Lenz's Law pictorial 3

[Bloozntooz7868]

I need to roll out an electrical generation presentation to groups of Technicians to help underpin their understanding of generation principals. I was separately discussing with one of them about why Generators tend to slow down when load is applied, and how the Governor is increased to correct the frequency in Isoch mode etc etc. I was using the explanation of Lenz’s Law (along with Flemming’s Right Hand Rule and Maxwells Corkscrew Rule) for why this happens, but he was not understanding, and I ended up running out of ideas for ways of getting the phenomenon across to him, and he asked if I had any pictorials (which I didn’t).

I searched the internet for suitable pictorials (which speak a thousand words) but failed to find anything that clearly represented this, so my question is, does anyone have pictorials of this (not narratives – they are in abundance on the internet)?

I attempted to sketch it out myself, and found that it was more difficult that is first seemed, and actually led to some confusion on my part. I have attached my drawing (I’m sorry, yes, it’s pretty bad) and want to verify that what I am thinking is actually correct. [Let me know if the link is not working!]:

Lenz's drawing

It represents the Rotor outside the Stator so I could clearly show the Field Winding and Stator Winding directions, and I have shown just a single Stator Coil for clarity.

Using the sketch, this is my explanation:

1) The direction of Field current around the salient pole heads means a North is on top and South is on the bottom, by way of Maxwell’s Corkscrew rule (MCR).
2) This Field cuts the Stator coil and, using Flemming’s Right Hand Rule (FRHR), a Stator current is induced in the direction indicated with the arrow.
3) Using MCR, this sets up a clockwise field around the Stator conductor (when viewed from the left).
4) Changing to the small picture (left had side end on view), with the Rotor being driven clockwise by the Prime Mover, this clockwise Stator field interacts negatively with the rotating North Pole of the Rotor, and hence creates drag on it (tending to slow the Rotor down).

Could anyone please confirm or correct (probably the latter) what I am thinking, and also maybe provide some better pictorials that I could use.

Thank you in advance for any help.

 

[logbook]

I think you are overcomplicating things involving Lenz's law. Increased load slows anything down.

Try running with a marine on your back, that will slow you down! Ever had a car? Try letting the clutch out without pressing the accelerator (gas peddle for USA). The engine slows. (Of course modern fuel injected cars automatically compensate, just to confuse the issue)

 

[electricpete]

2) This Field cuts the Stator coil and, using Flemming’s Right Hand Rule (FRHR), a Stator current is induced in the direction indicated with the arrow.

As small clarification, I’d prefer to break that into two steps:
2A – a voltage is induced in stator coil by Faraday’s law
2B – that voltage results in a current, depending on load connected. Assuming a resistive load, then the current is in phase with the voltage. And a little thought shows that the maximum current in the loop you showed occurs roughly at the time the rotor is in the position you show (when rate of change of flux through the loop is at its highest).

3) Using MCR, this sets up a clockwise field around the Stator conductor (when viewed from the left).
4) Changing to the small picture (left had side end on view), with the Rotor being driven clockwise by the Prime Mover, this clockwise Stator field interacts negatively with the rotating North Pole of the Rotor, and hence creates drag on it (tending to slow the Rotor down).

#4 leaves me flat. It is as you suggest interaction of rotor and stator fields that results in the force, but that can be tough to visualize. If you draw field lines it may give a hint (the field lines act like springs) but again not really intuitive to draw.

You could instead apply F = q V x B = i L x B to the stator conductors.
Look at the top stator conductor from the end view (lower right hand corner)
q V =i L is into the page
B is straight up
Resulting force is to the right. This falls out of the well known right-hand rule or cross product rule. Note that i must be defined as conventional current (if your audience thinks of electron current, it doesn’t work).

Similarly looking at bottom stator conductor from the end view (lower right hand corner)
q V =i L is out of the page
B is still straight up
Resulting force is to the Left.

The result of both of the above is a torque on the stator clockwise (in direction of rotation).

But assuming steady state condition (angular acceleration is zero), Newton tells us there must be an equal/opposite reaction on the rotor. The electromagnetic torque on the rotor is CCW (opposite direction of rotation). Mechanical power must be exerted to keep the rotor turning at constant speed against the electromoagnetic torque. Perhaps we already knew that from conservation of energy: Neglecting losses in the machine, the amount of mechanical power in (w*T) input must equal the amount of electrical power out (I * V).

The idea of forces acting directly on conductor can be intuitive. But if conductors are embedded in slots, the torque producing force acts primarily on the iron core (not the copper conductors). I have some whitepapers that delve into that aspect. I’m sure it’s not an essential aspect for your audience, but some of the figures showing flux lines etc might help somewhere along the line.

http://electricpete1.tripod.com/torque_web/attach/LinkToTheShortVersion.htm

http://electricpete1.tripod.com/torque_web/attach/LinkToTheLongVersion.htm

Here is a more direct link to the first one:

http://electricpete1.tripod.com/torque_web/attach/TheShortVersion091207.pdf

=====================================
(2B)+(2B)' ?

 

[stevenal]

[URL unfurl="true"]http://video.mit.edu/watch/physics-demo-lenzs-law-with-copper-pipe-10268/[/url]

 

[2dye4]

Like Pete said.

A magnet approaching a coil will induce current in it which makes it's own magnetic field of the same polarity.
So you have a north pole moving toward a north pole or a south moving toward another south and this has to be forced
to occur.

Similarly as the magnet pulls away from the coil the polarity of the coil current reverses and now its pole switches
so that it attracts the magnet as it is forced to pull away from it.

