SSC JE 2015 Electrical question paper with solution

SSC JE 2015 Electrical question paper Fully Solved

 

Ques 1. How much energy is stored in a 100 mH inductance when a current of 1A is flowing through it?

  1. 5.0 J
  2. 0.05
  3. 0.5 J
  4. 0.005 J 

The energy stored in the magnetic field of an inductor can be expressed as

E = 1/2 L I2

where

E = energy stored (joules, J)

L = inductance (henrys, H)

I = current (amps, A)

1/2(100 x 10-3 x 1)

50 x 10-3 J

0.05 J

 

Ques 2. For the circuit shown below, find the resistance between points P & Q

  1. 1 Ω  
  2. 2 Ω
  3. 3 Ω
  4. 4 Ω

 

Ques 3. The rate of change of current in a 4 H inductor is 2 Amps/sec. Find the voltage across inductor

  1. 8 V 
  2. 16 V
  3. 2 V
  4. 0.8 V
Here L = 4H and di/dt = 2 A

Hence voltage Across inductor

= V = L(di/dt)

= 4 x 2 = 8 V

 

Ques 4. Find the node voltage VA

  1. 6 V 
  2. 5.66 V
  3. 6.66 V
  4. 5 V

 

Ques 5. In a pure inductive circuit if the supply frequency is reduced to half the current will?

  1. Be four times as high
  2. Be doubled
  3. Be reduced to half
  4. Be reduced to one fourth
The amplitude of the current in a pure inductive circuit is given by

Im = Vm/ωl

And ω = 2πf

Im = Vm/2πfl

Im ∝ 1/f

Hence, when the frequency is halved, the amplitude of the current is doubled in a pure inductive circuit.

 

Ques 6. A 10 pole 25 Hz alternator is directly coupled to and is driven by 60 Hz synchronous motor then the number of poles in a synchronous motor is?

  1. 24 poles 
  2. 48 poles
  3. 12 Poles
  4. None of the above

Number of poles of alternator Pa = 10

F = 25 Hz (alternator)

F = 60 Hz (motor)

Then the number of poles of motor Pm =?

Since synchronous motor is directly coupled hence

Synchronous speed of an alternator = Synchronous speed of the motor

(120 x 25)/10 = (120 x 60)/ Pm

Pm = 24

 

Ques 7. When a source is delivering maximum power to the load the efficiency will be?

  1. Below 50 %
  2. Above 50%
  3. 50 %
  4. Maximum

Maximum Power Transfer theorem

The maximum power is transferred when the load resistance RL is equal to internal resistance Rth or the equivalent resistance Rth.

RL = Rth

Under this condition, the same amount of power is dissipated in the internal resistance Rth and hence the efficiency is 50%.

 

Ques 8. In the Maxwell Bridge as shown in the figure below the value of resistance Rx and inductance Lx of a coil is to be calculated after balancing the bridge. The component value is shown in the figure at balance the value of Rx and Lx will respectively be

  1. 37.5 ohms, 75 mH
  2. 75 ohm, 75 mH
  3. 375 ohm, 75 mH 
  4. 75 ohm, 150 mH

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 balanced condition for Maxwell bridge is given as

Z1Z4 = Z2Z3

Here Z1 = Rx + jωLx

Z2 = 200Ω

Z3 = 750Ω

Z4 = R4 ||(1/ωC4)

 

Ques 9. The internal resistance of a voltage source is 10 ohm and has 10 volts and its terminals. Find the maximum power that can be transferred to the load

  1. 25 W
  2. 5 W
  3. 0.25 W
  4. 2.5 W 

For maximum power to be transferred from a source to a load resistance, the value of load resistance should be equal to the internal resistance of the source.

RL = R = 10Ω

current through the circuit = 10/(10+10) = 0.5 A

power transferred = I2R = 0.25 x 10 = 2.5 W

 

Ques 10. As the load is increased the speed of the DC shunt motor is

  1. Remain constant 
  2. Increase Proportionately
  3. Reduces Slightly
  4. Increase Slightly

For a dc shunt motor. field current is practically constant and thus the torque is directly proportional to the armature current. Hence the torque-armature current characteristic is a straight line passing through the origin. However, due to the armature reaction under the loaded conditions of the machine. the flux decreases slightly with the load.
Since flux is practically constant for DC shunt motor.The speed of DC shunt motor is

N = K (V – IaRa)

Armature current increases as the load on the motor is increased. Thus, the speed of the dc shunt motor will fall slightly with the armature current. As the speed characteristic is only slightly drooping, the dc shunt motor is normally regarded as a constant speed motor.

Control System Objective Question And Answer with Explanation

Most Important Control system Multiple Choice Question (MCQ) With Explanation

Ques1.  In an open loop control system

  1. Output is independent of control input
  2. Output is dependent on control input
  3. Only system parameters have effect on the control output
  4.  None of the above
Answer.1. Output is independent of control system

Explanation:

When the action of the system is independent of the output i.e the output has no influence or effect on the control action of the input signal then the system is called as the open loop control system.An Open-loop system is also called as the non-feedback system. 

 

Ques 2. For open control system which of the following statements is incorrect?

  1.  Less expensive
  2.  Recalibration is not required for maintaining the required quality of the output
  3. Construction is simple and maintenance easy
  4.  Errors are caused by disturbance
Answer.2. Recalibration is not required for maintaining the required quality of the output

Explanation:

There are several limitations of open-loop systems which include: slow system response, poor disturbance rejection, poor tracking under uncertainties, high sensitivity to system parameter errors (e.g., errors in plant or controller gains), and high sensitivity to changes in calibration errors (hence recalibration is usually necessary).

 

Ques 3. A control system in which the control action is somehow dependent on the output is known as

  1. Closed loop system
  2. Semiclosed loop system
  3. Open system
  4. None of the above
Answer.1. Closed loop control system

Explanation:

A closed-loop control system compares a measurement of the actual output with the desired output (reference or command input). The measure of the output is called the feedback signal.  The difference between the two quantities (the error signal) is then used to drive the output closer to the reference input through the controller and actuator.

 

Ques 4. In closed loop control system, with positive value of feedback gain, the overall gain of the system will

  1. Decrease
  2. Increase
  3. Be unaffected
  4. Any of the above
Answer.1. Decrease

Explanation:

In an automated process, positive feedback would result in the system becoming unstable because an increase in the output leads to an increase in the input which creates an increase in the output and so the loop starts again.

Negative Feedback is often used in automated Control processes because it works to change the input in such a way that the output is decreased to provide stable operating conditions.

 

Ques 5. Which of the following is an open loop control system?

  1. Field controlled D.C. servomotor
  2. Ward Leonard control
  3. Metadyne
  4. Stroboscope
Answer.1. Field controlled D.C. servomotor

Explanation:

  • In this motor the control signal obtained by the servo amplifier is applied to the field winding and with the constant current source, the armature current is maintained constant.
  • This type of motor has large reactance to resistance ratio (L/R ratio). Hence the time constant of the motor is high. Therefore it can’t give the rapid response to the quick changing control signals.
  • It is an open loop control system because any change in the output has no effect on the input i.e speed will not be constant for load variations on the motor since There is not any feedback to the controller to indicate the change in speed due to load.

Block diagram of field-control DC motor

 

Ques 6. Which of the following statements is not necessarily correct for open control system?

  1. Input command is the sole factor responsible for providing the control action
  2. Presence of non-linearities causes malfunctioning
  3. Less expensive
  4. Less stable than closed loop system

Answer.4. Less stable than closed loop system

Explanation:

Advantages of Open Loop Control System

  1. Better reliability and stability
  2. More suitable where accurate positioning of the tools is not required
  3. Easily buildable
  4. Less costly

Disadvantages of Closed Loop Control system

  1. Error cannot be corrected
  2. Control action depend upon input command
  3. Presence of non-linearities can cause malfunction.

Why Open Loop control system is more accurate

  • Open loop systems are set up to achieve desired output for particular input in given conditions and in general, gives stable bounded output for bounded input.
  • Mathematically, all the open loop poles(in Laplace domain) need to be to the left of the s- plane for dampening response. Any pole to the right might cause the system to become unstable.

 

Ques 7. In open loop system

  1. Control action depends on the size of the system
  2. Control action depends on system variables
  3. Control action depends on the input signal
  4. Control action is independent of the output
Answer.4. Control action is independent of the output

Explanation:

Control action is the quantity responsible for activating the system to produce the output.

