three contactors integrated into one control system

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copper123

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I have a design question that I need some advice on.
I need to drive three panel boards with three different contactors. I have two different control criteria. One is temperature and the other is time. At or below 20 degrees, the contactors are supposed to pull in. Here is the kicker: Only two of the panels will be on at the same time. The contactors are going to be on 20-minute cycles. For lack of a better term, lets call the contactors and the panels that they feed, A, B, C. So here is the mode of operation. At 20F. Degrees or lower, the control side needs to energize. Panel and contactor A and B become active. After 20 minutes, Panel A denergizes, B stays energized, and C becomes powered. After another 20, B shuts off, C stays on and A is back on again. So basically, each panel stays on for 40 minutes and is only off for 20 minutes of a 60-minute hour. Do you guys know of any controlling device that can harmonize these three contactors? A basic time clock on each contactor does not have the accuracy that I need. Also, if all three were to energize at the same time, it would trip the OCPD. I have driven tons of contactors with many sorts of control devices, but just never tried to integrate three into the same system. I have been looking at PLC devices, but have zero experience with what I am looking at. Also, this needs to be in an outdoor setting. It will be fun to do this work, but the control device or the electrical control components are giving me some major roadblocks.
Thanks in advance for any suggestions that you might have.
 
Re: three contactors integrated into one control system

A PLC would be the way to go. You are going to get an education doing this anyway, learning PLC technology can be useful for other things down the road.

-Hal
 
Re: three contactors integrated into one control system

Thanks for the reply, I have been looking at information on the web and am amazed at the solid state electronics/chips that have tons of functions. Such tiny devices and they can handle up to 10 amps at 120 volts of control power going through them!!
 
Re: three contactors integrated into one control system

A simple cam timer in series with the thermostat would be a simple solution to your project. Ideally the cam timer would have one revolution per hour and 3 cams. The cam motor and line side of the cam switches would be energized by the thermostat. Cam A would be set for 0-40 minutes, Cam B for 20-60 and Cam C for 40-20.
Don
 
Re: three contactors integrated into one control system

Thanks Don!

That is really cool and I have never heard of one before. The old bulls always have something to teach the young bulls.
 
Re: three contactors integrated into one control system

You could also use relay logic, with time delay relays. There are many on the market, and there are plenty of programmable ones that could meet your needs. However, I'd have to go with the PLC solution. Programmable timing relays can easily run $50 to $120 each depending on models and options. A really small and cheap PLC can be had for around $120 or so. The only problem with that is that you also have a one time purchase of the programming software and usually a cable to connect the PLC to a computer. But once you have the cable and software, the next project that comes along only requires another PLC.

There are many PLC brands out there, and I've used a lot of them. I'm partial to the Siemens S7 microcontrollers myself. Siemens also makes a tiny little PLC that can be programmed from the keypad called the LOGO series. It would probably be perfect for this application.

Whatever I've said about Siemens applies to other brands too. I just happen to like these best for quick and cheap jobs.

John

[ November 12, 2005, 09:54 PM: Message edited by: flick ]
 
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