Moving iron instruments are used for measuring AC and DC current or voltage, with types classified into repulsion and attraction types. The working principle involves the movement of a pointer due to the magnetic field produced by a current-carrying coil, with torque provided by springs or weights. While offering high accuracy and low cost for AC measurements, they have limitations such as non-linear scales and sensitivity to frequency variations.

1. Moving Iron (MI) Instruments
• Generally used to measure AC current
or voltage (but can measure DC current
and voltage without any external circuit)
• Pointer is connected to specially
designed soft iron that moves
according to the intensity of magnetic
field acting on it
• The magnetic field is produced by
current carrying coil

2. 2
Classification of MI
1) Repulsion (or double iron) type
2) Attraction (or single iron) type

3. 3
Types of torque in MI
• Deflecting torque produces movement on an
aluminum pointer over a calibrated scale
• Control torque: provided by spring or weight
• Damping torque: Pneumatic (mechanical
damping) aluminum vane attached to the shaft

4. 4
Constructional details of
attraction type MI
• Moving element : Soft
iron
• Coil: Produces magnetic
field & magnetize the iron
piece
• Control springs or
weights, aluminum vane,
aluminum pointer, shaft,
calibrated scale, mirror, etc

5. 5
Constructional details of
Repulsion type MI

6. 6
Constructional details of
Repulsion type MI

7. 7
Constructional details of
Repulsion type MI
• Two concentric iron vanes (Fixed & Movable)
• Solenoid coil (stationary)
• Pointer attached to movable vane
• Movable vane is curved rectangular in shape
• Fixed vane is tampered
• Controls springs, Aluminum pointer,
Aluminum vane, calibrated scale, mirror etc

8. 8
Working principle
• Current flow magnetizes the coil
• The two iron vanes become magnetized with
north poles at their upper ends and south poles
at their lower ends for one direction of current
through the coil
• Due to repulsion, the unbalanced component
of force, tangent to the movable element,
causes it to turn against the force exerted by
the springs

9. 9
Working principle
• When no current flows through the coil, the
movable vane is positioned so that it is
opposite the larger portion of the tapered
fixed vane, and the scale reading is zero
• The amount of magnetization of the vanes
depends on the strength of the field, which,
in turn, depends on the amount of current
flowing through the coil.

10. 10
Design for MI Ammeter and MI
Voltmeter
Ammeter: Coil with few turns of large wire
Voltmeter:
1)Solenoid with many turns of small wire
2)Portable size: a) self contained series
resistance b)ranges up to 750 V
3) Higher ranges are obtained by the use of
additional external multipliers

11. 11
• Error due to residual magnetism in the
vanes
• The error may be minimized by reversing
the meter connections and averaging the
readings
MI for DC purpose

12. 12
• Low cost
• used for both ac and dc purpose
• High accuracy for AC
• Robust
• Torque to weight ratio is high (less friction)
Advantages

13. 13
• Non linear scale
• Draw more power
• Shielding (laminated iron cylinder)
should be provided to protect from
external magnetic fields
• Deflecting torque is not exactly
proportional to I2
• Variation of frequency introduce
error
Limitations

14. 14
Comparison between the scale of
MI and MC