RE: [CAD_CAM_EDM_DRO] Re: panel meters

To: "'CAD_CAM_EDM_DRO@yahoogroups.com'"
<CAD_CAM_EDM_DRO@yahoogroups.com>
From: Andy Wander <awander@...>
Date sent: Thu, 11 Mar 2004 09:50:16 -0500
Subject: RE: [CAD_CAM_EDM_DRO] Re: panel meters
Send reply to: CAD_CAM_EDM_DRO@yahoogroups.com


Andy

Thanks for the explanation.
I guess the question I really have regards matching the meter
to the transformer.
In analog meter movements there is a rating f.s. which is full
scale meter movement or the voltage required to swing the meter
needle across the scale to maximum voltage.
There is another rating which is the resistance of the meter
movement. My memory is that I thought I had a good meter
in the old Simpson 260? multimeter at 20,000 ohms/volt.
So the question is
Does the resistance of the meter movement make much of a
difference in the reading you get across the Current Transformer.
I always thought it did so used the old panel meters from substation
panels to make sure I got a fairly accurate reading.

Bob McKnight
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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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