Repair old KWH meter

[soldar]

Spurred by this YouTube video... I have an old KWH energy meter which sort of works but not perfectly. It seems to measure OK with large loads but with small loads the disc does not spin. I do not really need it for anything but I thought it might be interesting to adjust so that it works right.  I plugged in a 100 W incandescent lamp (@230V) and the disc does not move.

If there is an expert on this type of device willing to provide me with guidance and hand-holding then I would attempt the repair. Otherwise I will give it a pass because, as I say, I do not really need it for anything in particular.

[coppice]

There isn't much to go wrong with those things, apart from bearing the disc runs on. With a high quality bearing they run for many decades with barely any shift in their accuracy. With a poorer bearing, dirt build up. lubricant drying out, or corrosion means the torque needed to get the disc turning rises,

[Gyro]

The gear that couples with the worm at the top of the disc shaft could also wear, maybe starting to jam.

[soldar]

No, none of those two things. It is only that some times it has difficulty getting started. I believe it has to do with adjusting the magnets that provide "magnetic friction" as explained in minute 5:00 of the video. But they are different in my unit. It has a screw that looks like it would be for that purpose but I do not want to just start changing adjustments if I do not know what I am doing.

[coppice]

No, none of those two things. It is only that some times it has difficulty getting started. I believe it has to do with adjusting the magnets that provide "magnetic friction" as explained in minute 5:00 of the video. But they are different in my unit. It has a screw that looks like it would be for that purpose but I do not want to just start changing adjustments if I do not know what I am doing.

Both our descriptions are things which cause difficulty in the disc getting started, but are generally no problem once it gets turning. Unless the meter has suffered physical damage its very rare for the magnetic friction calibration to change.

[soldar]

Definitely not the worm gear which is very smooth and loose. I don't know how I could check the bearing but I figure adding a drip of oil would probably make it worse.

[Gyro]

I took one apart many years ago and the bottom bearing was a hardened point running in a jewelled cup. You could try inspecting the point under a microscope, but I think cleaning would be a much better option than oiling.

P.S. The magnetic adjustment brake didn't ought to have any effect because it works on eddy current - no motion (at start), no drag.

[TimFox]

A permanent magnet surrounding a rotating conductive metal disk is a common way to get damping:  a retarding force proportional to velocity, to damp resonance.

[Gyro]

From an old electrical engineering book I have, the magnet also provides the calibration adjustment and, through a bimetal strip arrangement, temperature compensation.