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Moving coil instrument vs Moving Iron Instrument | Difference between Moving coil instrument and Moving Iron Instrument

This page compares Moving coil instrument vs Moving Iron Instrument and mentions difference between Moving coil (M.C.) instrument and Moving Iron (M.I.) Instrument.

Moving Coil (M.C.) instrument

• These instruments are of two types viz. Permanent magnet moving coil (used for DC only) and dynamometer type moving coil instrument (For AC and DC both).

• In the Permanent magnet moving coil instrument, when a current carrying conductor is placed in a magnetic field, it experiences a force. The direction of the force is given by "Fleming's left hand rule". A coil placed in a magnetic field and carrying operating current is attached to the moving system. With the movement of the coil, pointer moves over the scale.

• Dynamometer type instruments are based on the principle that "mechanical force exists between current carrying conductors". It consists of fixed coil and moving coil. The Moving coil is pivoted between the two fixed coils. Bith are excited separately or together if connected in series. The moving coil is attached to the moving system so that under the action of deflecting torque, the pointer moves over the scale.

Moving Iron (M.I.) instrument

• These instruments are of two types viz. attraction type moving iron instrument and Moving Iron Repulsion type instrument.

• In the attraction type moving iron instrument, current is passed through the coil. When this happens, coil produces magnetic field which attracts soft iron cam placed near to it.
• The cam is fixed on the spindle which in turn gives motion to the pointer on the scale.
• Whatever may be the direction of the current, iron disc would always be pulled in. Hence this instrument can be used for measurement of both AC and DC.
• Here, θ ∝ I2, Where θ is deflection of the disc.
• For AC, θ ∝ I2 r.m.f.
• The scale is not linear in moving iron instrument.

• Moving iron repulsion type instrument works on the principle that when two pieces of soft iron rods or strips are magnetised in the same direction by current carrying coil, they will experience a force of repulsion.
• In this type, one strip (say 'F') is fixed where as the other strip or vane (say 'M') is moveable to which spindle is attached. This spindle rotates the pointer on the scale when current is passed through the coil.

Following table summarizes difference between M.C. instrument and M.I. Instrument types.

Moving Coil (M.C.) Instrument Moving Iron (M.I.) Instrument
More accurate Less accurate
Costly Cheaper
Uniform scale non-uniform scale
Sensitive in construction Robust in construction
Low power consumption High power consumption
Eddy current damping is used Air friction damping is used
It is used for DC measurements. It is used for both AC as well as DC measurements.
Controlling torque is provided by spring. Controlling torque is provided by gravity and spring.
Deflection in moving coil instrument is proportional to current i.e. θ ∝ I. Deflection in moving iron instrument is proportional to square of the current i.e. θ ∝ I2.
Errors are set due to aging of control springs, permanent magnet (i.e. No hysteresis loss). Errors are set due to hysteresis and stray fields (i.e. hysteresis loss takes place).


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