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Re: Off Line - DRSSTC (OL-DRSSTC) Concept....



Original poster: Terry Fritz <vardin@xxxxxxxxxxxxxxxxxxxxxxx>

Hi DC,

When we rectify the AC line we basically get a 120Hz signal so the system should simply resonant at 120 Hz.

F = 1 / (2 x pi x SQRT(L x C)

For 120Hz and 185uF that works to 9.51mH.

Any modern fast IGBT should be fine...

Cheers,

        Terry


At 11:52 AM 9/24/2005, you wrote:


Great idea Terry. What is the value of the blocking inductor and can you suggest an IGBT part number for this current level. I would prefer Semkron if you have their datasheets.

Dr. Resonance



The OLTC is very simple and tough, the DRSSTC is fairly complex and not quite as tough... I think there is a system (concept only!) that might marry the two in a very simple, cheap, and hardy configuration that any coiler should be able to put together...


I direct you attention to this schematic:

http://hot-streamer.com/temp/OL-DRSSTC.gif

Allow me to explain...

From the top right, the AC line voltage (probably from a variac) is rectified and charges a capacitor (Cresonant) through a blocking inductor to say 300VDC. This capacitor is similar to the ~3000uF buss caps we use in the DRSSTC. However, this capacitor is significantly smaller in value since it is limiting system power in this "fixed" 120 BPS coil. For a 1000W 120 BPS coil:

1000W = 120BPS x Ecap Ecap = 8.33Joules 8.33 = 1/2 x C x 300^2 C = 185uF

Note that with the proper selection of blocking inductor, the capacitor voltage can be "resonantly doubled" off the 60Hz AC line. I think "my" OLTC is the only one that ever used that proven concept...

Note that if the IGBTs obnoxiously decide to cross conduct, the 8.33 joules could be absorbed directly by the thermal mass of the IGBTs and the blocking inductor would provide time for them to correct. - It should never blow up...

So every 120th of a second, the system is provided with a say 185uF cap charged to 300VDC to "run the thing"... The duration of the cycle will be determined by how long it takes to use up the 185uF cap's energy.

Lprimary and Cprimary are typical per DRSSTC standards as is the secondary coil and all...

The H-bridge has four typical IGBTs with anti-parallel diodes.

The gate drive to the IGBTS is derived directly from the primary current now! There are four current transformers that provide isolated gate drive directly to each of the IGBTS from the primary current loop. Back to back Zener and Schottky diodes provide voltage control and very fast switching control of the current from the CTs to control the IGBTs. Of course, the "phasing" needs to be correct...

So the 185uF cap will discharge thought the IGBT H-Bridge and drive the coil. The direct connected, but voltage limited, CTs will feedback the primary current to the IGBTs providing synchronized timing control.

The firing MOVs "start" the cycle. Once the voltage reaches their level, they will conduct and "turn on" the system so the CT and IGBT system will start running.

As the 185uF cap drains, the system will fall out of conduction. When the primary current and drive signals collapse, the system's power will be almost all drained, so no harm should come...

This is all purely untested "pie in the sky stuff"... But if the concept is valid, it would bring together the best of the OLTC and DRSSTC together in a very simple, cheap, and robust system.

You can hold the AC line - to primary tank drive system in the palm of your hand!!!

If the concept holds, everything just changed... Again ;-)))

Cheers,

Terry