Mechanically driven generator & inductive resistance to torque

[MasonJ1989]

I have found research from multiple sources and probably shouldn't question but I want to be clear.
Inside a generator you have a fixed number of poles and a fixed number of induction coils, the particulars are not important. As a pole rotates past the "inductor" which holds the coil there is an inductive friction between the magnet and ferrous material, the friction is a part of the overall resistance to the driving mechanical torque.
Through their research it's suggested that the drag on the rotor is actually decreased as rpm is increased?
If true perhaps not much at a "lower" rpm but significant at a much greater rpm.

 

[jim hardy]

Words are an attempt to convey an image that's in somebody's mind.
The words get translated and paint an image in the listener's mind.

It is impossible to figure out from a secondhand conversation that's severely truncated what was in the mind of the original speaker.

Have you ever played with magnets on opposite sides of a tabletop? By moving the one underneath you can drag the one on top around.
That's magnetic attraction not friction because friction makes heat.

There are eddy currents induced in armature iron that do make heat , one might consider them analogous to friction.

Through their research it's suggested that the drag on the rotor is actually decreased as rpm is increased?

That's a statement and should end with a period.
A question well phrased is halfway answered. What are you asking ?

 

[MasonJ1989]

Yes so without antiquated speech let's keep it simple because we are discussing a concept of simplicity. As the magnet is attracted to the induction coil and ferrous core the rotor moves in that direction after it passes the core the magnets are still attracted to the core thereby creating a drag force which is analogous to resistance considering it is resisting the motion of torque and resistance could be interpreted as friction just depends on thesaurus

 

[jim hardy]

You must have something in mind. Perhaps you'll decide to convey it. Sounds as if it might be related to reluctance effect in motors.

 

[Dr.D]

antiquated speech

Just what "antiquated speech" did you have in mind? I have not seen anything above that fits that description.

You might want to be careful because it appears pretty clear that you don't have a real good understanding of this topic. You will not find anyone here or elsewhere that knows more about motors and generators than Jim Hardy, so listen up and pay attention.

 

[MasonJ1989]

Let me try being more punctual with terminology. Inside of every rotary generator AC current is developed and depending on slip ring setup you can derive either AC or DC. Inside the same generator you have a rotor and a stator and stator coils. For a permanent magnet generator where the rotor holds the permanent magnets there is a magnetic attraction to the stator coils which are wound around a core usually of iron however any ferrous material could be included. This magnetic attraction will cause a friction or a frictional force or a force that is resistant or resisting the momentum or speed of the rotor. So the question is and has been the same; if the rotor moves faster or with more RPM will that attractive magnetic force that resists the movement of the rotor be equal or less then at a lower speed or RPM?

 

[Tom.G]

Can we try this approach?

Given:
Some ferrous material fixed in position
A permanent magnet approaching, nearing, then receding from but never touching, the ferrous material

Question:
Does the travel speed of the magnet affect the net attractive force between the magnet and the ferrous material?
(The question is to be interpreted in the context of speeds and distances 'typical' of a 'small' alternator, perhaps less than 1,000 Watts.)

p.s. There seems to be a difference in native languages that is impacting our usually helpful responses.

 

[Dr.D]

This magnetic attraction will cause a friction or a frictional force or a force that is resistant or resisting the momentum or speed of the rotor.

This is simply incorrect.

 

[jim hardy]

So the question is and has been the same; if the rotor moves faster or with more RPM will that attractive magnetic force that resists the movement of the rotor be equal or less then at a lower speed or RPM?

Doesn't that depend on what current is flowing in the coils surrounding the poles?

There is more than one effect in play.

With zero current reluctance effect is enough that indeed in some motors you can feel with your fingers. Grab a stepper motor and try it.
With a synchronous generator there may or may not be such an effect depending on power factor and pole-slot arrangement.

Magnetization of iron is not instantaneous, it propagates from outside in. So there may be some frequency dependence in effects of cogging from reluctance..
You'll have to define your experiment to discuss it.

I'm no longer young enough to know everything so am quitting this thread.

Good luck.

old jim

 

[MasonJ1989]

<< Mentor Note -- Post edited to remove insult >>

You guys don't do concepts well. I can interpret your jargon but you act as if my language is a bottomless pit.

A magnet near any ferrous material will have an attraction, a stronger attraction as you approach the material, considering the enormous range of materials, forces, and distances, I would not think you guys would nit pick small details. I just want to know if relative motion of a fixed permanent magnet at a fixed distance from a ferrous material feels less attraction as it moves faster.

 

[Dr.D]

relative motion of a fixed permanent magnet at a fixed distance from a ferrous material feels less attraction as it moves faster.

What is the relative motion between two fixed objects (the fixed permanent magnet and the ferrous material at a fixed distance)? If both are fixed, how can there be relative motion?

We are not speaking "jargon." We are attempting to speak in a way that correctly describes what we are talking about. You might try it sometime; it does wonders for clarity of thought.

