Re: Coupling coefficient of industrial transformers

On 13 Apr 2007 09:54:07 -0700, "orvillefpike" <orvillefpike@xxxxxxxxx>
wrote:

M. Bruhns

I thought of doing like you suggested, using the simulation and
comparing with the actual circuit, but I didn't think it could work
because I want to know "k" to figure out what kind of voltage kick-
back the leakage inductance would send back to the "H" bridge. Since
that kick-back might be large enough to destroy the bridge, I was
using the simulation to avoid destroying expensive IGBTs. In the
simulation, if "k" is equal to 1 there is no kick-back, if it is .99,
the kick back is quite large.

M. Phantom

I read a few of the post you recommended and I read other post on
"Leakage Inductance" and a lot of them revolve around shorting the
secondary. By doing this, how will it not destroy the transformer

The idea is to short one winding and *measure* the inductance and
resistance at the other winding with a meter, such as this:
http://www.bellnw.com/products/0650/


unless you use lower voltage than its nominal voltage. I could connect
the 10 Kva transformer I have on 125 Vac on the primary and measure
the OCV of the secondary and vice versa if this method is good enough.

You will get better results if you use this method rather than trying to
measure the leakage inductances.

I have a microwave transformer, that I rewound the secondary with 40
turns of # 10 wire, to make some test. I would like to know "k" for
that transformer but I couldn't feed the secondary with 125 Vac, the
current would be too high and if I use a lower voltage coming from a
small transformer, like lets say a 16 Vac, 40 Va transformer, it will
need to be able to supply more current than it was designed for if
it's connected to the secondary of the MOT.

Are you doing all this for a hobby purpose, or is it for your job? If it's
for your job, they your employers should buy (or maybe they already have) a
variable transformer (http://www.variac.com/) so that you can apply a low
variable voltage to any winding of a transformer under test.

I have a question about MOT, when I connected the transformer to 125
Vac, with the secondary disconnected it drew about 3 Amps., if I
removed the magnetic shunt, I believe it would be called, between the
primary and secondary winding, the current goes to 5 Amps. How would I
know if it's saturating or close to saturating.

You can use a variac to apply a voltage to one winding of the transformer
(with *none* of the other windings shorted, of course). Slowly turn up the
voltage while measuring the current, as John Popelish described. When you
begin to saturate the core, the current will rise more rapidly.

Another way to detect saturation is this: if you have an oscilloscope,
put a 5 or 10 watt, 1 ohm resistor in series with the winding that you are
driving. Look at the voltage across the resistor with the scope. When the
voltage applied to the winding is low, the current (as measured by the
voltage across the resistor) will have a sinusoidal wave shape. Slowly
turn up the voltage, and when you reach saturation, the wave shape will
begin to have a distorted, "peaky" wave shape.



Thanks

.

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