Competitive DC Machines MCQs – Efficiency and Testing ( DC Machines ) MCQs
Latest DC Machines MCQs
By practicing these MCQs of Efficiency and Testing ( DC Machines ) MCQs – Latest Competitive MCQs , an individual for exams performs better than before. This post comprising of objective questions and answers related to “ Efficiency and Testing ( DC Machines ) Mcqs “. As wise people believe “Perfect Practice make a Man Perfect”. It is therefore practice these mcqs of DC Machines to approach the success. Tab this page to check ” Efficiency and Testing ( DC Machines )” for the preparation of competitive mcqs, FPSC mcqs, PPSC mcqs, SPSC mcqs, KPPSC mcqs, AJKPSC mcqs, BPSC mcqs, NTS mcqs, PTS mcqs, OTS mcqs, Atomic Energy mcqs, Pak Army mcqs, Pak Navy mcqs, CTS mcqs, ETEA mcqs and others.
DC Machines MCQs – Efficiency and Testing ( DC Machines ) MCQs
The most occurred mcqs of Efficiency and Testing ( ) in past papers. Past papers of Efficiency and Testing ( DC Machines ) Mcqs. Past papers of Efficiency and Testing ( DC Machines ) Mcqs . Mcqs are the necessary part of any competitive / job related exams. The Mcqs having specific numbers in any written test. It is therefore everyone have to learn / remember the related Efficiency and Testing ( DC Machines ) Mcqs. The Important series of Efficiency and Testing ( DC Machines ) Mcqs are given below:
Efficiency of DC Machine- 1
1. The efficiency of the DC motor at maximum power will be ___________________
a) 100%
b) Around 90%
c) Anywhere between 75% and 90%
d) Less than 50%
Answer: d
Explanation: For getting maximum power, derivative of power with respect to current is equal to 0. This is practically impossible to achieve as, current required is much more than its normal rated value. Large heat will be produced in a machine and efficiency of motor will be less than 50 %.
2. The hysteresis loss in a DC machine least depends on _____________
a) Frequency of magnetic reversals
b) Maximum value of flux density
c) Volume and grade of iron
d) Rate of flow of ventilating air
Answer: d
Explanation: As iron core of the armature is rotating in magnetic field, some losses occurs in the core which is called core losses. These losses are categorized as Hysteresis loss and Eddy current loss. They depend on all quantities listed above.
3. Which of the following is not the effect of iron loss?
a) Loss of efficiency
b) Excessive heating of core
c) Increase in terminal voltage
d) Rise in temperature of ventilating air
Answer: c
Explanation: Iron loss causes excessive heat production in the core of a machine, which will rise the temperature of ventilating air, as it acts as heat exchanger. Thus, terminal voltage rise is not an effect of any loss.
4. Which of the following loss is likely to have highest proportion at rated load of the DC generator?
a) Hysteresis loss
b) Field copper loss
c) Armature copper loss
d) Eddy current loss
Answer: c
Explanation: Armature copper loss is directly proportional to the square of armature current multiplied by the armature resistance and also the series field resistance if present any. As, at loaded condition armature current is very high.
5. Which of the following loss in a DC generator varies significantly with the load current?
a) Field copper loss
b) Windage loss
c) Armature copper loss
d) Cannot be determined
Answer: c
Explanation: Armature copper loss is directly proportional to the square of armature current, as load current varies armature current varies, which is reflected significantly in loss as a square of it. Thus, armature copper loss can be detected.
6. Which of the following methods is likely to result in reduction of hysteresis loss in a DC generator?
a) Providing laminations in armature core
b) Providing laminations in stator
c) Using non-magnetic material for frame
d) Using material of low hysteresis co-efficient for armature core material
Answer: d
Explanation: Providing laminations will work for reducing eddy current losses, but hysteresis loss is dependent on the material chosen. Thus, using different material for core of armature will definitely work.
7. Which of the following loss/losses in a DC generator is dissipated in the form of heat?
a) Mechanical loss
b) Core loss
c) Copper loss
d) Mechanical, Copper and Core
Answer: d
Explanation: All the losses listed above dissipate the heat. This, dissipated heat due to various losses results in increasing the temperature of ventilating air. These losses are dangerous in long running of a machine, can reduce efficiency also.
