Compulsator Discussion
Dr. Shark, Wed Jan 31 2007, 05:57PMI've had it with my washing machine motor, it just kept confusing the **** out of me. Different windings for two-speed operation, high resitance startup windings, and lots of other hidden secrets - too much for me.
The idea of building a simple compulsator is not so easy to shake off though. It seems so simple, yet not a single amateur has succeded in building one so far. I don't know whether this is for lack of trying - it seems to be extraordinarily hard to find information about compulsators - or if there are really inherent difficulties which make even a small scale demonstration device (i.e. NOT suitable for powering something) infeasible I don't know. But with this thread I would like to find out.
The best information about the operation principle of a comulsator I could find was this:
The generator has a static field winding and a rotating armature winding. The latter is connected in series with the (also stationary) compensating winding. Every time the generator completes a turn the current that flows through the armature flows back through the compensation, therefore canceling out the inductance.
This happens to coincide with the voltage maximum, so a large current pulse can be extracted.
The greatest difficulty I can see here is the following: Both the armature and the compensation carry the same huge current. The two have to be moving relative to another so the field cancels out once per turn. Thus one of the high-current windings needs to be moving and the full current has to be commutated via slip-rings. In an ordinary generator the armature is stationary, so only the small magnetizing current needs to be commutated.
Does this make sense? If I got the principle correctly, then why do I keep reading about short-circuited compensation windings (or rather copper sleeves) between the exciter and armature coil? This would be an entirely different priciple than the one I sketched above.
Assuming my theory is correct, the only "hard" part of building a Compulsator would be to build a brush system that could take current in the high kA range from the sliprings. This does not seem to be an impossible task, considering that any railgun already contains a sliding contact carrying the same currents. Apart from that I can only see the massive magnetic forces ripping the coils apart, but that could be dealt with by impregnating the windings with graphite composite or so.
Re: Compulsator Discussion
Madgyver, Thu Feb 01 2007, 10:13AM
What you said makes sence so far. Although you don't need commutation. A sliding contact on a coper ring should be enough. That way compensation happens every full turn. What you descibed sounds more like every half turn.
Armature Shielding or shorted compensation windigs are a completly different approach.
The real trouble with compulsators is the mechnical construction. The design has to have very high RPM and has to withstand the forces upon discharge.
Madgyver, Thu Feb 01 2007, 10:13AM
What you said makes sence so far. Although you don't need commutation. A sliding contact on a coper ring should be enough. That way compensation happens every full turn. What you descibed sounds more like every half turn.
Armature Shielding or shorted compensation windigs are a completly different approach.
The real trouble with compulsators is the mechnical construction. The design has to have very high RPM and has to withstand the forces upon discharge.
Re: Compulsator Discussion
Dr. Shark, Thu Feb 01 2007, 12:01PM
Hey Minh, thanks for your input! You are certainly right about the commutation vs. slip ring issue, I just got confused and thought every brush contact is automatically a commutator.
You seem to know something that I would like to know, which is the issue of shorted compensation. I this a way to achive the same (or similar) results without the need for slip ring contacts? That would make amateur construction much easier!
Regarding the mechanical construction you are certainly right once we start talking about serios energy leves - 10s or even 100s of kJ - but for a small demonstration device with only a few J pulse energy this should be managable.
Dr. Shark, Thu Feb 01 2007, 12:01PM
Hey Minh, thanks for your input! You are certainly right about the commutation vs. slip ring issue, I just got confused and thought every brush contact is automatically a commutator.
You seem to know something that I would like to know, which is the issue of shorted compensation. I this a way to achive the same (or similar) results without the need for slip ring contacts? That would make amateur construction much easier!
Regarding the mechanical construction you are certainly right once we start talking about serios energy leves - 10s or even 100s of kJ - but for a small demonstration device with only a few J pulse energy this should be managable.
Re: Compulsator Discussion
Madgyver, Mon Feb 05 2007, 02:21PM
Hi, I am sorry I was a little bit busy with learning, an exam and machine designing practices for the University.
But, I was able to dig up some infos about passive compensation for you.
You have understood the principle of active compensation, so I wil start from there. As you know the inductance is compensated by a stator winding wound in opposite direction as the armature winding. Both coils will produce magnetic fields during a discharge which will cancel each other out, effectivly reducing inductance.
