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Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
Location:
Posts: 86
Hello, a simple questions puzzles me.say you have a solenoid , basically a core of low reluctance metal with a coil on it. now you move a N-S pole pass its ends you get induction into the coil.
what happens if one pole is permanently attached to one end of the solenoid and all you do is just move the other pole pass the other end of the solenoid.?Like the S pole for example of a magnet is attached to the solenoid permanently and you take another magnet and just move it's N pole near the solenoid, do you get as much induced current as if you would move both magnets pass both ends of the solenoid simultaneously?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
It all depends on the change in total flux through the solenoid.
The total flux through the solenoid is given by the magnetic equivalent of Ohm's Law, where you add up the mmf (magnetomotive force) of the all the series magnets (their 'voltage'), divide by the path reluctance (its 'resistance'), to end up with the magnetic field (equivalent to current, although you can't identify anything physical flowing).
It's not clear from your question whether you have one flexible magnet with the N attached to the solenoid core, and are waving the S end around, or whether you have two magnets. The situation is essentially the same, except that in the two magnet case, the total path reluctance includes the air gap between the two magnets.
This is why quantitative predictions of magnetic field are so difficult, except in situations with very well defined geometry, and well defined permeability.
If you do electrical ohm's law, then the conductivity of copper wire is 10 to 15 orders of magnitude better than air, or the plastic insulation on the wire. It is very easy to cleanly separate batteries, conductors, resistors, just totally neglect current flow except in the copper.
If you do magnetic 'Ohms Law', permeability of iron is only a few orders of magnitude higher than air. It's like trying to do quantitative electrical work with bare copper wire in a bucket of salt-water.
You will get better results for your thought experiment if you have a cleaner geometry. Consider a solenoid with an iron core, projecting beyond each end of the core. Now attach the N pole of a short magnet to one end at right angles, and the S pole of another magnet to the other end at right angles. In this position, the flux through the core is quite low, due to the large airgap between the ends of the magnets. Now bring a soft iron bar into contact with the free ends of the two small magnets to complete the magnetic circuit, eliminating the airgaps, reducing the total path reluctance. The flux will increase, which will induce a voltage (not a current) into the solenoid. A current will flow if a conductor is connecting the ends of the solenoid winding, and the current will be proportional to the induced voltage.
Now replace one magnet with a soft iron bar, and repeat the experiment. Half the mmf, more or less the same path reluctance, about half the voltage generated.
Now space one magnet away from the core ...
You get the idea, figure out the total flux from the geometry, total change in flux gives voltage.seconds, so rate of change of flux gives voltage. Which means if you wave the magnets about faster, you get more voltage, but for less time.
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
Location:
Posts: 86
I must say I do understand all of what you say Dr.Slack , but I didn't understand one thing , when one pole is fixed to one end of the solenoid and the other one is not , is the flux less of what it could be if both magnet were moving or not? theoretically the way it seems to me is that if one magnet is fixed to the solenoid core then its flux can't vary , it can only extend to loop into the other magnet when brought near and then loop back into itself. So to me it kinda seems that both magnets moving would double the induction while one fixed and one moving would be roughly 1/2, although I;m really not sure.
the reason I ask is because I was thinking about a brushless alternator of a specific sort. Just imagine say 5 cylindrical cores with coils on them , all the say lower sides of the cores are then attached to a disc , so technically you have a disc with five equal in spacing solenoids around the perimeter of the disc , in the middle the disc forms a larger middle core that goes up similary to the outer solenoids. the inner core also has a coil around it , the field coil. say you now took a specific rotor that has an airgap with the middle core field coil and close airgaps with the outer solenoid cores, but the rotor is made such that its like a fan blade at the upper side in that it can pass by no more than two solenoid core ends in any time as it rotates around. in other words the middle central core having the field coil on it creates a flux which from one sides loops around and splits into each of the five cores then goes up and comes back into the middle core but through the rotor and hence the rotor rotates it can only come back from any two cores at a time thus the cores keep changing as the rotor rotates by each next one. so the coils on those cores should see induction right?
the other option would be to have a stationary field coil and have both ends like the rotor one i described just,
now a few tought questions i face here , if this works ,assuming output coils on all cores are connected in series, could the field coil be driven by an AC current , and if the rotor rpm is sufficiently high could the induced output follow the field coil input in terms of frequency and waveform ?
