Cutoff (not transfer) switch questions

[jtodd]

I'm not terribly well-versed in the electrical arts, but I have a pretty good idea of what I would like to implement.  I'm sure that there exist solutions for what I'm describing, but I'm hoping that what I want exists in an inexpensive package instead of as a sub-set of features to some other, larger and more expensive system which I don't need. 

Here's the problem: if for some reason my diesel motive plant (12/2) fails, then I will experience a situation where the flywheels and generator slowly spin down to a stop.  During this spin down, voltage drops in a curve over time, which is VERY unfriendly to most equipment that I'm familiar with and tends to fry things in unexpected ways.  I'd like to have a system downstream from my generator that just chops off my output voltage and keeps it off in the event of some upstream problem (generator, motor, wiring, etc.)  "100% no power" is better than "sort-of no power."

Think of this as a transfer switch that doesn't transfer.  Does such a thing exist as a 1/2 transfer switch?  If so, who makes them?  Or should I just buy a transfer switch?  Does anyone have a website or pointers for a vendor for either direction?  While I appreciate technical descriptions, I expect that I won't probably build this myself and will end up buying it.  I know what I'm good at, and there are too many strange variables in electrical systems for me to say that I'll become an expert at the cost of my computers, televisions, refrigerators, or life.

Here are some possible solutions.  Most of them are probably flat out wrong, but I'll take comments on what would work if anyone knows of a product or method that is along these lines:

  - a power conditioner.  Not only would this provide a more clean power output from the genset, but perhaps it would just drop to zero volts if it was unable to provide it's rated output.  The downside is that these things suck quite a bit of power (not sure how much, but I recall at least 20% from my days spent with computing gear that used them.)

  - a voltage-detecting breaker.  I don't know if this exists.  If voltage moves between 0 and 110vac and stays up for more than X seconds, then the breaker "sets" itself.  If after that time, input voltage input drops below 110vac for more than X seconds, then the breaker trips and requires that voltage drops to 0 for X seconds before resetting.  (Of course, 110vac might be 120vac, or some other value.)

JT

[Jim Mc]

Check out the Potter & Brumfield 'CS' series of voltage sensing relays here:

http://relays.tycoelectronics.com/pnb.asp

Since their contacts are not rated high enough for your application, you'd have to use the CS relay to operate a beefier contactor. 

[Doug]

Look for an ac contactor with the current rating you need and see what voltage it drops out at under these conditions. This can be adjusted some with a dropping resistor wired in series with the holding contact. Perhaps it would be better to try this with a cheap 10 amp off the shelf relay first.

I have all kinds of parts here, I'll try and hoble something simple and cheap together, however the frequency drop isn't something I can simulate on a bench.....

I'll get back too you.

Doug

[jtodd]

Check out the Potter & Brumfield 'CS' series of voltage sensing relays here:

http://relays.tycoelectronics.com/pnb.asp

Since their contacts are not rated high enough for your application, you'd have to use the CS relay to operate a beefier contactor. 

Thanks for the link!  That's what I was looking for, I think, at least from a commercial package.  Two of the units (since they have both NC and NO contacts and settable trigger points) will even work in series for an over/under voltage configuration, so that would be perfect!  They're a bit on the pricey side, but I'll bet I can find some used.  This is a really handy list of all of the P&B relays; I've never seen the catalog in a full format.

I still need a larger breaker/relay to drive with these smaller triggers - any references for a larger unit?  Ideally, I'd like it to be driven by 12vdc, so that I can operate it from my 12vdc control circuits.  I guess this is just a request for a heavy-duty 12vdc relay...  I looked at the "heavy duty" relays on that Tyco/Potter & Brumsfeild page, but they stop at "1HP @120VAC" which I think is less than I need.  I actually have a bunch of the PRD-11DG0-12 relays, which are the maximum amperage relays on the list, but it seems that might fall short of a ~7kva output.  Or am I mis-reading those specs?  (It says 30A at 240VAC, so maybe it's adequate?  I have an ST-10 with a 12/2.)

Look for an ac contactor with the current rating you need and see what voltage it drops out at under these conditions. This can be adjusted some with a dropping resistor wired in series with the holding contact. Perhaps it would be better to try this with a cheap 10 amp off the shelf relay first.

I have all kinds of parts here, I'll try and hoble something simple and cheap together, however the frequency drop isn't something I can simulate on a bench.....

I'll get back too you.

Doug

Doug -
   This is probably what the CS relays do, but your idea sounds a lot less expensive.  I'm guessing it would take a little fiddling with combinations of resistors to find the right range where power should drop off.  Perhaps a variable resistor in series to allow for adjustment?  With the right relay, this would be an "all-in-one" package that wouldn't require a secondary breaker/relay arrangement like the CS relay would, but at the price of possibly learning about unexpected consequences of such designs.   

[Jim Mc]

... however the frequency drop isn't something I can simulate on a bench.....

It'll be important.  Relay coils are inductive and as the frequency drops, their reactance drops as well.  Thus at a frequency lower than that for which they were designed, they'll need lower voltage.  IMHO, using a normal contactor coil is not a good plan for an undervoltage detector.

The PB CS relays are super common - I'd be surprised if you can't find one on eBay for a lot less than their $150 list price.

[Doug]

Yup thats that problem cooking up something cheap....
As I said before I need to tinker and think about this a while. Maybe its as simple as using a 120 VDC coil wiha dropping resistor?

Doug

[Jim Mc]

That'd be better.  Or perhaps a 24 V AC coil, with a capacitive dropping element.  That way a lower frequency would help out, as long as the LC combination didn't pass through resonance as the generator winds down (ie series RLC circuit would need to be analyzed to make sure resonant freq is well above 60 Hz).

But, yeah it's a design problem that's not gonna be solved without expending some time...

(PS, another complicating factor is the fact that a relay coil's inductance changes a lot as the relay pulls in, since the magnetic path changes - so good luck with this one... I still like the P&B CS relay.)

[christurner]

The best off-the-shelf / easiest solution would be to buy a UPS capable of handling your electrical load - APC SmartUPS's can be had for $100 or less without batteries.  Buy new batteries either to fit the UPS or wire up longer lasting lead acid batteries (marine, car, deep discharge).  The good thing about the APC's is you can modify what voltage it cuts mains at with apcupsd, and with the same program can set a controlled shutdown of your systems.  If all you need is something to go from 120v to 0v, buy one, keep the batteries if bad, and it when it cuts mains it will go to 0 pretty quickly.