Analog Ammeters and Voltmeters

Permanent Magnet Moving Coil Instruments (PMMC)

  1. The working principle of these instruments is the same as that of the d’ Arsonval type of galvanometers.
  2. The flux densities used in PMMC instruments vary from 0.1 Wb/m­2 to 1 Wb/m2.
  3. These instruments can only be used on
  4. These instruments are very accurate and sensitive.
  5. Controlling torque is provided by two phosphor bronze hair springs.
  6. These springs also serve to lead current in and out of the coil.
  7. Eddy current damping is provided by the aluminium former.
  8. PMMC Instruments have uniform scale.
  9. Low power consumption (25μW to 200μW) because driving power is small.
  10. High torque to weight ratio.

Range of PMMC Instruments

  1. Ammeter: – 0 – 5μA to 0-50mA (without shunt).
  2. Voltmeter: – 0 – 50 mV (without multiplier).

Moving Iron Instruments

  1. These instruments are used either as voltmeter or ammeter only.
  2. These instruments can be used on AC as well as on
  3. Moving iron instruments are of two types, namely; Attraction type and Repulsion type.
  4. The controlling torque is provided by spring control method.
  5. The damping torque is provided by Air friction.
  6. Moving Iron instruments have non-uniform scale.
  7. They are cheap, robust and simple in construction.

Ranges of Moving Iron Instruments

  1. Ammeter: – from about 0-20mA to 0-800A maximum without current transformer.
  2. Voltmeter: – from about 0 – 800V maximum with potential transformer.

Dynamometer type instruments

  1. These are known as electrodynamic instruments.
  2. The basic principle of this instrument is that when a current carrying moving coil is placed in the magnetic field produced by the current carrying fixed coil, a force is exerted on the coil sides on the moving coil and deflection takes place.
  3. These instruments can be used on AC as well as on
  4. Controlling torque is provided by two phosphor bronze hair springs.
  5. These springs also serve to lead current in and out of the coil.
  6. Air friction damping is employed in such instruments.
  7. At low power factors, the inductances of the potential coil cause serious errors.

Ranges

  1. Ammeter:- 0 – 0.25 A to 0-100A
  2. Ammeter:- 0- 5V to 0-750V.