# In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will?

In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will remain unchanged.

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Answer : The armature may burn if the back e.m.f. of a D.C. motor vanishes suddenly.

Answer : If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will increase.

Description : As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit?

Answer : As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit a resistance is connected in series with armature.

Answer : The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is 0.5.

Answer : In case of D.C. shunt motors the speed is dependent on back e.m.f. only because flux is practically constant in D:C. shunt motors.

Answer : In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to 25% of the previous value.

Answer : In case the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be less than 50%.

Description : In a 2 kW, 200 V, 1000 rpm. DC series motor the torque at full load was found to be 0.3 N -m. The torque at half full load in N -m is :

Answer : In a 2 kW, 200 V, 1000 rpm. DC series motor the torque at full load was found to be 0.3 N -m. The torque at half full load in N -m is : 0.075

Description : The counter e.m.f. of a d.c. motor?

Answer : The counter e.m.f. of a d.c. motor helps in energy conversion.

Answer : When the armature of a D.C. motor rotates, e.m.f. induced is back e.m.f.

Answer : Torque developed by a D.C. motor depends upon magnetic field, active length of the conductor, current flow through the conductors, number of conductors, radius of armature.

Description : Torque developed by a three-phase, 400V, induction motor is 100 N -m. If the applied voltage is reduced to 200-V, the d'veloped torque will be

Answer : Torque developed by a three-phase, 400V, induction motor is 100 N -m. If the applied voltage is reduced to 200-V, the d'veloped torque will be 25 N -m

Answer : As the load is increased the speed of D.C. shunt motor will reduce slightly.

Description : The speed of a D.C. motor can be controlled by varying  1. Its flux per pole 2. Resistance of armature circuit 3. Applied voltage 4. All of the above

Answer : The speed of a D.C. motor can be controlled by varying  1. Its flux per pole 2. Resistance of armature circuit 3. Applied voltage 4. All of the above

Answer : The speed of a D.C. shunt motor can be increased by increasing the resistance in field circuit.

Answer : The speed of a D.C. shunt motor more than its full-load speed can be obtained by decreasing the field current.

Answer : The armature torque of the D.C. shunt motor is proportional to armature current.

Description : If the back emf of a dc motor is doubled and its speed is also doubled, the torque developed in the machine will  (1) get doubled (2) become 4 times (3) get halved (4) remain unaltered

Answer : Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have to be cumulatively compounded.

Description : What are the speed control methods of dc motor?

Answer : Field control of DC motor Armature control of DC motor

Answer : In a D.C. shunt motor, speed is independent of armature current.

Answer : The speed of a D.C. series motor is inversely proportional to the armature current.

Description : If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be?

Answer : If residual magnetism is present in a D.C. generator, the induced e.m.f. at zero speed will be zero.

Answer : Cumulatively compounded D.C. motor will be suitable along with flywheel for intermittent light and heavy loads.

Description : A 240 V, dc shunt motor draws 15 A while supplying the rated load at a speed of 80 rad/s. The armature resistance is 0.5 W and the field winding resistance is 80 W. The net Voltage across armature resistance at the time of plugging will be

Answer : A 240 V, dc shunt motor draws 15 A while supplying the rated load at a speed of 80 rad/s. The armature resistance is 0.5 W and the field winding resistance is 80 W. The net Voltage across armature resistance at the time of plugging will be 474 V

Answer : In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be more than full-load current.

Answer : If a D.C. motor is connected across the A.C. supply it will burn due to heat produced in the field winding by eddy currents.

Answer : If a D.C. series motor is operated on A.C. supply, it will spark excessively, have poor efficiency, have poor power factor.

Description : The torque developed by any 3-phase induction motor at 0.8 p.u. slip is (A) Full-load torque (B) unstable torque (C) starting torque (D) break down

Answer : The torque developed by any 3-phase induction motor at 0.8 p.u. slip is unstable torque

Answer : In case of a reluctance motor, when the load is increased so that it cannot maintain synchronous speed the motor will run as induction motor.

Answer : The current drawn by the armature of D.C. motor is directly proportional to the torque required.

Answer : By looking at Commutator part of the motor, it can be easily confirmed that a particular motor is D.C. motor.

Answer : The direction of rotation of a D.C. series motor can be changed by interchanging field terminals.

Description : In a d.c. motor, unidirectional torque is produced with the help of

Answer : Commutator provides unidirectional torque and pulsating dc

Answer : In repulsion motor, zero torque is developed when brush axis coincides with the field axis and brush axis is 90° electrical to field axis.

Answer : In repulsion motor, maximum torque is developed when brush axis is at 45° electrical to the field axis.

Answer : D.C. Series motor is generally preferred for cranes and hoists.

Answer : If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses.

Answer : The type of D.C. motor used for shears and punches is cumulative compound D.C. motor.

Answer : Speed torque characteristic of a repulsion induction motor is similar to that of a D.C. compound motor.

Answer : In a differentially compounded D.C. motor, if shunt field suddenly opens the motor will first stop and then run in opposite direction as series motor.

Answer : Sparking, is discouraged in a D.C. motor because commutator gets damaged.

Answer : Sparking at the commutator of a D.C. motor may result in damage to commutator segments, damage to commutator insulation, increased power consumption.

Description : DC motor gross torque example

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Description : Why a dc series motor has a high starting torque?

Answer : If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor is to be derated by a factor recommended by manufacturer and select the next higher H.P. motor.

Answer : For a D.C. shunt motor if the excitation is changed torque will change but power will remain constant.

Answer : At the instant of starting when a D.C. motor is put on supply, it behaves like a low resistance circuit.

Answer : The armature voltage control of D.C. motor provides constant torque drive.

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