Max We used to be Westinghouse power distribution components division, now we are part of Cutler- Hammer. Most of our embedded designs have to do with power metering and systems monitoring. I am involved primarily with the communications aspects. Recently I have been drafted into the EMC/safety CE mark compliance arena.
Be fore warned that I know a reasonable amount on the topic but some may consider it just enough to be dangerous. We have some designs that power right off the mains with voltages up to 600 volts. In review of these designs I see that we don't have MOVs across the front end. The diodes of the front end bridge were chosen with a very high peak inverse voltage. after the bridge there is a MOV that clamps at approx 1000 volts! this front end feeds a DC to DC converter. The bridge does have 15 ohms of limiting impedance feeding it. In practice the MOV does not have to clamp that much power and the resistors help limit it. The point here is that one can design a front end to withstand some of the abuse. Take a look at designs of Adustable Speed Drives for electric Motors. They rectify AC to DC off the mains and then recreate a different AC frequency to vary the speed of the motor. Or the manufactures of Uniterruptable power supplies for large systems. I could be wrong but I don't think you'll see MOVs across the mains here. If they do then they have to be sized joule ratting wise to fit the application. Think of the mains source impedance!! It is very low. It can source a lot of power without a limiting impedance in the way. A very good trade magazine on this general topic is "Power Conversion and Intelligent Motion" published by Intertec International Inc. out of Ventura California (ph 805-650-7070). If the mains are very noisy or surge prone one can put Transient Voltage Surge Suppression "TVSS" right on the mains!! Cutler-Hammer sells power distribution gear with TVSS built right in. The TVSS is built by Tycor and it is bolted directly to the mains. These are capacitive filters and MOVs built for the application. They are very large - too large to consider in some embedded product. I noted that you stated a 10KW load. Is this really that large of a computer? Perhaps your product could have them built in but I think it would be easier to have them specified as required built into the distribution gear or added as a retrofit. Cutler-Hammer and many other suppliers make this type of product. Another relevant trade magazine is "Power Quality Assurance" published again by Intertec International. Most of my designs are much smaller embedded designs with a power budget of maybe 25 W and typically powered off of a step down control power transformer (CPT). Because I am isolated from the mains through the CPT it reduces the levels I must test to for surge withstand and lessens the joule ratting issues on the MOV selection. Typical vendors include Siemens GE Harris Panasonic All of these vendors make MOVs that fit my application with PCB mount types and much larger units that look like they can bolt right to the bus bar. Most of these vendors have catalogs that feature application notes that should be studied for intelligent application. I believe this should help you out. You mentioned attachment to an EMI filter. It sounds like you would like to use off the shelf components to make this work? I have one product where I am trying to retrofit in some EMC solutions and have been puzzling over this same issue. > How do I use an external EMI filter when I will need surge protection in front of it as well? > I have looked at Schaftner, Corcom, Spectrum and Okaya. Okaya is something new to me but they did have a suppression block that fits on the front that got my attention. Some of these vendors had 3 phase EMI filters but I don't remember any up to 10kW without it being custom built. You might want to look at some of the filters used on the Adjustable Motor drives. I have one reference vendor reference here "Emission Control Ltd" in Butler Wi 414-790-0092. I am still struggling with some of the agency issues and the proper manor to apply the MOVs from an engineering perspective. I hope this helps some. Chris Wells Sr Des Eng. Cutler-Hammer [email protected] ------------- Original Text From: C=US/A=INTERNET/DDA=ID/mkelson(a)chekov.corp.es.com, on 5/14/97 12:25 PM: Chris, I also have a potential problem with power-line disturbances with a product presently under development. The computer system that we are working on will operate from 3-phase voltage, up to about 415 volts at maybe 10KW. Can you tell me how you would handle the mechanical issues involved with installing MOVs in a system? Would you for instance, simply install them on the terminals of the EMI filter? Also, do you have any suggestions for MOV vendors? I'm unable to post to pstc right now due to a glitch in our UNIX system, but would appreciate any response that you might have time to make. Max Max Kelson Peripherals Engineer Evans & Sutherland Computer Corp. [email protected] o
