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Electrical Instruments
Measuring instruments can be classified into two broad categories :
(i) Absolute instruments
(ii)Secondary instruments
Absolute Instruments : Such instruments give the magnitude of the quantity
being measured in terms of the constants of the instruments. For example, a
tangent galvanometer, as it measures current in terms of the tangent of the angle
of deflection produced by the current, radius and number of turns of the
galvanometer and the horizontal component of the earth’s magnetic field. In
practice, we seldom use an absolute instrument.
Secondary Instruments : The deflection of such instruments directly give us the
magnitude of the quantity being measured. These have to be calibrated by comparison
with an absolute instrument or with another secondary instrument which has already
been calibrated beforehand. Such instruments are in most common use in laboratories,
industries and power stations, etc. Secondary instruments are further divided into three
groups :
(i) Indicating instruments (ii) Recording instruments ( iii) Integrating instruments
Indicating Instruments : These instruments given the magnitude of the quantity at the
time when it is being measured. The measurement is indicated by a pointer moving over
a marked (graduated) dial. Ordinary voltmeters, ammeters, wattmeters belong to this
group.
Recording Instruments : These instruments give a continuous record of the quantity
being measured over a given period. The variations of the quantity being measured are
recorded by a pen. This inked pen lightly touches a sheet of paper wrapped over a drum.
The drum rotates slowly at uniform speed. The pen is deflected by the magnitude of the
quantity being measured. Thus, a curve is traced. Such an instrument is used by doctors
to give ECG (Electro-Cardio-Gram) of a patient.
Integrating Instruments : Such instruments give total amount of quantity being
measured over a period of time. The summation given by such an instrument is the
product of time and an electrical quantity. Ampere-hour meter, watt-hour (energy) meter
and odometer in a car (which measures the total distance covered) are examples of this
category. The summation value is generally given by a register consisting of a set of
pointers and dials, or an electronic digital display.
ESSENTIALS OF AN INSTRUMENT
1. Deflecting Torque
When the instrument is connected in a circuit, the pointer should move from its zero position. The
pointer is made to move because of a force or a torque produced. This deflecting torque can be
produced by any of the effects of current (or of voltage) given in Table 1. The deflecting torque
works on the moving system to which the pointer is attached. Obviously, the magnitude of
deflecting torque produced is proportional to the magnitude of the quantity being measured, say,
the current ‘I’ flowing through the instrument.
S.No.
Effect
Instrument
Suitable for measuring of
1
Magnetic effect
Ammeters, voltmeters,
wattmeters, integrating
meters
Current, Voltage, power, energy (ac and
dc systems)
2
Thermal effect
Ammeters, voltmeters
Current, voltage (ac and dc systems)
3
Chemical effect
Integrating meters
DC ampere hour
4
Electrostatic effect
Voltmeters
Voltage (ac and dc systems)
5
Electromagnetic
induction effect
Ammeters, voltmeters,
wattmeters, energy meters
Current, voltage, power, energy (ac and
dc systems)
2. Controlling Torque :This torque, also called restraining torque or restoring torque,
opposes the deflecting torque. The magnitude of the controlling torque is proportional to the
deflection of the pointer. When the pointer is at its zero position, the controlling torque is
zero. But, as the deflection increases, the magnitude of the controlling torque also increases.
At some position of the pointer, the controlling torque becomes equal to the deflecting
torque. The pointer then stops moving further. Thus, the final position of the pointer gives
the magnitude of the quantity being measured.
When the instrument is disconnected from the circuit, the deflecting torque vanishes. It is
due to the controlling torque that the pointer comes back to its zero position. If the
controlling torque were not there, the pointer would have remained in the deflected position
even after the instrument was disconnected from the circuit.
Thus, the controlling torque serves two functions :
1. The pointer stops moving beyond the final deflection
2. The pointer comes back to its zero position when the instrument is disconnected
There are following two ways of obtaining controlling torque:
1. Spring Control : This is the most commonly used method of obtaining the controlling Torque . In cheaper
instruments, only one hairspring made of phosphor bronze is used (Fig. 1). The outer end of this spring is
fixed and the inner end is attached with the spindle. When the pointer is at zero of the scale, the spring is
normal. As the pointer moves, the spring winds and produces an opposing torque. This torque is proportional
to the angle of deflection. The figure also shows a balance weight attached to the pointer. This is meant to
balance the moving system so that its centre of gravity coincides with the axis of rotation, thereby reducing
the friction between the pivot and bearings.
A better arrangement utilizes two flat spiral hairsprings, A and B, as shown in Fig. 2. The inner ends of these
springs are attached to the spindle S. The outer end of B is fixed, whereas that of A is attached to one end of
lever L, pivoted at P. By moving sideways the other end of lever L, the zero of the pointer can be easily
adjusted.
Single Spring
Double Spring
The two springs A and B are wound in opposite directions so that when the moving
system is deflected, one spring winds while the other unwinds. The controlling
torque produced is due to the combined torsions of the two springs. To make the
controlling torque directly proportional to the angle of deflection, the springs should
have fairly large number of turns so that deformation per unit length remains small.
The springs should be stressed well below the elastic limit at the maximum
deflection of the instrument in order to avoid occurrence of zero-shift.