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Induction Motor Interview Questions with Answers | Part-1

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Moving Iron Instruments and Moving Coil Instruments | Electrical Measurement Theory

Moving Iron Instruments and Moving Coil Instruments | Electrical Measurement Theory


Moving Iron Instruments


Moving Iron Instruments
Moving Iron Instruments and Moving Coil Instruments | Electrical Measurement Theory


Moving-iron instruments are commonly used in laboratories and switch boards at commercial frequencies because they are very cheap and can be manufactured with required accuracy.


Moving-iron instruments can be divided into two types:


(1)Attraction type

In this type a sheet of soft iron is attracted towards a solenoid.

(2)Repulsion type

In which two parallel rods or strips of soft iron, magnetised inside a solenoid are regarded as repelling each other.

Moving Coil Instruments


The moving-coil instruments have a following two types:

(1)Permanent-magnet type

It can be used for D.C.only.

(2)Dynamometer type

It can be used both for A.C.and D.C.

PMMC - Permanent-magnet Moving-Coil Type Instruments


Figure of Permanent Magnet Moving Coil type instrument is show below:

Permanent Magnet Moving Coil type instrument
Permanent Magnet Moving Coil type instrument


A permanent-magnet moving coil-type instrument works on the principle that when a
current-carrying conductor is placed in a magnetic field, it is acted upon by a force which tends to move it to one side and out of the field.


Electrodynamic or Dynamometer Instruments
In an electrodynamic instrument the operating field is produced by another fixed coil and not by permanent magnet. 


This instrument can be used as an ammeter or as voltmeter but is generally used as a wattmeter.


The deflecting torque is due to interaction of
magnetic fields produced by currents in fixed and moving coil.

Td ∝ I1 × I2

So, Td = K I1 × I2

where K is a constant.

Since the instrument is spring controlled, the restoring or contral torque(Td) is proportional
to the angular deflection θ

Tc ∝ θ

As Td=Tc for final deflection

So, θ ∝ I1 I2

Figer of Electrodynamic or dynamometer
instrument are shown below:


Dynamometer type instruments
Dynamometer type instruments



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