Test: Electrical & Electronic Measurements- 2 - Electrical Engineering (EE) MCQ

In automatic control of processes and operations, measurement is required to control and provide the desired response

In engineering experimental analysis, measurement is required to find out the response of system to inputs

To monitor a system or process we need measurement

Induction type of instruments ae used to measure AC quantities only. We can use these instruments as an ammeter, voltmeter, wattmeter and an energy meter.

Sensitivity: It is the change in output to the unit change in input

Linearity:The property of an instrument, if output varies linearly according to input is known as linearity. It is the ability to produce the input characteristics symmetrically and linearly. The graphical relationship between input and output is a straight line. It is also a supporting characteristic of accuracy.

Sensitivity: It is the change in output to the unit change in input

Change in the output = 6 mm

Change in input = 3 A

Sensitivity (S) = 6/3 = 2 mm/A

In measuring instruments, the effect of unknown quantity is converted into a mechanical force which is transmitted to the pointer, which indicates on a scale. This moving system has to satisfy following requirements.

(1) Mechanism for producing deflecting torque

(2) Mechanism for producing controlling torque

(3) Mechanism for producing damping torque

Hence gravitational force is not needed for the satisfactory operation of any indicated instrument.

For moving iron voltmeter,

Deflection (θ) is directly proportional to square of the current

Given that,

The deflection in Moving iron instrument is given by,

Deflection is directly proportional to square of rms value of operating current

Given that,

Standard voltage E_s = 1.0185V

The balance obtained at 60 cm.

Resistance R_s = 60Ω

Working current 

Voltage corresponding to 82 cm,

V = 0.0169 × 82 = 1.39V =

Dynamometer type instrument as wattmeter is used for the measurement of power in AC circuits

Moving iron type, rectifier type and thermocouple type instruments are mostly used as ammeter or voltmeter

According to Blondel’s theorem the number of wattmeters to be required to measure the total power in n-phase system is either N or(N-1).

When separate neutral wire is available in the system the no. of watt meters to be required is N.

When the neutral wire is not available in the system, then the no. of watt meters to be required is (N-1). One line is acts as a common line for return path.

The total power is the sum of the all wattmeter readings.

The disc of domestic supply energy meter is made of aluminium.

Energy meter consists of rotating aluminium disc mounted on a spindle between two electro magnets.

In a single phase induction type energy meter, the deflecting torque is directly proportional to the true power in the circuit.

Dead Time: It is the time before the instrument begins to response after the quantity has altered

Dead Space (or) Dead Zone: In some instruments, for some range of input values, there is no change in output. This range of input is called dead space (or) dead zone.

A piezometer is a device used to measure liquid pressure in a system by measuring the height to which a column of the liquid rises against gravity. It is used to measure very low pressures.

The potentiometer wires should have high resistivity (specific resistance) and low temperature coefficient. The alloys constantan and manganin are traditionally used for this purpose.

Deflection sensitivity is defined as,

Deflection factor is the reciprocal of deflection sensitivity.

CRO is used to measure the quantities at higher frequencies. Hence CRO is suitable for the measurement of 10 mV at 50 MHz.

Kelvin’s double bridge is used for the measurement of very low resistances. Using potentiometer method and Ammeter Voltmeter method we can measure low resistance.

Wheatstone bridge is suitable for the measurement of medium resistance

Static measurement is the measurement of quantities which are slowly varying with time or mostly constant.

Dynamic measurement is the measurement of quantities which changes rapidly with time.

Capacitive transducers are used for dynamic measurements, used for measuring the displacement, pressure and other physical quantities.