Transformer Inductance Addition

[Manny]

Two questions.

Can I just add an inductor in series to a transformer
primary and secondary to increase the inductive reactance
of both (rather than adding more turns)? Will this affect the voltage??
I wonder if adding an inductor and adding more
turns would produce similar behavior in the circuit. What
do you think.

I tried using my multi-meter to measure the inductance
of a transfomer. How come I can't measure any, as if there
is open connections. What's the proper procedure. Thanks.

Manny

 

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[Nick Kennedy]

It's a bad idea. The added external inductance would be equivalent to
what's called *leakage* inductance, which is generally the bad kind. The
other kind is *mutual* inductance, which is what causes transformer action.
The mutual inductance is generally also what is seen as *magnetizing*
inductance in a transformer.

Leakage inductance causes a voltage drop proportional to load and so causes
the transformer's output voltage to deviate from the ideal transformation
ratio.

A simple but fairly good model of a transformer consists of a leakage
inductance term in series with the source, a magnetizing inductance term
after that, in paralle, and an "ideal" transformer following that. If the
leakage inductance approaches zero and the magnetizing inductance becomes
very high, then the model approaches an ideal transformer, which is usually
what you want.

Not sure why you can't measure your inductance. It might be relatively high
and so may be off scale for the inductance meter built in to your DVM.

Regards,

Nick

 

[Manny]

It's a bad idea. The added external inductance would be equivalent to
what's called *leakage* inductance, which is generally the bad kind. The
other kind is *mutual* inductance, which is what causes transformer action.
The mutual inductance is generally also what is seen as *magnetizing*
inductance in a transformer.

Leakage inductance causes a voltage drop proportional to load and so causes
the transformer's output voltage to deviate from the ideal transformation
ratio.

Thanks for this important information. So what I should do instead
is to increase the number of turns of both primary and secondary.
For example.. the input is 9 volts, the output is 90 volts..
and there 10 turns at the primary and 100 turns at the secondary.
What would happen if I'd increase the turns of the 10 to 50 (in
the primary) and the 100 to 500 (in the secondary). I presume it
would still output a voltage of 90 volts given a 9 volt input
right. Also I presume the increase inductance can caused more
current wasted right? So the bad effect of this is only current
losses and nothing else unlike leakage inductance which can so
distort the waveform. Agree?

I want to change the inductance because my circuit would take
longer time for a certain cycle such that I don't want the increase
current to destroy the components in my circuit.

Many thanks.

Manny.

 

[Nick Kennedy]

(cut my earlier info)

Thanks for this important information. So what I should do instead
is to increase the number of turns of both primary and secondary.
For example.. the input is 9 volts, the output is 90 volts..
and there 10 turns at the primary and 100 turns at the secondary.
What would happen if I'd increase the turns of the 10 to 50 (in
the primary) and the 100 to 500 (in the secondary). I presume it
would still output a voltage of 90 volts given a 9 volt input
right.

That's right. The voltage ratio would stay the same, but higher magnetizing
inductance would put less load on the source and make the transformer look
more like an "ideal" transformer.

Also I presume the increase inductance can caused more
current wasted right? So the bad effect of this is only current
losses and nothing else unlike leakage inductance which can so
distort the waveform. Agree?

Well, I'm not sure what you're saying here. The increased inductance you
described above is a *good* thing, in general. The increase you proposed
earlier isn't good because it's just a series impedance that doesn't really
change the transformer.

Increasing the turns of the transformer results in *less* wasted current.
It won't distort the waveform either way.

I want to change the inductance because my circuit would take
longer time for a certain cycle such that I don't want the increase
current to destroy the components in my circuit.

Sorry, I'm not sure I follow--we may have a bit of a language barrier. When
you speak of a longer time, are you referring to using the transformer for
something other than transforming sinusoidal voltages & currents?

Regards,

Nick

Many thanks.

Trimmed earlier stuff