Heavy-duty relays or contactors: question for the electricians

[jtodd]

I'm in the extremely slow process of building an automated system for auto-switchover that will remove load from the generator if the engine stops.  (well, really, it's a "failsafe" to keep engine failures from killing my generator and my household appliances as the system spins down.)  To this end, I need a BIG relay or contactor.  I'm a 12vdc control plane, so the relay/contactor needs to be powered by that low voltage.  But then I need 200-300 amps of throughput on the connections for my A/C load.

So... my options are limited, right?  Or is there stuff "off-the-shelf" that will do this? 

I found a Westinghouse relay that is for aviation systems, but is operated on 12vdc for the coil.  The stats are "16-30 D.C. Coil voltage, main contactor 220 amps - 230/460 volts 400Hz, P/N 9002001-4, 6 power terminals, 7 additional double pole, double-throw sets of contacts".   Before I buy it, I'd like to ensure that it's the Right Thing.  Or maybe there are cheaper alternatives (this is close to $200 - ouch!)

Is the fact that this is 400hz a problem?  Is that a functional requirement specification, or just that it CAN handle 400hz and it can also handle 60hz?  A relay is a relay... or am I supremely ignorant of more delicate workings of switches?

JT

[mike90045]

If that switch is listed for 400Hz, and not listed for 60, I would pass.    I don't know what the differences would be, but the 400hz has a much shorter cycle, and the arc will self quench, 60 hz will still be arcing when 400 would have shut off.

300 amps is a pretty big load, most house transfer switches stop at 200A

look at item 2:
http://www.o-digital.com/wholesale-products/2226/2228-2/XSSR-DA-60A~400A-Single-phese-AC-Solid-State-Relay-Module-Type-123444.html

[jtodd]

Thanks!  I'll look at those that you sent.  Any idea how I'd go about ordering that kind of kit?  I can  press "inquire" on the form, of course, but have you ordered from them?  China is hit-and-miss for supply of good stuff.

I know that 300 amps is a big load, but I expect there will be 12kw (@220VAC) that will be going through it at one point or another, and I like a nice 100% margin for safety.  Granted, 300 is a lot more than double, but if it's cheaper than alternatives that are lower amperate, then OK, bigger is fine.  The step functions on the ones you reference sound pretty good - thanks for the link.

JT

[mike90045]

There are many SSR mfg's out there, you may have a rig a 12V relay to another relay to get the voltage needed to trigger some.

[Jim Mc]

...well, really, it's a "failsafe" to keep engine failures from killing my generator and my household appliances as the system spins down.)...

Just curious, how does this spin-down effect kill generators and household appliances?  How big a generator are you talking about?

[jtodd]

Digging deep in the threads on this board (sorry, no references at hand) you can find that the ST type heads will sometimes de-magnetize themselves if left under load while they're spinning down, which means you have to re-magnetize them to get the minimal coil voltage to "bootstrap" the generator.  I don't recall what this is called - again, there are threads about what you need to do, and I"m sorry I don't have any bookmarked.

As far as other equipment: running things like refrigerator compressors on less-than-normal voltage will cause them to have problems.  Some switching power supplies will overcompensate trying to bring back voltage to normal levels, and the magic smoke comes out.   

I've been in several office buildings during "brown outs" in California (unintentional ones) where voltage drops to 60 to 80 volts and we've lost photocopiers, A/C compressors, and alarm systems.  Perversely, I've also had UPS systems go south during voltage drops that aren't "on/off" events.  I can't tell you exactly what goes wrong in each case, but "Bad Things" happen when you lose voltage over a longer window of time.

I'd prefer everything does dark, immediately.  I have an RPM gauge with a relay output, and I have some timer relays as well to allow the system to get up to speed before I switch over to looking at RPM as the failsafe.  Also: the system is also self-regulating - if the RPM (or voltage) indicator(s) goes above or below the threshold, it shuts down the fuel supply and will engage the compression releases so the engine doesn't suddenly go into a "no load" overspin.  I haven't implemented the cue-ball-in-the-intake-pipe system that was discussed here some years ago in the event of a blow-by runaway, but I think the compression releases will be sufficient to overcome that potential problem.

JT

[mike90045]

"Spin down" also lowers the power line frequency, from 60, to 50 hz  to 40 hz and the lower frequencies, as they approach DC,  start causing large current surges through wound coils.

[Jim Mc]

"Spin down" also lowers the power line frequency, from 60, to 50 hz  to 40 hz and the lower frequencies, as they approach DC,  start causing large current surges through wound coils.

