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I don't understand what the following paragraph is trying to tell:

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I know how PMMC works. But why does the text say PMMC cannot be used to measure AC? It says action of iron varies with frequency. Which iron? Why is error introduced? I'm completely lost. I would be glad if someone could explain what mean here in a plain way.

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And what are these resistors for?

Here is another text:

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Why does the text say measurement of ac is not possible with PMMC?

Edit:

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I think the reason of rectifying AC to pulsating DC is given by the above paragraph:

Due to the inertia of the movable coil, the meter indicates a steady deflection

(?)

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The reason you can't use a PMMC with AC is because a PMMC, since it's a moving mass, cant measure the instantaneous current, but rather measures the average current. The average current of an AC waveform is, of course, 0, so your meter will just read 0 the whole time. In fact, I'm pretty sure it's not possible to directly use a moving coil meter of any type for AC.

The following was assuming a different type of meter, but the OP is using a moving coil meter. (This is due to the hysteresis of iron. When you magnetise and then demagnetise iron, you lose energy (represented by the purple area below). When you magnetise iron, it wants to stay magnetised. This affects the current waveform, turning it from a nice ac wave into a different shape (a more pointy one, if I remember correctly).

edit: removed incorrect info)

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    \$\begingroup\$ +1 need to add.. resistors are there because voltmeters are really ammeters in disguise measuring the current through a known resistance = volts. \$\endgroup\$ – Trevor_G Oct 20 '17 at 17:56
  • \$\begingroup\$ Where is the iron? How does this hysteresis interact and affect the current's mean value or waveform? Can you illustrate it a bit. \$\endgroup\$ – Genzo Oct 20 '17 at 18:12
  • \$\begingroup\$ I really dont get why ac is not used with PMMC \$\endgroup\$ – Genzo Oct 20 '17 at 19:09
  • \$\begingroup\$ Oops, I made a mistake, please ignore the second paragraph. PMMC still has the iron in it and there are still energy losses. \$\endgroup\$ – BeB00 Oct 20 '17 at 19:17
  • \$\begingroup\$ I dont see anything about hysteresis in texts about the reasons why ac is not measured by PMMC. see my edit. \$\endgroup\$ – Genzo Oct 20 '17 at 19:28
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Basically if you excite a structure "back and forth" with a very high frequency comparing to its natural frequency the structure will not move.

The movable coil will not move in PMMC when the current is AC. In PMMC instrument if you pass an AC current, the current alternates/reverses very quick so the torque on the coils reveres too rapidly and the coil will not deflect. Only if AC frequency is too low you can see a deflection. That's why to measure AC with a PMMC you need to rectify the current.

On the other hand, if the permanent magnet's field is replaced by a magnetic field which is created by a coil through which alternating current passes, the field will reverse as the current reverses and the torque will be unidirectional so the coil will deflect. And this will be called an electrodynamometer instrument.

For the need of resistors(left circuit voltmeter, right circuit ammeter):

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The coil will have some internal resistance Ri. And the voltage required for a full deflection will be Vo = RixIo. But for measurements with higher voltages a series multiplier resistor Rm is added. So we can write for the voltmeter:

V/Vo = (Rm+Ri) / Ri

Rm = Rix(V/Vo -1) ≈ Rix(V/Vo)

Same movement can be used to measure currents larger than Io, , by paralleling the instrument with a resistive shunt as Rs. With full deflection Vo across the meter will pass Io so we can write for the ammeter:

RixIo = Rsx(I-Io) then

Rs = RixIo/(I-Io) = Vo/(I-Io) ≈ Vo/I

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