is813cs@pyr.gatech.EDU (Cris Simpson) (10/25/89)
We need a way to measure inductance at fairly high voltages,
around 120 volts. Wayne Kerr sells an LCR analyzer that only goes
to 5 volts. HP has one up to 38 volts. Does anyone know of a box
that can do 120? Alternately, does anyone have experience with
a home-brew system to do it? We will be working in the milliHenry
range.
Eternal gratitude, etc, etc. ...
cris
--
|| Gee, do you think it'd help if I plugged in both ends of this cable? ||
Cris Simpson Computer Engineer VA Rehab R&D Center
GATech Atlanta,GA
is813cs@pyr.gatech.edu ...!{Almost Anywhere}!gatech!gitpyr!is813csjk3k+@andrew.cmu.edu (Joe Keane) (10/26/89)
As long as the magnetic field in the inductor is weak, inductance shouldn't change much with voltage. You may be OK just using the value from a lower voltage. What i'd do is to put the inductor in series with a small resistor, the value depending on what frequency you're using. Hook them up to a sinusoidal source at the voltage and frequency you're interested in. Then look at the voltages across both on a scope, making sure you don't ground something that isn't. Given the ratio and phase angle between the two voltages, you can figure out both the inductance and resistance of your inductor at the given voltage and frequency. I assume of course that the capacitance is negligible.
larry@kitty.UUCP (Larry Lippman) (10/26/89)
In article <9450@pyr.gatech.EDU>, is813cs@pyr.gatech.EDU (Cris Simpson) writes: > We need a way to measure inductance at fairly high voltages, > around 120 volts. Wayne Kerr sells an LCR analyzer that only goes > to 5 volts. HP has one up to 38 volts. Does anyone know of a box > that can do 120? Alternately, does anyone have experience with > a home-brew system to do it? We will be working in the milliHenry > range. Are you referring to superimposed DC voltage, open-circuit AC execitation voltage, a common mode voltage, or what? Inductors don't generally care about voltage, but they sure do care about *current* (both AC and DC), however. Also, other considerations in selecting an inductance bridge are: (1) do you want to measure Q, (2) do you want to measure R; and (3) at what *frequency* do you want to measure? I haven't seen any Wayne-Kerr apparatus in years. Come to think of it, I haven't seen any Freed Transformer Co. inductance bridges in a good many years, either. <> Larry Lippman @ Recognition Research Corp. - Uniquex Corp. - Viatran Corp. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"
berryh@udel.edu (John Berryhill) (10/26/89)
In article <0ZFW62y00XcRM3bGAl@andrew.cmu.edu> jk3k+@andrew.cmu.edu (Joe Keane) writes: >Then look at the voltages across both on a scope, making sure you don't >ground something that isn't. Given the ratio and phase angle between >the two voltages, you can figure out both the inductance and resistance >of your inductor at the given voltage and frequency. I assume of course >that the capacitance is negligible. Why make it tough on yourself (and risk damaging the scope with big AC)? If you have an AC voltmeter and ammeter, just measure V and I to get the magnitude of the impedance Z. Calculate L from |Z|^2= R^2 + w^2 L^2 where w=2(pi) 60 Hz -- John Berryhill "Tasty Snacks in Cellophane Packs" 143 King William, Newark DE 19711
is813cs@pyr.gatech.EDU (Cris Simpson) (10/26/89)
>>From: jk3k+@andrew.cmu.edu (Joe Keane) >>As long as the magnetic field in the inductor is weak, inductance >>shouldn't change much with voltage. You may be OK just using the value >>from a lower voltage. The field will not be weak, we will be looking at saturation effects. Also the resistance is high, 30 Ohms. (These are motor windings.) >>What i'd do is to put the inductor in series with a small resistor, the >>value depending on what frequency you're using. Hook them up to a >>sinusoidal source at the voltage and frequency you're interested in. >>Then look at the voltages across both on a scope, making sure you don't >>ground something that isn't. Given the ratio and phase angle between >>the two voltages, you can figure out both the inductance and resistance >>of your inductor at the given voltage and frequency. I assume of course >>that the capacitance is negligible. 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.) In article <3467@kitty.UUCP> larry@kitty.UUCP (Larry Lippman) writes: > > Are you referring to superimposed DC voltage, open-circuit AC >execitation voltage, a common mode voltage, or what? Inductors don't >generally care about voltage, but they sure do care about *current* >(both AC and DC), however. Also, other considerations in selecting an >inductance bridge are: (1) do you want to measure Q, (2) do you want >to measure R; and (3) at what *frequency* do you want to measure? > > I haven't seen any Wayne-Kerr apparatus in years. Come to >think of it, I haven't seen any Freed Transformer Co. inductance >bridges in a good many years, either. > From above, AC excitation. We borrowed a Wayne-Kerr, it was very nice, with a video screen, 20A bias unit, etc. Unfortunately, only 5v. Any ideas now? Thanks, cris -- || Gee, do you think it'd help if I plugged in both ends of this cable? || Cris Simpson Computer Engineer VA Rehab R&D Center GATech Atlanta,GA is813cs@pyr.gatech.edu ...!{Almost Anywhere}!gatech!gitpyr!is813cs
larry@kitty.UUCP (Larry Lippman) (10/27/89)
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. <> UUCP {allegra|boulder|decvax|rutgers|watmath}!sunybcs!kitty!larry <> TEL 716/688-1231 | 716/773-1700 {hplabs|utzoo|uunet}!/ \uniquex!larry <> FAX 716/741-9635 | 716/773-2488 "Have you hugged your cat today?"
jk3k+@andrew.cmu.edu (Joe Keane) (10/29/89)
John Berryhill writes: >Calculate L from |Z|^2= R^2 + w^2 L^2 One equation, two unknowns. How would you find R at the given frequency? We're probably dealing with `high frequency' in terms of skin effect. Cris Simpson writes: >Any ideas now? Given that you're dealing with saturation and `high' frequencies, i don't think you can characterize it by a single number. So i'm not sure exactly what you want. The reactance over some frequency range? Harmonic generation? Current ramp response? Let me ask: why do you want this?
is813cs@pyr.gatech.EDU (Cris Simpson) (10/30/89)
>Given that you're dealing with saturation and `high' frequencies, i >don't think you can characterize it by a single number. So i'm not sure >exactly what you want. The reactance over some frequency range? >Harmonic generation? Current ramp response? Let me ask: why do you >want this? We need to be able to characterize the inductance of some high resistance motor windings. These inductances vary depending on a number of factors. I would be happy with the inductance at any frequency ( say 100 Hz.) at (.5, 1, 2, 3, 4, 5 Amps). Current ramp response is mostly what I want. Cris -- || Gee, do you think it'd help if I plugged in both ends of this cable? || Cris Simpson Computer Engineer VA Rehab R&D Center GATech Atlanta,GA is813cs@pyr.gatech.edu ...!{Almost Anywhere}!gatech!gitpyr!is813cs