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Why is the coil in a permanent magnet moving coil (PMMC) instrument wound on a soft iron core, even though we are dealing with direct current?

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Is it to avoid sudden increases in current, or because the iron core increases the inductance of the coil, or some other reason?

On the other hand there are disadvantages such as eddy current and core loss will increase.

What is the actual purpose of the soft iron core?

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    \$\begingroup\$ To reduce the reluctance of the magnetic circuit. \$\endgroup\$
    – user16324
    Commented Aug 10, 2021 at 17:38

1 Answer 1

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The high permeability of the soft iron core and fixed air gap to the magnets surrounding it, offers several benefits:

  1. Independent of core permeability variations relative to the air gap when high enough.
  2. No hysteresis with the DC operation of the meter.
  3. High mutual coupling of winding current flux through the air gap.
  4. High immunity to nearby magnetic fields
  5. Low aging or thermal effects as the fixed gap determines the flux and torque.
  6. Damping ratio due to L/R=T ratio when used as a low current meter such as 250mV/10k
  7. or even slower with a shunt R <1 ohm has a mechanical damping affect and since it is DC responding ,
  8. Eddy current losses have no effect on steady state.
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  • \$\begingroup\$ Is increase in B field of magnet due to iron core causes increase in torque is also a reason? \$\endgroup\$
    – user215805
    Commented Aug 10, 2021 at 18:30
  • \$\begingroup\$ No it is the increase in air gap that counts \$\endgroup\$ Commented Aug 10, 2021 at 19:36

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