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Right choice is (a) in the FORM of a magnetic shunt

To ELABORATE: Magnetic shunt reaches SATURATION at overloads. As a result, its permeability reduces. Hence the overload compensating device takes the form of a magnetic shunt.

Correct CHOICE is (c) proportional to square of load current

Best explanation: In an energy meter, the self braking torque is dependent on the square of the load current. As a result the disc ROTATES at a slightly slower SPEED at HIGH value of LOADS.

Right answer is (B) continuously REVOLVES

For explanation: When an ENERGY meter is operated in FULL load condition, disc revolves continuously due to the field of the series magnet. As a result, an e.m.f is induced in the disc.

The correct CHOICE is (a) True

Explanation: Interaction between the parts of the shaded and unshaded fluxes, OBTAINED through a shading loop leads to a small driving torque. The VALUE of the torque can be ADJUSTED through lateral movement of the loop.

The CORRECT option is (c) by ADJUSTING the position of limb

The explanation: We can ELIMINATE the friction torque COMPLETELY by adjusting the position of the shading loop. This ENABLES in providing compensation for the frictional torque.

Correct choice is (B) by a shading LOOP

To explain I WOULD say: Friction can be compensated in an energy meter by making use of a shading loop. It is placed between the central limb of the shunt MAGNET and a DISC. Figure below illustrates the same.

Right ANSWER is (a) TORQUE is small

Easy EXPLANATION: The reading in an energy meter is OBTAINED correctly when the torque value is small at low loads. It is independent of the load on the meter an acts in the same DIRECTION as the driving torque.

Correct choice is (b) diverts series flux

For explanation I would say: As the magnetic SHUNT reaches saturation at OVERLOADS, magnetic shunt diverts the flux due to series MAGNET. As a result a large portion of the flux appears in the gap of the air disc. This compensates the SELF BRAKING torque.

The correct option is (a) more flux is INDUCED

To explain: When the copper shading bands are moved up the CENTRAL limb, more flux is induced. As a result the difference in angle between the PHASE and voltage increases. The angle can be MADE approximately equal to 90 DEGREES.

Correct CHOICE is (d) placed around central limb

The BEST explanation: The central limb of a shunt magnet consists of copper SHADING bands. They can be moved up or down by making use of the central limb. FIGURE below illustrates the use of copper shading bands.

Right choice is (b) False

Explanation: On the central limb of a shunt magnet we can make use of a SHADING COIL. The shading coil CONSISTS of a few fairly thick TURNS. The figure below SHOWS a shading coil on the central limb of the shunt magnet.

Right OPTION is (a) low resistance and iron losses

The explanation: In ORDER to obtain an exact reading for the energy consumed by a load using an energy METER, we require the pressure coil to have a low VALUE of resistance and low iron losses.

The correct OPTION is (B) False

Explanation: When the difference in phase angles between the shunt magnetic FLUX and the applied VOLTAGE is 90 degrees (lagging), the READING of the load energy obtained from an energy meter is correct.

Right ANSWER is (a) True

Explanation: Flux ∅ is produced in an ENERGY METER when it is connected to a supply voltage of magnitude V volts. Since the potential COIL is highly inductive, current and flux lag the voltage by 90°.

The correct CHOICE is (d) proportional to the DISC speed

The explanation: The EDDY currents induced in an ALUMINIUM disc vary in proportion to the speed of the disc. As a result the braking torque exerted on the disc VARIES in proportion to the speed.

Correct CHOICE is (c) permanent magnet

The best I can EXPLAIN: In an induction type single phase energy METER, edge of the aluminium disc consists of a permanent magnet also KNOWN as a brake magnet. E.m.f is induced in the aluminium disc when it rotates as a result of the magnetic field produced by the brake magnet.

The CORRECT choice is (b) light aluminium disc

For explanation I would say: Aluminium disc is provided in the air gap between the series and shunt MAGNETS. JEWEL bearings support the spindle. Hence the moving SYSTEM in an induction type single phase ENERGY meter consists of light aluminium disc.

Right choice is (a) large turns of wire

Easy explanation: The CENTRAL limb of the shunt magnet has a large number of turns. It connected ACROSS the SUPPLY and is known as the voltage coil or potential coil. It is excited USING a current value that is PROPORTIONAL to the applied voltage.

Right answer is (c) U shaped laminations

To elaborate: The driving system of an induction type SINGLE PHASE energy meter consists of U shaped laminations made of silicon steel. The laminations are insulated from one ANOTHER and pressed to FORM the core.

Right choice is (B) two electromagnets

To explain: In an induction TYPE single phase energy METER, the driving system CONSISTS of two electromagnets, namely SERIES electromagnet and shunt electromagnet.

Right choice is (a) A.C. measurements

For explanation I would say: A.C. measurements are MADE using Induction type INSTRUMENTS. Induction type ENERGY METER is used to MEASURE the energy that is consumed in A.C. circuits only.

The correct option is (a) True

To EXPLAIN: In a low power FACTOR wattmeter, by connecting a capacitor of value C ACROSS a PORTION of the resistance R in the circuit the error in the pressure COIL can be compensated.

Correct OPTION is (b) False

To explain: PRESSURE coil has an ERROR induced DUE to inductance. It is given by the relation EI sinΦ tanβ. As value of Φ is large for low value of power factor, the error is high in a pressure coil.

The correct CHOICE is (b) more

The explanation: Low power factor circuits have low power. Current coil CONSTITUTES a high current. As a result, the power loss in the current coil is high. The reading OBTAINED from the WATTMETER is prone to errors.

