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Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
*This Post Was Edited On 8/28/09*
I have been doing some research for my next DRSSTC build. Below are the calculated parameters from JAVATC for my MMC, Primary, Secondary, And Topload. This is going to be a big coil running off a 240 line @ 30 amps. I will be using Daniel's Resonant board to generate the gate single. And I will be using Daniels Iso Drivers to drive the HUGE cm600's.
A few questions... - Is the MMC Capacitance and voltage rating adequate? - 40kHz a good resonant frequency for the cm600s? - I wanted to go with a pancake style Primary however I felt the OD of the primary would increase the risk of strikes from the secondary... would you agree? - 26awg to thin for this kind of coil? - Powerex has multiple cm600 model numbers. I have selected 2 that I think would work, the Cm600HA-24H or the CM600HB-24A. The only diffrence between the two (I think) is the Maximum Collector Dissipation. Which one should I choose? (attachment at bottom of page)
Units = Inches
Secondary Specs Secondary length = 40" Secondary Diameter = 10" #26 AWG turns = 2210 <--- I think this wire might be a bit thin? Topload = 10" x 45" secondary resonant freq = 40kHz Aspect ratio = 4:1 Coupling = .2k # of 1/2 cycles for energy transfer = 5 Transfer time = 61uS (not sure if this acceptable) Secondary Q = 228 (not sure if this is acceptable)
MMC Specs Capacitance = 1uf (BIG) Capacitance Reactance = 3.979ohms Voltage peak at 1200amps = 4775v
12 caps in series x (500Vac x 1.41) = 8484votls (almost double the calculated voltage peak) 8 caps in parallel = 1uf exactly Total number of caps = 96... (expensive but not bad)
High Voltage Specs (Have not decided on half bridge or full.... any advice?) Half-bridge = 2 cm600s DC Power = 677volts (Fullwave rectified, I would love some advice on cap size and voltage rating) GDT driver = Daniel's ISO drivers Modulator / driver = Daniel's resonant board
Would Steve Ward's universal driver work with this setup?
Registered Member #1911
Joined: Mon Jan 05 2009, 06:30PM
Location: Salem, Oregon, USA
Posts: 165
I'm not the greatest with SSTCs, but I'm pretty sure that TeslaMap won't tell you the proper parameters for an SSTC. Sure, it'll tell you Fres of the secondary, but you won't get the proper bank capacitance, since you'll be using your own frequency to drive it, rather than the 60Hz line frequency. If I'm wrong, please correct me, but this is just what I have observed.
Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
Plasma Lover wrote ...
I'm not the greatest with SSTCs, but I'm pretty sure that TeslaMap won't tell you the proper parameters for an SSTC. Sure, it'll tell you Fres of the secondary, but you won't get the proper bank capacitance, since you'll be using your own frequency to drive it, rather than the 60Hz line frequency. If I'm wrong, please correct me, but this is just what I have observed.
you may actually be right... the mmc tank capacitance was based off Steve Ward's DRSSTC 2. Since mine will be close to 8kW I thought the same MMC design might work, however if what you say is true than my primary needs some re-thinking.
Registered Member #1107
Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792
Tesla map is not very good for calculating DRSSTC's. Use JAVATC ithat program has never let me down and the calculations are spot on. One obvious thing i see is the tank capacitor is not nearly up to the task voltage wise. Running at 1500A will have about 9kvP going through the capacitor and it will not last very long that way. You should design the primary cap to be rated at at least 8kvac instead of 8kvdc.
Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
teslacoolguy wrote ...
Tesla map is not very good for calculating DRSSTC's. Use JAVATC ithat program has never let me down and the calculations are spot on. One obvious thing i see is the tank capacitor is not nearly up to the task voltage wise. Running at 1500A will have about 9kvP going through the capacitor and it will not last very long that way. You should design the primary cap to be rated at at least 8kvac instead of 8kvdc.
forgive my ignorance, but why do i want the caps rated for 8kvac? the HV supply will be DC. very similar to Daniel's Minibrute Voltage Doubler.
I attached an image of the current minibrute schematic. Ill go ahead and use JAVATC and see what I come up with.
