Theory test question

[gotshot]

I am taking the washington admin. exam. One of the questions is if you have a circuit pulling 4 amps and you wrap the wire around the jaws of an ampmeter 4 times what will it read 16 amps, 4 amps, etc. I say it will still read 4 amps. Sorry I am rusty on theory, it has been a long time. I have passed the calc. section and code section. And am running out of time on the wac. and theory section. Any help would be appreciated. Thanks.

 

[al hildenbrand]

Here, I'll give you a little nudge. See if you can develop the answer from this hint:

An ampprobe is comprised of two of the three parts of a simple transformer, with a display circuit added.

The jaws of the ampprobe are the transformer core.

Inside the body of the ampprobe a winding around the transformer core connects to the display circuit.

The ampprobe, by itself, is not a complete transformer until a conductor is run through the center of the jaws and the jaws are closed, thus providing the other winding on the transformer.

 

[charlie b]

I'll nudge in a different way.

Current flowing in a wire creates a magnetic field around itself. Wrapping a wire into a set of two coils creates a magnetic field that is twice as large, because the field from the first loop is added to the field from the second loop. What an amp probe is doing, as Al suggested, is measuring the presence of a magnetic field within the jaws.

So you tell me: If you have more loops of wire grasped by the jaws, how is the amp probe going to respond to the field that it is measuring?

 

[gotshot]

so if the amp probe is like a transformer with the probe being the primary side with one winding and the conductor is the secondary, with four windings it should read 4 times the primary, or 16 amps. Is that what I am getting out of this? Thanks for the replys and help.

 

[gotshot]

I think I found it. Ampere-turn----- is a unit of magnetizing force equal to the number of amps in a circuit multiplied by the number of turns in the winding in which it flows. That sounds correct to me how about you?
4amps times 4 windings should read 16 amps.

 

[al hildenbrand]

By Jove, I think he's got it!

Transformer turns ratio.

The winding connected to the display is unchanged. The ratio changes as the number of wraps of conductor around the ampprobe jaws increases.

16 Amps is correct.

 

[chevyx92]

So then you are saying if you take 2 different wires from lets say from the same phase you will read a combined field for a reading??

 

[al hildenbrand]

Yes, that's what will happen.

 

[iwire]

This is a trick I have used on occasion to make my amprobe more sensitive to small currents.

Make a nice coil of wire with 10 wraps and place that into the circuit clamp on with the amprobe and divide the displayed result by 10.

Amprobe also makes a accessory tool for doing this same thing.

 

[charlie b]

so if the amp probe is like a transformer with the probe being the primary side with one winding and the conductor is the secondary. . . .

It's the other way around. The conductor (i.e., the one whose current you are trying to measure) is the primary. As current flows in that conductor, it creates a magnetic field. That field is detected by the amp probe, and so the amp probe is functioning as the secondary.

 

[hillbilly]

I've got a old analog (clamp on) amp meter. As far as I know, it's pretty accurate. What's the best way to check calibration (other than using another meter)?
steve

 

[Kessler4130]

The simple terms I remember for some reason went something along the lines of "ampacity is the measurement of electrons passing any given point" I may have something back asswards.

 

[al hildenbrand]

The simple terms I remember for some reason went something along the lines of "ampacity is the measurement of electrons passing any given point".

Yeah, that's a good view point, and correct as stated.

One can change the view point. . .depending upon the basic definition one wants to use for "current" itself. Physicists and Electrical Engineers will differ.

For the guy doing power & light in buildings, mostly making solid connections and fastening stuff to stuff. . .the change of view point on what current is doesn't have context beyond intellectual.

 

[gotshot]

Well all I know is that I just passed the theory and wac section of the washington admin. test. That gives me all three sections and I am good to go. Thanks for the help.

 

[al hildenbrand]

Well all I know is that I just passed the theory and wac section of the washington admin. test.

Hey!!!

Congradulations!!!