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-   -   Short circuit current for an HVAC 24 v transformer? (http://www.diychatroom.com/f17/short-circuit-current-hvac-24-v-transformer-76458/)

Yoyizit 07-17-2010 09:08 PM

Short circuit current for an HVAC 24 v transformer?
 
"On paper" it's about 25 A.
Anybody measured this?

hvaclover 07-18-2010 04:38 AM

Quote:

Originally Posted by Yoyizit (Post 471104)
"On paper" it's about 25 A.
Anybody measured this?

Clearify, Yowzie.

beenthere 07-18-2010 08:34 AM

Hope that was suppose to be 2.5 amp.

Yoyizit 07-18-2010 11:38 AM

For 40 VA @ 24 vac the current is 1.7 A. The rated 24.0 v only applies when the transformer is loaded to its rated value of 40 VA.

If the open circuit voltage for one of these is, say, 26 vac then the internal impedance of the secondary is (26-24)/1.7 = 1.2 ohms.
26/1.2 = 22 A of short circuit current.

If it's 27 V o-c then it's 1.8 ohms and 15 A short circuit current.

I've never tested one of these HVAC transformers.

And this arith. doesn't apply to bell transformers because they shut down if they're shorted.

beenthere 07-18-2010 12:06 PM

Go to radio shack. And get yourself some resistors. And test a 40 VA transformer.

It will burn up before you reach 25 amps.

beenthere 07-18-2010 12:18 PM

While a dead short amp draw, will be higher then a load increase burn up.

Your applying a DC formula. It is used on A/C transformers, as many DC formulas are applied to A/C.
There is always a difference is actual outcome though.

If you are supplying 120 volts to the primary, as measured by a standard meter/or true RMS meter. The transformer primary is actually subjected to 170 VOLTS at peak and bottom sine.

yuri 07-18-2010 01:38 PM

I've never tested one of these HVAC transformers.


You have one in your furnace. Let us know how it turns out, Yoyzie.:)

Yoyizit 07-18-2010 03:32 PM

Quote:

Originally Posted by yuri (Post 471369)
I've never tested one of these HVAC transformers.


You have one in your furnace. Let us know how it turns out, Yoyzie.:)

You try it first.
Can you imagine how mad my frau would be if I were somehow remotely responsible for our AC to suddenly stop working after I went into the basement?
She's already asking questions about the catapult that I'm testing in the backyard [powered by garage door springs] and is openly wondering where the puppy went.
http://www.youtube.com/watch?v=F5Y_4d9UhEA
Now, really, how would I know where the puppy went?

I e-mailed an HVAC manuf. whose name kind of rhymes with. . .dunno'. . .warrior/carrion/Vikky Carr/sumpin' . . .with my question.

Scuba_Dave 07-18-2010 05:49 PM

Quote:

Originally Posted by Yoyizit (Post 471411)
She's already asking questions about the catapult that I'm testing in the backyard [powered by garage door springs] and is openly wondering where the puppy went.
http://www.youtube.com/watch?v=F5Y_4d9UhEA
Now, really, how would I know where the puppy went?

No trebuchet ?

hvaclover 07-18-2010 05:50 PM

Quote:

Originally Posted by beenthere (Post 471342)
While a dead short amp draw, will be higher then a load increase burn up.

Your applying a DC formula.

That makes the second time Yowzie has been busted applying DC rules of electricty to AC practical application.

You gotta use Impedance and Reactance for Alternatin Current Yowzie.
But that is an interesting topic for discussion. We all have TRANSFORMER FROM HELL horror stories we could share I'm sure.

Yoyizit 07-18-2010 05:52 PM

Quote:

Originally Posted by Scuba_Dave (Post 471462)
No trebuchet ?

It's kind of the same.

There's a YouTube video of a young girl setting one of these off. I'm sorry to say the makers of that video have no idea what they are messing with and the danger she is in.

hvaclover 07-18-2010 05:53 PM

Quote:

Originally Posted by Scuba_Dave (Post 471462)
No trebuchet ?


Dude don't get him started! The next thing we'll see is Da Vinci style drawings and lonf drawn out equations!

hvaclover 07-18-2010 05:55 PM

Quote:

Originally Posted by Yoyizit (Post 471466)
It's kind of the same.

There's a YouTube video of a young girl setting one of these off. I'm sorry to say the makers of that video have no idea what they are messing with and the danger she is in.

You wouldn't say that if she landed in your lap....:eek:

Yoyizit 07-18-2010 06:02 PM

Quote:

Originally Posted by hvaclover (Post 471471)
You wouldn't say that if she landed in your lap....:eek:

Look to your east - I'm sending you a cinder block at Mach 1. Allow 40 minutes for the flight time.

hvaclover 07-19-2010 03:17 AM

Quote:

Originally Posted by Yoyizit (Post 471474)
Look to your east - I'm sending you a cinder block at Mach 1. Allow 40 minutes for the flight time.

My wife said it would not match our existing stone work----better make it a hard baked red brick instead.


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