Passive compensation (in this case shorted compensation) works similar. Upon discharge the armature winding produces a strong magnetic field, as you know according to Lenz's law the shorted winding will counter react with a opposing magnetic field, this will reduce the inductance of the circuit.
Of course the compensation will be maximal if armature winding and shorted winding are coaxial to each other. Therefore inductance will be a sinusodial function.
If a conductive shield is used, the induced magnetic fields are always the same, no matter how the armature is orientated, so inductance is constant.
The effect of active compensation will be magnitudes larger then shorted winding compensation and the effect shorted winding compensation will be magnitudes larger then the conductive shield.
BTW: Try to get a copy of
"COMPULSATOR RESEARCH AT THE UNIVERSITY OF TEXAS AT AUSTIN--AN OVERVIEW"
by M. L. Spann, S. B. Pratap, M. D. Werst, A. W. Walls, and W. G. Fulcher
published in:
IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 1, JANUARY 1989
Madgyver, Mon Feb 05 2007, 02:21PM
Hi, I am sorry I was a little bit busy with learning, an exam and machine designing practices for the University.
But, I was able to dig up some infos about passive compensation for you.
You have understood the principle of active compensation, so I wil start from there. As you know the inductance is compensated by a stator winding wound in opposite direction as the armature winding. Both coils will produce magnetic fields during a discharge which will cancel each other out, effectivly reducing inductance.
Passive compensation (in this case shorted compensation) works similar. Upon discharge the armature winding produces a strong magnetic field, as you know according to Lenz's law the shorted winding will counter react with a opposing magnetic field, this will reduce the inductance of the circuit.
Of course the compensation will be maximal if armature winding and shorted winding are coaxial to each other. Therefore inductance will be a sinusodial function.
If a conductive shield is used, the induced magnetic fields are always the same, no matter how the armature is orientated, so inductance is constant.
The effect of active compensation will be magnitudes larger then shorted winding compensation and the effect shorted winding compensation will be magnitudes larger then the conductive shield.
BTW: Try to get a copy of
"COMPULSATOR RESEARCH AT THE UNIVERSITY OF TEXAS AT AUSTIN--AN OVERVIEW"
by M. L. Spann, S. B. Pratap, M. D. Werst, A. W. Walls, and W. G. Fulcher
published in:
IEEE TRANSACTIONS ON MAGNETICS, VOL. 25, NO. 1, JANUARY 1989
Re: Compulsator Discussion
Jasonr, Mon Feb 05 2007, 07:58PM
WIKIPEDIA Compulsator page
Rollette's Compulsator page
Compulsators are needlessly complicated. First off many use the compulsator to start the flywheel/rotor spinning(Dual primary). This can be simplified by using your washing machine motor to spin a flywheel.
There are huge complications in phase angles and timing in which you need to draw power to get perfect efficiency however for you building one at home, all of this makes it so complicated you will never build one.
If you take the approach of spinning up with a washing machine motor, you can use a standard car alternator as a start for a compulsator. The slip rings are only used the on the primary coil. ( the only part that is spinning). The outside secondarys are part of the frame, can easily be rewound once you blow them up from playing around. You need to rip out the regulator and connect the cap bank right to the primary, and replace the secondary rectifier diodes with diodes that are external diodes that can rectify the pulse.
If you spin the fly wheel with a washing machine motor. And directly attach the fly wheel to the alternator. You can then use a belt drive to take the 3600rpm washing machine motor and gear it up to 7200rpm using a 2:1 pulley system. You should have a working model to play with. But this a dangerous thing you need a very robust enclosure!!!!! As is everything we do on this board I guess..........
Jason
Rollette's Compulsator schematic sort of?
Jasonr, Mon Feb 05 2007, 07:58PM
WIKIPEDIA Compulsator page
Rollette's Compulsator page
Compulsators are needlessly complicated. First off many use the compulsator to start the flywheel/rotor spinning(Dual primary). This can be simplified by using your washing machine motor to spin a flywheel.
There are huge complications in phase angles and timing in which you need to draw power to get perfect efficiency however for you building one at home, all of this makes it so complicated you will never build one.
If you take the approach of spinning up with a washing machine motor, you can use a standard car alternator as a start for a compulsator. The slip rings are only used the on the primary coil. ( the only part that is spinning). The outside secondarys are part of the frame, can easily be rewound once you blow them up from playing around. You need to rip out the regulator and connect the cap bank right to the primary, and replace the secondary rectifier diodes with diodes that are external diodes that can rectify the pulse.
