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# Multiple choice question for engineering

## Set 1

1. Errors are introduced in Current Transformers.
a) True
b) False

Answer: a [Reason:] A current transformer is used for the measurement of very high currents. The use of a C.T. leads to the introduction of two errors in power measurement.

2. Ratio error is defined as
a) Ratio error = KnR
b) Ratio error = Kn – RR
c) Ratio error = Kn – R
d) Ratio error = 1R

Answer: b [Reason:] Ratio error of a C.T. is defined as the ratio of the magnitude of the difference between the nominal and actual ratio with respect to the actual ratio.

3. Phase angle in a C.T. is defined as

Answer: c [Reason:] During power measurement, there exists phase angle error in a C.T. The phase angle is defined as where. Im is the magnetising component of the excitation current Ic is the core loss component of the excitation current Is is the secondary winding current.

4. Phase angle error is given by

Answer: d [Reason:] The power measurement in a C.T. leads to phase angle error. Phase angle error is given by the relation where, θ is the phase angle error Im is the magnetising component of the excitation current Is is the secondary winding current.

5. Ratio error is due to _________
a) iron loss
b) C.T.
c) magnetising component
d) supply voltage

Answer: a [Reason:] We know that the ratio error in a C.T. is given by the relation Ratio error = Kn – RR = n + IeIs where, Ie is the iron loss component of the excitation current n is the turns ratio.

6. Phase angle error is due to _________
a) C.T.
b) magnetising component
c) iron loss
d) supply voltage

Answer: b [Reason:] We know that the phase angle error in a C.T. is given by the relation where, θ is the phase angle error Im is the magnetising component of the excitation current Is is the secondary winding current It is observed from the equation for the phase angle error that it depends on the magnetising component of the excitation current.

7. In power measurements 180° phase shift is required.
a) True
b) False

Answer: a [Reason:] For eliminating errors in power measurement, there must be a phase difference of 180° between the primary and the secondary currents.

8. Errors in a C.T. can be minimised by _________
a) making use of laminations
b) having low reactance
c) increasing the secondary winding turns
d) decreasing the primary winding turns

Answer: b [Reason:] The excitation current Io can be minimised thus eliminating the errors in a C.T. by minimising the iron loss. The core must have a low iron loss and a minimum value of leakage reactance.

## Set 2

1. Pressure coil of a wattmeter
a) has capacitance and inductance
b) has inductance and resistance
c) has resistance and capacitance
d) has only inductance

Answer: a [Reason:] Series resistance R consists of inter turn capacitance. As a result, the pressure coil of the wattmeter consists of capacitance in addition to inductance.

2. What is the effect of capacitance on wattmeter reading?
a) aiding the inductance
b) opposite to that of inductance
c) aiding the capacitance
d) opposite to that of resistance

Answer: b [Reason:] 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.

3. Wattmeter reading is not affected by temperature.
a) True
b) False

Answer: b [Reason:] 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.

4. Dynamometer type wattmeter has _________
a) strong magnetic field
b) intermediate magnetic field
c) weak magnetic field
d) no magnetic field

Answer: c [Reason:] 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.

5. Wattmeter reading has errors induced by _________
a) resistance
b) self-capacitance
c) self-inductance
d) mutual inductance

Answer: d [Reason:] 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.

6. Current in a pressure coil of the Dynamometer type wattmeter
a) lags the applied voltage
c) is in phase with the applied voltage
d) there is a phase difference of 90 degrees

Answer: a [Reason:] 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.

7. Wattmeters are compensated for errors due to inductance by _________
a) using a series capacitor
b) using a parallel capacitor
c) using a series resistance
d) using a parallel resistance

Answer: b [Reason:] 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.

8. Eddy currents are induced in solid metal parts within the thickness of the conductor.
a) True
b) False

Answer: a [Reason:] 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.

9. What is the effect of frequency on the torque of a moving system?
a) torque is half of the frequency
b) torque is twice the frequency
c) torque is thrice the frequency
d) torque is four times the frequency

Answer: b [Reason:] 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.

## Set 3

1. In a C.T., the primary ampere turns is variable.
a) True
b) False

Answer: b [Reason:] In a current transformer, the primary ampere turns is fixed. When the secondary circuit is opened, assuming a constant primary current we get a fixed number of primary ampere turns.

2. When the secondary circuit of a C.T. is open,
a) a very high flux density is produced
b) a very low flux density is produced
c) a moderate flux density is produced
d) no flux density is produced

Answer: a [Reason:] When current is flowing in the secondary circuit which is kept open in a C.T., a very high flux density is produced in the core as a result of the absence of demagnetizing ampere turns.

3. High flux density leads to _________
a) a decreased voltage in the secondary
b) a decreased voltage in the primary
c) a increased voltage in the secondary
d) a decreased voltage in the primary

Answer: c [Reason:] High flux density in the secondary winding of a C.T. leads to an increase in the induced voltage in the secondary winding. This may damage the insulation.

4. When primary current flows, secondary circuit should be open.
a) True
b) False

Answer: b [Reason:] In a current transformer, even when it is not in use for any measurement purposes, the secondary circuit must be closed when the primary current is flowing.

