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,
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.
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.
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.
Thanks
A couple of comments:
First, I think you will find it easiest to measure your transformer's
coupling coefficient in the way that I believe "The Phantom" posted
originally. Quoting what he wrote:
"To measure the coupling coefficient (of an iron core transformer)
without
making an inductance measurement, do this:
Apply rated voltage (sine wave) at one winding, and measure the open
circuit voltage at the other, getting the ratio V2/V1'. V1' means
that
winding 1 was excited.
Now excite winding two and measure the open circuit voltage at the
other
winding, getting the ratio V1/V2'.
The coupling coefficient is very nearly SQRT(V2/V1' * V1/V2')
The turns ratio is very nearly SQRT(V2/V1' / V1/V2') "
Second, your H-bridge design should be such that it can withstand a
moderate amount of leakage inductance in the transformer it drives.
Why would you not want it to be able to do that? And is it really so
difficult? I assume the kickback is at turn-off of an H-bridge
device, and is largely from an inductance that has considerable
capacitance associated with it, being part of the windings of a line-
frequency transformer, so dV/dt should not be 100V/nanosecond as it
can be when power mosfets are driving a high frequency inductive load
with very little capacitance. If that's the case, don't either the
substrate diodes in your H-bridge devices, or diodes you've purpously
put into the circuit, handle it nicely? You do want to have a large
enough capacitance on the DC supply to the H-bridge so that the
kickback can be absorbed into it. It's certainly not as big a problem
as trying to use the driver bridge to dump the stored inertial energy
in a large DC motor and its rotating load, where you might have to
absorb hundreds of joules.
Perhaps you could provide more information about the problems you see
in your simulation. Perhaps the solution is not to demand an
unreasonably high coupling coefficient in the transformer, but rather
in the proper design of the driving circuit.
Cheers,
Tom