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Registered Member #3302
Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42
So i have this big SCR that i have been using for my induction launcher to shoot some aluminum discs, the SCR is rated for 8000A pulse at 50Hz and 8380A pulse at 60Hz. The pulse for my current coil is about 0.5ms long and peaks at just under 7kA, that is well below the rating, but i am thinking of getting a bigger cap bank and the simulation says that it would peak at 10.2kA that is above the rating. So i am asking how can i calculate how much current the SCR can handle for a lets say 1ms pulse? 1ms pulse is over 8 times shorter than a 60Hz pulse that is 8.3ms, so i would guess it could handle a 10kA pulse easily but if someone can provide me some sort of equation to calculate this it would be great.
My current cap bank is 3300uF at 380V and the new one would be 6600uF at 450V. Currently the launcher shoots a 30g aluminum disc 5mm thick and 60mm diameter about 20m/s so its about 6J of kinetic energy but it doesnt even go through a bit thicker cardboard so i want some more power.
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1714
Wizzup wrote ...
So i have this big SCR that i have been using for my induction launcher to shoot some aluminum discs, the SCR is rated for 8000A pulse at 50Hz and 8380A pulse at 60Hz. ... 1ms pulse is over 8 times shorter than a 60Hz pulse that is 8.3ms, so i would guess it could handle a 10kA pulse easily but if someone can provide me some sort of equation to calculate this it would be great.
Traditionally, and within limits, the single-pulse current capability can be expressed as "I squared t" or I^2t , the time integral of the square of the current. So the device can handle 2x the current for 1/4x the time, e.g. 8000 A peak for 10 ms (half cycle of sine wave) is like 16000 peak for 2.5 ms. Numerically, and considering the RMS current, I2T = 8000^2/2*0.010 = 320,000 ampere-squared seconds. Is that in the datasheet?
The numbers you gave for 60 Hz, compared to 50 Hz, show that the mfr is much more conservative about increasing current as the pulse width shrinks. You might plot the pulse width versus max current (or vs. computed datasheet I2t) on a log-log scale, and extrapolate to your intended pulse width.
You might find it interesting to look up or calculate the minimum gauge of copper wire that can handle 320,000 A^2s with less than, say, 100 degrees C of temperature rise per shot.
I expect that the peak current in your induction launcher will be significantly higher WITH a projectile than without. Are you equipped to measure that? The inductively coupled shorted turn will increase R and reduce L looking into the primary winding. How far do you figure the projectile moves away during the current pulse?
Registered Member #3302
Joined: Sun Oct 10 2010, 02:21PM
Location: Finland
Posts: 42
Thanks a lot for the clear-up, i don't have any equipment to measure the current during a shot but the SCR will for sure survive the new cap bank, at 0.625ms pulse it should handle over 30kA regarding to what you said and i dont think that the current will triple when the projectile is placed in front of the coil so it should be safe. Also if the current increases the pulse time will shorten a bit atleast so it compensates it a bit.
EDIT: I see you edited your post, i calculated that if the I^2t value would decrease linearry as the pulse width decreases, it would be 157kAs at 1.3ms pulse, that would give about 15kA max pulse current for that time. So with 0.5ms pulse it would still handle about 23kA with the same formula and that is over twice what is needed for my purpose. Even if it would decrease even more than that, it should be still over 15kA for sure that should be enough also. If someone thinks it will go below 15kA and my math is totally wrong then please correct me here.
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SCR peak current calculation