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
Hi Austin. The primary capacitor (the MMC) is what needs to have a high voltage rating. Remeber, the primary coil and MMC bank have impedances on their own, so they will see high voltage during operation. Lets say you are driving the system at 46 KHz like you are. Well, at 46 KHz a .5625uf cap bank will have a capacitive reactance of about 6 Ohms. (google capacitive reactance calculator) So what voltage is required to push 1500Amps through 6 Ohms,,,, well, around 9 KV. So your MMC will see 9KV in the positive and negative directions. Now, if you get ambitious and decide to run 2000 amps peak, then your cap bank will need to have an even higher voltage rating, because a higher voltage would be required to push more current through the system. What if you double the resonant frequency of your system, then your cap bank will need to have an even higher voltage rating, because capacitive reactance goes up with frequency. Since the Voltage the mmc will see is AC, you probably want to design the bank to withstand much more than 9KV. As far as I know, quick voltage reversals from +9Kv to -9Kv call for a bank that can withstand much closer to 18Kv.
forgive my ignorance, but why do i want the caps rated for 8kvac? the HV supply will be DC. very similar to Daniel's Minibrute Voltage Doubler.
You posted a picture of Dan's DC supply to the bridge. Those caps simply provide 400VDC for the input to the bridge. They don't see high voltage. Remeber, each one of those caps will see a max of 200V DC (assuming 140VAC input) On the positive half cycle of the AC one cap will charge to 200VDC, and then the negative half cycle will charge the other cap to 200VDC. Put them in series, as Dan did, and then you get 400VDC across the two. These caps store the energy that will be transfered into the tank circuit (mmc, and primary) on each half cycle of operation.
Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
thanks for the reply Steve it was very helpful.
Ok so what you said about about capacitive reactance made a lot of sense. but I still have a few questions.
-how did you determine peak-to-peak currents of 1500 amps? I thought each cm600 from emitter to collect was rated for 600amps -I am still not sure how to determine an adequate capacitance for my bank. As I understand it, the capacitance should be a function of the amount of current flowing every second or in this case @ 46khz. However all the calculators seem to based off NSTs
1 farad = 1 coulomb / volt 1 amp = 1 coulomb / second Voltage = 18,000
If I have 1500 amps peak-topeak flowing want is the needed capacitance (in farads). The system runs at a rate of 46khz or 46,000 times a second.
So that means in 1 second .032609 coulombs would have passed through the circuit right?
And .032609 coulombs / 18000 volts = .000002 farads or .2uf? this is most likely wrong, but it is my understanding thus far.
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
Well, lets just go over it all! Warning: This will be very long.
Before we talk about choosing capacitors, lets analyze what happens in an LC and what happens when you change the components in the LC. (this seems to be your main concern) As you know, an LC consists of a capacitive and inductive component that are both capable of storing energy. The capacitor stores energy in an electric field and the inductor stores energy in a magnetic field. So what determines the resonant frequency, well, the values of L and C of course. Now heres something that is critical to understand. If you increase the value of the capacitor (meaning a larger capacitance, not neccesarily a larger capacitor), or if you increase the value of the inductance, then the Fo (resonant frequency) goes down. So what happens if you decrease the value of the L or C? Then the resonant frequency goes up.
Ok, now imagine that we are going to charge a capacitor with a 20V source. What happens when you connect the capacitor to the V source. The largest current flows right at the start, and then the current decreases as time elapses. Once the current reaches 0, the capacitor is then charged up to 20V. The voltage from the supply source would still be 20V, and the voltage from the capacitor would be negative 20V so no more current would flow. (the voltages cancel out) In other words, if you graphed the current, it would be entirely concave up.
Now, lets imagine we are trying to charge that same capacitor with an inductor in series. Do you think the largest current would occur right at the start when you connect the circuit to a V source. WRONG. Remeber, inductance is the property that opposes changes in current. So when the capacitor will try to pull current, the inductor will be reluctant to let it do so at the start. (they have a love hate relationship) The current will slowly rise up, because the inductor is OPPOSING the increase, and then once the capacitor is near full charge, the current will drop back to zero. But just as the inductor opposes the increase in current, it will oppose the decrease, so the current will fall back to zero slowly. So the current slowly climbed up, peaked, and then went back down. This is what causes a sinusoidal primary current in a drsstc tank circuit.
