fan relay: dry contact vs. mercury vs. solid state

[semenzato]

Greetings experts,

I am retrofitting a poorly designed multiple-zone hydronics/forced air system at my house in Oakland, CA. In its original conception, the air handler fans are always on. That is, there is a manual switch for every fan. And the handlers are not that quiet either. I am adding relays to control fan on/off. I bought a few 30A mechanical contactors (the fan motors range from 1/3 HP to 3/4 HP) for about $10 each and installed one, and it works fine, but it is wicked noisy! THUNK!

This one isn't close to a bedroom so it doesn't matter too much, but I have to do better for the bedrooms. I am considering these three choices.

1. Small 10A relays. They click too, but they are much quieter. However, I don't know if the contacts can stand the inrush current. I bought 3-pole relays and connected the poles in parallel, so that gives me 30A in theory. Is this a good idea?

1. Mercury relays. These can handle high currents and cost about , but are they quieter than the small relays? Does anybody know?

2. Solid-state relays. I am looking at the Carlo Gavazzi models, which seem very nice. They have models with 24VAC input, and a 50A relay costs about . The voltage drop across the relay is 1.6V, and at steady state the currents I am dealing with are low enough that I won't need to install heat sinks.

Thanks for any help! Luigi Semenzato

[Speedy Jim]

Mercury is way overkill. The 10A relays will probably work just fine. But I think I'd go with solid-state ones on this.

Jim

[gfretwell]

Go solid state. There are lots of Solid State Relays. They can be ac or dc trigger. I like the 3-30v DC. You can drive them directly from CMOS 4xxx chips

[semenzato]

Thanks for the suggestions. I saw the DC trigger relays, but the rest of the system (the solenoid valves, pretty much) run at 24VAC, so I would need an additional AC/DC converter, not worth the trouble I think.

[Chris Lewis]

According to :

You have to consider the fact that motor switching requires far beefier switch contactors than their ampere requirements alone would suggest.

Relays and switches used for motors _should_ carry "horse power ratings" in addition to the ampacity ratings.

While a 30A relay _should_ be beefy enough to handle 1/4-1/3 HP at

120 (which'll draw 3-7A steady state, 20A or more on startup), I personally would prefer to see a HP rating.

I bought a RF controlled AC relay, intended to drive a 1HP dust collector. It was rated for about 1000W, not enough. So I used it to control a massive old HP-rated relay I had laying around. Massive: it's rated at 15HP at 277V. Enclosed it in the case from a fried PC power supply ;-)

You don't need something that big. Look for relays rated at

15A/1HP or so.

No. Fractional variations in contact resistance will cause drastic current imbalances, and generally speaking, it buys you little.

I'm not familiar with high current mercury relays.

It's _particularly_ critical with solid state relays to look for some sort of HP rating or allowance for high surge loads - see the specs. Semiconductors blow a lot faster than mechanical contactors or fuses.

Note that running a solid state relay with a DC actuator on a 24VAC circuit is pretty easy. A small bridge rectifier (a buck or two) (and perhaps a small electrolytic capacitor) will do the trick. Depends on how far you want to DIY this. Of course, there are solid state relays designed for 24V AC control too.

[gfretwell]

Good point. You do want to match the SSR to the system you are using it in. That is why they make several styles. Be generous in chosing the size, at least twice what you think the load will be. It is cheap insurance. Also watch out for which ones require a heat sink. (usually

10a or greater).

[semenzato]

Thank you very much for your comments!

This may be truly overkill, but I did a quick calculation of the effects of the variation of resistivity with temperature. The idea is that if there is an imbalance, the pole carrying more current will heat up more, and the resistance will increase relatively to the other poles, equalizing the currents.

For common metals (Al, Cu, Fe) relative-delta(R) = alpha * delta(T) with alpha approximately 0.4 * 10^-3 near room temperature. That means that a temperature increase of 25C causes a 10% increase in resistance. I measured the contact resistance in my relay, it appears to be only 7 mOhm with 1A current, and the variation between the poles is about 10%. So I don't think I need to worry about that kind of temperature difference. Things could change at higher temperature, but alpha also increases (it's a function of some power of T greater than 1) and that should help equalize the currents.

If there is a flaw in this reasoning I will sure be glad to know before I fry my relay!

[Chris Lewis]

According to :

It's not contactor temperature you have to worry about per-se. It's contactor erosion.

The contacts won't make contact at exactly the same time, so, one of them takes the brunt of the make/break arcs and erodes quicker than the others.

Make/break arcs are what destroys non-HP-rated relays or switches with motors.

Secondly, due to erosion, the contact area (and hence resistance) will vary over time. A tiny difference in absolute terms will make a _huge_ difference in current balance.

Paralleling conductors is generally simply not done _unless_ a single conductor is in itself enough for the current flow. Paralleling conductors then is only a means to reduce the resistance of a conductor (voltage drop), not increasing its ampacity.

Same thing for relays where it might give you a level of redundancy, but not much more.

[semenzato]

I'll buy that. It will arc on a single pole first, and last. In that case, the resistance *decreases* with the current (that is, zero current = no ionization = high resistance). Then I don't know what happens. If erosion increases the distance between the contacts, then the closing arc will favor the less eroded contacts. But the opening arcs?

Trying to reason this out is just too messy. Thank you for the clear analysis. You don't happen to live near the San Francisco Bay Area, do you?

[Chris Lewis]

According to :

Life's like that ;-)

Which is part of the reason you avoid the situation. There are a lot of factors, minor variations in which make the results not necessarily predictable.

Don't get me wrong, relay contacts are often put in parallel. But they should _not_ be to improve the "contact rating". A 10A, non-HP rated relay is not adequate for a 1/3 HP motor. I'm sure it'd work for quite a while, but you only want to do this once, right? ;-)

I was in SF for a week two weeks ago, but I live on almost the other coast, in the great white north ;-)

[semenzato]

Well thanks for all the help. I hate not being able to reciprocate. I guess I will just help random people when I can, like you do.

Are you the Chris Lewis of the wiring FAQ (I actually read parts of that, before wiring the alarm system in my addition) as well as the famous canadian despammer as well as author of "how to run your usenet web site"? Nah, they can't be all the same person :-)

[Chris Lewis]

According to :

And some other things.

Guilty as charged. Sorry about that ;-)