SSC JE Measurement & Instrumentation Solved Questions (2018-2009) Part-2

Answer.1. A shunt connected across moving the coil

Explanation:-

The use of D-Arsonval galvanometer very common in the variety of measuring instrument. The galvanometer is basically used in an instrument for detecting the presence of small voltages or currents in a circuit or to indicate zero current in applications lilts bridge circuits. Thus galvanometer has to be very much sensitive.

Damping: The damping is eddy current damping. The eddy currents developed in the metal former on which coil is mounted, are responsible to produce damping torque. For effective damping, low resistance is connected across the galvanometer terminals. By adjusting the value of this resistance damping can be changed and critical damping can be achieved.

Answer.1. θ

Explanation:-

In indicating instrument the controlling torque, also called as a restoring or balancing torque is obtained by two methods

1. Spring control Method
2. Gravity control Method

Spring Control Method

The springs used in measuring instruments for providing the controlling torque must have the following properties.

1. These should be of non-magnetic material.
2. Specific resistance should be low.
3. The resistance temperature coefficient should be low.
4. These should not be affected much by mechanical fatigue.

Phosphor Bronze is the most suitable for the springs of an indicating instrument as it satisfies most of the above properties. ln this method, two spiral hair-springs are used and spiraled in the opposite direction. The use of two springs also avoids error due to temperature variations. One end of both the springs is attached to the body of the instrument and another end is attached to the moving system.

When the instrument is not in use, the two springs are in their natural position without any tension or compression( deflecting torque = Controlling Torque). When the instrument is connected to the circuit for the measurement, deflecting torque acts and the pointer moves on the calibrated scale. One of the springs is unwound while the other gets twisted. The resultant movement in the springs provides the controlling torque.

More the deflection more is the twist and hence greater will be the controlling torque. Thus the controlling torque is directly proportional to the deflection of the moving system, i.e.,

Tc ∝ θ

And it is also clear that the pointer comes to rest when controlling torque becomes numerically equal to deflecting torque i.e

T_c = T_d

But the deflection or the deflection torque depends upon the current flowing through it, hence

I ∝ θ

Answer.2. Low resistance shunt

Explanation:-

The Range of DC milliammeter can be extended by using resistance in parallel. The shunts are low resistances used in ammeters for range extension. The shunt is made of Manganin or Constantan depending on whether it is used for DC or AC.

With Im as the current flow through the meter capable of producing the maximum deflection, precautions are to be taken to ensure that the current through the meter coil is limited to Im. However, when the current to be measured is large, the alternative path is provided via, the low resistance shunt R_sh.

Answer.1. Resistive

Explanation:-

Electrodynamic Wattmeter

Electrodynamic Wattmeter consists of a pair of fixed coils, known as current coils, and a movable coil is known as the potential coil. The fixed coils are made up of a few turns of a comparatively large conductor. The potential coil consists of many turns of fine wire. The current coils are connected in series with the circuit, while the potential coil is connected in parallel.

When line current flows through the current coil of a wattmeter, a field is set up around the coil. Since for  AC power, current and voltage may not be in phase, owing to the delaying effects of circuit inductance or capacitance. Therefore the potential coil of the wattmeter generally has a high-resistance resistor connected in series with it. This is for the purpose of making the potential-coil circuit of the meter as purely resistive as possible. As a result, current in the potential circuit is practically in phase with line voltage.

Answer.1. 15Ω

Explanation:-

The value of the shunt resistance required to convert the galvanometer into an ammeter

Where Ig =  the current for full-scale deflection in the galvanometer

G = Resistance of the galvanometer

Answer.4. Temperature variations

Explanation:-

The basic sources of error in instruments are friction, temperature, and aging of various parts. The swamping resistance is a resistor with a negative temperature coefficient connected in series with the instrument(moving coil) in order to reduce the error due to the variation of resistance of the moving coil with the variation of temperature.

The resistance of the swamping resistor may be taken nearly three times that of the coil so that the possible error of the instrument will be reduced by about 75%.

Conducting materials having the negligible temperature coefficient of resistance like Manganin can be used as shunts for these instruments.

Answer.4. Electro-Dynamometer

Explanation:-

A current measuring instrument that did not depend on the vagaries of magnetic materials was the electro-dynamometer. This measured the forces between current-carrying conductors, so it was really only suitable for high current work. Being very reliable, however, it was frequently used in laboratories and for calibrating other instruments.

The meter is mechanically damped by means of aluminum vanes that move in enclosed air chambers. Although very accurate, electro-dynamometer-type meters do not have the sensitivity of the D’Arsonval-type meter movement. For this reason, you will not find them used outside of the laboratory environment to a large extent.

Answer.3. 990 RPM

Explanation:-

The ammeter has 30 oscillations in one minute. So, rotor current has a frequency

f” = 30/60 = 0.5 Hz

Rotor frequency f” = s×f

s= f”/f = 0.5/50 = 0.01 Hz.

Synchronous speed Ns = 120f/p = 120 x 50/6 = 1000 RPM

Slip s = (Ns – Nr)/Ns

0.01 = (1000 – Nr)/1000

Nr = 990 RPM

Answer.1. Ammeter

Explanation:-

An ideal Ammeter has zero resistance. An Ammeter, connected in series, is supposed to have as little current as possible to indicate the reading otherwise, the indication will be wrong by a large margin. This means it must allow as much as possible current to pass through. Thus, by definition, it should possess a resistance as close to zero as possible.

Answer.2. Across the supply

Explanation:-

A dynamometer type wattmeter is most commonly employed for measurement of power in a.c as well as d.c circuits. It is based on the principle that mechanical force exists between two current-carrying conductors. It essentially consists of two coils, namely a fixed coil and moving coil.

These two coils are connected in a different arrangement. The fixed coils or current coil’ are connected in series with the load and moving coil is connected across the Voltage and therefore, carries a current proportional to the voltage. Since the moving coil carries a current proportional to the voltage, it is called the ‘pressure coil’ or ‘voltage coil’ of the wattmeter.

The current coils which are fixed are divided into two halves. The fixed coils are wound with heavy wire (fewer turns) as they have to offer small resistance to the load current flowing through them. The reason for dividing the current coil into two halves is simply to produce the uniform magnetic field for the moving coil. The ‘pressure coil’ which is free to move is mounted on a pivoted spindle and carries a pointer.

Answer.1. Maxwell and Hay bridge

Explanation:-

A Maxwell bridge is a modification to a Wheatstone bridge used to measure an unknown inductance (usually of low Q value) in terms of calibrated resistance and inductance or resistance and capacitance.

The Hay’s bridge is the modification of Maxwell’s bridge. Hay’s bridge uses a resistor in series with a standard capacitor (unlike Maxwell’s bridge which uses resistance in parallel with the capacitor). The Hay’s bridge is suited for the measurement of high Q inductors, mostly those inductors having Q > 10.

Answer.3. Wien bridge

Explanation:-

The Wien bridge is one of many common bridges. Wien’s bridge is used for precision measurement of capacitance in terms of resistance and frequency. It was also used to measure audio frequencies.