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Meter accuracy

I recently purchased 2 little voltmeters they look like the sort that would go in a control or instrument panel they are connected with just 2 wires which provide the operating supply ( they light up green and red) however the green  one states it will work between 20and 500 volts and the red one between 60 and 480 volts. When they are both on the green one indicates normally around 241 volts the red one shows 235 volts why the discrepancy I know it's not much but makes you wonder if one of them is lying. Secondly I've noticed that the green one tracks voltage changes faster than the red one and that a few times the green one jumps down to 238 then up to 241 multiple times while the red one stays the same and I think can see a slight flicker in my filament lamps when this is happening incidentally both meters are connected to the same plug  a 2 pin 5 amp one
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  • flat tops and over steep sides are typical of the voltage on rectifiers driving capacitor input circuits or charging batteries. The associated current is lower than you might expect on the flanks, and then higher on the peaks.

    This is sometimes referred to as  a "3rd harmonic" -  and if you sketch a sine wave, and then another one at 3 times the rate, and add a bit of the 3F to the basic wave, you will see depending on the phase you can get different effects. So if the zero-crossing of the main sine wave is in phase with a zero crossing on the 3F, that they re-enforce at the flanks, giving over steep sides, but by the main cycle is coming to the top, the 3F is on the way down again, so when you add the top gets either a flat top or a slightly bouncy depression.

    If you start the 3F in anti-phase with the main sine wave and repeat the sketching exercise, you will see that the flanks are now delayed, and then the main wave peak is added to the 3F peak  giving a ripply flat near the zero crossings and a significant up-tick at the peaks.


    So it is often said in passing, that the rectifier wave-forms can be mathematically modeled as a sine wave plus a 3F component,  that is in phase on the voltage, but the 3F current flows the opposite direction to the main current.


    The problem with 3F is that shifted by  1/3 of a cycle it looks the same on all 3 phases, so with identical loads on all phases the neutral currents do not cancel, causing odd effects at star-delta transformers.

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  • flat tops and over steep sides are typical of the voltage on rectifiers driving capacitor input circuits or charging batteries. The associated current is lower than you might expect on the flanks, and then higher on the peaks.

    This is sometimes referred to as  a "3rd harmonic" -  and if you sketch a sine wave, and then another one at 3 times the rate, and add a bit of the 3F to the basic wave, you will see depending on the phase you can get different effects. So if the zero-crossing of the main sine wave is in phase with a zero crossing on the 3F, that they re-enforce at the flanks, giving over steep sides, but by the main cycle is coming to the top, the 3F is on the way down again, so when you add the top gets either a flat top or a slightly bouncy depression.

    If you start the 3F in anti-phase with the main sine wave and repeat the sketching exercise, you will see that the flanks are now delayed, and then the main wave peak is added to the 3F peak  giving a ripply flat near the zero crossings and a significant up-tick at the peaks.


    So it is often said in passing, that the rectifier wave-forms can be mathematically modeled as a sine wave plus a 3F component,  that is in phase on the voltage, but the 3F current flows the opposite direction to the main current.


    The problem with 3F is that shifted by  1/3 of a cycle it looks the same on all 3 phases, so with identical loads on all phases the neutral currents do not cancel, causing odd effects at star-delta transformers.

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