Re: Pig current limiting

-----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 diode
>bridge to charge a filter capacitor of 15 uF. After that there is a reactor
>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 three
phase bridge supply as the ripple and the percent regulation are very good
This is the design used in automotive alternators because of the high
the low ripple factor, and the excellent voltage regulation.
>Of course, every time the primary capacitor gets charged the filter (15 uF)
>capacitor voltage drops a little down: I need a device to limit the current
>used to re-charge the big capacitor.
Probably not. If you use a filter cap you can expect a surge when you power
This surge may overload the diodes and or the transformers while the cap.
However if the diodes can handle the surge, the worst thing that will likely
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 like
>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
out the charging current over a period of time much greater than one cycle of
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 the
filter capacitor
the voltage will never drop on the filter enough to worry about. It would be
nice to be able to control the
input voltage to the "pig" though, so the autotransformer is probably a good
Hope this helps a little bit-