If you spin the fly wheel with a washing machine motor. And directly attach the fly wheel to the alternator. You can then use a belt drive to take the 3600rpm washing machine motor and gear it up to 7200rpm using a 2:1 pulley system. You should have a working model to play with. But this a dangerous thing you need a very robust enclosure!!!!! As is everything we do on this board I guess..........
Jason
Rollette's Compulsator schematic sort of?
Re: Compulsator Discussion
AndrewM, Tue Feb 06 2007, 12:35AM
Sorry to bring the level of discussion down, but why do you have it connected to a cap bank? I didn't think high current pulses were necesary or desired for cap charging?
AndrewM, Tue Feb 06 2007, 12:35AM
Sorry to bring the level of discussion down, but why do you have it connected to a cap bank? I didn't think high current pulses were necesary or desired for cap charging?
Re: Compulsator Discussion
Bjørn, Tue Feb 06 2007, 12:50AM
In a car alternator there are no permanent magnets. It uses current from the battery to set up the magnetic field, the regulator adjusts this current to keep the voltage correct.
By connecting a capacitor bank directly instead you can pulse it with a huge current to set up a very strong magnetic field and be able to extract a lot higher currents than normal.
Bjørn, Tue Feb 06 2007, 12:50AM
In a car alternator there are no permanent magnets. It uses current from the battery to set up the magnetic field, the regulator adjusts this current to keep the voltage correct.
By connecting a capacitor bank directly instead you can pulse it with a huge current to set up a very strong magnetic field and be able to extract a lot higher currents than normal.
Re: Compulsator Discussion
Jasonr, Tue Feb 06 2007, 02:37PM
Compulsator is not for charging caps, it is what you would use to power a railgun. This greatly differs from how an alternaotr works.
by pulsing a small cap bank into an already spinning alternator, you would be able to magnitudes higher current than the cap bank alone could produce.
Jasonr, Tue Feb 06 2007, 02:37PM
Compulsator is not for charging caps, it is what you would use to power a railgun. This greatly differs from how an alternaotr works.
by pulsing a small cap bank into an already spinning alternator, you would be able to magnitudes higher current than the cap bank alone could produce.
Re: Compulsator Discussion
Madgyver, Tue Feb 06 2007, 09:25PM
Well as Joe doesn't want it to power anything the design can be reduced to the most simplest form. For example exication by a pulsed field coil seems far to complicated and only makes sense with high power devices. Also single phase should be preferred.
I also guess that for starters, it will be difficult enough to build the compensation windings close enough to the armature without blocking it.
Madgyver, Tue Feb 06 2007, 09:25PM
Well as Joe doesn't want it to power anything the design can be reduced to the most simplest form. For example exication by a pulsed field coil seems far to complicated and only makes sense with high power devices. Also single phase should be preferred.
I also guess that for starters, it will be difficult enough to build the compensation windings close enough to the armature without blocking it.
Re: Compulsator Discussion
Steve Conner, Tue Feb 06 2007, 10:32PM
+Many on madgyver's original reply. I never realised the difference between a shorted compensating winding and a plain conducting shield till I read it, nor why the compensating winding needs to be positioned at 90 degrees to the field coils.
I still think pulsed excitation is a good idea. The reason being that iron losses slow the rotor down remarkably when the field is energized, as I found out one day when playing with a car alternator driven by a weed whacker engine. If the field is only on for as long as necessary, there will be less wastage.
Steve Conner, Tue Feb 06 2007, 10:32PM
+Many on madgyver's original reply. I never realised the difference between a shorted compensating winding and a plain conducting shield till I read it, nor why the compensating winding needs to be positioned at 90 degrees to the field coils.
I still think pulsed excitation is a good idea. The reason being that iron losses slow the rotor down remarkably when the field is energized, as I found out one day when playing with a car alternator driven by a weed whacker engine. If the field is only on for as long as necessary, there will be less wastage.
Re: Compulsator Discussion
Marko, Wed Feb 07 2007, 12:15PM
I needed time to digest this thread and all links.. and I... am still boggled with some things here
Looking at jasonr's schematic, it isn't really compulsator but rather just a pulsed alternator..?
I can imagine how could a dirty, demonstrative active compulsator, or whatever it is, be mocked up from a car alternator: (please correct me if it's complete nonsense):
- One would need to remove all windings, both stator and rotor, and replace them with much fatter wire, and only a single-phase stator winding.