8. Which of the following losses are significantly reduced by laminating the core of a DC generator?
a) Hysteresis losses
b) Eddy current losses
c) Copper losses
d) Windage losses
Answer: b
Explanation: Hysteresis losses can be minimized by using material with lower hysteresis coefficient. Eddy current losses can be minimized by using laminated sheets of core structed together. To reduce other losses, current should be minimized, can’t be reduced as it also has lower limit.
9. The total losses in a well-designed DC generator of 10 kW will be nearly equal to ________
a) 100 W
b) 500 W
c) 1000 W
d) 1500 W
Answer: b
Explanation: Total losses in a DC machine can be approximated to 4-5% of its rating from the experimental observations. Thus, 5% of 10 kW is equal to 500 W. It’s an approximation formed on various observations.
Efficiency of DC Machine- 2
1. The condition for maximum efficiency for a DC generator is __________
a) Eddy current losses = stray losses
b) Hysteresis losses = eddy current losses
c) Copper losses = 0
d) Variable losses = constant losses
Answer: d
Explanation: All losses in a given DC machine can be categorized into variable losses and constant losses. Variable losses are proportional to the square of armature current while constant losses are almost constant for a given DC machine throughout its application.
2. DC generators are normally designed for maximum efficiency at or near ____________
a) Full-load
b) Minimum load
c) Rated voltage
d) At all loads
Answer: c
Explanation: The efficiency of a machine is different at different values of power output. All electrical machines are generally designed to give maximum efficiency at or near the rated output of the machine. Thus, maximum efficiency occurs at rated voltage.
3. In a DC generator, the iron losses mainly take place in ____________
a) Yoke
b) Commutator
c) Armature conductors
d) Armature rotor
Answer: d
Explanation: Iron losses take place in the form of hysteresis loss and eddy current loss. These losses are maximum, where field is maximum. Thus, when armature is rotated in presence of flux we get maximum iron loss.
4. If DC generators are located near load centres, which losses can be minimised?
a) Iron losses
b) Eddy current losses
c) Line losses
d) Corona losses
Answer: c
Explanation: Line losses occur in long transmission lines while sending output power to the loading stations. Thus, by locating the generators near loading stations losses occurring due transmission line can be eliminated.
5. Nature of efficiency curve of DC machine is _________________
a) First decreases then increases
b) First constant then decreases
c) First constant then increases
d) First increases then decreases
Answer: d
Explanation: The efficiency of a DC machine is different at different values of power output. As the output increases, the efficiency increases till it reaches to a maximum value. As the output is further increased, the efficiency starts decreasing slowly.
6. Why retardation test is carried on DC machine?
a) To find stray losses
b) To find eddy current losses
c) To find field copper losses
d) To find windage losses
Answer: a
Explanation: This test is generally employed to shunt generators and shunt motors. From this method we can get stray losses of a machine. Thus, if armature and shunt copper losses at any given load current are known then efficiency of a DC machine can be easily estimated.
7. In the DC motor the iron losses occur in __________
a) Field
b) Rotor
c) Brushes
d) Commutator
Answer: b
Explanation: Armature winding in a DC machine is located on rotor. Iron losses occur in an armature, hence in rotor. Iron losses are hysteresis loss and eddy current loss, which are seen prominently in armature rotor.
Characteristics Of DC Motors MCQs
8. Which of the following losses is not under constant losses?
a) Friction and windage losses
b) No load core losses
c) Shunt field losses
d) Hysteresis losses
Answer: d
Explanation: All the losses that is friction and windage losses, no load core losses, shunt field core losses in shunt field and compound motors come under the category of constant losses, while iron losses come under category of variable losses.
9. Variable losses are proportional to ________
a) Armature current
b) Square of armature current
c) Inverse of armature current
d) Inverse of square of armature current
Answer: b
Explanation: Variable losses include losses in armature resistance and losses in series resistance, which are directly proportional to the square of armature current. It also includes stray load loss (iron plus copper) which is proportional to square of armature current.
Swinburne’s Test
1.Swinburne’s test can be carried out on all DC motors.
a) True
b) False
Answer: b
Explanation: Swinburne’s test is a no-load test; thus, it is performed only on shunt and compound motors. DC series motor is not advisable to start at no-load so, this test can’t be performed on series motor.
