Indicators and display

devices

Types of instruments

• PMMC:

DC measurements only. Most accurate

• Moving iron:

magnitude effect of current, most commonly used and the

cheapest, very accurate & can be used for both AC & DC.

• Electrodynamometer:

calibration is same for both ac & dc

• Hot wire:
• Thermocouple:

suitable for ac b’coz deflection depends

directly upon the heating effect of ac (rms value of current). Readings
are independent of frequency.

• Induction type:

ac measurements only

• Electrostatic:

used as voltmeter. Power consumption is small.

Cover a large range of voltage. Substandard accuracy

Various forces/torques required in

measuring instruments

• Deflecting torque/force: The defection of any instrument is determined

by the combined effect of the deflecting torque/force, control torque and
damping torque. The value of deflecting torque must depend on the
electrical signal to be measured; this torque causes the instrument
movement to rotate from its zero position.

• Controlling torque/force: This torque must act in the opposite sense to

the deflecting torque, and the movement will take up an equilibrium or
definite position when the deflecting and controlling torque are equal in
magnitude. Spiral springs or gravity usually provides the controlling
torque.

• Damping torque/force: A damping force is required to act in a direction

opposite to the movement of the moving system. This brings the moving
system to rest at the deflected position reasonably quickly without any
oscillation or very small oscillation. This is provided by i) air friction ii) fluid
friction iii) eddy current. It should be pointed out that any damping force
shall not influence the steady state deflection produced by a given
deflecting force or torque. Damping force increases with the angular
velocity of the moving system, so that its effect is greatest when the
rotation is rapid and zero when the system rotation is zero.

D’Arsonval Movement

The most commonly used sensing mechanism used in
DC ammeters, voltmeters, and ohm meters is a
current-sensing device called a D’Arsonval meter
movement (Figure 1). The D’Arsonval movement is a
DC moving coil-type movement in which an
electromagnetic core is suspended between the poles
of a permanent magnet. The    current    measured    is
   directed D’Arsonval Meter Movement through the
coils of the electromagnet so that the magnetic field
produced by the  current  opposes  the  field  of  the
permanent magnet and causes rotation of the core.
The core is restrained by springs so that the needle
will deflect or  move  in  proportion  to  the  current
intensity. The more current applied to the  core, the
 stronger  the  opposing field, and the larger the
deflection, upto the limit of the current capacity of the
   coil. When    the    current    is interrupted, the
opposing field collapses, and the needle is returned to
zero  by  the  restraining  springs. The limit of the
current that can be applied to  this  type  movement
 is  usually  less than one milli-ampere.

Modern D’Arsonval

movement

Principle of operation

• It has been mentioned that the

interaction between the induced field
and the field produced by the
permanent magnet causes a
deflecting torque, which results in
rotation of the coil.

PMMC movement showing ‘Y-

member’

• Advantages:
– They have low power consumption
– Their scales are – uniform and can be

designed to extend over an arch of2700 or so.

– They can be modified with the help of shunts

and resistances to cover a wide range of
currents and voltages

– They have very effective and efficient eddy-

current damping

• Disadvantages:
– Some errors are set in due to the ageing of

control springs and the permanent magnets