RE: [CAD_CAM_EDM_DRO] Re: panel meters

A current transformer and a potential(or voltage) transformer are the same
thing. Anytime you transform current, you will also transform potential.
In a theoretically perfect(no losses) transformer, Power out = Power in, and
power is Voltage * current(again, in the perfect case, neglecting power
factor), so as one goes up, the other goes down.

The reason that one may be CALLED a current transformer, I believe, is that
the current is the thing you are actually using, measuring, or desiring a
specific level of. The voltage is also transformed, but you "don't care" for
the application.

The way a transformer works is the same whether it is a called current
transformer or a potential transformer-the voltage across(or current
through) one winding causes a changing magnetic field which causes a voltage
to be induced across the other winding, which causes a current in the other
winding(through the load).

The process of converting the amps flowing in a conductor to a voltage is
done by measuring across a known load(such as the shunt resistors mentioned
previously in this thread) and using ohms law (E=I*R) or Potential in Volts
= Current in Amperes * Resistance in ohms.

Analog meters are usually specified by current, but this current needs a
voltage to cause it, so in essence, once you know the meter specs, if you
know either voltage or current, you know the other.



Andy Wander
Verrex Corporation


-----Original Message-----
From: Bob McKnight [mailto:bobmcknight@...]
Sent: Thursday, March 11, 2004 9:40 AM
To: CAD_CAM_EDM_DRO@yahoogroups.com
Subject: Re: [CAD_CAM_EDM_DRO] Re: panel meters

To: CAD_CAM_EDM_DRO@yahoogroups.com
From: "caudlet" <info@...>
Date sent: Wed, 10 Mar 2004 23:11:46 -0000
Subject: [CAD_CAM_EDM_DRO] Re: panel meters
Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com


Panel Meters and Current Transformers

The setup I have consists of

a Panel Meter which says it has a basic meter movement of 5 VAC (5
volts alternating current)

a Current Transformer which is 20:1 to be used with a 5VAC meter

I run my welding wire thru the center of the current transformer
and when I am welding, someone reads the meter and I multiply
the reading times 20.

A simpler way which I have used for years is with a clamp on
ammeter which used to be quite expensive but are now available
from Harbor Freight for about $10 for a multi-meter with the
clampon ammeterr function or $20 for the meter with a peak
hold feature. I bought one with the peak hold feature so I wouldn't
need someone to read the meter. The problem has been that
the initial in rush current when I strike the arc is not the value
I wanted, I wanted to save a value while I was welding and also
on my ancient EDM sinker machine (electron drill by Elox).

Maybe some one out there can explain how a Current Transformer
works. My understanding is that it converts the amps flowing in
conductor to a difference in potential that the meter reads.
In the electric meter shop that my dad ran in the 30s there were
racks of potential transformers and Current Transformers. I
never used a potential transformer. The Current Transformer
and meter that I have came off a metering panel at a substation
probably 50 years ago.

Bob

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<SNIP> You pass the "big wire"

> carrying the current you want to measure through the "donut" (CT).

That is then

> a "one turn primary", and a voltage proportional to the current

flowing in

> that wire appears between the lead-wires coming out of the CT.

Not to split hairs but if you just pass the wire through the center of
the CT and don't take a wrap you actually have a 1/2 turn Primary.



This voltage is

> measured by a panel-meter which will "read" the amps flowing in

that wire

> through the CT. In many cases, this "big wire" might be passed

through the "donut"

> more than once, to DOUBLE or TRIPLE (etc.) the resulting voltage

from the

> CT's secondary.

The current in the secondary is porportional to the current in the
primary divided by the turns ratio. For voltage to develop the
current needs a load to work against. A resistor across the secondary
becomes a load and the voltage then can be determined by simple
ohms
law. You can change the value of the resistor to get different
voltages but always be aware that the size of the resistor (wattage)
goes up with the square of the current in the secondary. Let's say you
have 20A flowing through the primary wire and you have just 1/2 turn
(wire goes straight through) and the transformer ratio is 20:1 so you
would have .5A flowing in the secondary. If you want 10VDC at that
primary current you would need a 20 ohm resistor across the
secondary.
The power flowing would be 5 Watts. The more voltage you want, the
larger the value of the resistor and the higher its dissipation.
Adding turns in the primary will give you more current in the
secondary and you can use a lower value resistor to get the same
voltage and a resulting smaller size for the power.


Sometimes you can spot CT's around the individual power lines

> entering an industrial building, say, a good-sized machine-shop.

These are

> often located overhead, but generally not on the "loose" part of

the "drop" that

> comes from the pole, but just these wires enter the

weatherhead/conduit-down.

> There will be smaller (say #14 or #12) wires from each CT (there'd

be THREE if

> the service-entry is 3-phase!), these leading down to a "box" next

to or

> right-behind the KWH meter. Connections in that "box" also read

the AC Voltage,

> and the meter "does the math" and figures what you owe Mr. Edison

(or whomever).

>
>

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sister site to the CCED group, as many of the same members are there, for OT
subjects, that are not allowed on the CCED list.

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