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Salvador wrote ...
Just imagine say 5 cylindrical cores with coils on them , all the say lower sides of the cores are then attached to a disc , so technically you have a disc with five equal in spacing solenoids around the perimeter of the disc , in the middle the disc forms a larger middle core that goes up similary to the outer solenoids. the inner core also has a coil around it , the field coil. say you now took a specific rotor that has an airgap with the middle core field coil and close airgaps with the outer solenoid cores, but the rotor is made such that its like a fan blade at the upper side in that it can pass by no more than two solenoid core ends in any time as it rotates around. in other words the middle central core having the field coil on it creates a flux which from one sides loops around and splits into each of the five cores then goes up and comes back into the middle core but through the rotor and hence the rotor rotates it can only come back from any two cores at a time thus the cores keep changing as the rotor rotates by each next one. so the coils on those cores should see induction right?
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
Location:
Posts: 86
well i attached a drawing , because my drawing skills are bad imagine you are looking at this generator from right above , the big black circle , disc is the very base , then there are five coils attached to it , from number 1 to 4 are the output cores/coils, number five is the center field coil. now the blue thingy i drawed is supposed top be the moving part , the rotor the base of the rotor is round and fully covers the area of the center field coil and has an airgap with it , the other side of the rotor is as the blue line suggests , in such a shape and it also has an airgap with all the cores around the perimeter it travels by. so the field coil creates the flux , since the one side were the cores are attached to the disc is permanent the flux already goes into all the core but the upper part rotates so the flux from each core can only be fully looped back to the middle coil when the rotor passes by.
as i asked before, I can sort of imagine this working on DC excitation but what if I would excite the field coil with AC , how would the output look like if all the output coils are connected in series?
P.S. I think I found a generator that is the closest thing to what I;m talking about here , it's called reluctance alternator, an alternator that has no moving coils or contacts and the moving part the rotor just serves to switch flux paths.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Yay for drawings! Your drawing skills are perfectly up to the task of illustrating this configuration.
First, the simpler, version. You can already imagine this working for DC excitation of coil 5, and indeed it does. You could replace coil 5 by a permanent magnet, and the behaviour would be the same. As you've drawn it, the odd numbered output coils 1 and 3 see high flux, the evens see low flux. As the armature rotates in the direction of the arrows, the flux decreases in the odds and increases in the evens. The peak rate of change of flux will be after a 45 degree turn, at which point the voltage generated in all the coils will be maximum. As the odd coil flux is decreasing and the even is increasing, the polarity will be opposite between the odds and the evens. This will always be the case, so it will be easy to wire them in series in the correct sense so that they all add up, rather than cancel out.
The output will be like a sine wave, so having a zero mean voltage, and swinging to positive and negative peaks.
Now to the question of AC excitation of the field coil 5. You don't specify what the respective frequencies are.
As a physicist, I always like looking for the limiting cases first
a) Let's assume the AC field is at a very low frequency compared to the rate of rotation.
As far as the rotating armature is concerned, the field is more or less static, it just changes steadily from rotation to rotation. When the field is peak, the output of the generator will be highest. As the field passes through zero, the output of the generator will be zero. As the field peaks in the opposite direction, the output of the generator will again be maximum, but this time the phase of the output voltage will be shifted by 180 degrees with respect to the armature position. The duration of the very low output will be essentially dictated by the AC excitation frequency.
b) Assume the AC field is at a very high frequency compared to the rate of rotation.
As far as the magnetically coupled coils are concerned, this is a transformer, whose configuration is changing slowly. When the armature is over the odd coils, the output phasing of the transformer will be opposite to that when coupled to the even coils. When the armature is at 45 degrees between the odd and even coils, the transformer coupling will be zero, as the coupling will be equal to pairs of coils connected in anti-phase. The duration of the very low output will be essentially dictated by the generator rotation frequency.