An open loop system is one in which the control action is independent of the output.

Examples of open-loop control systems are a toaster, in which the amount of heat is set by a human, and an automatic washing machine, in which the controls for water temperature, spin-cycle time, etc are preset by a human. ln both these cases, the control actions are not a function of the output of the toaster or the Washing machine.

Examples of feedback control are a room temperature thermostat, which senses room temperature and activates a heating or cooling unit when a certain threshold temperature is reached.

 

Ques 8.  Which system has the tendency to oscillate?

  1. Open loop system
  2. Closed-loop System
  3. Both 1 & 2
  4. None of the above
Answer.2. Closed-loop system

Explanation:

Due to the presence of feedback action, the closed-loop system has the tendency towards oscillation or instability.

 

Ques 9. A good control system has all the following features except

  1. Good stability
  2. Slow Response
  3. Good Accuracy
  4. Sufficient power handling capacity
Answer.2. Slow Response

Explanation:

For any control system, it is desirable that the system should react quickly to a control input or excitation.

 

Ques 10. A car is rising at a constant speed of 50 km/h, which of the following is the feedback element for the driver?

  1. Clutch
  2. Eyes
  3. Needle of the speedometer
  4. Steering wheel
Answer.3. Needle of speedometer

Explanation:

A speedometer or a speed meter is a gauge that measures and displays the instantaneous speed of a vehicle.

Here the change in speed will be sense by speedometer thus speedometer is used as a feedback for  the variation of the speed of the car

50 Important Basic Electrical MCQ| Objective Type Question Of Basic Electrical

50 Most Important Basic Electrical Objective Type Question with solution| MCQ Of Basic Electrical with explanation

 

Ques 1. A sine wave has a frequency of 50 Hz. Its angular frequency is _______radian/second.

  1. 100 π
  2. 50 π
  3. 25 π
  4. 5 π

Answer.1. 100 π

Explanation:

The relation between angular velocity and frequency is given as

ω = 2πf rad/sec

ω = 2xπx50

= 100π

 

Ques 2. The reactance offered by a capacitor to alternating current of frequency 50 Hz is 20 Q. If frequency is increased to 100 Hz, reactance becomes_____ohms.

  1. 5
  2. 5
  3. 10
  4. 15

Answer.3. 10

Explanation:

Capacitive reactance

Xc = 1/2πf

Therefore XC1/XC2 = 2πf2/2πf1

= 20/XC2 =100/50

= XC2 = 10

 

Ques 3. The period of a wave is

  1. The same as frequency
  2. Time required to complete one cycle
  3. Expressed in amperes
  4. None of the above

Answer.3. Time required to complete one cycle

Explanation:

The period of a wave is the time for a particle on a medium to make one complete vibrational cycle.

 

Ques 4. The form factor is the ratio of

  1. Peak value to r.m.s. value
  2. r.m.s. value to average value
  3. Average value to r.m.s. value
  4. None of the above

Answer.2. r.m.s value to average value

Explanation:

The form factor of an alternating current waveform (signal) is the ratio of the RMS (root mean square) value to the average value (mathematical mean of absolute values of all points on the waveform).

 

Ques 5.  The period of a sine wave is 0.02 seconds. Its frequency is

  1. 20 Hz
  2. 30 Hz
  3. 40 Hz
  4. 50 Hz

Answer.4. 50 Hz

Explanation:

Frequency = 1/period

= 1/0.02

= 50 Hz

 

Ques 6. A heater is rated as 230 V, 10 kW, A.C. The value 230 V refers to

  1. Average voltage
  2. R.M.S voltage
  3. Peak voltage
  4. None of the above

Answer.2. R.M.S voltage

Explanation:

In AC system, we can’t take average value because average value is equal to zero over a period in AC.

So now to calculate different parameters like voltage, current, etc we need RMS value. This is the reason why we use RMS quantities while dealing with AC.

 

Ques 7.  If two sinusoids of the same frequency but of different amplitudes and phase angles are subtracted, the resultant is

  1. A sinusoid of the same frequency
  2. A sinusoid of half the original frequency
  3. A sinusoid of double the frequency
  4. Not a sinusoid

Answer.1. A sinusoid of the same frequency

Explanation:

  • Adding two sinusoids of the same frequency but with possibly different amplitudes and phases, produces another sinusoid at that frequency.
  • When alternating waveforms have the same frequency but different phase, they are said to be “Out of phase”.

