Re: panel meters

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Bob McKnight"
<bobmcknight@c...> wrote:

> To: CAD_CAM_EDM_DRO@yahoogroups.com
> From: "caudlet" <info@t...>
> 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

If they specify the output voltage then the CT was a built-in load
resistor calibrated to give that voltage out. The raw CT
transformers like sold by Jameco have to have an external resistor
but they are rated up to 200A primary current. They are sold as 20:1
ratio CT transformers but if you just pass a wire through the middle
you will get 1/2 of the current you expect in the secondary. We know
this from emperical data from a design we just finished on the THC
project for MACH2 where we used said transformer to sense the AC
primary current of the plasma unit to trigger an "arc Good" signal.
Not all plasmas have an arc good that can be accessed easily.

>
> 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.

I don't know how you do that if your welder is pulsed DC so the
current waveform would be one sided. Transformers don't like
unbalanced waveforms since the magnetic flux in the core tends to
have no demagnitization path other than load. If it is one of the
new PWM inverter types I would think it would be even worse. Are you
getting accurate readings ?

>