5. High magnetizing forces in the core when removed suddenly lead to
a) no residue
b) partial residual magnetism
c) rupture of the device
d) residual magnetism

Answer: d [Reason:] In a C.T, the high magnetizing forces on the core may leave behind residual magnetism in the core and ratio and phase angle errors.

6. The secondary winding of a C.T.
a) can be shorted
b) can’t be shorted
c) can be opened
d) can’t be opened

Answer: a [Reason:] When a C.T. is used in measurements, it acts as a short circuit. This is because the impedance of the load is negligible due to the ammeter or the wattmeter coil. Thus the secondary winding of a C.T. can be shorted.

7. When the secondary winding of a C.T. is opened
a) C.T. is magnetised
b) C.T. is demagnetised
c) C.T. is shorted
d) C.T. is opened

Answer: b [Reason:] In a C.T., when the secondary winding is opened by mistake when the primary current is flowing, the C.T. must be demagnetized first fully and then only must be used again in the circuit for any measurements.

8. The rectangular type of C.T. is built of _________
a) L shaped windings
b) T shaped windings
c) A shaped windings
d) Z shaped windings

Answer: a [Reason:] In a rectangular type of C.T., the windings are placed on the shorter limbs. The primary winding is wound over the secondary. The rectangular type of C.T. is built of L shaped windings.

## Set 4

1. A Schering bridge can be used for capacitance measurements at low voltages.
a) True
b) False

Answer: b [Reason:] When a Schering bridge is used for the measurement of small capacitances at low voltages, it comprises of errors. In order to avoid this a high voltage Schering bridge is used.

2. How is high voltage obtained?
a) by using a step up transformer
b) by using a high voltage source
c) by using a step down transformer
d) by using a high current source

Answer: a [Reason:] High voltage supply for a Schering bridge is obtained by making use of a step up transformer. The frequency for the high voltage supply used in a Schering bridge is 50 Hz.

3. The detector used in a high voltage Schering bridge is _________
a) tunable circuit
b) e.m.f source
c) vibration galvanometer

Answer: c [Reason:] A high voltage Schering bridge is used for the measurement of low capacitance values at high voltages. A vibration galvanometer is used as a detector in a high voltage Schering bridge.

4. Detector is very sensitive in a high voltage Schering bridge.
a) True
b) False

Answer: a [Reason:] In a high voltage Schering bridge the ratio arms have high impedance values. As a result, a very small current is drawn by the ratio arms. Hence the detector must be very sensitive in a high voltage Schering bridge.

5. How are the inter-capacitance errors minimized?
a) by separating the plates of the capacitance
b) by using earthing
c) by reducing the area of cross section of the capacitors
d) by increasing the distance between the capacitor plates

Answer: b [Reason:] In a high voltage Schering bridge, errors due to inter-capacitance between the high and low impedance ratio arms can be eliminated through earthing screens.

6. How to minimize the effect of earth capacitance?
a) by increasing the supply voltage
b) by using a series resistance
c) by using a Wagner ground connection
d) by using an inductor across the circuit

Answer: c [Reason:] In a high voltage Schering bridge, the effect of earth capacitance on the circuit including the galvanometer and the contact leads is minimized by making use of a Wagner ground connection.

7. What is the effect of breakdown of high voltage capacitor?
a) No effect
b) voltage drop across the circuit reduces
c) circuit components rupture
d) a high voltage appears across the branches

Answer: d [Reason:] In a high voltage Schering bridge, a very high voltage appears across the branches when the breakdown of high voltage capacitor occurs. This is prevented by making use of a spark gap across the branches involved.

8. What is dependence of frequency on the balance equation?
a) independent
b) varies by a factor of 2
c) depends on the supply magnitude
d) depends on the detector used

Answer: a [Reason:] In a high voltage Schering bridge, the balance equation does not depend on the frequency of operation of the circuit. A detector is only used for detecting the balance condition in a bridge circuit.

## Set 5

1. Inductance comparison bridge is used to compute
a) unknown inductance and resistance
b) unknown resistance
c) unknown inductance
d) unknown capacitance

Answer: a [Reason:] By making use of an inductance comparison bridge, the values of unknown inductance and its internal resistance can be determined.

2. Ratio arms of the bridge consists of _________
a) pure inductances
b) pure resistances
c) pure capacitances
d) inductance and capacitance

Answer: b [Reason:] An inductance comparison bridge is basically used to compute the unknown resistance and inductance values. Ratio arms of an inductance bridge consists of pure resistances.

3. Value of unknown resistance is found by using the relation

Answer: c [Reason:] The inductance comparison bridge mainly consists of pure resistances in its ratio arm. The value of unknown resistance is given by the relation Rx = R2 R3R1.

4. The value of unknown inductance is found by using the relation

Answer: d [Reason:] At balance condition in an inductance comparison bridge, the value of unknown inductance is found by using the relation Lx = R2 L3R1.

5. Inductance control is obtained by _________
a) using R2
b) using R1
c) using R3
d) using Lx

Answer: a [Reason:] In an inductance comparison bridge, the resistance R2 and R3 are variable. The value of resistance R2 is varied so as to control the inductance of the bridge.

6. Bridge depends on frequency.
a) True
b) False

Answer: b [Reason:] In an inductance comparison bridge, the balance equation is independent of frequency. As a result the bridge balance condition remains unaffected by variation in the value of frequency.

7. Bridge can be used at audio frequency.
a) True
b) False