rust on the shaft will appear to stand almost still.
If you have a strobe light(Strobotac) that you can synchronize to the line so much the better.

When you get the hang of it, you can watch slip. A lightly loaded motor might take almost a minute to lose a whole turn with respect to the power line.

That's slip.
This curve shows that torque reverses when rotor field overtakes stator field and it becomes a generator.
n_s is line frequency rpm - for 60 hz 1800 or 3600 or 1200 or 720 or whatever.
http://www.expertsmind.com/topic/induction-motors/torque-slip-characteristics-917822.aspx

Figure: Torque-Slip Characteristics


The torque is zero While slip is zero; (Nr = Ns). We have already discussed that induction motor torque is zero at synchronous speed. At slip Sm, T; the motor gives the maximum torque that is also called as break down torque. While the rotor is stand still (S = 1), the torque corresponds to beginning torque Ts. In normally designed motor Ts is much less than Tmax (Break down torque). The torque slip characteristics from no-load to rather beyond full-load are almost linear.

old jim

 

[jim hardy]

updated slip animation

https://en.wikipedia.org/?title=Induction_motor

note it takes only a little slip for stator field to cut rotor bars
rotor current induced thereby is low frequency but not DC.

 

[cnh1995]

updated slip animation

https://en.wikipedia.org/?title=Induction_motor

note it takes only a little slip for stator field to cut rotor bars
rotor current induced thereby is low frequency but not DC.

Slip decides torque..So the motor slows down to provide more torque..When the load is mechanical, the shaft will feel opposition and slow down. But when the load is electrical (as per my previous post), how does the motor realize "it's time to slow down"? What feedback goes to the motor from the generator? When electrical load cosumes power,
1) it draws current from the generator and
2)motor slows down at the same time.
What is the link between these two? What synchronizes them? Is it the Lorentz force on the generator conductors?

 

[jim hardy]

What is the link between these two? What synchronizes them?

I don't understand the question.
are they not connected by a mechanical shaft ?

Draw a free body diagram ot the two rotors.
They comprise one rigid structure ,
for which sum of torques = angular acceleration/moment of inertiaLoading the generator raises the torque required to spin it

any machine with more power flowing out than in will slow down
until the sum of torques is zero

horsepower = 2pi X TORQUE X (RPM/33,000)

The feedback is Newton's laws of motion

 

[jim hardy]

we are still on this system, yes ?

I was studying indirect loading test on induction motor coupled to a dc generator. When the electrical load (lamp bank) was switched on, the motor on the other end slowed down. How does the motor know that it's been loaded?

 

[stedwards]

View attachment 84979

The problem with this diagram is that once the motor is rotating the free charges in the wire are moving, for the most part, in the direction shown for the force. Anyone else notice that? The force on the wire is now rotated a bit less than 90 from that shown. The typical drift velocity of a free electron in a wire is typically about one meter per hour. Wouldn't it seem that the majority of force on the wire is axial?

 

[jim hardy]

The typical drift velocity of a free electron in a wire is typically about one meter per hour. Wouldn't it seem that the majority of force on the wire is axial?

perhaps. But the wire is not free to move that direction

And the force is exerted on the charges inside the wire.
What's charge density in copper?
A lot of charges are pushing sideways against the considerable area of the insulation, not so many against the substantially smaller area of wire ends.What path do the charges take in a Faraday disk?

 

[cnh1995]

I don't understand the question.
are they not connected by a mechanical shaft ?

Draw a free body diagram ot the two rotors.
They comprise one rigid structure ,
for which sum of torques = angular acceleration/moment of inertiaLoading the generator raises the torque required to spin it

any machine with more power flowing out than in will slow down
until the sum of torques is zero

horsepower = 2pi X TORQUE X (RPM/33,000)

The feedback is Newton's laws of motion

I know the mechanics of the motor according to Neweton's laws but my problem is loading of the generator. In direct mechanical loading of the motor, rotor slows down and I understand it well. But when an electrical load is applied at the generator end, how does it load the motor on the other end?? How does the armature current in the generator make the motor slow down?? Motor responds to mechanical load because it "feels" it. So here,what is the link between electrical loading(which is actually done) and mechanical loading(which is felt by the motor)??

 

[jim hardy]

How does the armature current in the generator make the motor slow down??

Is energy conserved in the generator?
When the lamp bank is turned on, does the generator supply that energy?
Must not that energy go into the generator as mechanical so it can come out as electrical?
What is the formula for mechanical power into the generator?
Does not shaft horsepower = 2pi X torque X rpm/33000 irrespective of whether the machine is an electric motor, generator, waterwheel or steam engine?

If the generator suddenly becomes harder to turn will the shaft slow down ?

 

[cnh1995]

Is energy conserved in the generator?
When the lamp bank is turned on, does the generator supply that energy?
Must not that energy go into the generator as mechanical so it can come out as electrical?
What is the formula for mechanical power into the generator?
Does not shaft horsepower = 2pi X torque X rpm/33000 irrespective of whether the machine is an electric motor, generator, waterwheel or steam engine?

