Re: Measuring inductive limits (was Re: Rotary Sparkgap)
Subject: Re: Measuring inductive limits (was Re: Rotary Sparkgap)
From: "Malcolm Watts" <MALCOLM-at-directorate.wnp.ac.nz>
Date: Tue, 27 Feb 1996 12:49:11 +1200
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Replying to Wes' queries on my post..
> Yes, I did see that posting; very clever to do the driving thru
> the primary. But for a power transformer, doesn't that add more
> measurement problems?
A number of issues here for sure. From an impedance point of view,
no, if the generator impedance is suitably low. In fact, driving
the transformer that way mirrors conditions that exist when hooked
to the mains but at a much lower power level.
> There's a certain minimum primary current
> needed to make a power transformer work properly; this level of
> current is drawn even when there's no load on the secondary.
I guess the issue is : what is needed to overcome core hysteresis?
But if the core doesn't start out near saturation it can still
swing, albeit on one side of the B-H curve perhaps, yes? In that
case, it is still acting as a transformer. Would measurements at
low signal levels be possible at all if this were not so?
Core flux is only a function of the applied voltage as I see
> the signal generator is not supplying that minimum current,
> you'll get measurements, but will they reflect the way the
> transformer will perform under real power?
Two things make me think that it is. (1) My measurements correspond
to those expected if the manufacturer's ratings are to be taken at
face value, and (2) the transformer appears to behave the way it is
measured when under power.
For example, core saturation caused by the primary can't be an
issue under power because you'll be blowing fuses if it did saturate
- its inductance would drop to a very low value. However, I'll temper
that by saying that I haven't checked the primary resistance.
Other than that, I do have confidence in the measurements.
However, there may be something I've overlooked. Anybody?