Re: Pig current limiting

to: Marco

A good "rule of thumb" for filter cap size in this resonant charging
scenario is 10 x resonant cap size.  This will provide adequate energy for
the tank circuit.


> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com
> Subject: Re: Pig current limiting
> Date: Saturday, January 09, 1999 9:12 AM
> Original Poster: "David Dean" <deano-at-corridor-dot-net> 
> -----Original Message-----
> From: Tesla List <tesla-at-pupman-dot-com>
> To: tesla-at-pupman-dot-com <tesla-at-pupman-dot-com>
> Date: Friday, January 08, 1999 1:06 AM
> Subject: Pig current limiting
> >Original Poster: "Marco Denicolai" <Marco.Denicolai-at-tellabs.fi>
> >
> >The tank supply I am designing uses a 3-phase pig transformer and a 6
> >bridge to charge a filter capacitor of 15 uF. After that there is a
> >that charges the primary capacitor usign resonant charging.
> As for the three phase bridge rectifier-
> from Power Supplies Switching Regulators Inverters and Converters
> by Irving M. Gottlieb copyright 1984
> ISBN 0-8306-0665-3
> Published by TAB Books
> Blue Ridge Summit, PA 17294-0850
> (note that factors are normalised with respect to Average DC)
> Average output volts DC---1.0
> RMS volts output------------1.0
> Peak volts output------------1.05
> Peak Reverse Rectifier Voltage-1.05
> Ripple factor------------------4.2%
> Average DC output current-1.0
> Average DC output current (per element) 0.333
> RMS current/element (resistive load)-0.579
> RMS current/element (inductive load)-0.578
> Peak current/element (resistive load)-1.05
> Peak current/element (inductive load)-1.00
> >From this it can be seen that a filter capacitor is not needed for a
> phase bridge supply as the ripple and the percent regulation are very
> already.
> This is the design used in automotive alternators because of the high
> efficiancy,
> the low ripple factor, and the excellent voltage regulation.
> >
> >Of course, every time the primary capacitor gets charged the filter (15
> >capacitor voltage drops a little down: I need a device to limit the
> >used to re-charge the big capacitor.
> >
> Probably not. If you use a filter cap you can expect a surge when you
> up.
> This surge may overload the diodes and or the transformers while the cap.
> charges.
> However if the diodes can handle the surge, the worst thing that will
> happen is you will
> blow a fuse or trip the breaker on the mains.
> >A suitable resistor will delay too much the recharge (and develop losses
> >and heat!) and the capacitor will never reach full voltage.
> >
> >I was thinking to some limiting reactor on the 400VAC (primary) side.
> >
> >My questions:
> >
> >- if I use a simple autotransformer, it will limit the voltage but the
> >current will be still orders of magnitude too high, right?
> >- what about using a non-linear reactor, one that allows the current to
> >rise until a value X and then will keep it about to that value. I mean
> >a MOT with secondary shorted? Who manufacture/sells these devices?
> >
> If you use an autotransformer to ramp up the voltage on power up you will
> spread
> out the charging current over a period of time much greater than one
cycle of
> the
> input current. This will effectivly eliminate the "power on surge" and no
> current limiting reactor will be required.
> As long as the tank capacitor is a reasonable fraction of the value of
> filter capacitor
> the voltage will never drop on the filter enough to worry about. It would
> nice to be able to control the
> input voltage to the "pig" though, so the autotransformer is probably a
> idea.
> >
> >
> Hope this helps a little bit-
> later
> deano