Exam Question
(a) Sketch and describe moving iron and moving coil instruments for measuring current. (10)
(b) How the moving iron instrument can be used to measure voltage? (6)
Class 4 (4th Engineer)
MET
📅 Oct 2023
Reference Answer
### Part (a): Moving Iron and Moving Coil Instruments
#### 1. Moving Iron (MI) Instrument
**Principle of Operation:**
The operation of a moving iron instrument is based on the magnetic effect of electric current. When a current flows through a coil, it produces a magnetic field. A piece of soft iron (vane) placed in this field gets magnetized and experiences a force, causing it to move. This movement is used to deflect a pointer over a calibrated scale. The deflecting torque ($$T_d$$) is proportional to the square of the current flowing through the coil ($$T_d \propto I^2$$). Since the torque is proportional to the square of the current, the instrument can be used for both DC and AC measurements. For AC, it measures the RMS value.
#### 1. Moving Iron (MI) Instrument
**Principle of Operation:**
The operation of a moving iron instrument is based on the magnetic effect of electric current. When a current flows through a coil, it produces a magnetic field. A piece of soft iron (vane) placed in this field gets magnetized and experiences a force, causing it to move. This movement is used to deflect a pointer over a calibrated scale. The deflecting torque ($$T_d$$) is proportional to the square of the current flowing through the coil ($$T_d \propto I^2$$). Since the torque is proportional to the square of the current, the instrument can be used for both DC and AC measurements. For AC, it measures the RMS value.
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