Right answer is (a) have a low torque

Easy explanation: Low power factor wattmeters have a low value of CONTROL torque. They provide full scale deflection for low values of power factors of the ORDER of 0.1.

The correct answer is (d) ten times

The best explanation: In a low power factor dynamometer TYPE wattmeter, PRESSURE coil has a resistance VALUE that is ONE tenth of the actual with respect to unity power wattmeters. This is done in order to ENSURE a reasonable amount of torque at low power factors.

Right answer is (C) resistance

The best I can explain: In a low power FACTOR dynamometer TYPE wattmeter, the PRESSURE coil is designed in order to have a low VALUE of resistance. The current flowing through the pressure coil is increased in order to raise the operating torque.

Right CHOICE is (b) INDUCTANCE

The best I can explain: At low POWER factors in an ordinary dynamometer type WATTMETER, the errors are CAUSED due to inductance of the pressure coil.

Correct answer is (a) small

Best explanation: The deflecting torque in an ordinary DYNAMOMETER is small. The CURRENT and pressure COILS are excited. The measurement of power FACTOR is inaccurate.

Right option is (b) torque is twice the frequency

The best explanation: In a WATTMETER, the torque of the moving system is affected by the frequency. Torque is twice the natural frequency with respect to the supply VOLTAGE. As a result of this, pointer in the MEASUREMENT SCALE vibrates causing difficulty in READING.

Correct option is (a) True

To explain: EDDY currents produce their own field. They affect the magnitude and phase of the current flowing through the current COIL. This leads to errors. As a result, the WATTMETER reads low values for lagging POWER FACTORS and high values for leading power factors.

Correct choice is (b) using a parallel capacitor

To EXPLAIN I WOULD say: Wattmeters are compensated for ERRORS caused by the inductance of pressure coil through the connection of a suitable parallel capacitor with multiplier resistance R.

Fig illustrates that the total impedance of the pressure coil circuit Z2 EQUALS the total resistance of the pressure coil circuit namely R2.

The correct ANSWER is (c) weak magnetic field

To explain I would SAY: Dynamometer type wattmeter consists of a weak magnetic field. It is affected by stray magnetic fields. As a RESULT, wattmeters are shielded to OFFSET EFFECTS of stray magnetic fields.

The CORRECT choice is (d) mutual inductance

For explanation: Due to mutual inductance between the current and pressure coil in a wattmeter, ERRORS are introduced. Errors are negligible at POWER frequencies whereas they have a considerable VALUE at higher frequencies.

Correct choice is (a) lags the applied voltage

To ELABORATE: The current through the pressure coil is in phase with the applied voltage in an ideal WATTMETER. The pressure coil is ASSUMED to be purely RESISTIVE. Practically, the pressure coil has a small inductance L. As a result the current lags the applied voltage by a certain angle.

where, R2 = r2 + R = total RESISTANCE of the pressure coil

r2 = resistance of pressure coil

R = series resistance with the pressure coil

I1 = load current.

The correct answer is (b) False

The best I can explain: Wattmeter reading is AFFECTED by change in temperature. Resistance of the pressure coil and the spring stiffness are affected by the changes in temperature. EFFECTS caused by these two effects OPPOSE each other and NEUTRALIZE.

The correct option is (b) opposite to that of INDUCTANCE

The best I can explain: For lagging POWER FACTOR of the LOAD, the wattmeter reading is more. As a result, the wattmeter shows a reading opposite to that of the inductance.

Right choice is (a) has CAPACITANCE and inductance

Best explanation: Series RESISTANCE R consists of INTER turn capacitance. As a result, the pressure coil of the wattmeter consists of capacitance in addition to inductance.

Right answer is (a) POINTER moves

For explanation: In a Dynamometer type wattmeter, when the MOVING coil deflects the pointer moves over the scale. The pointer then comes back to rest at a point where the deflecting torque EQUALS the controlling torque.

Right choice is (d) a FORCE is exerted

The explanation: When the current carrying COIL of a Dynamometer type WATTMETER is placed in the magnetic field of ANOTHER current carrying coil, the moving coil experiences a force. As a result a deflection torque is generated and the moving coil UNDERGOES deflection.

The correct choice is (c) 2

To explain: When a Dynamometer TYPE wattmeter is used for the measurement of A.C. power, the fixed coil is split into two equal PARTS. The two parts are air-cored to AVOID hysteresis LOSS.

The correct choice is (B) False

The explanation: In a DYNAMOMETER type wattmeter, controlling torque is provided by two phosphor bronze hair springs. They act as leads to the current FLOWING through the coil. Also AIR friction DAMPING is used.

Correct answer is (b) POTENTIAL and current coils

The BEST I can explain: Dynamometer type wattmeter is USED for the MEASUREMENT of A.C. as well as D.C. power. It consists of a fixed coil forming the current coil WHEREAS the moving coil forms the potential coil.

The correct answer is (a) True

The explanation: Power in SINGLE phase A.C. circuits are measured by making USE of a WATTMETER. It consists of a current COIL (CC) and a potential coil (pc).Current coil is connected with the load while the potential coil is connected across the supply.

Right CHOICE is (d) wattmeter

The best I can explain: A.C. circuits make USE of power FACTOR of the circuit in addition to the current FLOWING through the circuit and the VOLTAGE across the circuit. As a result, a wattmeter is used to measure A.C. power.