Re: Help with a inductor problem

Hello, I have no problems measuring DC resistance of my large reactors with
a DMM. The inductive reactance is a AC phenomena, and should not influence
the small DC current placed across it by the meter. A 15 henry reactor I
have (rated at 1750 V), measures only a couple ohms off of the 50 or so, of
the DC name plate value.


David Trimmell

At 11:49 AM 9/30/1999 , you wrote:
>Original Poster: "Eric Davidson" <edavidson-at-icva.gov> 
>IMO you sent it back prematurely.  You will NOT be able to measure the
>resistance with a high input impedance ohmmeter like the Flukes, Beckmans
>etc. A 60's vintage Simpson may do the job.  The huge inductance of the
>choke causes it to buck when the ohmmeter tries to measure the resistance.
>The counter emf generated by the inductance opposes the voltage from the
>meter.  Your resistance reading will likely fluctuate wildly and never
>stabilize.  The fix is easy.  Simply place a 100 ohm 1/2 W resistor (or any
>known resistance under say, 10 kohms)  in PARALLEL with the choke and
>measure the resistance of the two in parallel.  Calculate the resistance of
>the choke.  The resistor will damp the inductive kick of the choke and your
>resistance reading will be rock stable and very accurate.  I've used this
>method to measure the DC resistance of the control coil on my saturable
>reactor, it works great.  Hope this helps.
>PS  I'd try to get that choke back...sounds like a nice find..just my
>> Original Poster: "Lee Marsh" <lmm-at-flash-dot-net>
>> Hi All    I found an old Radar modulator charging inductor.  On the can it
>> has a list of specs.
>> as follows:   15 Henrys,  17.5 kvrms,   0.110 A ,     DC res  nom.   296
>> ohms.  A diagram
>> on the can indicates that it is a simple Iron core choke.  When I use my
>> DVM to measure
>> the resistance it tells me that it is over one meg ohm. Does this make
>> sense? The guy I
>> got it from says you cant measure it with on simple ohm meter that it
>> a high voltage
>> to do it  (like  HV diodes) and he FAXed me a diagram of how to measure it
>> This consists
>> of   a 100W  200000 ohm resistor and another 1 ohm in series with a 1570dc
>> supply.
>> When  he measure  across the 1 ohm resistor he obtains a reading of 7.9mv
>> or .0079 A
>> then  he adds the inductor and measurs again  this time he gets 6.9mv or
>> .0069 A which
>> if true tells me that that the choke resistance isn't in the meg ohms
>> range. But its still pretty high more than 296 ohms listed on the can.
>> I sent it back.  Did I do the right thing?