[Prev][Next][Index][Thread]

Re: Current transformers



Jeffery,

If you look carefully at the bottom-most portion of your ammeter's
faceplate, you will probably see a "50:5" printed in tiny letters. If
you're using a matching 50:5 current transformer (CT), you should be OK.
However, where you're measuring current, and which side of the variac
your ballast is connected, are also important relative to your current
readings. 

Let's assume that your system has the welder's primary (ballast)
connected in series with the variac's INPUT side. If your CT/ammeter is
also connected in series with the line side of your variac, you will
indeed be measuring the current drawn through the ballast. However,
suppose your CT/ammeter is connected on the OUTPUT side of the variac.
In this case you likely measure a much larger current than that flowing
through the ballast, particulalry for low variac settings. This is
because a variac passes power from the input to the output at virtually
100% efficiency:

        Vin*Iin = Vout*Iout

   Or, rearranging:

        Iout = Iin*(Vin/Vout)
      
At low variac settings and with a short-circuited pig, you'll see variac
output currents that are MUCH higher than the current flowing
through the ballast. For example, suupose your variac setting is 20% of
the incoming line voltage, the pig's secondary is short-circuited, and
your ballast current is 5A. The current flowing from the variac's output
and through pig's primary winding will actually be 25A! Now, if you
cranked up your ballast's current limit to 50A, you could easily see
250A flowing through the output brush of your variac (until it smoked or
you popped the output fuse). It's this current multiplying effect that
sometimes accounts for the lower portion of a variac getting "fried",
even though the variac input current was well within the its ratings.

The moral of the story: 
You can design your system to run with the ballast inductor on either
side of the variac. However, to prevent possible damage to the variac
under short-circuit conditions under all variac settings, you may choose
to connect it in series with the OUTPUT side. The ballast current and
pig current will only be the same for all variac settings and pig load
conditions if the ballast, CT/ammeter, and pig are all connected in
series on the output side of the variac. 

Hope this helps!

-- Bert --

Tesla List wrote:
> 
> Original Poster: kd4lyh-at-webtv-dot-net (Jeffery Pederson)
> 
> Im using an a.c ammeter with a current transformer to moniter my primary
> current to my pig the meter has multiple scales im using the 0-50 amp
> scale (0-5amp full scale deflection) My question is what ratio of
> current transformer to use ? Im using a 50:5 ratio current transformer
> right know but i think the meter isnt reading the correct current. IT
> says im drawing 20 amp with the welder on it lowest setting. I dont have
> an a.c amprobe to see if this is corr  ect , Im going to invest in one
> to investigate this matter further. Is the 50:5 ratio correct. My
> current transformer is a instruments  transformer inc. cat.#2SHT500,
> 50-400 Hz, the arc welder is a Lincoln A.C. 225s, the transformer is an
> Epoxy Cast High Voltage type 23,000Volts at  37.5K.V.A
> I have a dedicated circuit just for running tesla coils up to about
> 14KVa. wish I had more room. Any help would be appreciated. Thankyou