electrical measuring instruments

1. ELECTRICAL
MEASURING
INSTRUMENTS
BY KIRUTHIKA
III B.SC PHYSICS ‘C’ SECTION

2. Moving coil instruments
Moving coil instruments can be classified into two
types:
Permanent magnet moving coil instrument (for DC)
Dynamometer type (for both AC and DC)

3. PERMANENT MAGNET MOVING COIL
PRINCIPLE OF OPERATION:
The operation of a permanent magnet moving coil
instrument is based upon the principle that when a
current carrying conductor is placed in a magnetic
field, it experiences a force or torque which moves
it.

4. CONSTRUCTION
A PMMC meter (or D’Arsonval meters) is
constructed of 5 main components:
 Stationary Part or Magnet System
 Moving Coil
 Control System
 Damping System
 Meter

5. Stationary Part or Magnet System
In the present time we use magnets of high field intensities, high coercive
force instead of using U shaped permanent magnet having soft iron pole
pieces. The magnets which we are using nowadays are made up of materials
like alcomax and alnico which provide high field strength.
Moving Coil
The moving coil can freely moves between the two permanent magnets as
shown in the figure given below. The coil is wound with many turns of
copper wire and is placed on rectangular aluminium which is pivoted on
jeweled bearings.
Control System
The spring generally acts as control system for PMMC instruments. The
spring also serves another important function by providing the path to lead
current in and out of the coil.

6. Damping System
The damping force hence torque is provided by
movement of aluminium former in the magnetic field
created by the permanent magnets.
Meter
Meter of these instruments consists of light weight
pointer to have free movement and scale which is
linear or uniform and varies with angle.

7. Advantages of Permanent Magnet
Moving Coil Instruments
1.The scale is uniformly divided as the current is directly proportional to deflection of
the pointer. Hence it is very easy to measure quantities from these instruments.
2.Power consumption is also very low in these types of instruments.
3.A high torque to weight ratio.
4.These are having multiple advantages, a single instrument can be used for
measuring various quantities by using different values of shunts and multipliers

8. Disadvantages of Permanent Magnet
Moving Coil Instruments
1.These instruments cannot measure AC quantities.
2.The cost of these instruments is high as compared
to moving iron instruments

9. VOLTMETER
 It is used for measuring the voltages of both the AC and
DC devices.
 The electronic voltmeter gives the accurate reading
because of high input resistance.
In all electronic voltmeter circuits the principle involved is that
an indication on a permanent magnet moving coil instrument
(normally abbreviated as PMMC or D’Arsonval movement)
proportional to the input voltage is obtained by means of
amplification in one or more stages with a high input impedance.

10. Working Principle of Electronic
Voltmeter