Then, both windings should be seriesed, rotor spun up, and then connected to a power supply at right moment.
In one part of a cycle, coils should be aligned that way that currents flow in exactly opposite directions, and big 'discharge' SCR is fired exactly at that point.
I can also see some problems in the beggining, for example, that in car alternator, some 'air gap' is left between rotor and stator armatures for mechanical reasons. This will probably cause coupling between windings to be too low and high leakage inductance will mess things up.
Another is, providing an initial current pulse to coils, wich I'm unsure how to do properly.
I guess coils would need to be pulsed while their inductance is at 'maximum' in order not to burn any power or slow the flywheel down?
And third... already figured by others... is triggering the SCR exactly at required time, wich I quess can be done using optoisolators.
And... the slip ring, wich normally carries only small exitation current, will now be torched. It will probably just set itself on fire after few firings.
\brainfart
Marko, Wed Feb 07 2007, 12:15PM
I needed time to digest this thread and all links.. and I... am still boggled with some things here
Looking at jasonr's schematic, it isn't really compulsator but rather just a pulsed alternator..?
I can imagine how could a dirty, demonstrative active compulsator, or whatever it is, be mocked up from a car alternator: (please correct me if it's complete nonsense):
- One would need to remove all windings, both stator and rotor, and replace them with much fatter wire, and only a single-phase stator winding.
Then, both windings should be seriesed, rotor spun up, and then connected to a power supply at right moment.
In one part of a cycle, coils should be aligned that way that currents flow in exactly opposite directions, and big 'discharge' SCR is fired exactly at that point.
I can also see some problems in the beggining, for example, that in car alternator, some 'air gap' is left between rotor and stator armatures for mechanical reasons. This will probably cause coupling between windings to be too low and high leakage inductance will mess things up.
Another is, providing an initial current pulse to coils, wich I'm unsure how to do properly.
I guess coils would need to be pulsed while their inductance is at 'maximum' in order not to burn any power or slow the flywheel down?
And third... already figured by others... is triggering the SCR exactly at required time, wich I quess can be done using optoisolators.
And... the slip ring, wich normally carries only small exitation current, will now be torched. It will probably just set itself on fire after few firings.
\brainfart
Re: Compulsator Discussion
Jasonr, Wed Feb 07 2007, 04:43PM
I guess my point is if you wanted to build a home built compulsator this is where “I†would start. If you wanted to talk theory and never really build one, that is a different story. Is a car alternator the perfect platform no, is it a good starting point for an amateur who can’t build a huge custom motor, sure. Is it a better platform and more common that a washing machine motor, yes.
(and reading the first post I would say he wants practical)
First "I" would
build a 3kJ cap bank~500volts
I would use a car disc brake rotor for a flywheel. And spin the alternator at 7200rpm with a belt and a washing machine motor geared up.(rotor is easily found, heavy and already balanced)
Get the platform working until I blew the windings or brushes, and do as much testing as possible. measure currents, vs rpm, play with different timings, all of the easy things
second
If it were me I would rewind the rotor with double or triple runs of 12 ga magnet wire. ( or ¼†copper water pipe covered in heat shrink)
I would add a second or third set of brushes to get more surface area on the slip rings.
I would rewind the stator with as large of ga wire as possible.
I would optically fire the scr for the cap bank from a hole in the flywheel.
I would use a disc brake rotor for a flywheel. And spin the alternator at 7200rpm.
third
build a real one.
Jason
I will update this page tonight with ideas I have. http://www.rollette.com/railgun/compulsator/
Jasonr, Wed Feb 07 2007, 04:43PM
I guess my point is if you wanted to build a home built compulsator this is where “I†would start. If you wanted to talk theory and never really build one, that is a different story. Is a car alternator the perfect platform no, is it a good starting point for an amateur who can’t build a huge custom motor, sure. Is it a better platform and more common that a washing machine motor, yes.
(and reading the first post I would say he wants practical)
First "I" would
build a 3kJ cap bank~500volts
I would use a car disc brake rotor for a flywheel. And spin the alternator at 7200rpm with a belt and a washing machine motor geared up.(rotor is easily found, heavy and already balanced)
Get the platform working until I blew the windings or brushes, and do as much testing as possible. measure currents, vs rpm, play with different timings, all of the easy things
second
If it were me I would rewind the rotor with double or triple runs of 12 ga magnet wire. ( or ¼†copper water pipe covered in heat shrink)
I would add a second or third set of brushes to get more surface area on the slip rings.