Now let me explain this a bit more so you can gain an intuitive sense of what happens when you CHANGE L or C within a drsstc primary circuit. (remeber, the DC supply source is trying to charge C through L)
Increasing L: Having a large L would mean way more opposition to current change. The inductor would take forever to let the current rise to charge the capacitor, and then take forever to let it fall. The current waveform would be much flatter, but longer, like a hill, rather than a sharp peak that is short and tall. Increasing L would DECREASE the resonant frequency.
Decreasing L: The inductor would let the current rise quickly, thus charging the MMC quickly, and then current would fall quickly. Lowering L would make the resonant frequency higher.
Increasing C: A larger capacitor simply takes longer to charge. This would decrease the resonant frequency. However, a large C means a higher current, because a larger capacitor will pull more current than a smaller one from the same voltage source.
Decreasing C: Smaller capacitors pull less current, and are faster to charge. This increases the resonant frequency.
So now you know what happens when we change L or C when we are trying to charge (DC input to bridge) a capacitor bank (the MMC primary caps) with an inductor (your primary coil) in series.
I am tired of typing at the moment. Later tonight I will explain a bunch more stuff to you.
Registered Member #1169
Joined: Wed Dec 12 2007, 09:16AM
Location: Portland OR
Posts: 251
hey Steve,
Thank you for taking the time to write such a through and good explanation of an LC circuit. To supplement your lecture I went on youtube to to find illustrations of what you were explaining.
I think I have a good conceptual understanding of how and why and LC circuit oscillates and how different parameters such as the capacitance or inductance can affect the voltage or frequency of the circuit.
I am still having trouble computing a capacitance and voltage rating for my MMC bank however.
should i determine the capacitance based on the resonating frequency of the secondary? Or should the resonating frequency of my primary determine the size of my secondary?
If I have a 240 volts input and I put two 400 volt capacitors in series i should get 800VDC to play with correct?
How do i insure that that capacitance of my mmc does not draw to much current resulting overheating / failure or just the opposite scenario where I draw too little current resulting in smaller sparks?
I think the fact that this is an DC LC circuit and not an AC LC circuit like a spark gap is confusing me :(
Registered Member #1845
Joined: Fri Dec 05 2008, 05:38AM
Location: California
Posts: 211
I am still having trouble computing a capacitance and voltage rating for my MMC bank however.
Well, the tank circuit values will dictate what your surge impedance will be. High surge impedance mean that it takes more RF cycles to reach a given current level, and a lower surge impedance means energy transfer will be quick, and the current will build up fast. If I recall, Chris' "Fatboy" DRSSTC used an MMC of .3uf? See if you can find out what fatobys tank values are. Its driven by cm600's also. I don't really know what surge impedance is adequate for a given drsstc. There is probably a lot to consider when it comes to choosing a good Z value. Maybe someone could enlighten us here.
should i determine the capacitance based on the resonating frequency of the secondary? Or should the resonating frequency of my primary determine the size of my secondary?
I think it is generally better to first decide what your secondary parameters will be, because it is much easier to "tweak" the number of primary turns, than to adjust the Secondary and Topload.
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.
How do i insure that that capacitance of my mmc does not draw to much current resulting overheating / failure or just the opposite scenario where I draw too little current resulting in smaller sparks?
Well, you will start with a small bus voltage and work your way up. If you going to basically duplicate Steve's drsstc 2 you should probably just use his value for the mmc.
I think the fact that this is an DC LC circuit and not an AC LC circuit like a spark gap is confusing me :(
The current within the tank circuit alternates. It is an AC LC circuit. The DC input to the bridge adds energy to the tank circuit (which has, of course, an alternating current) on each half cycle of operation. It adds energy to the positive half cycle by placing a positive voltage across the tank, and then adds energy to the negative half cycle by placing a negative voltage across the tank.
This brings up a question of mine. If one builds a drsstc that hits 200amps on the first half cycle of operation, then in a perfect world, (100% efficiency) it should hit 400amps on the negative half cycle right? Wait, maybe I should be starting a new thread, this is not on topic?
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DRSSTC-2 Design Review (Updated Specs 8/28/09)
