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Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
I'm not sure what Steve was talking about. Initially you had a .5625uf mmc and an operational frequency of 46.39KHz. Thats a capacitive reactance of about 6.099 Ohms. Then with your new setup you have a .75uf mmc with a new frequency of 39.14KHz. That a capacitive reactance of 5.422 Ohms. The fact that you increased the mmc value dropped the resonant frequency of the system.
Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
SteveC wrote ...
I'm not sure what Steve was talking about. Initially you had a .5625uf mmc and an operational frequency of 46.39KHz. Thats a capacitive reactance of about 6.099 Ohms. Then with your new setup you have a .75uf mmc with a new frequency of 39.14KHz. That a capacitive reactance of 5.422 Ohms. The fact that you increased the mmc value dropped the resonant frequency of the system.
Right?
I changed the physical dimensions of the secondary to lower the resonant frequency for the CM600 IGBTs. I'll be using a 10" diameter tube instead of 8" this lowered the resonant frequency to about 37 khz and then I added 5" to the secondary which brought it back up to about 39khz. (calculated using JavaTC)
I let javaTC auto tune the primary coil to to the secondary (which auto changed the physical dimensions and turns of the primary) with a coupling of about .147k and an MMC capacitance of .75uf @ 10kv.
I have really learned so much about DRSSTCs in the last few days. Thank you so much guys. KEEP EM COMING!
The next step will be designing and constructing a low inductance Half Bridge. I ordered the parts and they should be coming very soon. Thanks Daniel.
For the Fullwave Rectified voltage doubler will two 12,000uf @ 350volt Electrolytic Caps in series work?. will that be adequate for my system? What should the rectifyer diodes be rated at? >900Volt @ 50amps?
Registered Member #1900
Joined: Fri Jan 02 2009, 06:44PM
Location: Texas
Posts: 29
SteveC wrote ...
I'm not sure what Steve was talking about. Initially you had a .5625uf mmc and an operational frequency of 46.39KHz. Thats a capacitive reactance of about 6.099 Ohms. Then with your new setup you have a .75uf mmc with a new frequency of 39.14KHz. That a capacitive reactance of 5.422 Ohms. The fact that you increased the mmc value dropped the resonant frequency of the system.
Right?
I think he meant that in general the impedance of a capacitor goes down with increasing operating frequency (think high pass filter)... but in our special case (DRSSTC resonant circuit) you are altering the properties of the capacitor (capacitance) which changes the resonant frequency which, due to feedback, changes the operating frequency in the opposite direction. The impedance also goes in the opposite direction.
|Zc| = 1/(2pi*f*C)
The general case would be true if we were changing the frequency elsewhere (such as primary coil).
I was confused about your statement too... but I get it now.
Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
teslacoolguy wrote ...
SteveC wrote ...
If I have a 240 volts input and I put two 400 volt capacitors in series i should get 800VDC to play with correct?
If its the standard doubler arrangement then yes.
huh? According to my calculations, you will have a maximum of 672V across the bus.
yah your right, 240volts X 2 X 1.414 RMS = 672 peak voltage or 650 volts due to resistance...
I am still unsure how to design a full wave rectified voltage doubler. I have sample schematics but not sure how to determine the capacitance / voltage rating of my capacitors or the voltage / current rating of the diodes. Obviously the system needs to be designed in a way that I store enough energy to supply the MMC but also that they have enough time to recharge for the next pulse. I am going to get Pspice to today and see what kind of simulations i can run. I think this will help me gain a better understanding what parameters affect what.
But if someone would be willing to educate me and the rest of the world on how to design a full-wave rectified voltage doubler, it would be greatly appreciated!
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
huh? According to my calculations, you will have a maximum of 672V across the bus.
You are absolutely right Teslacoolguy, my mistake. With 240VAC it would be something around 672. He could only get up to 800VDC if he had a 240V variac that could step up the mains voltage to 280VAC on its secondary side. (not sure if most variacs can do this)
I think he meant that in general the impedance of a capacitor goes down with increasing operating frequency (think high pass filter)... but in our special case (DRSSTC resonant circuit) you are altering the properties of the capacitor (capacitance) which changes the resonant frequency which, due to feedback, changes the operating frequency in the opposite direction. The impedance also goes in the opposite direction.
|Zc| = 1/(2pi*f*C)
The general case would be true if we were changing the frequency elsewhere (such as primary coil).
I was confused about your statement too... but I get it now.
Yes, if you were to just connect a cap to a HF AC source, its impedance would decrease with increasing frequency. Like you said, in the drsstc case, if you doubled the mmc cap value, that would seemingly decrease its reactance, but at the same time it would drop the resonant frequency of the system and that would increase its reactance.
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DRSSTC-2 Design Review (Updated Specs 8/28/09)
