Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!mailrus!wuarchive!gem.mps.ohio-state.edu!sunybcs!kitty!larry
From: larry@kitty.UUCP (Larry Lippman)
Newsgroups: sci.electronics
Subject: Re: Inductance Measurement (Big V)
Summary: Indirect measurement of inductance...
Message-ID: <3469@kitty.UUCP>
Date: 27 Oct 89 04:52:32 GMT
References: <9450@pyr.gatech.EDU> <3467@kitty.UUCP> <9465@pyr.gatech.EDU>
Distribution: usa
Organization: Recognition Research Corp., Clarence, NY
Lines: 56

In article <9465@pyr.gatech.EDU>, is813cs@pyr.gatech.EDU (Cris Simpson) writes:
> The field will not be weak, we will be looking at saturation effects.
> Also the resistance is high, 30 Ohms.  (These are motor windings.)
> ... 
> From above, you see why this is difficult.  We don't have any frequency
> generators that can push 4A through 30 Ohms.  (At up to 20khz.)

	It ain't likely that any vendor will have an off-the-shelf product
to meet your requirements.  It also ain't likely that you are going to
solve your problem inexpensively.

	If you are *running* the motor, you also have complicated the issue
by adding the effects of back-EMF.

	I would start with a variable-frequency power source, such as
one made by California Instruments, Elgar, etc.  A 500 VA variable
frequency power supply, which will probably meet your requirements,
will set you back between $ 3K to $ 4K.

	Since it appears that you are interested in magnetic saturation
effects, I would suggest placing one or more Hall effect gaussmeter
sensors (such as made by F. W. Bell Inc.) at strategic locations on
the motor.  Such Hall effect sensors will respond in realtime to 20 kHz
magnetic signals.  A calibrated off-the-shelf Hall effect gaussmeter 
with a suitable probe will cost between $ 1K and 1.5 K.

	I would then instrument the motor for volts, amperes (using a CT),
and reactive VA.  I would then take these channels plus the magnetic field
signals and feed them to a realtime data acquisition system.  I would
then ramp the variable power supply and look at the relationship between
observed magnetic field, current, VA and RVA to ascertain what is happening
to the *impedance* of the motor winding.  Since you can measure the winding
resistance before excitation is applied, you will have sufficient data to
calculate the inductive reactance, and hence inductance.  If the winding is
going to undergo any significant heating as a result of the excitation
current, you will also have to measure its temperature to compensate for
the change in DC resistance as the winding rises above ambient temperature.

	I would be real concerned about temperature errors if you are
going to load this winding.

	If you are going to study these motor windings for saturation
effects, then you are going to have to examine magnetic field, current
and voltage *waveforms*.

	If you are really serious about this effort and have the budget,
your instrumentation requirements can be readily implemented.  However,
the effort is not trivial; I would estimate between 10 to 20 man-days,
depending upon your facilities and qualifications, along with an expense
of at least $ 5K (assuming that you already have a realtime data acquisition
system with a computer for analysis).

<> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp.
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