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Dr. Shark

Wed Jan 31 2007, 05:57PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

I'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:
Compulsator

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.
20compulsator

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.

Madgyver

Thu Feb 01 2007, 10:13AM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

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.

Dr. Shark

Thu Feb 01 2007, 12:01PM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

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.

Madgyver

Mon Feb 05 2007, 02:21PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

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

Jasonr

Mon Feb 05 2007, 07:58PM

Registered Member #167 Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54

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?

AndrewM

Tue Feb 06 2007, 12:35AM

Registered Member #49 Joined: Thu Feb 09 2006, 04:05AM
Location: Bigass Pile of Penguins
Posts: 362

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?

Bjørn

Tue Feb 06 2007, 12:50AM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

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.

Jasonr

Tue Feb 06 2007, 02:37PM

Registered Member #167 Joined: Mon Feb 13 2006, 06:41PM
Location: Waterloo, WI
Posts: 54

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.

Madgyver

Tue Feb 06 2007, 09:25PM

Registered Member #177 Joined: Wed Feb 15 2006, 02:16PM
Location: Munich, Germany
Posts: 214

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.

Steve Conner

Tue Feb 06 2007, 10:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

+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.

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