"NT" wrote in message
I disagree.
Since the relay is open when power is applied to the coil, it's the
open inductance (and the resistance, of course) which will determine
how much current will flow through the coil, that current being what
generates the magnetic field to start the armature on its way.
yup
Then when the relay closes, the closed inductance comes into play and
holds the armature in place until the current through the coil is
reduced to a point where the armature's return spring overcomes the
weakened magnetic field, allowing the armature to open.
yes. I guess in theory both matter, one determines closing behaviour,
the other ensures the relay doesnt overheat. In practice though the
margins are very large, and its normal to simply fix holding current
to suit the relay, and not worry about closing current, which will be
so close as to make no real world difference in all but exceptional
circumstances. But yes, we can consider both if need be.
true with all relays under all ac conditions. Theyre designed to work
that way.
No, its exactly how theyre designed to operate.
Where's the conjecture? I get the feeling you could do with bringing
your skills up to speed on relays.
---
Perhaps.
---
Something like this?
+-----+
120AC>--|~ +|----+
| | |
| | [COIL]
| | |
120AC>--|~ -|----+
+-----+
Not in all cases, certainly.
I'd like to see you find one single electromechanical relay that wont
work for.
---
The P&B MR5A I talked about in an earlier post, which has a 240V
50/60Hz coil, a coil resistance of 4800 ohms, an impedance of ~ 6600
ohms at 50 Hz, an open inductance of 14.5 henrys, and a closed
inductance of 16 henrys
But I don't think "typical" is what we're after since we want
something that will _always_ work.
This 2:1 ratio normally is good for relays, and the OP can check his
to see if it conforms to that. If it does, the thing will always work
when subject to this formula.
FWIW, when ac is applied you get puling force plus vibration. With dc
there is no vibration component when its closed, so less holding
current is needed. How much less I've really no idea.
Some relays are fast movers capable of 100s of Hz, some are slow. Ac
relays can always work on dc, but dc ones often dont work ok on ac.
Since current is what's doing the work, my real-world example shows
that 240V 50 Hz RMS impressed across a load with an impedance of 6600
ohms will force 36mA RMS of current through the load.
Then, since it's current that's doing the work, 36mA of DC through the
coil should accomplish the same thing.
Re ripple: If the relay is designed to run on ac 50 or 60Hz, its
designed and rated to live with the current and force variations that
go along with that, 100-120 times a second. Running it on rectified
mains will only serve to reduce the current variations through the
cycle, it wont cause the relay any issues.
Re rms voltage: With my 2:1 figures, rectified 120v is spot on. With
your 174v figure, 120v is well within the 50% margin. Of course for
some uses that margin would need to be confirmed by testing before
production, and reconfirmed if a new relay type is used. Or as you
say, a cap could be added. Or for off brand consumer goods, in it
goes, relays are good for it.
There's no such thing as "rms dc voltage",
RMS can be applied to any and every waveform, dc included. Its very
relevant when working with rectified ac, semismoothed or unsmoothed.
and if the relay is
designed to operate on AC with a certain RMS current in its coil, how
can it possibly overheat if that current is DC?
With the same current it wont, with higher curren ti will. IIRC you
proposed using 174v rms, that would be ok on your specific relay, but
not a universal solution.
NT
-----------------------
I sense a bit of cross confusion in this thread.
a)Do not the AC/DC ratings refer to the contact rating rather than the coil
rating? As with any switch, there is a big derating of contacts designed for
AC use but applied to DC- Example a typical 120V 15 A light switch would
fail at 15A 120V DC- it might work at 15A, 12V. Older switches with good
snap contacts do much better.
You indicate experience for the factor of 2:1- but is this something that
translates across the AC/DC barrier?
b) John indicates 174VDC giving 0.036A would be OK- for closing. However
when closed, the holding current will be 0.031A and this is the steady state
current that is involved in heating when the relay is closed. For DC the
voltage would need to be about 150VDC [ (174*0.031/0.036)^2]. So it
remains to be seen if that is sufficient to close the relay. According to
you- it would be more than adequate. From John's 80% criterion it is
inadequate.- so point (c).
c) The peak force is related to the square of peak flux For AC, this is
proportional to (Vrms/f)^2 independent of the magnetic medium. The magnetic
medium determines the corresponding peak current. While a DC current of
0.036A corresponds to an rms current of the same magnitude, and the average
force is the same, at that current, as the average force in the AC case, it
is well below the peak force (about a factor of 2, ). This may have have a
bearing on the relay's operation- just a conjecture.
Don Kelly
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