I suppose that'd be true if the voltage didn't diminish at the same rate as the frequency.  I think it does. So, I'm not convinced there'd be large current surges...

And even if there were surges of some degree, I can't imagine what coils (transformers, motors, etc) would be destroyed by a 'spin-down' surge lasting a few seconds.

Yes, I've also read that leaving a load on a generator as it spins down can destroy residual magnetism, requiring a re-flash.  I guess I'm inclined to see value in an under-freq or under-voltage lock out - mainly to disconnect the load if the engine can't maintain 60Hz for extended periods.  THAT can certainly cook induction motors, and transformers.  But I don't see how 'spin-down' is going to kill anything other than residual magnetism...

[mobile_bob]

the only thing i am aware of getting damaged is an unprotected avr
which will try to boost the field current to compensate for falling stator voltage
on spin down, some of the lower quality units as applied can burn out and reportedly
can over time burn out a field winding.

as for loss of residual magnetism, don't tell me st7.5
i routinely forget to disconnect while testing and have never had to reflash the thing,
however i do have to wait sometime 15 or 20 seconds for the thing to excite after spinning back up to 1800rpm at startup.

maybe folks are flashing when all they need to do is wait a bit longer while it is spinning at speed, and give it a chance to build back up?

i probably have stopped under load 50 times, and even restarted with a load attached and the thing will still come back up to life,, it just takes a while.

bob g

[Doug]

Hi Bob:
Long time since I read a post from you here.
I see you have been hidding out in the otherpower forum.
How is life treating you these days?

Contactors:
Run a pony relay to something large and industrial with a 120V coil.

I am very fond of CCL products, they made some very robust DC contactors in the days before Silvania bought them and run the line into the ground. Some I serviced quite recently where over 50 years old with plenty of life left in them. Best place to find somethng like that would be a crane company that services large DC cranes. Your going to say to me But Doug I want an AC unit.... These old DC contactors don't care they are tough as nails and will tollerate anything.

[rleonard]

I agree with the other Bob.  Use a standard NEMA industrial contactor.  NEMA 5 is rated for 270 A.  Coils will usually be 120vac 

http://www.engineeringtoolbox.com/nema-starters-d_918.html

You will probably spend more than $200 for one in a housing though.

Bob

[Thob]

Here's an idea: put a lower current relay in the field winding of the generator - if you kill the field, you should have no output from the generator, right?  It seems that it would be a much cheaper solution.  I'm assuming you'll power the relay from a 12v battery so you don't have a catch-22 on start-up (no power to turn on relay).

[jtodd]

Here's an idea: put a lower current relay in the field winding of the generator - if you kill the field, you should have no output from the generator, right?  It seems that it would be a much cheaper solution.  I'm assuming you'll power the relay from a 12v battery so you don't have a catch-22 on start-up (no power to turn on relay).

That's a pretty good idea, actually.  I could just power the relay via a 12v timed relay that latches upon start and then drops out after 15 or so seconds, giving the engine time to get up to speed and therefore power whatever the "active" portions of the circuit are.  Hmm.... 

JT

[kingsgully]

I'm in the extremely slow process of building an automated system for auto-switchover that will remove load from the generator if the engine stops.  (well, really, it's a "failsafe" to keep engine failures from killing my generator and my household appliances as the system spins down.)  To this end, I need a BIG relay or contactor.  I'm a 12vdc control plane, so the relay/contactor needs to be powered by that low voltage.  But then I need 200-300 amps of throughput on the connections for my A/C load.

So... my options are limited, right?  Or is there stuff "off-the-shelf" that will do this? 

I found a Westinghouse relay that is for aviation systems, but is operated on 12vdc for the coil.  The stats are "16-30 D.C. Coil voltage, main contactor 220 amps - 230/460 volts 400Hz, P/N 9002001-4, 6 power terminals, 7 additional double pole, double-throw sets of contacts".   Before I buy it, I'd like to ensure that it's the Right Thing.  Or maybe there are cheaper alternatives (this is close to $200 - ouch!)

Is the fact that this is 400hz a problem?  Is that a functional requirement specification, or just that it CAN handle 400hz and it can also handle 60hz?  A relay is a relay... or am I supremely ignorant of more delicate workings of switches?

JT

http://selectequipment.net/search/category/double%20throw-175

[sailawayrb]

Solid state relays like (W6240AXXSZS-AC90):

http://www.bb-elec.com/product_family.asp?FamilyId=224&TrailType=Sub&Trail=907

to build something like this (Engine Speed & Voltage Protection System):

http://listerenginegallery.com/main.php?g2_itemId=351

Bob B.