Test: Measurements & Instruments- 3 (Hindi) - Electrical Engineering (EE) MCQ

Integrating instruments totalize events over a specified period of time. The summation, which they give is the product of time and electrical quantity. Ampere hour and watt hour (energy) meters are examples of this category.

Voltmeter is not an integrating type of instrument

Indicating instruments are those instruments which indicate the magnitude of a quantity being measured. They generally make use of a dial and a pointer for this purpose. Ordinary voltmeters, ammeters and wattmeters belong to this category

Range is the most important specification of an instrument. It indicates the range of values which the instrument can measure. We will discuss the importance of range with an example.

Suppose we need to measure 10 mA quantity. So we need an ammeter with the full scale range of at least 10 mA. Hence range is useful when selecting the meter to measure the quantity

Given that, GAE = 1% of full scale reading

Full scale reading = 25 A

Error value = 0.01 × 25 = 0.25 A

When a voltmeter having an internal resistance of R_m is connected in parallel with load resistance R_L of circuit under test, the circuit conditions will be altered. The effective resistance will be the parallel combination of R_L and R_m. The voltmeter indicates the voltage across this effective resistance, where the indicated voltage will always be less than true voltage. This is known as loading effect.

Hence the instrument must possess high input impedance to reduce loading effect.

The essential requirements for instrument for instrument springs are:

1) They should be non-magnetic

2) They should be proof from mechanical fatigue

3) Where springs are used to lead current into moving system they should have a small resistance, their cross-sectional area must be sufficient to carry the current without temperature rise affecting their constant. They should also have a low resistance temperature coefficient.

For most applications phosphor bronze has been found to be the most suitable material except in instruments low in resistance (like multi-voltmeters).

In gravity control, a small weight is placed on an arm attached to the moving system. The position of this weight is adjustable. This weight produces a controlling torque due to gravity.

This controlling torque is given by,

T_c = K_gsinθ

T_d ∝ I

At equilibrium position, T_C = T_D

⇒ I ∝ sinθ

The damping torque should be of such a magnitude that the pointer quickly comes to its final steady state position without overshooting.

If the instrument is underdamped, the moving system will oscillate about the final steady position with a decreasing amplitude and will take some time before it comes to rest.

When the system moves rapidly but smoothly to its final steady position, the instrument is said to be critically damped.

If the damping torque is more than what is required for critical damping, the instrument is said to be overdamped. In an overdamped instrument, the moving system moves slowly to its final steady state position in a lethargic fashion.

In spring control instrument, the controlling torque is given by,

T_c = K_cθ

T_d ∝ I

At equilibrium position, T_C = T_D

⇒ I ∝ θ

Hence in spring control, the scale is uniform

In gravity control, a small weight is placed on an arm attached to the moving system. The position of this weight is adjustable. This weight produces a controlling torque due to gravity.

This controlling torque is given by,

T_c = K_gsinθ

T_d ∝ I

At equilibrium position, T_C = T_D

⇒ I ∝ sinθ

Hence in gravity control, the scale is non uniform

Given that,

R_m = 100 Ω

I_m = 10 mA

I = 100 μA

Two wattmeter method can be used to measure the total power delivered to both start as well as delta connected loads.

It has two fixed coils and one moving coil. The field is produced by these fixed coils which is required for the operation of the instrument. A uniform field is obtained at the centre of the coil due to division of coil in two sections. It is used as current coil.

The moving coil is mounted on an aluminium spindle. It used as pressure coil or voltage coil.

Single or one wattmeter method is used for only balanced three phase loads. It is best method for the measurement of 3 phase reactive power compared to 2 wattmeter method since error can be minimized.

The loss of charge method is used to measure high values of resistance. This method is best suitable for measurement of insulation resistance in case of cable.

The copper shading bands are also called the power factor compensators. These rings are provided on the central limb of the shunt magnet to make the phase angle displacement between magnetic field set up by shunt magnet and supply voltage is approximately 90 degrees.

A thermocouple is a device used extensively for measuring temperature. It comprises of two dissimilar metals, A and B. These are joined together to form two junctions, p and q, which are maintained at the temperatures T₁ and T₂ respectively. Since the two junctions are maintained at different temperatures the Peltier emf is generated within the circuit and it is the function of the temperatures of two junctions.

If the temperature of both the junctions is same, equal and opposite emf will be generated at both junctions and the net current flowing through the junction is zero.

If the junctions are maintained at different temperatures, the emf’s will not become zero and there will be a net current flowing through the circuit.

The total emf flowing through this circuit depends on the metals used within the circuit as well as the temperature of the two junctions.

The ramp type DVM uses a linear ramp or staircase ramp technique. The staircase ramp technique is a simpler version of linear ramp technique. It is called as null balancing technique.

LVDT (Linear Variable Differential Transformer) is the most widely used inductive transducer that coverts the linear motion into the electrical signal. Sensitivity of LVDT is mainly due to exact cancellation of secondary voltages

The horizontal and vertical plates are placed between the electron gun and the screen, thus it can detect the beam according to the input signal. Generally these horizontal deflection plates are placed vertically.

Sensitivity (S) of a Wheatstone bridge is expressed as,

S = D/I

Where: D is Deflection and I is current

The Wheatstone bridge is more sensitive when all their resistances are equal, or their ratio is unity. Their sensitivity decreases when their ratio is less than unity. The reduction in sensitivity reduces the accuracy of the bridge.

In Anderson bridge, the unknown inductance and resistance values are,

The unknown inductance is measured in terms of known resistance and capacitance.