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Old 16th Jan 2020, 7:12 pm   #1
mark_in_manc
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Default Calibrating moving iron ammeter

Hi folks.

I have a somewhat aged 5A Philip Harris moving iron ammeter. I want to use it on a 50Hz ac supply. As things stand it reads about 2/3 of the value shown on my Avo 8 (actually two Avo 8s, which more or less agree). Inside there is a bobbin of tinned wire wrapped around the needle pivot, and that's about it - no shunt as far as I can see, the addition of which would, I guess, make it read even further under. Any suggestions for how to deal with it - re-wind the bobbin with more turns, or...?
Thanks
Mark
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Old 16th Jan 2020, 8:40 pm   #2
golexica
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Default Re: Calibrating moving iron ammeter

Isn't the current/torque response of a moving iron quadratic, and has to be corrected for with clever weights/a nonlinear scale, if you just wound more turns, i'm not sure it would end up lining up anymore.

being 2/3 off consistently seems weird, wouldn't you expect the amount that it's off to change across the scale? one thought is that 2/3 seems awfully close to 1/sqrt(2)- ~67% and ~70% i.e. what would happen if the needle was moving in a magnetic field that was 1/2 as strong. Are there coils/magnets on both sides of the needle? if there were, and one had failed, wouldn't that cause something similar?

sorry if stupid question, am new here
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Old 16th Jan 2020, 10:08 pm   #3
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Default Re: Calibrating moving iron ammeter

Well, you prompted me to have a proper go at it, for (avo) rms currents between 1.6A and 3.6A. The cal factor between the two is about 0.63, right the way across. Yes, the scale on the MI meter is very non-linear, compressed at the high end. I understand what you mean about the possible significance of the factor - as far as I can see there's only one coil. The load was pretty reactive - I guess I could have another go with a resistive load (variac, kettle...) and see what happens. Can you tell I'm grabbing at straws?
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Old 16th Jan 2020, 11:26 pm   #4
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Default Re: Calibrating moving iron ammeter

Is the reading any different with DC?

I'd have expected springs to soften with age, but could they have hardened? No magnets to weaken.

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Old 17th Jan 2020, 12:07 am   #5
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Default Re: Calibrating moving iron ammeter

Talking of clutching at straws, what voltage are you using when measuring the current?
If it is a very low voltage- have you tried a higher voltage..?

(golexica- your questions were not stoopid)

A shorted turn in the coil might produce low results..?

As per David's comment, i have known coilsprings to weaken giving optimistic meter readings (having gone green with corrosion) but not encountered one getting stiffer.

If one turn of a coilspring had got hooked up on the adjacent turn (due to rough handling or overload) i would expect the under-read to be more subtle (perhaps 12%) but my experience of this particular fault is limited to PMMC movements so it could be worth getting a jeweller's loupe out and checking the coilsprings for tangle/s.

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Old 17th Jan 2020, 2:05 am   #6
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Default Re: Calibrating moving iron ammeter

Thinking about it more, if there was a short, you could indeed compensate for it by wrapping extra turns around the magnet, but at 63% of the correct value, you'd have to add an extra 60% worth of turns (1-0.63^2) to get it back in alignment, might be easier to just rewrap the entire coil at that point?
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Old 17th Jan 2020, 1:10 pm   #7
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Default Re: Calibrating moving iron ammeter

In retrospect- sorry- my last 2 paras. about the coilspring/s won't apply here as the consistent error on a non-linear scale says this is not a mechanical problem- rather an electrical or electromagnetic one.

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Old 17th Jan 2020, 4:31 pm   #8
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Default Re: Calibrating moving iron ammeter

I also reconsider, what i just said, forgot to think about the coil inductance, which is going to change as you add turns, so not a square, might actually be linear, but you'd probably have to fine tune it by adding them one at a time.
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Old 17th Jan 2020, 4:46 pm   #9
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Default Re: Calibrating moving iron ammeter

Moving-iron meters are somewhat strange - apart from the non-linear scale.

I've seen some extremely-odd readings from M.I. meters when used to measure current/voltage that is non-sinusoidal (as, for example, when feeding AC into large inverter/SMPS/motor-drive-type circuits). There again, moving-coil meters can be tricked in such situations too.
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Old 17th Jan 2020, 6:39 pm   #10
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Default Re: Calibrating moving iron ammeter

Ah, this is a motor drive (VFD, only 750W full tilt) - but it's hard to imagine I am taking bites out of the mains unless I guess there is a lot of voltage drop in the feed to the shed? It's not a long way from the electric meter. The voltage was nominally mains 240v for all readings. I wonder if I can measure the waveform across the meter, if I use an isolating transformer to drive the scope. And all that said, I guess I need to try a comparative calibration with DC, and also on ac with a resistive load. Kettle, variac, here we come
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Old 17th Jan 2020, 8:19 pm   #11
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Default Re: Calibrating moving iron ammeter

A VFD? that might cause issues, weird harmonics, on both input and output.

output because of the switching. But also on the input if it's not built super well, especially if it's just a single phase- with a three phase it'd be much less of a problem.

it's gonna need to charge capacitors to make the output. Capacitors charged through a bridge rectifier produce an absolutely appaling current waveform, close to a square wave ( the bridge rectifier blocks all current untill the voltage on the mains risies above the voltage on the capacitor, so the current suddenly switches on from almost nothing, and then switches off suddenly when the voltage drops below the input caps )

the iron can only magnetise so fast, it's gonna have trouble reading those high frequency components in something like a square wave.

Last edited by golexica; 17th Jan 2020 at 8:21 pm. Reason: add something about 3 phase
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