So inside the generator there are poles approaching and retreating from each other constantly and when the generator is supplying
power this causes the magnets to force each other.

This is the very very simple explanation that I use in my mind to think about generator loading.

 

[Bloozntooz7868]

Gents,

Thank you very much for your insights and clear explanations.

@electricpete: Very interesting paper you have published there - like you say, the convention is to show (like I simplistically have) Stator coils in the air-gap, and I have actually wondered in the past what effect there is in the real world by having them in slots. Your paper has admirably cleared that up - thanks again!

@stevenal: Appreciate the visual representation of induction principals - this is a great video that I will incorporate. Visual tools always work the best to get the point across (exactly what I was after!).

@2dye4: A brief and to the point summary that was the key to making sense of it all! This captures everything I was after, because as I was thinking of drawing the field lines, as electricpete suggested, you posted, and the light suddenly came on. Your explanation has helped immensely.

I really appreciate your time responding - LPS's all round!

Regards,
David.

 

[waross]

I thought that I understood generator action very well until I read this thread.
Have you ever driven an automobile with cruise control?
When traveling with cruise control operating, the controller (governor) monitors the vehicle speed. If the power required to maintain the set speed increases due to a hill or a head wind, the car will start to slow down. The governor will sense the speed drop and open the throttle to supply more power. The speed will stabilize at slightly below the set point. If the hill gets steeper, the car will slow a little more and the governor will open the throttle more, and the sped will stabilize at a slightly lower speed. In control terminology the difference between the set speed and the speed under full load may be expressed as a percentage and is called the proportional band.
The governor of a diesel generator works exactly the same way. Rather than proportional band the speed change is called "Droop".
3% proportional band or 3% droop is a standard setting for a diesel generator.
An islanded generator will run at 61.8 Hz or 51.5 Hz at no load and "droop" to 60 Hz or 50 Hz at full load. For the purists 3% droop is actually 3% proportional band with a 3% offset. (That means that it is 3% of 60 Hz and not 3% of 61.8 Hz. Not much difference.)

Second chapter; Isochronous control;
I have described the old classic cruise control action.
Newer vehicles may have an added feature. This is a circuit that detects that the speed has stabilized below the set point due to an increase in load. The circuit then makes a slow adjustment to bring the speed up to the setpoint. In control terminology this is called "Reset" or "Integral" control. In a generator governor this is called isochronous control.

I don't have any graphics but I suggest that if you can find a presentation describing the action of cruise control, you may use that. It may be easier to explain the action by referring to a similar device that most of the audience are already familiar with.

That is why a generator slows down when a load is applied and the method to cope with the slow down.
I have probably misunderstood but it seems to me that we have been struggling with the reasons and explanations as to how a generator converts mechanical energy into electrical energy under conditions of varying load.
Probably my mistake.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[waross]

Not a pictorial but this may help:

http://www.pondlucier.com/Company/TIPS/2002/February 2002.pdf

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[electricpete]

Bill is right that speed droop of a generator in isoc mode is a function of the governor/control and absolutely nothing to do with electromagnetic theory.

I may have got fooled by your title and the remainder of your op if that was intended to be an explanation for speed droop. I was simply explaining the mechanism/polarity for electromagnetic torque at steady state constant frequency.


=====================================
(2B)+(2B)' ?

 

[Bloozntooz7868]

@electricalpete: no you were not fooled by my title. I was indeed after the electromagnetic influences that tend to slow a Rotor down when load current is increased on the Stator of a Generator - that's it. I was not after an explanation on Isoch/Droop or Proportional/Integral control loop corrections a Governor system.

@Waross: Thank you for your explanation, and I fully concur with what you said (and I am actually using nearly the exact same model to explain it in the presentation I am building), but it was not what I was after from my original question.

I got out of the earlier posts what I needed, because it spoke in terms of electrical fields and torque, and they helped represent the 'drag' effect of electrical load current increasing. In terms of cause and effect, then this phenomenon is the cause, and the effect is what Waross eluded to with the control system sensing this and acting on the Governor.

Thanks.

 

[catserveng]

Does this help any?

http://www.engines.republika.pl/podstawy.pdf

I used this (and the previous publications) for many years teaching generator set technicians.

I also used training products from this company with good feedback from our technicians,

http://www.hpcnet.com/index.php?opt...nd-generators&catid=74:interactive&Itemid=240

MikeL

 

[Brad1979]

electricpete,

Very nice write-ups on how force in electric machines acts on the magnetic material instead of the conductors. You would be happy to know that "Electric Machinery" by Fitzgerald, Kingsley, and Umans makes this point in their textbook. This textbook seems to be a pretty standard introductory textbook to electric machines in many universities. Here is a relevant quote from p. 115 of the 6th edition of the book:

For situations in which the forces act only on current-carrying elements and which are of simple geometry (such as that of Example 3.1), Eq. 3.6 is generally the simplest and easiest way to calculate the forces acting on the system. Unfortunately, very few practical situations fall into this class. In fact, as discussed in Chapter 1, most electromechanical-energy-conversion devices contain magnetic material; in these systems, forces act directly on the magnetic material and clearly cannot be calculated from Eq. 3.6.

Techniques for calculating the detailed, localized forces acting on magnetic materials
are extremely complex and require detailed knowledge of the field distribution
throughout the structure.

They then go on to describe a couple techniques for calculating forces in electromechanical-energy-conversion devices. Equation 3.6 is just Lorentz's Force Law for magnetic fields only (that is, F = J x B).

 

[Bloozntooz7868]

catserveng,

Thanks for the postings. Can never have enough good material