If the generator suddenly becomes harder to turn will the shaft slow down ?

Yes, it will slow down. But what makes the generator harder to turn? How does electrical loading make the shaft mechanically slow?? Does current in the generator exhibit motoring effect and oppose the rotation? If yes, what's the mechanism? Is the Lorentz force or magnetic field distortion(as seen in the diagram I've posted earlier)?

 

[jim hardy]

Does current in the generator exhibit motoring effect and oppose the rotation?

Exactly.
You can apply the right hand rule and see that one machine will both generate and motor.
In a DC machine all that happens is armature current and torque both reverse directions.

If yes, what's the mechanism?

I use Lorentz . In a traditional DC machine's armature the force is exerted on the conductors.

I acknowledge there are vector calculus ways to analyze using magnetic stress tensors , Poynting vectors etc

but for everyday work one does not have to be so expert at higher math to have a working mental image of the motor.

]Is the Lorentz force or magnetic field distortion(as seen in the diagram I've posted earlier)?

This author expresses torque in terms of magnetic fields
https://en.wikipedia.org/wiki/Maxwell_stress_tensor

If the field is only magnetic (which is largely true in motors, for instance), some of the terms drop out, and the equation in SI units becomes:

For cylindrical objects, such as the rotor of a motor, this is further simplified to:

where r is the shear in the radial (outward from the cylinder) direction, and t is the shear in the tangential (around the cylinder) direction. It is the tangential force which spins the motor. Br is the flux density in the radial direction, and Bt is the flux density in the tangential direction.

But what makes the generator harder to turn?

I guess with the spring loaded belt tensioners of modern cars it is no longer intuitive that a generator gets hard to turn when you load it.
I'm from the days when a loose fan belt would screech until the battery quit accepting high current from the generator.

So i apologize if i seemed short with you...

 

[cnh1995]

Exactly.
You can apply the right hand rule and see that one machine will both generate and motor.
In a DC machine all that happens is armature current and torque both reverse directions.I use Lorentz . In a traditional DC machine's armature the force is exerted on the conductors.

I acknowledge there are vector calculus ways to analyze using magnetic stress tensors , Poynting vectors etc

but for everyday work one does not have to be so expert at higher math to have a working mental image of the motor.

This author expresses torque in terms of magnetic fields
https://en.wikipedia.org/wiki/Maxwell_stress_tensor

I guess with the spring loaded belt tensioners of modern cars it is no longer intuitive that a generator gets hard to turn when you load it.
I'm from the days when a loose fan belt would screech until the battery quit accepting high current from the generator.

So i apologize if i seemed short with you...

Seems that it's not as straightforward as it looks in the diagram.. Thank you very mucha for your patient replies.. I hope I didn't bore you with this long thread..

 

[jim hardy]

Seems that it's not as straightforward as it looks in the diagram.. Thank you very mucha for your patient replies.. I hope I didn't bore you with this long thread..

not at all, i wondered what you were driving at.

There's always several ways to teach a subject.
My vector calculus is rusty .
Somebody who's fluent in it might prefer that approach.
old jim

 

[cnh1995]

not at all, i wondered what you were driving at.

There's always several ways to teach a subject.
My vector calculus is rusty .
Somebody who's fluent in it might prefer that approach.
old jim

Just one last question..If I placed another wire very close to the original wire in the diagram and sent current through it in the opposite direction, the resultant magnetic field due to both the currents will be almost 0. Will there be a force on the wire?? There won't be any distortion of flux as seen in the original diagram. Will Lorentz force act on them??

 

[jim hardy]

Not trying to duck your question here...

I assume your two wires are physically kept close together by something like glue , or their innate rigidity ? If not the geometry will change and the problem becomes dynamic.

I'll answer that each wire will experience a vertical force from its Lorentz force effect and those forces of course sum to zero. (ignore the torque couple for now)

and there's an additional force on each wire from the other one
http://hyperphysics.phy-astr.gsu.edu/hbase/magnetic/wirfor.html
of course you postulated current in opposite directions not same direction as in this figure - so wouldn't that force be repel not attract ?

I've been thinking nights about your questions

and they make me ponder what is nature of space that force is transmitted between objects over a distance by magnetic and electric and gravity fields?
Higher math is able to describe the interactions out there in free space not just at the objects.

But we can only sense the resulting force at an object like a wire or a charged piece of matter where we can attach a force measuring device..
I need an aether.

I think that's why i stick with my 19th century analogies - my algebra is so prone to mistakes that i have to visualize the forces before i'll believe the math.
That is a handicap. I really advise you to master Vector Calculus and Maxwell's equations, they've become the coin of the realm.

old jim

 

[jim hardy]

ps thanks for your patience with me .

 

[cnh1995]

ps thanks for your patience with me .

Thanks a lot..:-)I think its time for me to start working on higher math..

 

[jim hardy]

I think its time for me to start working on higher math..

There are plenty of extraordinarily capable people here at PF ...

I'll not pretend I'm qualified to teach vector calculus.

Still learning something every day --

old jim