I think Jim had the right idea. We are too old to know everything, so it is time to leave this thread. I'm out as well.

 

[RonL]

This might be what the OP was trying to talk about ?
I think the thread is done, but thought this might be of interest to someone.

https://en.wikipedia.org/wiki/Brushed_DC_electric_motor

Compensation for stator field distortion[edit]
In a real dynamo, the field is never perfectly uniform. Instead, as the rotor spins it induces field effects which drag and distort the magnetic lines of the outer non-rotating stator.

Exaggerated example of how the field is distorted by the rotor.

Iron filings show the distorted field across the rotor.
The faster the rotor spins, the further the degree of field distortion. Because the dynamo operates most efficiently with the rotor field at right angles to the stator field, it is necessary to either retard or advance the brush position to put the rotor's field into the correct position to be at a right angle to the distorted field.

Centered position of the commutating plane if there were no field distortion effects.

Actual position of the commutating plane to compensate for field distortion.
These field effects are reversed when the direction of spin is reversed. It is therefore difficult to build an efficient reversible commutated dynamo, since for highest field strength it is necessary to move the brushes to the opposite side of the normal neutral plane.

The effect can be considered to be somewhat similar to timing advance in an internal combustion engine. Generally a dynamo that has been designed to run at a certain fixed speed will have its brushes permanently fixed to align the field for highest efficiency at that speed.[1]

 

[jim hardy]

So the question is and has been the same; if the rotor moves faster or with more RPM will that attractive magnetic force that resists the movement of the rotor be equal or less then at a lower speed or RPM?

At last, a question.

I held my strong fridge magnet close to the fridge and waved it back and forth. Couldn't feel any difference in attractive force.

In a magnetic brake drag at higher relative speed will be greater.
https://www.princeton.edu/ssp/joseph-henry-project/eddy-currents/eddy_wiki.pdf

Eddy current loss goes up with frequency.
In a generator the field strength would be reduced as speed increases so as to keep terminal volts within reason, countering the braking force from eddy currents..
Generators are laminated so as to minimize eddy current loss and thickness of the laminations determines to what frequency are they effective.. So which effect wins out, increased frequency or reduced field, depends on how the machine is built and operated.

I recommend to you Lavoisier's Preface to Treatise on Chemistry.. It's less about chemistry than about how to communicate thoughts.

https://web.lemoyne.edu/giunta/lavpref.html

"Instead of applying observation to the things we wished to know, we have chosen rather to imagine them. Advancing from one ill founded supposition to another, we have at last bewildered ourselves amidst a multitude of errors. These errors becoming prejudices, are, of course, adopted as principles, and we thus bewilder ourselves more and more. The method, too, by which we conduct our reasonings is as absurd; we abuse words which we do not understand, and call this the art of reasoning. When matters have been brought this length, when errors have been thus accumulated, there is but one remedy by which order can be restored to the faculty of thinking; this is, to forget all that we have learned, to trace back our ideas to their source, to follow the train in which they rise, and, as my Lord Bacon says, to frame the human understanding anew.

This remedy becomes the more difficult in proportion as we think ourselves more learned..."

 

[berkeman]

Thread closed temporarily for Moderation...

 

[berkeman]

Thread re-opened after some edits.

 

[Problemsolver]

Words are an attempt to convey an image that's in somebody's mind.
The words get translated and paint an image in the listener's mind.

It is impossible to figure out from a secondhand conversation that's severely truncated what was in the mind of the original speaker.

Have you ever played with magnets on opposite sides of a tabletop? By moving the one underneath you can drag the one on top around.
That's magnetic attraction not friction because friction makes heat.

There are eddy currents induced in armature iron that do make heat , one might consider them analogous to friction. That's a statement and should end with a period.
A question well phrased is halfway answered. What are you asking ?

Well Jim, actually the magnets rubbing against the table does produce friction.

 

[jim hardy]

Well Jim, actually the magnets rubbing against the table does produce friction.

I think OP was inquiring about the force that's behind electro-mechanical energy conversion.
Without resorting to high math or machine theory
that force is in proportion to the strength of the magnetic fields involved. He calls it 'friction' but it's more akin to the torque that a dipole 'feels' in a magnetic field, or the force you feel when bringing magnets close to one another as in a tabletop tug-of-war.

He didn't describe his thought experiment in enough detail to give a meaningful answer. That's why i tried to drop this hint:

In a generator the field strength would be reduced as speed increases

If he holds field strength constant and raises speed,
then voltage and current into a resistive load will both increase in proportion, and torque being in proportion to armature current will increase as well.
If instead he reduces field strength to hold voltage and power constant, then torque must decrease because power is product of torque and speed.

That's why i wouldn't say "yes" or "no" .

old jim

 

[Dr.D]

Well Jim, actually the magnets rubbing against the table does produce friction.

Friction is only produced when you have two surfaces in contact. Magnetic forces act across an air gap (or a vacuum) with no contact involved.