2. Which of the following test will be suitable for testing two similar DC series motors of large capacity?
a) Swinburne’s test
b) Hopkinson’s test
c) Field test
d) Brake test
Answer: c
Explanation: As Swinburne’s test and Hopkinson’s test are no-load test, one can’t perform these tests on DC series motor. For DC series motor with large capacities, we conduct field test to find various losses occurring in a machine.
3. Which losses can be identified from Swinburne’s test?
a) No-load core loss
b) Windage and friction loss
c) No-load and windage and friction loss
d) Stray load loss
Answer: c
Explanation: We get total rotational losses occurring in a machine, which are equal to no-load core losses (iron + copper) and windage and friction losses. We also get shunt field losses and variable loss occurring in armature resistance.
4. While carrying out Swinburne’s test at rated armature voltage motor will run at ____________
a) Speed equal to rated speed
b) Speed greater than rated speed
c) Speed less than rated speed
d) Can run anywhere
Answer: b
Explanation: The motor is set to run on a rated speed while conducting Swinburne’s test by adjusting field current to rated value. When rated armature voltage is applied, motor runs with slightly greater speed than the rated one as some o the losses are not taken into account.
5. In order to run motor on rated speed while carrying out Swinburne’s test we add ______________
a) Resistance in parallel with armature
b) Resistance in series with armature
c) Inductor in series with armature
d) Capacitor in parallel with armature
Answer: b
Explanation: The machine would run at higher than rated speed with a rated armature voltage. Therefore, a series in the armature circuit is employed to reduce voltage applied to the motor armature such that it runs at rated speed.
6. What is the purpose of performing retardation test after Swinburne’s test?
a) To find stray load loss
b) To find variable losses
c) To separate out windage and friction losses
d) To find shunt field losses
Answer: c
Explanation: While performing Swinburne’s test we get addition of all losses. So, if we want to treat each loss individually, one needs to separate them. Retardation test is thus used by disconnecting both armature and field of the running motor.
7. Efficiency calculated by Swinburne’s test is _____________
a) Exactly equal
b) Over-estimated
c) Under-estimated
d) Depends on the manual errors
Answer: b
Explanation: The stray-load loss cannot be determined by this test and hence efficiency is over-estimated. Correction can be applied after assuming the stray-load loss to be half the no-load loss, which is done generally. Also, temperature may effect on resistance value which is not considered in this test.
8. Which of the following is not a disadvantage of a Swinburne’s test?
a) The stray-load losses can’t be determined by this test
b) Steady temperature rise can’t be determined
c) Does not give results about satisfactory commutation
d) Machine gets damaged
Answer: d
Explanation: For increasing life and durability of a machine we carry out such tests. But stray load losses, steady temperature rise is not determined by this method. Also, results about satisfactory commutation are not given by this method.
9. While carrying out retardation test, if t is equal to time constant then _____________
a) Speed increases to 36.8% of its initial value
b) Speed reduces to 36.8% of its initial value
c) Speed reduces to 26.8% of its initial value
d) Speed reduces to 46.8% of its initial value
Answer: b
Explanation: While carrying out retardation test, speed of the motor decreases. At time t equal to time constant then speed reduced is given by 36.8% of its initial value. Retardation test result is used to determine initial slope of ω(t).
10. In retardation test _____________
a) Motor switch is made ON and various speed readings are taken
b) At rated speed various speed readings are taken out at different times
c) Motor switch is made OFF at rated speed and various speed readings are taken
d) Some readings are taken while speed is building up and some readings while speed is lowering down
Answer: c
Explanation: The motor is run to rated speed (or any high speed) and the supply is switched-off. As the motor decelerates (retards), several speed-time readings are taken, by a speedometer and watch with seconds hand. Initial readings are taken at small time intervals and the time interval is increased as the motor slows down.
11. Retardation curve is ______________
a) Starting from origin
b) Starts from some positive value and increasing
c) Starts from some positive curve and stays constant
d) Starts from some positive value and decreases
Answer: d
Explanation: Retardation curve is a plot of speed vs. time at various retarding speeds. Thus, curve starts at some positive value and shows exponentially decaying nature with time. From various values of speed we calculate windage and friction loss at each point.
