[TCML] Power Factor Correction, current transformers and current waveform monitoring.

[Skip Malley <skip@xxxxxxxxxxxxxxx>]

If you wanted to monitor the current input for your coil, you should 
read the following.  I have answers to questions that I have seen 
asked in the past regarding Power Factor Correction and input current 
monitoring.  I know this post is larger than most, but PFC and input 
current monitoring is a big thing.

Since current monitoring and PFC seem to be recurring themes, I 
figured I would put in my 2 cents and experience.  I worked for a 
manufacturer of TV broadcast equipment for 28 years.  I designed many 
analog and switching supplies.  I needed an inexpensive way to 
monitor the current waveforms and needed to make accurate current 
measurements.  I did not find any adequate commercially available 
solutions so I made my own.

It turns out that the best start for a current transformer was a 
standard small size step down power transformer.  I simply took a 
small 6VA power transformer and removed the secondary winding.  I 
needed something to measure up to about 20 amps.  I took a piece of 
12 Gauge wire and wound it through the secondary opening as many 
turns as would fit.  A current transformer needs an appropriate 
load.  For example, I have two 100:5 current transformers monitoring 
the AC input to my house driving 5A current meters with a 100A scale.

In my case, I wanted to view the current waveform on a scope.  I 
loaded the secondary with a resistance that produced one Volt out for 
every one Amp through the primary.  I can now connect the output to a 
scope and simply view current directly on the scope.  One volt on the 
scope corresponds to one amp through the transformer.

This can be done very simply and cheaply for any desired input 
current and output voltage to measure.

I built mine into an electrical box with a line cord, power switch 
and duplex outlet with a BNC for measured signal output.  This BNC 
can connect directly to an oscilloscope or multimeter measuring AC volts.

See the photos linked below to see my transformer and some current 
waveforms of a computer, standard fluorescent 2x40Watt fixture and a 
medium compact fluorescent light.  The images consist of two 
waveforms together.  The large one is the AC voltage measured at the 
device.  The smaller one is the current.  The numbers on the top of 
the screen is the scale for the voltage (50V/div).  The number at the 
bottom is actually the Amps / division.  Remember, the current 
transformer unit outputs one volt per amp.

http://www.theweyside.com/photos/Tesla/CT_Complete.jpg  Complete assembled unit

http://www.theweyside.com/photos/Tesla/CT_Open.jpg  Opened unit 
showing transformer.  The transformer would normally be covered with 
tape or shrink tubing depending on requirements.

http://www.theweyside.com/photos/Tesla/CT_Computer.jpg  Computer / 
Monitor input current.  This is the current waveform of most 
electronic equipment.

http://www.theweyside.com/photos/Tesla/CT_FlTubes.jpg  Standard 
current from your regular 2 x 40 Watt fluorescent tube fixture that 
you would buy in Lowes or Home Depot for your work shop.  Note that 
the current waveform is a bit ahead of the voltage

http://www.theweyside.com/photos/Tesla/CT_CF.jpg   This is a nasty 
looking one.  This is the current of a 26 Watt Compact Fluorescent light.

Using a current transformer like this can easily help someone 
determine the best capacitance for power factor correction.  It can 
also be used as a simple way to display the current used by a 
coil.  This can be connected to an appropriate panel meter to display 
the system current.

If enough are interested in this, I would gladly document the details 
to make your own current transformer, or I can provide a complete 
current transformer with an appropriate load for a moderate price to 
cover my costs in transformers and wire.

Contact me off list at skip@xxxxxxxxxxxxxx if you are interested in 
this information or transformers.  If enough are interested in the 
construction details. I will make a later post with further details.

Skip Malley

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