The equation of A.C sin wave is given as:

Vst = Asin(ω x t)

Vst = Asin(2πft)

Where Vs is the A.C signal voltage with the peak amplitude of “A” volts

Let us take an example

Suppose we add the two sinusoidal with amplitude of A and B but having the same frequencies the equation can be written as

Vt = V1t + V2t

= Asin(ω x t) + Bsin(ω x t)

= (A +B) sin(ω x t)

Hence the amplitude is added up but the frequency remains same.

 

Ques 8. The peak value of a sine wave is 200 V. Its average value is

  1. 127.4 V
  2. 141.4 V
  3. 282.8 V
  4. 200V

Answer.1. 127. 4 V

Explanation:

For sinusoidal waveform

Peak Factor = peak value/R.M.S value = 141.44

Hence R.M.S value = 200/1.414 = 141.44

Form factor = R.M.S value/ Average value

Hence average value = 141.44/1.11

=127.42 V

 

Ques 9 If two sine waves of the same frequency have a phase difference of π radians, then

  1. Both will reach their minimum values at the same instant
  2. Both will reach their maximum values at the same instant
  3. When one wave reaches its maximum value, the other will reach its minimum value
  4. None of the above

Answer.3. When one wave reaches its maximum value, the other will reach its minimum value

Explanation:
If the phase difference is 180 degrees (π radians), then the two sine waves are said to be in antiphase or Phase opposition i.e if one wave reaches its maximum value, then other will reach its minimum value.

 

Ques 10. The voltage of domestic supply is 230V. This figure represents

  1. Mean value
  2. R.M.S value
  3. Peak value
  4. Average value

Answer.2. R.M.S value

Explanation:
Root Mean Square is the actual value of an alternating quantity which tells us an energy transfer capability of an AC source. The domestic single phase AC supply is 230 V, 50 hertz, where 230 V is the R.M.S value of alternating voltage

50 Important mcq of Power System with Explanation

50 Most Important MCQ OF Power System | Objective Type Question OF Power system with Explanation

 

Ques 1. A shunt reactor at 100 MVAr is operated at 98% of its rated voltage and at 96% of its rated frequency. The reactive power absorbed by the reactor is

  1. 98 MVAr
  2. 104.02 MVAr
  3. 96.04 MVAr
  4. 100.04 MVAr
Answer.4. 100.04 MVAr

Explanation:

The reactive power Q is given as

 A shunt reactor at 100 MVAr is operated at 98% of its rated voltage and at 96% of its rated frequency. The reactive power absorbed by the reactor is

 

Ques 2. A single-phase transformer with a 2-kVA rating has a 480-V primary and a 120-V secondary. Determine the primary and secondary full-load currents of the transformer.

  1. 4.17 A and 16.67 A
  2. 10 A and 13. 2 A
  3. 6 A and 16.67 A
  4. Insufficient Data
Answer.1. 4.17 A and 16.67 A

Explanation:

Primary full-load current = VA rating / Primary Voltage

Primary full-load current = ( 2 kVA * 1 000 ) / 480 V = 4.17 A

Secondary full-load current = VA rating / Secondary Voltage

Secondary full-load current = ( 2 kVA * 1 000 ) / 120 V = 16.67 A

 

Ques 3. With the help of a reactive compensator, it is possible to have

  1. Constant voltage operation only
  2. Unity p.f. operation only
  3. Both constant voltage and unity p.f.
  4. Either constant voltage or unity p.f.

Answer.3.Both constant voltage and unity p.f.

Explanation:

  • Reactive power (VAR) compensation is defined as the management of reactive power to improve the performance of ac systems.The important features of reactive compensation are:
    1. Load Compensation: The main objectives of load compensation are
      1. Increase the power factor of the system
      2. To balance the real power drawn from the system
      3.  Compensate voltage regulation
      4. Eliminate current harmonics
    2. Voltage Support: The main purpose of voltage support is to decrease the voltage fluctuation at a given terminal of the transmission line. Therefore the VAR compensation improves the stability of AC system by increasing the maximum active power that can be transmitted.
  • Methods of Reactive Power Compensation are
    1. Shunt compensation
    2. Series compensation
    3. Synchronous condensers
    4. Static VAR compensators
    5. Static compensators

