Electrical question

[280plus]

Soon as it cools off I'm out in the shed checking this all out. 

[Jim147]

I'm wondering if 280 has an old analog milliammeter setting around somewhere. Just to see what it would show. I need to go dig around in my old furnace parts box. This could be a learning experience for us.

And Nick if you just drop a wrench you could answer this pretty quick.

jim

[230RN]

^ Yeah, I was going to ask him to report the sensitivity of the meter(s) he used, but I decided to let it go for now.

[280plus]

I gots flukes. True rms, which means nothing to me because I forgot the whole story on that. All I know is it's supposed to be gooder.

I have a couple ideas. One is to simply try and pull in a 24v relay by connecting one leg to the transfomer and the other to ground. I may go fishing around for the parts today. I have everything.

[230RN]

Terry sticks his neck out with another prediction:

With a sensitive meter such as Flukes are, the  most you would read is 12 Volts, I would bet, going from either lead of the output of your 24 volt xfrmr to ground.  (1.414 times that (16.97V) if you're measuring peak-to-peak, which IIRC, most Fluke RMS meters will do if you want them to.)

Reason?  The secondary of the transformer is sitting in, so to speak, "free space," and while the voltage between the output leads would be 24 volts, you would be measuring each individual lead to a virtual center tap on the transformer, i.e., ground.

A relay will not operate on the very low currents available from either leg of the xfrmr to ground, as I said, in the microampere range.  It would sink the low currents available so that almost no voltage could build up, just as a low-impedance meter would sink that current and give almost no voltage reading. 

I notice the only ones bumping this thread are myself, 280plus, and Jim147, so I'm willing to either quit this or take it to PMs.

Terry "If It Works, It's a Fluke," 230RN

Root-Mean-Square measurements:

http://www.practicalphysics.org/go/Guidance_107.html

The True RMS meter actually takes rapid samples of the varying voltage and actually does this calculation within the meter's own software and comes up with a true RMS value regardless of the shape of the input waveform. 

The non-True RMS meter is just simply calibrated to "sorta" come up with an RMS voltage assuming that the input waveform is sinusoidal.  Which it might not be.  Other, old-fashioned RMS meters actually measured the heating value of the current by measuring the temperature change of a calibrated heating element and working from that.

[280plus]

Nah, there's always a few readers. I see what you are saying though. I'll keep you posted. 

[280plus]

Oh, missed the second part of your post. Thanks for the down and dirty on rms. I remember now.

[cosine]

I'm still reading. I'm enjoying the discussion and trying to learn from it.

[Hawkmoon]

I'm still reading. I'm enjoying the discussion and trying to learn from it.

It's hard for me to enjoy being reminded how little I know and how small are the chances that I might actually ever be able to understand any of what this thread is discussing.

[Jim147]

I found one of my analog ampclamp multimeters. It's a little too humid for me to want to be out playing with a transformer right now. I have one energized here the house but I don't think anyone would be happy if I took the furnace apart right now. It will go as low as 3A on the clamp or 150 mA in series. But it might not live long if it's used for any line-to-line voltage on Nicks setup.

Now what kind of readings can we get if we put a load between the short and the ground?  

Voltage would drop to zero? I think.

jim

[280plus]

I do believe that's all correct. Terry's take on 12.5V to ground is probably correct. I feel like I've sen that before. Question is amps to ground which I think I'll try with a direct shoet while readingamps. I haven't gotten to it yet but I will. probably tomorrow, it's supposed to be more tolerable outside.

[230RN]

^ Watch it.  Probably OK with a 24VAC transformer, but anything higher could get dangerous.

Reason?  If you short one end of the output coil of the xfrmr to ground, it is no longer an isolation transformer and the full output voltage will appear on the other (ungrounded) lead of the secondary, with the capability of delivering real current, up to the design output current of the transformer, and not just the microamps I was talking about before.  (Where any "ground" was through the low capacitance of the the whole setup to ground.)

Reason for that?  One side of the primary circuit is inherently grounded through the mains ground, so shorting one side of the output side to ground establishes a direct connection to one side of the primary.

And you end up with an autotransformer.  --Essentially, a single coil transformer with the tap on it grounded.  If you then measured the voltage between the ungrounded lead of the primary and the ungrounded lead of the secondary, what would would appear is the input voltage PLUS whatever voltage the secondary is supposed to deliver.  In the case of the 24 V transformer, 110 Volts plus 24 Volts, for a total of 134 Volts.

If the isolation transformer were a 1:1 (110VAC to 110VAC), you would get 220VAC between those ungrounded leads, with the full delivery amperage of the transformer behind it.

I wish to hell my scanner were working.  This would all be a lot more clear with diagrams.  I may take a few minutes out of my precious, long-overdue, and well-deserved breaks today to throw together a sketch of this, scan it with the company scanner, and post it.

Terry, 230RN

[230RN]

Double post due to my screwing around trying to get a pdf file to show directly.

[230RN]

Here ya go...




CASE I
Illustrates the situation posted by the OP, but without the motor.  Due to any small stray capacitances from the isolation transformer to ground, a small but indeteminate voltage will be sensed by a grounded sensitive (usually electronic) meter at either point C or D.  A less sensitive meter (such as a D'Arsonval moving-coil meter) will probably show nothing, since the few electrons available due to the coupling to ground through the stray capacitance will be drained off by the coil of the "less sensitive" meter without visibly actuating the coil-and-pointer assembly. 

What happens with the addition of the shorted motor depends on how "high up" on the motor's coil the short postulated by the OP is.  If the short is high up, the short will draw a lot of current through the secondary, which will be reflected back through the transformer, probably causing the fuses (or breakers) in the mains to blow.

However, if the OP's diagram reflects the true state of affairs, not much (if anything) will be felt by the grounded stick figure --depending on how large the stray capacitance is.  Typically, stray capacitances such as this are very tiny, and depend, as with all capacitances, on the insulation between the "plates" of the capacitor, the distance between the "plates" of the capacitor, and the area of the smaller "plate."

CASE II
Shows the situation if 280plus "shorts" one of the transformer's output leads to ground in an attempt to measure the current to ground.  In essense, an autotransformer will be created.

CASE III
The CASE II situation is redrawn here to illustrate more clearly the autotransformer configuration.  (I forgot to add the iron core, but I don't want to re-scan the drawing.)  The dotted line is merely to illustrate that there is a solid connection (through ground) from B to D.

For our British readers, substitute the word "earth" for the word "ground."  For our American readers, substitute the word "line" for "mains."

Donations accepted at the door.  Go in peace. <cue choir>

Terry, 230RN

[Ron]

I've encountered plenty of 24 volt control circuits that would read zero volts to ground at both legs of the secondary. Of course in every case the transformer did not have one leg grounded as you find in most commercial HVAC units. The control circuit was truly isolated.

Having said that I would never trust a transformer and ground myself like the OP suggests. A failure in the transformer windings could be catastrophic to your health.

[Headless Thompson Gunner]

We used to use isolation xformers back in school in the labs.  It was required procedure whenever working directly with mains AC, safer for both the user and for the mains.  Had racks of the things you could borrow.

In theory, the secondary side of the isolation xformer is floating and can't cause any potential to ground. Touching a single point of that side shouldn't cause any shock problems, it'll just make that side float so that the point you touch is at 0V compared to ground.  At most you'll get some parasitic leakage current, probably not even measurable.  

In practice, it's still dangerous.  The real world isn't so neat and tidy as a schematic.  When working with real parts and real wires on a real bench, you never know for sure that the transformer secondary is truly isolated from ground.  If any part of that system is ground referenced, even indirectly, any other part of it can shock you.

And you still have 120V of potential between the high side and the low side of the secondary, so you can still fry yourself that way.

[280plus]

I'm lost on this auto transformer thing. Because I do see transformers with grounded secondarys and that may be the part that is confusing me on the voltag to ground. Nut anyhoo, So you're saying if I were to grab A snd C I would be delivered 134V? Question is, why am I grabbing A? If I know it's hot, I ainta gonna be grabbing for it. I hope.  (We need a smilie getting zapped by a lightning bolt.  )

 

[sanglant]

  now you've done it, this has to be the grabbing a hot lead icon.

[280plus]

I guess I should get off my dead ass and go set this up. It's been a long summer for the A/C guy(s) so I'm a bit uh, lacking in motivation right now. Tomorrow suddenly became totally freed up so I may feel more ambitious in the AM.  I do believe I'll put that first thing on the schedule for tomorrow.

1. Screw around in shed for 30 minutes.
2. Climb on and off of a 20' flat roof several times, run out at least 130" of garden hose, take apart a rooftop A/C, clean the crap out of it, reverse process.

Who wants to come?  

[280plus]

now you've done it, this has to be the grabbing a hot lead icon.

Bingo! Ok, now make it really really small.

[230RN]

From:

http://en.wikipedia.org/wiki/Autotransformer

Autotransformer --a single-coil transformer where taps are provided to allow different voltages to be tapped, which may be either higher or lower voltage, depending on where the taps are:



My diagram in CASE III above, reply # 38, is sort of up-side down because of the layout of the previous diagrams, but the concept should be obvious.


Variac variable transformer  -an autotransformer where, instead of taps, there is a carbon brush which can be slid along the length of the coil to obtain varying voltages, and the coil is formed into a circle on a big toroidal core:



Damned handy devices.

[280plus]

Ohhh, I call that a "multi tap" transformer. 

[230RN]

^ And the Variac is an "infinite-tap" transformer. :)  (Well, at least in terms of one tap per turn.)

That "Don't Grab Hot Side Leads" icon becomes unscary when reduced to emoticon size.  I dood a .20 by .20 reduction on it to 600 bytes and it just looks like a colored blob.

Say, why don't we just color-code the transformer leads?  Like maybe black for the input side, red for high voltage output, yellow for 5 Volt output, red-yellow for HV center tap... and like that?



And then we could all promise not to stand in puddles or use more than one hand whilst mucking about with "real" electricity (as opposed to all the 5-volt TTL-level voltages common nowadays --which isn't "real" electricity.)  :)

Terry, 230RN

http://www.radioremembered.org/xfmr.htm