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

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?

2. Mercury relays. These can handle high currents and cost about $25,
but are they quieter than the small relays? Does anybody know?

3. 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 $30. 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

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

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

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.

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

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.

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?

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

2. Mercury relays. These can handle high currents and cost about $25,
but are they quieter than the small relays? Does anybody know?

I'm not familiar with high current mercury relays.

3. 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 $30. 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.

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.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

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

Thank you very much for your comments!

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?

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

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!

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

According to :

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

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.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.

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

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.

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?

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

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 :-)

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

According to :
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 :-)

And some other things.

Guilty as charged. Sorry about that ;-)
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.