Hopkinson’s Test
1. Hopkinson’s test of D.C. machines is conducted at _______________
a) No-load
b) Part load
c) Full-load
d) Overload
Answer: c
Explanation: Unlike Swinburne’s test Hopkinson’s test is carried out at loaded condition. Thus, we get stray load loss also, while finding out the efficiency. Hence, efficiency is not over-estimated like Swinburne’s test.
2. Hopkinson’s test requires ____________
a) One DC machine on which test is carried out
b) Two different DC machines
c) Two identical DC machines
d) Can be worked with one or two machines
Answer: c
Explanation: This is a regenerative test in which two identical dc shunt machines are coupled mechanically and tested simultaneously. One of the machines is used as a motor driving while the other one acts as a generator which supplies electric power to motor.
3. In Hopkinson’s test, two machines are connected in ______________
a) Series
b) Parallel
c) Can be connected in parallel or series
d) Two machines are not required
Answer: b
Explanation: The two machines are made parallel by means of switch S after checking that similar polarities of the machine are connected across the switch. Here, one machine is driving another machine.
4. What will happen if field current of generator in Hopkinson’s test is increased?
a) Current through motor armature will increase
b) Current through motor armature will decrease
c) Current through motor armature will remain constant
d) Motor armature current cannot be determined
Answer: a
Explanation: If field current through generator is increased, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.
5. What will happen if field current of motor in Hopkinson’s test is decreased?
a) Current through motor armature will increase
b) Current through motor armature will decrease
c) Current through motor armature will remain constant
d) Motor armature current can’t be determined
Answer: a
Explanation: If field current through motor is decreased, speed of the motor will increase due to inverse proportionality, back emf of generator will increase thus it’ll become greater than back emf of a motor, so to compensate this effect armature current in generator will increase thus, motor armature current will also increase.
6. Hopkinson’s test is a regenerative test.
a) True
b) False
Answer: a
Explanation: Hopkinson’s test is a regenerative test, because the power drawn from the mains is only that needed to supply losses. The test is, therefore, economical for long duration test like a “heat run”.
7. For carrying out load test on Hopkinson’s test setup _________________
a) Actual load is needed
b) By changing field currents in two machines load can be changed
c) Can’t carry out
d) By changing the armature current test is carried out
Answer: b
Explanation: There is no need to arrange for actual load (loading resistors) which apart from the cost of energy consumed, would be prohibitive in size for large-size machines. By merely adjusting the field currents of the two machines, the load can be easily changed and a load test conducted over the complete load range in a short time.
8. Hopkinson’s test gives _______________
a) Combined iron losses of two machines which can be separated
b) Combined iron losses of two machines which can’t be separated
c) Doesn’t include iron losses
d) Depends on actual setup
Answer: b
Explanation: From Hopkinson’s test both machines are not loaded equally and this crucial in small machines. Thus, it is important to know the separate iron losses for given machines. But, test gives combined losses which are different for different machines as excitation differs.
9. Hopkinson’s test is suitable for ______________
a) Small machines only
b) Small and medium machines
c) All machines
d) Only large machines
Answer: d
Explanation: A large variation of field currents is required for small machines, the full-load set speed is usually higher than the rated speed and the speed varies with load. The full load in small machines cannot obtained by cutting out all the external resistances present in the generator field. Sufficient reduction in the motor field current is necessary to achieve full-load conditions resulting in speeds greater than the rated value.
10. Why field test is conducted even if Hopkinson’s test is present?
a) Instability of an operation
b) Possibility of run-away speed
c) Both instability and possibility of run-away speed
d) Field test is not conducted
Answer: c
Explanation: Regenerative test on two identical series motors is not feasible because of instability of such an operation and the possibility of run-away speed. Therefore, we have toconduct a loading test.
11. In field’s test generator field and motor field are connected in ______________
a) Series
b) Parallel
c) Alternatively, series and parallel
d) Not connected
Answer: a
Explanation: The generator field is connected in series with motor field circuit. The generator is thus separately excited and its excitation is identical to that of motor at all loads. This ensures that the iron-loss of both the machines are always equal.