Re: Volt-amps BS


"Jon Slaughter" <Jon_Slaughter@xxxxxxxxxxx> wrote in message
news:YgxPh.10756$JZ3.6289@xxxxxxxxxxxxxxxxxxxxxxxxxxxxx

"Chris" <cfoley1064@xxxxxxxxx> wrote in message
news:1175357592.122214.249990@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
On Mar 31, 10:53 am, "Michael" <newsz...@xxxxxxxxx> wrote:
Some of these industrial ratings want to make me pull my hair
out.....horsepower....volt/amps.....I've never bothered to commit any
of it
to memory.

Could some please tell me...

If a relay has a 24vac coil and a "hold VA" of 15. what does that
translate
to in current draw at 24 volts? Who knows what they're talking
about.....I'm
sure the relay probably kicks in around 18 volts or so. I'm just trying
to
get an idea of the transformer I'll need.

Thanks. !

Hi, Michael. AC relays and solenoids are constructed differently than
their DC counterparts. If you'd like, try to run a 12VDC coil relay
with 12VAC. You'll just create a buzzer. AC relays have shading
rings to counter this effect.

An AC relay or solenoid draws a heck of a lot more current when it's
pulling in (sometimes called "pull current") than when the plunger or
relay actuator is in place. This lower current is called the "holding
current".

Can you explain a little about how the AC relays work a bit? I'm confused
because it would seem that only DC would work unless there was some sort
of rectification inside. What are these shading rings and stuff? (I tried
searching but really can only find info on "DC" relays)

Thanks,
Jon

There are some AC relays with built-in rectifiers to a DC coil, but they
are rare.

For AC devices with capacitance or inductance, VA does not equal W (watts).
However, for calculating current draw, you can use VA/V to get A. The
transformer needs to be sized for the current, not the power. When an AC
relay or solenoid operates, the "clapper" or plunger moves to a position
where the inductance increases, so the current drops.

The hold VA is determined by the current drawn at rated voltage with the
relay pulled in. The relay will drop out at a lower voltage, something like
70% of operating voltage, which would be about 1/2 the hold VA.

I found a good explanation for shading coils and shaded pole motors in the
following link:

http://www.everything2.com/index.pl?node=Shading%20ring

Paul


.

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