Re: [TCML] Advice on MMC

[David Dean <deano@xxxxxxxxxxxxxxxx>]

Hi Andrew

On Tuesday 29 December 2009 02:19:38 pm andrew wrote:
> ie how do you measure this? I have a fluke digital meter but I am a bit
> leary of shorting out the 9000v secondary lest there be smoke! Is it really
> ok to do that?
> 

I have done that, no problems, it worked just fine. I am *NOT* an advocate of 
that method however.

I have a cheap Radio Shack 0-500mA DC panel meter with a cheap Radio Shack 1A 
600V bridge rectifier with the DC terminals soldered directly across the meter 
terminals, and leads from the AC terminals to the secondary terminals of the 
transformer under test. If something went wrong, $20 vs $120 up for decent DMM.

If you have a digital clamp on ammeter, you can get the digital resolution 
without any connection to the transformer by winding a coil out of stiff wire. I 
usually use ten turns of #12 THHN solid wire, actually have a couple of coils on 
the shelf, just that one is most often used. The coil has enough opening that 
you can center the opening in the clamp around the coil on one side and the 
clamp does not extend past the center of the hole in the coil. 4" ID coil works 
for my clamp on Fluke meter. Then connect the wires from the ends of the coil to 
the terminals of the transformer you want to test, set the meter on an insulated 
block so that it can stay centered around but not touch the coil, power up, take 
measurement (don't touch meter during measurement for good luck) and power off. 
Done. If you do not have a clamp on meter, they do make clamp on attachments for 
"regular" meters.

By the way, you divide the reading on the meter by the number of turns passing 
through the clamp,
 ten turn coil-
meter reads 4A
divide by ten
400mA is passing through the coil.
You can use any number of turns to match the resolution of your meter.


> And yes regarding Ballast. I think I need something there. At the moment
>  for testing things and to stop it blowing fuses. I have just been using a
>  3 kw oil column heater in series with the primaries. (very big resistor!)
>
12.5A  19.2Ohms 
Assuming 10 to 1 turns ratio and all MOTs are the same, and assuming MOTs are 
"ideal" transformers, which they are not, your short circuit current should be 
312.5mA
> I don't have a variac but am keeping my eye out for one of suitable rating
> but there are not many for sale around here.
> 
> I like the idea of building a variable reator with iron wire in a tube but
> not quite sure how to do that.
That is a great solution, I have not built one myself. If you have an extra MOT, 
I have built a rough and ready ballast, adjustable and reasonably linear. Not 
adjustable "on the fly", but easy to make. It would be easier for you as your 
MOTs are already 240V.

I cut the welds holding the I to the E and removed the secondary and shunts. 
Added another primary from another MOT with same size core. Rigged clamp to hold 
I on E, put plastic shims between I and E and clamped tightly. Works like a 
charm. 

You only need one MOT and do not have to add a second primary.

I really wanted better linearity, so ended up splicing the two cores together 
side by side "double wide core" and winding my own coil to get a straight line 
from 0 to 240 V input. Saturation starts at 240 instead of 180 and is just a bit 
off the line at 260. 

No pictures, no text, only graphs at:
http://deanostoybox.com/projects/ballast/

later
deano
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