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The correct choice is (B) Coupling mounted on the SHAFT

Easy explanation: The output power of any electrical motor is always TAKEN from the coupling mounted on the shaft because the motor CONVERTS input electrical power into output MECHANICAL power. So, we get mechanical power at shaft coupling.

Right choice is (C) Helps in energy conversion

To explain I would SAY: Back emf or counter emf makes the DC a self-regulating machine i.e. it makes the DC motor to DRAW as much armature current as it is just enough to develop the required load TORQUE. Thus, it helps in energy conversion and it is always LESS than applied voltage.

Correct choice is (b) Below rated speed

The best explanation: In ARMATURE circuit resistance method armature resistance can be only increased, by adding another resistance in series. As a RESULT, the back emf will decrease. Speed is directly PROPORTIONAL to back emf so, it will also decrease. THEREFORE, this method is USED for below rated speed.

The correct choice is (a) Above rated speed

To explain I would SAY: By reducing of field CURRENT, FLUX will also REDUCE, because flux is directly proportional to the field current passing through the winding. As speed is inversely proportional to flux, so this method can be USED for only above rated speed.

The correct answer is (B) Ward-Leonard control

For EXPLANATION I would say: In ward-Leonard method, by using big CIRCUITRY, very fine speed control over the whole range from zero to NORMAL speed and that too in both directions can be obtained. The motor-generator set can PROVIDE speed both below and above the rated speed and in both direction, by this method.

Right ANSWER is (d) Current, active length, no. of conductors, magnetic FIELD all

The explanation: Torque PRODUCED by a DC motor is directly proportional to an armature current flowing through the machine. While it ALSO depends on some constant values like active length, no. OD armature conductors and magnetic field which are constant for a given machine.

Right choice is (d) Is to be de-rated by a factor recommended by manufacturer and SELECT the next higher H.P. motor

The EXPLANATION: The ambient temperature is one of a factor in derating.If the ambient temperature given is HIGH, the insulation may REACH its maximum temperature LIMIT quickly.Hence, the motor may have to be de-rated, and for application next motor must be used.

Right answer is (a) Low cost

To explain I WOULD SAY: Dc machine is very useful over AC when operation is concerned but its superior speed CONTROL system itself REQUIRES 2 more machines. DC machine requires high maintenance due to commutator and brushes so it is an expensive machine.

Correct answer is (b) Low resistance circuit

For explanation: At the starting when BACK emf is very low, machine will fetch large AMOUNT of armature current from SUPPLY acting as a low resistive circuit. In order to avoid such case, we use starters in DC machine which ADD some resistance to armature at the starting.

The correct answer is (a) REMAIN unchanged

Easy explanation: Torque DEPENDS on armature current, so as long as armature current remains constant torque will not change. Speed depends DIRECTLY with back EMF thus, it will definitely become double when back emf is doubled.

Right choice is (d) DC motor

Best explanation: DC series motor gives the MAXIMUM starting torque at starting. With the help of Ward Leonard speed control METHOD various speeds are ALSO available. It provides one mode to othe transmission also.

Correct choice is (a) Series resistance is connected with armature

Easy explanation: To prevent large armature current from damaging the armature circuit we connect series resistance with armature and in order to AVOID losses we reconnect the same resistance with field circuit once machine is started, i.e. the EXACT principle of STARTERS.

Correct option is (d) CONSTANT torque drive

The explanation is: By CHANGING the armature voltage we can run motor to GIVE various speeds at constant torque. While changing the field is ALSO another idea to change speed but it’ll be variable torque constant POWER operation.

The correct OPTION is (b) False

The best I can explain: Sparking at brushes and COMMUTATOR segments lead to damage of commutators, which is the main distinguishable COMPONENT in a DC MACHINE. So, it is advisable to fasten the commutation speed and avoid sparking.

Right CHOICE is (d) Bending MOMENT, unbalanced magnetic pull and twisting stains

To explain: The ARMATURE shaft must be able to WITHSTAND any unbalanced magnetic pull on the armature core, bending moment due to weight of the armature and commutator, twisting stains due to transmission of torque, for a good and long run application of motor.

Correct ANSWER is (c) Supply voltage = 2 X Ea

Explanation: For a MOTOR, from POWER EQUATION it is known that,

The correct ANSWER is (C) Full-load current

Easiest explanation: When SUPPLY VOLTAGE is increased full load current will decrease in order to keep output power CONSTANT, which will decrease torque at that moment, while starting torque will remain as it is, irrespective of any change in supply voltage.

The CORRECT choice is (a) TORQUE

Explanation: From the equation of torque GENERATED in a DC machine, we know that in both DC motor and DC GENERATOR, current drawn is directly proportional to the torque required by the machine.

The correct choice is (d) Output power AVAILABLE at the shaft

To explain: NAME PLATE of the motor shows rated values i.e. rated SPEED, rated current, rated voltage. It also shows output power available at shaft when all other QUANTITIES are set to rated values.

Right choice is (d) FLEMING’s left-hand rule

To explain: Flemings laws can be SUMMARIZED as WHENEVER, a current carrying conductor comes under a magnetic FIELD, there will be a force acting on the conductor and on the other hand, if a conductor is forcefully brought under a magnetic field, there will be an induced current in that conductor.

The correct answer is (a) Field WINDING

The best I can explain: Maximum temperature rise can be sustained by field winding, as it is not INVOLVED in ROTARY PARTS. Field winding is present AWAY from rotary parts of the machine, so temperature rise in the machine will not produce any effect in machine rotations.

The correct answer is (c) Commutator

Explanation: All other parts except brushes and commutator are same in AC MACHINE when outer LOOKS are only TAKEN in consideration. Commutator is USED only in DC machine for providing mechanical rectification and not in AC machine.

Right option is (a) Back emf INCREASE but LINE current falls.

The explanation: In case of DC MOTOR, the speed is proportional to the back emf (Ea ∝ N). So, with the increase in speed, the back emf ALSO increases. Therefore, armature current is also decreased, in case of series motor, armature current is equal to the line or load current.

Correct option is (C) The ARMATURE may burn

The explanation is: If back emf vanishes suddenly, motor circuit will try to retain back emf by drawing more current from SUPPLY. If SUPPLYING unit didn’t trip down by this TIME, excess current in armature may heat up the armature.

The CORRECT choice is (d) Burn due to heat produced in the field winding

The best I can explain: In case of parallel field connection, it won’t rotate at all and will start humming and will create vibrations, as a torque produced by POSITIVE and negative cycle will cancel out each other. DC MOTOR will be heated up and it may burn.