I would rewind the stator with as large of ga wire as possible.
I would optically fire the scr for the cap bank from a hole in the flywheel.
I would use a disc brake rotor for a flywheel. And spin the alternator at 7200rpm.
third
build a real one.
Jason
I will update this page tonight with ideas I have. http://www.rollette.com/railgun/compulsator/
Re: Compulsator Discussion
Steve Conner, Wed Feb 07 2007, 05:23PM
I agree with Jason. I think the hardest part for the amateur will be modifying the rotor to fit a compensating winding. You'd need access to a machine shop, and no small amount of precision machining skills, to modify the rotor well enough to keep it holding together at 7200rpm.
A while back, myself and some other folks did some rough calculations on this using standard power engineering theory. An average alternator has about 10% subtransient reactance, IIRC. That means it can supply 10 times its rated current into a short circuit for a few cycles, before armature reaction kicks in and the current starts to fall. 10 times the rated current of a car alternator is puny even by hobbyist pulsed power standards.
So, in modifying the alternator, you have to get the subtransient reactance down by at least an order of magnitude. That is what the compensating winding on the rotor does. If you place it right, it appears as a shorted turn in the case of passive compensation, or a mirror image of the stator winding if the compensation is active, hence lowering the reactance (=inductance) just when maximum current is needed, as explained in that earlier post with the nice diagrams.
From that point of view, a compulsator is really just a FCG that spins instead of exploding, and the field coils provide the seed flux to be compressed. With a car altenator, the existing field coil on the rotor would probably do fine to "seed" it.
Steve Conner, Wed Feb 07 2007, 05:23PM
I agree with Jason. I think the hardest part for the amateur will be modifying the rotor to fit a compensating winding. You'd need access to a machine shop, and no small amount of precision machining skills, to modify the rotor well enough to keep it holding together at 7200rpm.
A while back, myself and some other folks did some rough calculations on this using standard power engineering theory. An average alternator has about 10% subtransient reactance, IIRC. That means it can supply 10 times its rated current into a short circuit for a few cycles, before armature reaction kicks in and the current starts to fall. 10 times the rated current of a car alternator is puny even by hobbyist pulsed power standards.
So, in modifying the alternator, you have to get the subtransient reactance down by at least an order of magnitude. That is what the compensating winding on the rotor does. If you place it right, it appears as a shorted turn in the case of passive compensation, or a mirror image of the stator winding if the compensation is active, hence lowering the reactance (=inductance) just when maximum current is needed, as explained in that earlier post with the nice diagrams.
From that point of view, a compulsator is really just a FCG that spins instead of exploding, and the field coils provide the seed flux to be compressed. With a car altenator, the existing field coil on the rotor would probably do fine to "seed" it.
Re: Compulsator Discussion
Madgyver, Wed Feb 07 2007, 07:09PM
I admit, I haven't though about iron losses but I understand and agree to you points. I just thought that because he said:
that he just wants a small tabletop display, where the effect of compensation can be demonstrated.
Sure, a 3kJ cap bank sounds very interesting and I don't doubt that it would be a very impressive setup which could power a Railgun. But I think it is overkill, compared to what Joe seems to want.
Of course if we are talking about serious power levels suitable for amateur railguns, there are no other ways possible.
I wonder if it would be possible for an amateur to fabricate a new rotor out of composite material, making it air core.
Madgyver, Wed Feb 07 2007, 07:09PM
I admit, I haven't though about iron losses but I understand and agree to you points. I just thought that because he said:
wrote ...
small scale demonstration device (i.e. NOT suitable for powering something)
small scale demonstration device (i.e. NOT suitable for powering something)
that he just wants a small tabletop display, where the effect of compensation can be demonstrated.
Sure, a 3kJ cap bank sounds very interesting and I don't doubt that it would be a very impressive setup which could power a Railgun. But I think it is overkill, compared to what Joe seems to want.
Of course if we are talking about serious power levels suitable for amateur railguns, there are no other ways possible.
I wonder if it would be possible for an amateur to fabricate a new rotor out of composite material, making it air core.
Re: Compulsator Discussion
Marko, Wed Feb 07 2007, 07:49PM
Afaiu, the compensation winding is on stator in case of active compulsator:
so modifying is a bit easier, but still a lot of trouble.