What you describe, maybe even have invented, is a multiplier. Whether it's useful for anything these days remains to be seen. It might have been extremely useful 100 years ago, when rotating machinery ruled, semiconductors didn't exist, and even rectifiers were difficult components. Why?
c) Assume the rotation rates are identical.
If I can get my head round this properly, I think it makes a rectifier with a controllable output voltage*. If the armature is driven synchronously to the field excitation, then it will output any DC voltage from fully negative to fully positive, controlled by adjusting the phase of the armature with respect to the field excitation.
If you look back in the electrical control literature, you find things like the Ward-Leonard system, which was essentially a DC current amplifier based on brushed DC rotating machinery, usually used to speed control another DC motor. One of the criticisms leveled at this system is that if you want a controllable 10hp motor, you have to install 30hp of motors and generators. But people still did it, as it was the only game in town.
I think the variable reluctance generator, as you've drawn it, would not be as efficient as a brushed DC motor or generator. But, being brushless, and working as it does with variable phase as the control, it might have found a niche.
These days of course you'd use semiconductors to produce controllable DC from an AC input.
It doesn't seem to have any advantages as a pure motor or generator over a straight BLDC, so it wouldn't find any application there.
As a thought experiment into unusual magnetic geometries, well done, it's an interesting result.
* I'm hope I'm correct in theory, but I might be wrong in practice, if saturation effects reduce its gain to unusably low levels. I might even be wrong in theory, there's a nagging thought that the 45 degree position is peak output for fast field, but zero output for slow field. But, that's as far as I'm going to go in my analysis, it points to where to concentrate on looking to see what this thing does.
Registered Member #54402
Joined: Mon Feb 02 2015, 11:09PM
Location:
Posts: 86
well sad, most of the idea i get are either already in use and I find out that later on or like this one could have been okay but then I first need to invent a time machine.
to be honest the reason i thought about this is that I wanted to see wether a generator for a low rpm mechanical source could be made lightweight and smaller. talking about the application , lets take that low rpm case like in a hydro powerplant, they usually have very large generators were a rotor has many many poles on it so that once it rotates with those low rpm it could make til the 50hz output frequency needed for most local grids in the world. since these days we favour HVDC transmission over larger distances anyways , maybe one could take a generator like this use a high frequency excitation to make it smaller for the same power output , the frequency wouldn;t matter hence it would be rectified anyways.
well this is just a thought since im not sure how this generator would behave in the low rpm/high excitation frequency regime anyways. but if my thinking is right it could be then made out of ferrite and use less loops of wire for the same output
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
Salvador wrote ...
well sad, most of the idea i get are either already in use and I find out that later on or like this one could have been okay but then I first need to invent a time machine.
I feel your pain. In my time, I have invented the condom, the theodolite, the automatic flush toilet, and the sigma delta ADC, all too late to take the world by storm.
Salvador wrote ...
to be honest the reason i thought about this is that I wanted to see wether a generator for a low rpm mechanical source could be made lightweight and smaller. talking about the application , lets take that low rpm case like in a hydro powerplant, they usually have very large generators were a rotor has many many poles on it so that once it rotates with those low rpm it could make til the 50hz output frequency needed for most local grids in the world. since these days we favour HVDC transmission over larger distances anyways , maybe one could take a generator like this use a high frequency excitation to make it smaller for the same power output , the frequency wouldn;t matter hence it would be rectified anyways.
well this is just a thought since im not sure how this generator would behave in the low rpm/high excitation frequency regime anyways. but if my thinking is right it could be then made out of ferrite and use less loops of wire for the same output
One fatal flaw in your plan is the single phase excitation field, which means the output dips to zero frequently. You might try getting your head around multiphase excitation. On the other hand, a generator consisting of a PM BLDC machine + power semiconductors to rectify and invert would probably be smaller, simpler, cheaper, more mainstream. 'Rectify and invert' can be simplified further to a matrix converter, which completely does away with the DC bus and the associated bus capacitors.
Registered Member #3414
Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245
Actually, this 'could' have use as a 'controllable output' brushless alternator.
For a given RPM, the output will depend on the amount of DC exitation to the centre coil.
This could find applications where brushed alternators are not suitable, although I can't think of any at the moment, but they must have advantages somewhere.
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magnetic flux path
