SCR Drive Thermal Compound

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Thread Starter

Ted Isto

A couple of our books on SCR Drives around here (in the 50-200 hp range) recommend a silacone thermal joint compound (Alcoa #2 or equivalent) on the surfaces of the SCR when reinstalling a new one between the heatsink and the drive.I think the compound should be both thermally and electrically conductive but what I find in catalogs is only thermally conductive. Any ideas of sources for what you recommend? Many thanks, Ted
 
I would not recommend an electrically-conductive thermal heatsink paste. In 1989 I had to replace a hockey-puck GTO on a Square D (Ramsey) drive and their manual required the above compound. The only compound I found had powdered silver in it. I was uneasy using this due to the 600VDC application; what if this stuff drips or migrates? I called Square D and their engineers agreed with me and told me to use regular heatsink compound. I made certain that all bonding jumpers were in place and in good condition.
 
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Curt Wuollet

Heat Sink Compound at any Radio Shack store or electronics distributor. Thermally conductive metal oxides in a silicone grease. There is no need for electrical conductivity as the stud or the fasteners take care of that. Use as little as possible, the idea is to fill the voids and the points that make contact transfer heat better than the grease.

Regards

cww

 
I would not recommend an electrically-conductive thermal heatsink paste. In 1989 I had to replace a hockey-puck GTO on a Square D (Ramsey) drive
and their manual required the above compound. The only compound I found had powdered silver in it. I was uneasy using this due to the 600VDC
application; what if this stuff drips or migrates? I called Square D and their engineers agreed with me and told me to use regular heatsink
compound. I made certain that all bonding jumpers were in place and in good condition.
 
B
If the application is what I'm figuring ("hockey puck" SCRs clamped between heat sink extrusions) such that the conductive part is from "HS1-to_SCR
Anode Flat (thru SCR die) to SCR Cathode Flat-to-HS2" then I'm not certain Curt's advice will work. I don't know how well zinc oxide/silicone paste fares when putting it inline (twice) with a couple tens to hundreds of amps trying to flow through, but the spec sheets shows, for instance, volume resistivity per ASTM D-257 of 2.00 x 10^15 ohm-cm, so it's likely not very good.

If memory serves, Reliance Electric once packaged a small 'coin battery' sized container of 'special transfer compound' with each SCR they sold, but didn't see any the last time I bought a set of Reliance-brand SCR to rebuild one of our 250 HP stacks (so I just carefully cleaned the surfaces, and hoped for the best). On the other hand, did not notice evidence that it had any factory-applied thermal transfer compound in the first place. Don't know if this was because they don't supply or use the thermal grease anymore, or
simply don't use it on the smaller SCR pucks (the ones I'd changed in the past were for 300 and 400 HP drives).

Looked around on the web a bit (because I didn't have any luck a couple of months back when poking aorund the Reliance site for the same thing), and
didn't get any usable hits on the Alcoa #2 reference.

Found that prying info on thermal transfer compounds out of the search engines wasn't going very well ... learned CG Electronics is now called GC Waldom, and neither shows up as having a web site on Allied Electronics line card page.

This site http://www.melcor.com/tgrease.htm has a table of different thermal greases, but not much information of their specs.

CG Electronics type 44 specs are on, on all things, a computer sales site http://www.directron.com/type44.html

When I searched on the MIL spec number obtained from this reference (MIL-C-47113), came up with a number of useful hits.

http://www.aosco.com/sil_chart.shtml (AOS Thermal Compunds, Inc.)
http://www.polysi.com/technicaldata.htm (PolySi Tech)
http://wwww.arcticsilver.com/as2.htm (Silver-based, poorly electrically
conductive grease)
http://www.rbdc.com/~moreau/Chemplex1381.htm (Chemplex 1381)
http://www.d6industries.com/ (Various formulations; tech ref section)
http://www.chemtronics.com/catalog/catalog.cgi?action=list_products&category=7
(Chemtronics CircuitWorks Conductive Grease - claims to be electrically conductive)


Of these, only Chemtronics addresses a transfer grease that is both thermally and electrically conductive, which means this is a rare beast.

You might want to call the drive manufacturer, and see if they still use Alcoa #2 transfer compound, and, if so, where *they* get it from.

Bob
 
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Curt Wuollet

Hi Bob

You might be right for hockey puck packages. But for those, don't the finish, flatness, and pressure specs provide thermal transfer without the compound? I'd get an application note from the SCR manufacturer to be sure. These are kinda spendy.

Regards

cww
 
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David Erickson

I would recommend Burndy Penetrox. They have various formulations depending on the materials in the joint and the temperature range of the joint. The SCR to heatsink joint should use the higher temp. compound whereas a bus-bar joint can probably use the lower temp. compound.

We used Penetrox the 11+ years I worked at a drives company and they still use it today.

A word of warning - do not get it on your clothes - it will never come off.
 
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