With passive, copper sleeves on rotor rather than stator are a better idea.
I had an idea to use the stator winding both as compensator and field winding, but I see that it didn't make much sense.
I figure that those two coils need to be 90 degrees out of phase to each other, so compensation occurs exactly at the voltage peak.
Could maybe, a stator of a single phase motor be used for that application? It has two windings out of phase as such and would be perfect to be rewound for a small compulsator. But, now it would need a custom rotor.
With a passive compulsator, rotor could be used as a field winding, wich would greatly reduce stress on the slip ring, and make the stator carry the heavy output current.
(I guess that was what steve talked about).
I imagine this would be hard because short-circuit rings again need to be 90deg out of 'phase' with rotor windings, and I haven't seen such a rotor anywhere.
I'm sorry if I just repeated everything that has been said.. repeating just seems to make me understand things better..
I can imagine that air-cored rotor would simply have too much leakage inductance; coupling with compensation coil would be poor and so the output would.
What else except iron could be used anyways? What do those multi-megajoule compulsators use as their core material?
Marko, Wed Feb 07 2007, 07:49PM
I think the hardest part for the amateur will be modifying the rotor to fit a compensating winding. You'd need access to a machine shop, and no small amount of precision machining skills, to modify the rotor well enough to keep it holding together at 7200rpm.
Afaiu, the compensation winding is on stator in case of active compulsator:
connected in series with the (also stationary) compensating winding.
so modifying is a bit easier, but still a lot of trouble.
With passive, copper sleeves on rotor rather than stator are a better idea.
I had an idea to use the stator winding both as compensator and field winding, but I see that it didn't make much sense.
I figure that those two coils need to be 90 degrees out of phase to each other, so compensation occurs exactly at the voltage peak.
Could maybe, a stator of a single phase motor be used for that application? It has two windings out of phase as such and would be perfect to be rewound for a small compulsator. But, now it would need a custom rotor.
With a passive compulsator, rotor could be used as a field winding, wich would greatly reduce stress on the slip ring, and make the stator carry the heavy output current.
(I guess that was what steve talked about).
I imagine this would be hard because short-circuit rings again need to be 90deg out of 'phase' with rotor windings, and I haven't seen such a rotor anywhere.
I'm sorry if I just repeated everything that has been said.. repeating just seems to make me understand things better..
I wonder if it would be possible for an amateur to fabricate a new rotor out of composite material, making it air core.
I can imagine that air-cored rotor would simply have too much leakage inductance; coupling with compensation coil would be poor and so the output would.
What else except iron could be used anyways? What do those multi-megajoule compulsators use as their core material?
Re: Compulsator Discussion
Madgyver, Wed Feb 07 2007, 08:16PM
Well actually they use fiber composite rotors, I just don't know if an amateur can build something like that. The low coupling is the price to be paid for reduction of iron losses.
Madgyver, Wed Feb 07 2007, 08:16PM
Firkragg wrote ...
I can imagine that air-cored rotor would simply have too much leakage inductance; coupling with compensation coil would be poor and so the output would.
What else except iron could be used anyways? What do those multi-megajoule compulsators use as their core material?
I can imagine that air-cored rotor would simply have too much leakage inductance; coupling with compensation coil would be poor and so the output would.
What else except iron could be used anyways? What do those multi-megajoule compulsators use as their core material?
Well actually they use fiber composite rotors, I just don't know if an amateur can build something like that. The low coupling is the price to be paid for reduction of iron losses.
Re: Compulsator Discussion
Steve Conner, Wed Feb 07 2007, 11:41PM
The basic principle is that the flux gets caught and pinched between some rotating windings and some stationary windings. So, passive compensation needs the compensating winding to be on the rotor, if the output winding is on the stator.
In order for the flux to get smashed between them, the two sets of windings have to be carrying equal and opposite (and insanely large) amp-turns. The active design achieves this by connecting them in series, the passive design does it by induction.
The newer machines made for military railguns are all composite and air cored. The drawback of an air cored machine, AFAIK, is that it needs a much higher field current to excite it. I read about one machine that had about 2500A field current flowing for 1 second before the discharge, which was something like 500kA. It used the same winding for field and discharge.
Steve Conner, Wed Feb 07 2007, 11:41PM
The basic principle is that the flux gets caught and pinched between some rotating windings and some stationary windings. So, passive compensation needs the compensating winding to be on the rotor, if the output winding is on the stator.
