Friday, July 6, 2007

Power Supplies - Update #1

My thinking on the power supplies has evolved.

What I'm considering now is a 400 or 500 VDC power bus (maybe two of them) SCR regulated from a 3 phase power supply. 12 pulse for reduced filtering requirements.

The bus will supply the power for 100 V @ 25 A converters (parallelable) for the magnet supply, and 1,000 V @ 25A converters (series amenable) for the high voltage.

If each converter has its own phase trigger, the phases can be staggered so that the effective ripple frequency will be the drive freq. (around 30 KHz) times the number of converters, times 2 (for full wave rectification). For a 1,000 A @ 100 V magnet supply that puts the ripple up at about 2.4 MHz. Easy to filter. A 20 KV @ 25 A supply would have 20 converters. That would give a 1.2 MHz ripple. A full up 80 KV @ 25 A supply would have 4.8 MHz effective ripple.

Update: 08 July 007 1856z

On the HV side we will also need 24VDC (nominal) @ 25 A to power any controls on the HV side including UC2901 feedback modulators. Plus 120VAC @ 60 Hz 10 KW for misc power. The 24VDC will be battery backed to supply surge currents. There will also be quench SCRs (into resistive loads) for shorting the magnet supply and the HV supply. The resistors to be sized so that the supply output is discharged in .1 second or less to 1/10,000th of the maximum voltage or current.

Update: 09 July 007 0757z

I have been thinking about safety issues. Most notably what happens when utility power is lost? First off if the LN2 pump is pumping it should keep pumping for a few minutes after power loss. Which means battery backup. Second any water cooling must continue for a few minutes after power off. The water chiller need not be backed up. The control computer needs to be battery backed up. In fact it should operate from the battery bus at all times so power glitches do not affect its operation. The same will be true of the data collection computer. In addition a backup generator with a 48 hour gasoline supply should be provided. All important loads should have automatic transfer switches to the backup generator. Power transfer should be sequenced to minimize the surge load on the generator.


Roga said...

You can pre-pressurize the LN2 tank(s), or self press them by dumping some waste heat in through a tank heat exchanger, or even just have the LN2 gravity fed and pumped up to the reservoir, depending on the Dp you need through the coils. This will ensure passive coolant flow as long as the valves stay open, without the need for battery backup; or in addition to battery backup as an extra layer of safety.

M. Simon said...

I would actually like to operate the LN2 tanks at somewhat below atmospheric during runs in order to reduce the temperature to below 77°K.