alternating relay

[bill06]

I need to know how to change a float controlled lead-lag pump control into an alternating pump controller?

 

[dpc]

You'll need to add an electro-mechanical alternator if they are still made, or add the logic using latching relays or a PLC.

The basic idea is that you need something that can "remember" which pump was the last to run. The old alternators were specifically designed for this.

You can still have a lead-lag scheme if both pumps need to run at times. The alternator just changes which pump starts first.

Talk with any MCC or motor starter manufacturer - I'm sure they can provide typical schematics.

 

[Warpspeed]

You can get bistable relays.

These operate like a child's see-saw. There are two actuating coils and an armature rocks one way or the other. A small permanent bias magnet keeps the armature "stuck" in whatever position was last energized. The relay therefore "remembers" even after both coils are completely de-energized.

So you have an "on" coil, and an "off" coil and usually one or more sets of changeover contacts just like any normal relay. By cross coupling the contacts and coils it can be made to alternate every time it is energized.

 

[itsmoked]

Remember the alternators and relays "just" alternate they don't time balance.

 

[LionelHutz]

The cheapest and easiest way I've done this is to use a Telemecanique Zelio or a Moeller Easy relay. They're small, cheap programmable relays and come with a display and keypad on the unit so you program right at the unit. I just bring the floats to the unit and then program the alternating and lead/lag control in the unit. No other relays are required.

 

[jraef]

While I would agree that these programmable relays / picoPLCs are perfectly capable of the job, are relatively easy to program and can do plenty of other functions if you need them, you have to "consider the audience" for the most part. If the person servicing the controller is going to be "Toothless Bubba the 'Lectrician", those relays may be too much for them. I prefer to use the absolute simplest alternator relay on the market, the old Furnas Class 47 ALT, now sold by Siemens. It just doesn't get any simpler than this.
Link to brochure
Link to diagram
There are other simple electronic alternators made by Diversified Electronics, SSAC, Time Mark etc., all good as well, but this one is by far the least number of wires to do the job and the least expensive.

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[dpc]

I can't believe they still make these things. I like it. Thanks, jraef for a free trip down memory lane.

 

[jraef]

Unfortunately, I'm sure some marketing genius at Siemens will someday question that decision and eliminate them, but for now, I think they still sell a lot of them. Because the design and tooling is already paid for, I'd imagine they are cheap to make.

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