current transformers

["Tesla list" <tesla-at-pupman-dot-com>]

Original poster: "Mr Gregory Peters by way of Terry Fritz <twftesla-at-qwest-dot-net>" <s371034-at-student.uq.edu.au>

Current transformers are simply small toroidal transformers that lack a 
primary winding. You run a high current wire through the core, and 
depending on the stepdown ratio, number of turns you wind, etc, the 
high current will be reduced to a lower current (typically 5 volts) 
that is much more easily read by a cheap ammeter. This provides two 
advantages, firstly, you can get big current transformers and small 
ammeters much more cheaply than you can get a single big ammeter. 
Secondly, you are kept a further distance from any nasty HV or HC lines.
You really don't need anything special for TC work. I use an 80:5 ratio 
CT that I bought new from RS for about $15 US. I run my feed line 
through it once. This means that when my coil is drawing 80A, 5A will 
be going through the CT and ammeter. One thing you have to do is make 
sure the scale on the ammeter matches your CT and number of turns. My 
ammeter will show FSD (full scale deflection) when 5 amps is going 
through it. I bought an 80A scale for the meter, so now when the coil 
pulls 80A, the meter will indicate this. If I used, say, a 120:5 CT, 
there is no turns ratio that will produce 5A through the secondary with 
80A through the feed line (1 turn= 120A for FSD, 2 turns = 60A for FSD, 
3 turns = 40A for FSD). This means that I would have needed a scale 
that shows either 120, 60 or 40A at FSD. However, I intend to draw a 
maximum of 80A. So 40A and 60A scales are too small, and a 120A scale 
is just wasting meter space.

Hope this helps,  
Greg Peters
Department of Earth Sciences,
University of Queensland, Australia
Phone: 0402 841 677
http://www.geocities-dot-com/gregjpeters