 

Ques 4. In case of 3 phase short circuit in a system, the power fed into the system is

  1. Mostly active
  2. Mostly Reactive
  3. Both active and reactive power are equal
  4. None of the above

Answer.2. Mostly Reactive

Explanation:

  • The fault current is mostly reactive since the fault bypasses the load ( the largest resistive part)
  • Now the entire transmission line is loaded with a high reactance because of the high frequency ( harmonics especially for unsymmetrical faults) and large transient current ( higher flux- larger the reactance) flowing.
  • Due to voltage drop in the transmission line, the Capacitance effect decreases.So the fault current is the current drawn by the transmission line which makes this current mostly reactive.Due to voltage drop in the transmission line, the Capacitance effect decreases.

 

Ques 5. The percentage resistance and percentage reactance of a 10 kVA, 400 V/200 V, 3- phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%, the maximum possible percentage efficiency of the transformer is

  1. 98.32%
  2. 96.85%
  3. 97.25%
  4. 96.12%
Answer.3. 97.25%

Explanation:

If x is the fraction of full load than transformer efficiency is given as

The percentage resistance and percentage reactance of a 10 kVA, 400 V/200 V, 3- phase transformer are 2% and 10% respectively. If the constant losses in the machine are 1%, the maximum possible percentage efficiency of the transformer is

 

Ques 6. Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 kVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.

  1. 420 Amp
  2. 520 Amp
  3. 530 Amp
  4. 480 Amp
Answer.2. 520 Amp

Explanation:

 Determine a minimum circuit breaker trip rating and interrupting capacity for a 10 kVA single phase transformer with 4% impedance, to be operated from a 480 volt 60 Hz source.

The breaker or fuse would have a minimum interrupting rating of 520 amps at 480 volts.

 

Ques 7. A star connected 440V, the 50Hz alternator has per phase synchronous reactance of 10Ω. It supplies a balanced load current of 20A, as shown in the per phase equivalent circuit diagram below. It is desirable to have zero voltage regulation. The load power factor should be
A star connected 440V, 50Hz alternator has per phase synchronous reactance of 10Ω. It supplies a balanced load current of 20A, as shown in the per phase equivalent circuit diagram below. It is desirable to have zero voltage regulation. The load power factor should be

  1. 0.82
  2. 0.62
  3. 0.50
  4. 0.92
Answer.4. 0.92

Explanation:

A star connected 440V, 50Hz alternator has per phase synchronous reactance of 10Ω. It supplies a balanced load current of 20A, as shown in the per phase equivalent circuit diagram below. It is desirable to have zero voltage regulation. The load power factor should be

 

Ques8. The synchronous motor connected to an infinite bus takes power at a lag p.f. If its excitation is increased

  1. Terminal Voltage increase
  2. Load angle increase
  3. Power factor Increase
  4. None of the above

Answer.3.Power factor Increase

Explanation:

  • Overexcited synchronous motor having leading power factor are widely used for improving power factor for those power system which employs a large number of an induction motor.

 

Ques 9. The synchronous generator connected to an infinite bus. If its excitation is increased

  1. Supply reactive power
  2. Absorb Reactive
  3. Both 1 and 2
  4. None of the above

Answer.1. Supply reactive power

Explanation:

  • If we increase the excitation the synchronous generator voltage Vo will increase and the synchronous Reactance Xs will experience a difference of potential Vx.
  • A current I will, therefore, circulate in the circuit
    I = (Vo – V)/Xs Where
    V = system voltage
  • Since synchronous reactance is inductive in nature, therefore, the current lags 90o behind the Vo
  • Hence when we over-excite the synchronous generator it supplies the reactive power to the system bus and the reactive power increase by increasing the DC excitation.

 

Ques 10. An alternator has a phase sequence of RYB for its phase voltage. In case the direction of rotation of alternator is reversed, the phase sequence will become

  1. YRB
  2. YBR
  3. RYB
  4. RBY

Answer.4. RBY

Explanation:

  • The phase sequence of an alternator can be reversed by interchanging any one pair of lines without causing any change in the supply sequence i.e changing RYB to RBY. Reversal of sequence results in reversal of the direction of rotation in case of an alternator.

100 Most Important MCQ Of Transmission and Distribution | Objective Type Question Of Transmission and Distribution

100 Most Important MCQ Of Transmission and Distribution | Electrical Engineering Multiple choices Question

Ques 1. By which of the following method electric power may be transmitted from one location to another location?

  1. UnderGround System
  2. Overhead system
  3. Both 1 and 2
  4. None of the above
Answer 3. Both 1 & 2 

Explanation: Electrical power is generally transmitted by two methods i.e underground transmission and overhead transmission

 

Ques 2. Which of the following transmission line have more initial cost?

  1. Overhead Transmission
  2. Underground transmission
  3. Both have almost same initial cost
  4. None of the above

Answer 2. Underground Transmission line

Explanation:

  • The underground cable can be 2-3 times more expensive than the overhead cable.
  • As the voltage level increases the cost of insulation is increased therefore the underground cable is restricted to low and medium voltages.

 

Ques 3.  Name the cable or conductor which connects the distributor to the consumer terminals.

  1. Service Mains
  2. Distributor
  3. Feeders
  4. None of the above

Answer 1. Service Mains

Explanation:

  • Service Mains: The service mains conductors form connecting links between distributors and metering points at consumer terminal.
  • The area of connection of a sub-main conductor is greater than the service mains.

 

Ques 4. Which of the following materials are not used for transmission and distribution of electrical power?

  1. Copper
  2. Aluminium
  3. Tungsten
  4. Steel

Answer 3. Tungsten

Explanation:

  • The resistance of tungsten is very high, more the resistance more the losses, therefore, tungsten is never used for transmission and distribution purposes.

 

Ques 5.  The usual spans with R.C.C. poles are

  1. 40—50 metres
  2. 60—100 metres
  3. 200 – 300 meters
  4. 80 – 150 meters

Answer 4. 80 – 150 meters

Explanation:

Reinforced concrete pole(R.C.C Pole)

Reinforced concrete pole(R.C.C Pole)

R.C.C Pole

  • The Reinforced concrete pole (R.C.C) is usually called as Concrete pole and they are used for system voltage up to 33kV.
  • Minimum overall length of R.C.C pole should be six meters.
  • The span length of R.C.C is between 80 – 200 meter because they are stronger and more durable.
  • They are free from corrosion hence poles have the longer life but these are very bulky in sizes.

 

Ques 6. Which of the following are the constants of the transmission lines?

  1. Inductance
  2. Capacitance
  3. Resistance
  4. Conductance
  5. All of the above

Answer 5. All of the Above

Explanation:

a b c d constants of the transmission line( Primary line constants)

  • The line constants are parameters that describe the characteristics of conductive transmission lines.
  • The primary line has following constants
    • R = Resistance per unit length Ω
    • L = Inductance per unit length (Henry) H 
    • C = Capacitance per unit length (Henry) H
    • G = Conductance per unit length ℧
  • All these constants are independent of frequency, therefore, they are called as primary constants and these constants are measured by considering both the wires of the transmission lines.
  • R and L elements are in series with the line (because they are properties of the conductor) and C and G are elements shunting the line (because they are properties of the dielectric material between the conductors).
  • G represents leakage current through the dielectric and in most cables is very small.

 

Ques 7. The phenomenon of rising in voltage at the receiving end of the open-circuited or lightly loaded line is called as

  1. Roman Effect
  2. Skin Effect
  3. Corona Effect
  4. Ferranti Effect

Answer 4. Ferranti Effect

Explanation:

Ferranti Effect

  • The phenomenon of the rise of voltage at receiving-end of an open circuited or lightly loaded transmission line is called as the Ferranti effect.
  • The receiving end voltage becomes more than the sending end voltage.
  • The Ferranti effect occurs in medium and long transmission lines.
  • The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied.The relative voltage rise is proportional to the square of the line length.
  • The Ferranti effect is much more pronounced in underground cables, even in short lengths, because of their high capacitance.

 

Ques 8. Low tension cables are meant for use up to

  1. 1 kV
  2. 5 kV
  3. 10 kV
  4. 33 kV
Answer 1. 1 kV

Explanation: Low voltage cables with a range up to 1 kV are used for energy distribution from the last transformer station to the customer.

 

Ques 9. The operating voltage of high tension cables is up to

  1. 1 – 11 kV
  2. 11 – 20 kV
  3. 11 – 33 kV
  4. above 33 kV

Answer 1. 1 – 11 kV

Explanation:

  • High tension cables are used for three-phase medium. These consist of three core belted type cable with separated insulation for each core.
  • The paper insulation is provided for high-tension cables.
  • The operating voltage of high tension cable is upto 11kV. 

 

Ques 10. The operating voltage of super tension cable is

  1. 1 – 11 kV
  2. 11 – 33 kV
  3. 33 – 66 kV
  4. Above 66 kV

Answer 2. 11- 33 kV

Explanation:

  • Super tension cables are used upto the voltage level of 33 kV. They are further classified into 3 types
    1. H – type or screened cables
    2. SL type cables
    3. HSL types cables

50 Most Important 3-phase Induction Motor MCQ with Explanation | Polyphase induction motor MCQ With Explanation

50 Most Important 3-phase Induction Motor MCQ with Explanation| Polyphase induction motor MCQ With Explanation|objective type question of an induction motor

 

Ques 1. The frame of an induction motor is usually made of

  1. Silicon steel
  2. Cast iron
  3. Aluminium
  4. Bronze
Answer 2. Cast Iron

Explanation:

  • The frame provides the structural support for all other motor components.
  • The frame must be made up of the rigid material that can absorb vibration as well as noise in the induction motor.
  • The frame of the induction motor is made either of welded sheet steel or fabricated steel, cast iron or cast aluminum alloy.
  • The material is selected based on cost, heat transfer characteristics and manufacturing capability considering the size of the machine.
  • The frame of small AC induction motor is made up of cast iron but the frame of medium size induction motor is made up of welded sheet steel.

 

Ques 2. In an induction motor, no-load the slip is generally

  1. Less than 1%
  2. 5%
  3. 2%
  4. 4%

Answer 1. Less than 1%

Explanation:

  • Slip,  s, is defined as the difference between synchronous speed and operating speed, at the same frequency, expressed in rpm, or in percentage or ratio of synchronous speed.
    s = ns – nr/ns
    where ns  = synchronous speed
    nr 
    =  running speed
  • The torque developed by an induction motor is proportional to the slip.
  • At synchronous speed, the slip is zero and hence torque developed by the motor is zero.
  • In the case of an ideal motor having zero friction and zero losses, a motor can rotate at synchronous speed, because there is no need for a motor to develop any torque to sustain rotation.
  • But for the practical motor, there will be some friction and other losses, which motor has to overcome by developing some torque. And to develop that torque, the motor must have slip. Therefore Motor can not run at synchronous speed but at slightly lower speed.
  • At full rated load, slip varies from more than 5% for small or special purpose motors
  • For large motors, the slip is less than 1%  because an induction motor consists of short circuit rotor winding having small resistance therefore small slip will induce the large current in the rotor which can produce sufficient torque. 

 

Ques 3. The shaft of an induction motor is made of

  1. Stainless steel
  2. Carbon steel
  3. Cast iron
  4. Aluminium

Answer 2. Carbon Steel

Explanation:

  • A shaft is a metallic, rotating and oscillating rod that rotates continually to transfer movements or torque in the machine.
  • Shafts transfer movement and torque in machine hence they are subject to torsion and shear stress, therefore they must be strong enough to bear the stress.
  • For an Induction motor shaft, we need a material with high torsional rigidity as well as material that can take a lot of stress.
  • Carbon steel is the appropriate choice for making the shaft of an induction motor.
  • Steel is a mixture of iron and carbon. The more we add carbon in steel, the more it becomes harder and stronger.
  • The hollow shaft can resist more torsion load with the same resisting area (same weight) as compared to the solid shaft. Therefore shaft of the induction motor must be of the hollow type.

 

Ques 4. The shaft of an induction motor must be of

  1. Solid
  2. Hollow
  3. Flexible
  4. Any of the above

Answer 2. Hollow

Explanation:

  • The hollow shaft can resist more torsion load with the same resisting area (same weight) as compared to the solid shaft. Therefore shaft of the induction motor must be of the hollow type.
  • The advantage of hollow shafts:
    1. High tolerance to the rotational imbalance due to low weight.
    2. Low rotational inertia
    3. Lightweight comparable to the solid shaft of the same diameter
    4. Due to  low specific mass hollow shafts can tolerant to high-temperature
    5. Cheaper in cost

 

Ques 5. Which of the following component is usually fabricated out of silicon steel?

  1. Stator core
  2. Bearings
  3. Shaft
  4. None of the above

Answer 1. Stator core

Explanation:

  • The stator core is made of silicon because it possesses high magnetic permeability and low magnetic hysteresis.
  • Adding silicon to steel increase its resistivity and strength and it is easier to magnetize silicon added steel.
  • It is laminated to reduce eddy-current losses.

 

Ques 6. Slip ring of an induction motor is usually made up of

  1. Aluminium
  2. Copper
  3. Phosphorus Bronze
  4. Carbon

Answer 3. Phosphorus Bronze

Explanation:

  • A slip ring is an electromechanical device that allows the transmission of power and electrical signals from a stationary to a rotating structure.
  • It is used in case of AC supply, where  AC is required.
  • With slip ring, the voltage in the external circuit varies like a sine wave and the current alternates the direction.
  • Phosphor bronze is an alloy of copper, tin, and phosphorus. The tin increases the corrosion resistance and strength of the alloy. The phosphorus increases the wear resistance and stiffness of the alloy.
  • Phosphorous bronze provide good electrical conductivity and low thermal conductivity
  • Since slip ring induction motors used to accelerate heavy loads over a  long period of time and to withstand excessive heat phosphorous bronze slip is used.

 

Ques 7. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor current will be

  1. 50 Hz
  2. 25 Hz
  3. 5 Hz
  4. 2 Hz

Answer 4. 2 Hz

Explanation:

  • Frequency of the rotor current is given as
    fr = sf
    fr= 0.4 x 50 = 2 Hz

 

Ques 8. The starting torque of a squirrel-cage induction motor is

  1. Full-load torque
  2. Slightly more than full-load torque
  3. Low
  4. Negligible

Answer 3. Low

Explanation:

  • Starting torque is directly proportional to the rotor resistance of an induction motor.
  • The resistance of squirrel-cage induction motor can not be varied as compared to the slip ring induction motor.
  • In the squirrel-cage induction motor, the rotor has very low fixed resistance, therefore, the starting torque is low due to low power factor and high reactance.
  • The starting torque could be increased by increasing starting resistance but due to high current in the rotor during starting will increase the copper loss which will decrease the motor efficiency. 

 

Ques 9. The efficiency of an induction motor is about

  1. 100%
  2. 80-90%
  3. 50-60%
  4. Less than 50%

Answer 2. 80-90%

Explanation:

  • Induction motors are often compared to Transformers because both the machines work under the principle of “mutual induction”. An induction motor is also called as rotating transformer with secondary winding short-circuited.
  • As we know that the transformer’s efficiency is greater than 95%.
  • An Induction motor includes friction and windage(i.e., mechanical losses) losses addition to the no load and copper losses, hence efficiency comes down to 80% to 90%.

 

Ques 10. A double squirrel-cage induction motor has

  1. Two series winding in stator
  2. Two parallel windings in stator
  3. Two parallel winding in Rotor
  4. Two rotors moving in opposite direction

Answer 3. Two parallel winding in Rotor

Explanation:

  • In squirrel cage motor there is no provision made for adding external resistance because the end rings are permanently shorted with the rotor conductor, therefore, the starting torque of squirrel cage induction motor is very poor due to low rotor resistance.
  • The starting torque could be increased by increasing starting resistance but due to high current in the rotor during starting will increase the copper loss which will decrease the motor efficiency.
  • As the name suggests the double squirrel-cage induction motor has two sets of parallel squirrel-cage winding in the same rotor.
  • The outer cage bar have smaller cross-section area as compared to inner cage bar
  • The outer cage bar is made up of high resistive material such as aluminum, brass etc.
  • The outer cage has less leakage flux linkage because of their relatively open slots hence it has lower reactance.
  • The Lower cage bar has high cross-section area and it is made up of low resistance material like copper.
  • The upper and lower cage bar is separated by narrow slit or construction.
  • The inner cage has high leakage flux linkage due to the presence of slit hence it has high self-inductance.
  • Therefore during starting period, it gave high resistance means high starting torque whereas it gives low resistance during running condition.