In order for the flux to get smashed between them, the two sets of windings have to be carrying equal and opposite (and insanely large) amp-turns. The active design achieves this by connecting them in series, the passive design does it by induction.
The newer machines made for military railguns are all composite and air cored. The drawback of an air cored machine, AFAIK, is that it needs a much higher field current to excite it. I read about one machine that had about 2500A field current flowing for 1 second before the discharge, which was something like 500kA. It used the same winding for field and discharge.
Re: Compulsator Discussion
Dr. Shark, Thu Feb 08 2007, 09:57AM
Wow, it's good to see the amount of discussion that has developed here! The first time in the history of man, amateurs are beginning to challenge the hegemony of the US military. Nah, 99% of what I know still comes from the great source that Madgyver pointed out: "COMPULSATOR RESEARCH AT THE UNIVERSITY OF TEXAS AT AUSTIN--AN OVERVIEW", I'd definitely suggest everyone to go over it. If you don't have an IEEE subscription please PM me, maybe there is something I can do
Contrary to what some have said, I belive passive compulsation is the way to go for a first design: The Reduction in inductance is exactly the same as for an active circuit, the only difference lies in the additional "flux compression" that happens in active compulsation and still boosts the output somewhat, giving a shorter, higher power output pulse. Too short for EM-guns if you believe the Austin Texas guys.
I have now come to think of compensation in terms of a transformer: Short the secondary and the primary current goes through the roof, as the impedance the mains socket sees drops from the mutual inductance of the transformer to the leakage inductance. The same would happen if an identical winding was connected in anti-parallel to the primary (active compensation), and the effect would be exactly the same.
I have now sourced a small shaded pole motor that might be suitable to use for a small compulsator:
It only has a single pole which dramatically simplifies the design, and the magnetic window area of this small transformer, being 5cm^2 is larger than that of my 48pole washing machine motor! I am planning to implant some permanent magnets into the rotor, but leaving the "squirrel cage" intact to act as the compensation winding. That way no slip rings will be required at all, again simlifying the design for a proof-of-concept device.
The design parameters I am eyeing are something like this: I want to store about 2kJ of mechanical energy in a 10cm diameter aluminium flywheel, rotating at a whooping 30000rpm. I have spun up CD-ROMs to that speed with a Dremel tool and they hold together (unless you touch them that is - then they explode) so it should be ok for an aluminium disk.
This would give me a pulse-width of about 1ms, during which I hope to extract 5kA at 50V or so. While this is only a couple of J, it could still drive a small coilgun, and it would be an ideal basis for further experimentation.
Dr. Shark, Thu Feb 08 2007, 09:57AM
Wow, it's good to see the amount of discussion that has developed here! The first time in the history of man, amateurs are beginning to challenge the hegemony of the US military. Nah, 99% of what I know still comes from the great source that Madgyver pointed out: "COMPULSATOR RESEARCH AT THE UNIVERSITY OF TEXAS AT AUSTIN--AN OVERVIEW", I'd definitely suggest everyone to go over it. If you don't have an IEEE subscription please PM me, maybe there is something I can do
Contrary to what some have said, I belive passive compulsation is the way to go for a first design: The Reduction in inductance is exactly the same as for an active circuit, the only difference lies in the additional "flux compression" that happens in active compulsation and still boosts the output somewhat, giving a shorter, higher power output pulse. Too short for EM-guns if you believe the Austin Texas guys.
I have now come to think of compensation in terms of a transformer: Short the secondary and the primary current goes through the roof, as the impedance the mains socket sees drops from the mutual inductance of the transformer to the leakage inductance. The same would happen if an identical winding was connected in anti-parallel to the primary (active compensation), and the effect would be exactly the same.
I have now sourced a small shaded pole motor that might be suitable to use for a small compulsator:
It only has a single pole which dramatically simplifies the design, and the magnetic window area of this small transformer, being 5cm^2 is larger than that of my 48pole washing machine motor! I am planning to implant some permanent magnets into the rotor, but leaving the "squirrel cage" intact to act as the compensation winding. That way no slip rings will be required at all, again simlifying the design for a proof-of-concept device.
The design parameters I am eyeing are something like this: I want to store about 2kJ of mechanical energy in a 10cm diameter aluminium flywheel, rotating at a whooping 30000rpm. I have spun up CD-ROMs to that speed with a Dremel tool and they hold together (unless you touch them that is - then they explode) so it should be ok for an aluminium disk.
