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.

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.

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

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.

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

Dept of E&E ELE101/102 6

Measurement of POWER:

Construction:

Dynamo type Wattmeter

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: