1. Dept of E&E ELE101/102 1
Moving Iron type instruments
•Attraction type
•Repulsion type
Basic types:
Principle:
A soft iron vane (moving iron) which is mounted on the spindle
moves when the operating current flows through the coil.

2. Dept of E&E ELE101/102 2
Attraction typeMoving Iron type instruments:
Construction:
When current flows through the coil, a soft iron piece moves from
the weaker magnetic field towards the stronger field.

3. Dept of E&E ELE101/102 3
Attraction typeMoving Iron type instruments:
Working:
Td α mean of i2
over a cycle.
Tc α θ
In steady state, Tc=Td
θ α mean of i2
over a cycle
Scale is not uniform

4. Dept of E&E ELE101/102 4
Repulsion type
θ α mean of i2
over a cycle
Scale is not uniform
Moving Iron type instruments:
When current flows through the coil, Two soft iron rods get
magnetized similarly, causing them to repel each other.

5. Dept of E&E ELE101/102 5
•Cheap and sturdy.
•Possess a good degree of reliability.
•Less friction errors
Advantages:
Disadvantages:
•Degree of precision of calibration is not high for dc
measurements due to hysteresis in iron parts
Moving Iron type instruments

6. Dept of E&E ELE101/102 6
Measurement of POWER:
Construction:
Dynamo type Wattmeter

7. Dept of E&E ELE101/102 7
Measurement of POWER:
Tdmean α VI cos φ
Since Spring control is used ; Tc α θ
At steady state, Td = Tc
Hence, θ α VI cos φ
Dynamo type Wattmeter
Working: