Do Parallel SCR's fire off exactly at the same time?? Need drive ideas / Electromagnetic Projectile Accelerators / Forums

Forums

4hv.org :: Forums :: Electromagnetic Projectile Accelerators

« Previous topic | Next topic »

Do Parallel SCR's fire off exactly at the same time?? Need drive ideas

Move Thread

LAN_403

w1vlf

Mon Jun 02 2008, 02:42AM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Hello Again Folks.

Right now I am using a contactor to fire my induction launcher.

Some parameters 6000mFd 750 VDC
Flat wire coil, 12.7 uHy Sorry but I do not have a good number for the coil resistance.

Time to fire up the 4 wire ohm meter. but my calculations say it should be roughly 10 milli Ohms.

Recently replaced my transformer in the HV supply and will be able to charge to 1000 Volts.

So,,,,

Question 1) Can someone take those specs and run them through

and make sure I am using this calc. correctly? I am getting currents in excess of 11 K amps

Question 2) I have 4 giant SCR's Stud mount type.
Each SCR is good for 1200 volts and 8000 amps non rpetitive at 16 millisecs.

I have already blown up 2 of these.

What I would like to do is parallel 4 of this and switch them all the same.

But I am worried that they will not all turn on at the same instant, and will sucessively pop .

Right know I am going to stick with the contactor, but the SCR's would be so much nicer if its possible.

Any suggestions.

I ahve about 30 more 2400 uFd @ 450 vdc would love to press into service.

So has anyone really pushed scr's in parallel.

PauLC
W1VLF

teslacoolguy

Mon Jun 02 2008, 04:24AM

Registered Member #1107 Joined: Thu Nov 08 2007, 10:09PM
Location:
Posts: 792

TDU did something very similar with hockey puk scr's.

Electroholic

Mon Jun 02 2008, 04:57AM

Registered Member #191 Joined: Fri Feb 17 2006, 02:01AM
Location: Esbjerg Denmark
Posts: 720

i think he said parallel, not series.

and to answer the original question, yes, SCRs can be put in parallel pretty easily.

OZZY

Mon Jun 02 2008, 12:42PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

Hello again w1vlf

I have estimated the ESR for 1 cap at 60mOHM so the ESR for the bank is 24mOHM plus 10mOHM for the coil gives 34mOHM and 12.7uH. If I put this in Barry`s RLC simulation I get a pulse of 0.928mS with a peak of 12.2KA at 900v.

This pulse is underdamped and shows a peak negative voltage on the cap bank of -250v. When using the contactor the pulse will ring down naturally and the capacitors will not be damaged (much). The SCR will only conduct in one direction and will switch off when the current crosses zero, this will leave the cap bank charged to -250v and will result in damage to, or even explosion of the capacitors.

You can use SCRs in parallel and they will share current reasonably well but they are not guaranteed to share equally or to turn on at the same time. This may or may not be a problem depending on the application.

Ozzy

w1vlf

Mon Jun 02 2008, 03:45PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

OZZY,
I totally forgot about adding the ESR of the capacitor bank. Thank you for remembering that. So it might be best to stick with the contactor for now. It reliable and just needs to have the contacts cleaned up ever 20 or so shots.

Thank you
I am going to start another thread about launch speed and the pulse width

PauLC
W1VLF

OZZY

Mon Jun 02 2008, 06:26PM

Registered Member #511 Joined: Sat Feb 10 2007, 11:36AM
Location: Somerset UK
Posts: 55

w1vlf

The contactor is a simple and elegant solution, it has served you well so far. Using SCRs would be more efficent but at the power levels of your launcher it deserves a bit of proper design to make sure it`s reliable. Check out the launchers built by FastMHz and Quantum Singularity.

edit: Happy birthday

Ozzy

Barry

Tue Jun 03 2008, 01:40PM

Registered Member #90 Joined: Thu Feb 09 2006, 02:44PM
Location: Seattle, Washington
Posts: 301

A good contactor may have lower resistance than an SCR, and this is usually very important.

You may be able to reduce pitting by speeding up the contact closure. In a circuit like this with series inductance, a faster closure can shorten the contact bounce before the current builds up. You might drive it with a little over-voltage, or there are relay drive circuits with a capacitor to improve pull-in time.

Can you post specs and a photo of your contactor for us? I'm considering something like this for my next (higher-power) coilgun. I might even build my own using a copper wedge. Thanks!

Barry

w1vlf

Thu Jun 05 2008, 04:27PM

Registered Member #1329 Joined: Mon Feb 18 2008, 07:31PM
Location: Harwinton Connecticut
Posts: 53

Hi Barry, Sorry about the delay in answering, I got a little behind on emails and the like.

Hoping this works there will be 2 attachment. 1 is the contactor itself and the other is a sorta close up of the contacts.

This an old.. I'll bet 30 years anyway 24 volt contactor from a fork lift. I use an 80 volt supply with 15,000 uf capacitor to fire the contactor.
The switch that charges the HV supply also charges this 80 volt cap.
The contents of that cap is used to pull in the contactor.

The contactor is pretty dirty and old but it does not seems to get leaky at least until up to 1000 volts. I have a megger on which I will check the open resitance of the contactor some day.
The picture of the contacts are after 15 aunches or so.

The contacts did stick in the beginning, until i was able to compensate for the mismatch in alignment due to the arc length of the contact arm.

PaulC
W1VLF

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob