Rewiring old AC ampmeter to DC?

[Simen]

Just got hold of an old AC Ampmeter with the range of 0 to 500Amp, and i'm trying to rebuild it to show 0 to 50Amp DC. There is a massive '1 turn' Coppercoil there, which i've now removed, and replaced with 9 turns of #12AWG, in hope of getting the meter to go up to the '500 amp' mark on the scale when fed with 50Amp.

So far, i've just tested with a 10amp load, and it looks good so far (goes up to '100Amp' mark), but i doesn't have any more loads to test it with right now. (Will do that later...)

Any comments about linearity etc? See pictures of the meter here:

(Large pictures; 1-2MB)

http://www.mwlmf.net/100_2096.jpg

http://www.mwlmf.net/100_2097.jpg

http://www.mwlmf.net/100_2098.jpg

http://www.mwlmf.net/100_2099.jpg

Regards,

Simen

[hiker]

heres an easy homebuilt ampmeter.....

     http://www.otherpower.com/otherpower_experiments_ammeter.html

[Nando]

This ampere meter has the shunt part of the movement and the movement seems to be vane type.

To reduce the reading the ring mass has to be reduced to a symmetric 1/5, but this reduction is critical and dependent on the vane type, and balancing spring, which seems to the torsion type.

A big and difficult job, and you do not need to have 100 amps of current, 20, or so amps is sufficient, of course higher current better setting of the accuracy.

Conversion to DC, I do not know -- too many unknowns.

[Flux]

I haven't looked at your pictures, but what you have done should give you a working device.

You will need to re-calibrate, as Nando said, there are too many variables to attempt to use the original scale with a multiplying factor. In theory you should have used 10 turns, but even then it is highly unlikely that the field from a coil of totally different construction will be anything like close enough to keep the original scale.

Moving iron movements are non linear and the position and shape of the coil has a drastic effect on the FSD and the scale shape.

For wind it may be a poor choice as the lower readings will all be crammed at the bottom of the scale and you may not see much for most of the time, but it will look impressive when you get the higher currents.

Flux

[Simen]

My goal is to use the original scale; just by adding a decimalpoint before the last zero in '100', 200' etc... (fyi: the meter are about 7 inches in diameter... )

I tried with 10 windings at first, but then the meter showed 12Amp and then some with an 10A load; with 9 windings, i'm pretty close. It still shows a bit too much, so i'll try to unwind 1/2 a turn or so later.

I have winded the #12 wire around an empty 35mm film plastic-cartridge, which have the same diameter as the original 'coil'

I have just tried adding another 10Amp of load; 20Amp in total, and the meter slided nicely up to the '200' mark, so it actually seems to be rather linear. (so far... )

I'm a bit confused about the workings of that half-circle of metalsheet attached to the axel of the needle; it doesn't seems much magnetical... But when inside a coil, it wouldn't need to be strong? Opposed to that hd-magnet ammeter on the Otherpower's experiment pages... (Yes, built that one... )

Regards,

Simen

[Simen]

BTW:

That large box underneath the needle contains just a plate, which are attached to the needleaxle, oriented along the axle, and the plate fits snugly inside that compartment without touching the sides. My guess is that it's just an 'air-dampener', to stabilize the movement of the needle at sudden peaks.

Regards,

Simen

[Flux]

That movement seems a little unusual. Normally there is a fixed and a moving iron that repel each other. From your photos I can't see a fixed iron. It seems to be a gravity controlled attracted iron version.

The actual shape and diameter of the coil and its position will be critical to maintaining the original scale shape. Moving the coil position may have more effect than removing a turn. There is really no way of knowing the mean path of the current in that original cast coil, you may need to distort your wound coil to simulate it.

It will be purely trial and error.

Yes the vane is an air damper and under wind conditions the damping may not be totally adequate, you will probably see the needle swinging about quite significantly but it will give you an idea. A well damped moving coil is far better for this job.

Flux

[Simen]

You're right; there is a small piece of iron on the brass-arm to the axle. The picture was shot with the needle in 'max' position; putting those two irons farthest away from each other. The angle the picture was shot with, also hides a small adjustable counterweight right beneath the circular ironsheet.

Yes, i will try and err... And eventually succeed But i think i'm on the right track now.

Regards,

Simen.

[Ungrounded Lightning Rod]

In theory you should have used 10 turns, but even then it is highly unlikely that the field from a coil of totally different construction will be anything like close enough to keep the original scale.

However welding 10 or 11 split washers together into a 10-turn coper helix with the same inside and outside diameters and total thickness should hit it close to dead on.

An N-turn coil of heavy copper wire spaced to take up the same volume would be in the ballpark (though the current distribution would be more even, probably making the central field slightly weaker).

[Nando]

Vane meters like this, have a field gap that infers calibration around certain currents.

This is one of the reasons the shunt appeared wit a low current meter.

Which at the same time lowered the price by several fold.

The solution for this case is to take the copper core and produce the 10 turns and at the gap position make another loop going high to continue the current but to minimize the field in that area.

In other words a 10 turns coil with the additional loop where the magnetic cut occurs.

Nando