This would give me a pulse-width of about 1ms, during which I hope to extract 5kA at 50V or so. While this is only a couple of J, it could still drive a small coilgun, and it would be an ideal basis for further experimentation.
Re: Compulsator Discussion
Marko, Thu Feb 08 2007, 11:24AM
I think it can be sumarrized as:
Passive compulsators - slip rings carry only small excitation current, output winding is stator,
but losses in copper sleeves on rotor will be high due to termedous currents flowing in them, so they can never actually fully compensate stator's inductance.
Active - need for a massive slip rings, but since exactly the same current flows in rotor and compensation winding, compensation should be better than of passive compy.
Now it all depends between tradeoff of between large slip rings or lossier short-circuit rings.
I also think that passive approach may give much more chance for a ghetto version of compulsator to work, simply due to problems with slip rings.
Magnets could be a good start, although probably not as powerful as exscitation winding would be.
I can imagine that biggest trouble is going to be to plant them at a correct phase in the rotor toj coincide minimal inductance and max voltage output...
Marko, Thu Feb 08 2007, 11:24AM
I think it can be sumarrized as:
Passive compulsators - slip rings carry only small excitation current, output winding is stator,
but losses in copper sleeves on rotor will be high due to termedous currents flowing in them, so they can never actually fully compensate stator's inductance.
Active - need for a massive slip rings, but since exactly the same current flows in rotor and compensation winding, compensation should be better than of passive compy.
Now it all depends between tradeoff of between large slip rings or lossier short-circuit rings.
I also think that passive approach may give much more chance for a ghetto version of compulsator to work, simply due to problems with slip rings.
I am planning to implant some permanent magnets into the rotor, but leaving the "squirrel cage" intact to act as the compensation winding. That way no slip rings will be required at all, again simlifying the design for a proof-of-concept device.
Magnets could be a good start, although probably not as powerful as exscitation winding would be.
I can imagine that biggest trouble is going to be to plant them at a correct phase in the rotor toj coincide minimal inductance and max voltage output...
Re: Compulsator Discussion
Dr. Shark, Sun Feb 11 2007, 01:09PM
Theory aside, I made some "not quite progress" on my model:
Firstly I implanted some small NIB magnets in the rotor, but then I decided that bigger is better, and went ahead with 20mm diameter magnets
which I epoxied into the rotor and heat-cured on my soldering iron
I proceeded to sand off the rough edges, so the rotor would still rotate, and put the thing back together:
Alas, when spinning the generator at 2500rpm with my power drill, not a single mV of EMF was induced!
I don't have the slightest clue as to what I did wrong: The permanent magnets are plenty powerfull, in fact cogging is so bad that I cannot turn the shaft by hand. The armature winding is only 5 turns of magnet wire now, put that should produce enough voltage to give a reding on my DMM! In fact, estimating the B-field to be 1T (yeah, maybe an order of magnitude less, I don't have a clue as to calculated this for a permanent magnet) and the window area at 5cm^2, I get a flux of 5e-4. Since the EMF is the time derivative of this, and I have a rotational frequency of 40Hz, the EMF should be around V=5e-4 * 40Hz = 20mV per turn, or something like that.
Mysterious....
Dr. Shark, Sun Feb 11 2007, 01:09PM
Theory aside, I made some "not quite progress" on my model:
Firstly I implanted some small NIB magnets in the rotor, but then I decided that bigger is better, and went ahead with 20mm diameter magnets
which I epoxied into the rotor and heat-cured on my soldering iron
I proceeded to sand off the rough edges, so the rotor would still rotate, and put the thing back together:
Alas, when spinning the generator at 2500rpm with my power drill, not a single mV of EMF was induced!
I don't have the slightest clue as to what I did wrong: The permanent magnets are plenty powerfull, in fact cogging is so bad that I cannot turn the shaft by hand. The armature winding is only 5 turns of magnet wire now, put that should produce enough voltage to give a reding on my DMM! In fact, estimating the B-field to be 1T (yeah, maybe an order of magnitude less, I don't have a clue as to calculated this for a permanent magnet) and the window area at 5cm^2, I get a flux of 5e-4. Since the EMF is the time derivative of this, and I have a rotational frequency of 40Hz, the EMF should be around V=5e-4 * 40Hz = 20mV per turn, or something like that.
Mysterious....
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