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The correct choice is (a) SERIES

Best 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.

Correct answer is (C) Both instability and possibility of run-away speed

To EXPLAIN: 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.

Correct CHOICE is (b) COMBINED iron losses of two machines which can’t be separated

Easiest 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.

The correct option is (b) By changing field currents in TWO machines load can be changed

The BEST 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.

The CORRECT option is (a) True

The explanation is: 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”.

The CORRECT option is (a) CURRENT through motor armature will increase

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.

Correct option is (B) Parallel

To explain: 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.

Right ANSWER is (c) Full-LOAD

The best I can explain: 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.

The correct answer is (d) Starts from some positive VALUE and decreases

To explain: 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.

The correct option is (c) Motor switch is MADE OFF at rated speed and various speed READINGS are TAKEN

For explanation I would say: 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.

Right option is (b) Over-estimated

To elaborate: 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.

Correct answer is (c) To separate out windage and friction losses

The best 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.

Right option is (b) Speed GREATER than rated speed

The explanation is: 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.

The correct answer is (b) Resistance in series with armature

For 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.

Correct OPTION is (C) No-load and windage and friction loss

To ELABORATE: 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.

Correct option is (C) Field TEST

To explain: 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.

The correct answer is (d) Hysteresis losses

Easy 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.

The CORRECT option is (B) Rotor

The 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.

The correct option is (a) To find STRAY losses

Easy 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.

Correct option is (d) First increases then decreases

The 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.

Right answer is (c) LINE losses

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.

Right choice is (d) ARMATURE rotor

Easy 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.

The CORRECT choice is (d) Variable losses = constant losses

The EXPLANATION is: 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.

Correct CHOICE is (b) 500 W

The 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.

The CORRECT choice is (b) EDDY current losses

The BEST 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.

The CORRECT answer is (d) Mechanical, Copper and Core

To elaborate: 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.

Correct answer is (c) Armature copper LOSS

For 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.

Right option is (c) Armature copper loss

The 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.

Correct choice is (c) Increase in terminal voltage

For 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.

The correct option is (d) Less than 50%

For 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 %.

Correct OPTION is (d) Rate of flow of ventilating air

The best 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.