Can anyone recommend a brand and model of an 220V DPST timer that I could put inline with a 100 amp sub panel? I have found some Intermatic Pool and Sauna timers that come very close to my load requirements but not quite. Besides I have to imagine that there are timers that are made specifically for such a scenario.
Thanks for any help!
M
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Replies
Exactly what is it that you are trying to do?
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming....
WOW!!! What a Ride!
I need to switch on and off an entire sub panel which contains circuits for thermal storage heating registers as opposed to a timer on each circuit.
M
Why not use a relay and separate timer?
Edited 3/23/2004 9:04 pm ET by Bill Hartmann
100A+ contactor and a regular timer.... Cheaper too.Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming.... WOW!!! What a Ride!
A contactor would certainly be availible in whatever amperage you need. And a simple (or digital) lamp timer could give you a lot of flexibility on the timing for only $15.
Sometimes I see big contactor for cheap as surplus equipment.
Another scheme would be a solid-state relay. I had a lot of 90-amp applications and got them from Omega Engineering. They can be ordered to switch on 0-5 volts, 4-20 mA or (I think) line voltage. It is important to put them on a big homping heat sink to get their full amp rating, but they are small and very reliable when install by the instructions.
David Thomas Overlooking Cook Inlet in Kenai, Alaska
>> Another scheme would be a solid-state relay.
Do you know if it's possible or advisable to parallel those if you can't find one rated for the current you need?
It's not a good practice to parallel solid state relays. No guarantee they will equally share the load current. Most likely one will hog more current than the other(s) and will be operating beyond its rated capacity. Also, I don't know the code issues here, but I suspect you need to have a breaker or disconnect prior to the relay. Keep in mind that solid state relays have leakage. Small, but not zero. Probably a couple milliamps at that size. A Crydom model A24125 takes 120 or 240 AC control input, and switches up to 280 volts at 125 amps. Price is about $80 brand new from Digikey. Loads of other types and brands available. Low or high AC and DC control voltage, etc. They also need a heat sink. A solid state relay that big will dissipate a few watts. Should be mounted to a metal plate with good air circulation.
I agree that paralleling two would be dicey. If one triggers just a few microseconds faster than the other - well, those electrons are pretty fast - so it would take all the initial load. And any slight difference in their impedence would result in one taking most of the on-going load. I'd only parallel them for reliability if the only size I could get was marginal due to the combo of amps/heat/heat dissiapation. Much better to get the next size up.
And, yes, use a good heat sink with good free air flow or with a fan. And put the stick white glue between the SSR and the heat sink for better conduction.
Never spec a SSR without looking at the chart for de-rating them for air temp / heat sink or not. A "90-amp" SSR might be a 40-amp one in your environment.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Timming should not be a problem.
One of the basic ratings is I**2*t. That is basically the surge rating or fusing current.
They are verry rugged in that way.
Ongoing sharing would be a problem. More concerned about the differences in characteristics from device to device, but line impendences would also be a problem.
Also you would want to put an inline fuse with each device. So that if one went the 2nd would not follow.
But if I remember correctly the likely failure mode (from overloading) is to short. But there are other failure modes.
Edited 3/24/2004 3:40 pm ET by Bill Hartmann
"But if I remember correctly the likely failure mode (from overloading) is to short"
Yes, that's what I've seen in some heating equipment (I didn't design it, but redesigned it because of that failure mode). Fails as a short, and stuff gets really hot (it was used to cycle the heater element).David Thomas Overlooking Cook Inlet in Kenai, Alaska
> A solid state relay that big will dissipate a few watts.
Perhaps someone else knows better than I on this, but if these things consist of a couple forward biased silicon junctions, each would drop about 0.65 volt, and at 100 Amps, that would be about 130 Watts.
-- J.S.
Yeah, but each side is only on half the time, so you're back down to 65.
No, most them by are either triacs (bi-directional) or both slugs are mounted in the same substrate.
If you're saying 130 watts total for two devices, one on each of two hot wires, I agree with you. I read your message as talking about one triac on one hot wire, in which case only one of the two transistors is conducting at a time, so only 65 watts for one triac.
Wow! Thanks everyone!
So from all of this a contactor with a 110 vac coil triggered with a timer would seem to be the best as most straight forward solutions.
As my sub panel is 100 amps I have found three contactors that will do the job.
Does derating apply here or should the contactors full load amps be = or > than the circuit rating?
"Does derating apply here or should the contactors full load amps be = or > than the circuit rating?"
Just rated at or above the circuit it is feeding. The derating applied to the solid-state relay, not the mechanical one.
