DC Machines Multiple Choice Questions on “Swinburne’s Test”.
1.Swinburne’s test can be carried out on all DC motors.
a) True
b) False
Answer: b
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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
Clarification: 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.