PMMC is the type of moving coil instruments. These instruments are very sensitive and accurate. DC voltage or current can measure by these instruments.

Principle of Operation of PMMC

The operation of PMMC instrument is based on the principle when a current carrying conductor placed in a magnetic field, a mechanical force experienced by the conductor.

Construction

It consists of a permanent shoe magnet. Light rectangular coil of having many turns of fine wire wound on a light aluminum former. The coil acts as a moving element. This coil is mounted on a spindle. Two phosphor bronze spiral hair springs are attached to the spindle. The springs provide the controlling torque as well as they act as incoming and outgoing leads for the current. Eddy current damping is used and it is provided by the aluminum former.

Working

When the instrument is connected in the circuit. The current flows through coil which is mounted on the spindle. Magnetic flux is set up in the coil. This magnetic field or flux interact with the field produced by permanent magnets, a force is exerted on the current carrying conductors of the coil which produces deflecting torque. This results pointer moves over the calibrated scale.
If current in the coil is reversed, the direction of deflecting torque will be reversed because field produced by the permanent magnets does not change.

Advantages

  • These instruments have uniform scale.
  • These instruments require low power for their operation.
  • High torque/weight ratio.
  • No hysteresis loss as the former is of aluminum
  • Reliable damping torque.
  • These instruments are very accurate and reliable.

Disadvantages

  • These instruments cannot be used for ac measurements.   
  • These instruments are costlier.                       

Errors in PMMC instruments

  • Change of resistance of the moving coil with temperature.
  • Due to ageing effects, wakening the stiffness of springs.