When working in single phase, some people like to wire contactors which (always?) have three poles, thusly: run L1 through pole 1, run L2 through pole 2 AND back around through pole 3. The logic is that you will wear down the three sets of contacts inside more evenly then if you never use pole 3.
I personally haven't seen a difference but my applications usually switch once an hour or less so they didn't have a lot of cycles. You will have even less cycles, right? It will be pretty infrquent that you switch that subpanel from on to off, I believe so this refinement probably adds little.David Thomas Overlooking Cook Inlet in Kenai, Alaska
Well you made a mistake and I just followed your lead <G>.
You are right each solid state relay would only lose 65 watts.
But you said that it was because the device was only on for have the time. That part was wrong, but I did not go back and read where the orginal 130 watts came from. That it was for 2 relays.
Latching relay. Coils are deenergized in both rest states. Coils are only used to change state. A momentary pulse triggers the relay. Very small in use I**R loss as only the contact resistance remains. Used mostly for large lighting circuits but they are available.
Pair of simple motor contactors. One for each leg...
View Image
Life is not a journey to the grave with the intention of arriving safely in a pretty and well preserved body, but rather to skid in broadside, thoroughly used up, totally worn out, and loudly proclaiming.... WOW!!! What a Ride!
Edited 3/24/2004 9:57 pm ET by IMERC
I looked it up. That particular unit (a dual SCR type) has a peak forward drop of 1.7 volts and dissipates about 110 watts at 100 amps, 240 VAC. There are more expensive units that have lower voltage drop and MOSFET types that have virtually none, but they're expensive.
For a 100 square inch aluminum heat sink in fairly still air, 20 watts will give a temperature rise of 30 degrees. You'd want to bolts that unit onto at least a 500 square inch plate; something like 2 by 2 feet. Steel is not as good a heat conductor as aluminum, so even bigger would be better. An extruded aluminum unit with fins would be smaller.
Given the amount of heat dissipated, you could think about conventional relays. I don't have data for bigger contactors handy, but a good industrial-quality relay is about $20 for a 25 amp unit. One issue here; noise. A relay is fairly quiet, just a click. But a big contactor makes a substantial "bang" every time it closes. You'd definitely hear it. Solid state are noiseless. I'd probably do a relay per circuit, after the breaker. It would be quiet and the whole system would not fail at once like you'd get if just one big relay ran the whole setup. Could also do several small solid state relays - after the breakers, not in parallel - but, the total heat dissipation would be similar. Around a hundred watts. Don't know how long they'd last in practical use, maybe a couple years, maybe 10. The sold state will last indefinitely.
As always, something that looks like it should be simple isn't really all that simple.
Depending on where this is in the house, maybe a copper cold water line could be routed through the heat sink....
-- J.S.
For 130 watts?
Yep. But, I try to avoid running water pipes through my wiring whenever possible. It is a fair number of watts to deal with for an otherwise simple problem. I wouldn't try to put a solid state relay that big into a subpanel. A 12" steel junction box has enough square inches of surface if it is entirely exposed. Steel isn't too great as a heat sink though. I'm still undecided whether its better to use a solid state relay that doesn't have contacts which wear out, or a relay that doesn't dump an extra hundred watts of heat. If the installation area will easily hold a nice big box to use for a heat sink, I'd probably go solid state. If not, use several small relays but make sure of access because they will wear out some time and need replacement.
Everyone,
Thanks again for all of your feedback!
All this discussion on mechanical vs SSR's led me to find a solution which is the best of both worlds.
Mercury Displacement Switches!
If you’re interested in more details see the following link.
http://www.mdius.com/relays.php#GENERAL%20INFORMATION
1/2 the price of a mechanical contactor, very quite and power efficient operation as well, all with very little heat. Perfect!
Thanks!
Michael
Those do look nice. 7 watts or so of heat to dissipate - which is easy. And little or no de-rating for DC use! Mechanical contactors are so bad on DC, in comparison.David Thomas Overlooking Cook Inlet in Kenai, Alaska
They contain quite a bit of mercury, so you must not dispose of them in the trash when they are shot.
You're going to have trouble finding one. Bill's suggestion is a good one.
Does it have to be a packaged solution? Shouldn't be too hard to find a timer to switch a low voltage signal to activate mechanical or solid state relays.
If you want to do the whole panel get a 100 amp 2 or 3 pole lighting contactor and control the coil with your timer.
You want a simple time clock, any intermatic or similar will work, 110v or 220v; use the clock to control a "contactor" (relay). The contactor will need to have an actuation coil of the same voltage as the clock switched circuit and the same # of load poles (typ 2, hot and hot) as you will be switching. It is assumed that you will not be opening the neutral and ground? Contactor must be sized (ampacity) to switch and carry the loads of your sub box.