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The correct answer is (b) Separately excited motors

Explanation: Dynamic braking is very effective for separately excited DC motors. As in separately excited motors the direction of field can be very EASILY ALTERED by ALTERING the terminals of the field, which is the CONDITION in dynamic braking.

Right CHOICE is (c) Both Reversing and Non-reversing

Best explanation: Dynamic BRAKING is employed to brake both reversing drives and non-reversing drives.

In dynamic braking, the ELECTRICAL energy GENERATED during stopping action is released as heat through a voltage regulated transistor and RESISTOR.

Correct option is (a) PLUGGING

Easiest explanation: Plugging braking provides maximum TORQUE in OPPOSITE direction at the instant of braking, this characteristic of braking suits perfectly with the application that is in ELEVATORS. If switch is kept ON, we GET reverse rotation also.

The correct ANSWER is (c) Back emf of rotor is more than the applied voltage

Explanation: The condition for REGENERATION is that the ROTATIONAL emf is more than the applied voltage so that the current is reversed and the MODE of operation changes from MOTORING to generating.

Right option is (a) Motor will run as a generator

For explanation: In regenerative method of electrical BRAKING, motor is suddenly FORCED to act as a generator, all the ENERGY then obtained is pushed back into the supply unlike in dynamic braking this energy is WASTED.

Correct answer is (c) Highest torque BRAKING

To EXPLAIN: In electrical braking called plugging direct REVERSAL of connections is done, which causes maximum torque to act on shaft but in OPPOSITE direction. As the speed decreases this torque ALSO starts decreasing.

The correct CHOICE is (c) Increasing supply voltage

For explanation: REGENERATION is achieved by either increasing field current, increasing armature speed, or by reducing supply voltage. Increasing supply voltage is not the METHOD which is EMPLOYED in regeneration process.

Correct option is (a) Requires BRAKING of machine

The explanation is: Regenerative braking is used specially where the DUTY cycle requires the braking or SLOWING of the machine more frequently and is most useful in holding a descending load of high POTENTIAL energy at a constant speed.

Right choice is (c) Generator

Explanation: The armature is disconnected from the supply and then a braking resistor Rb is immediately CONNECTED across it. The motor acts as a generator, DRIVEN by the inertia and STORED kinetic energy dissipating power in Rb. This is a simple method of BRINGING a motor nearly to a standstill.

The correct choice is (d) All- SYSTEM inertia, load TORQUE and motor rating

Best explanation: In dynamic BRAKING, when brakes are applied the armature is disconnected from machine CIRCUIT and connected to the braking resistor. Now, at this point motor is driven by kinetic energy GAINED earlier, dissipating power in braking resistor.

Right choice is (d) Removal of ARMATURE circuit from current machine circuit

Easiest EXPLANATION: Reversal of the connections of armature is the METHOD called plugging. In DYNAMIC braking we remove the armature circuit and connect it to different resistor, with FIELD circuit still connected to the external supply.

Right choice is (b) Speed decreases

The BEST explanation: Braking torque of the DC machine is given by n (ka2/Rb). Here, braking torque is directly proportional to the speed of the motor, so as the speed decreases the efficiency of ELECTRICAL brakes which is DEPENDENT on braking torque decreases.

Correct CHOICE is (a) n (ka^2/Rb)

Explanation: Braking TORQUE is equal to braking POWER divided by SPEED of the motor.

 T= [(nka )^2/Rb]/n. As, Braking power is equal to Ea^2/Rb/n. By solving for the braking torque from the above equation, we GET n (ka^2/Rb).

The correct option is (d) Eddy current

For explanation I would say: Eddy current is the ELECTRICAL effect or response of the system, which is reflected MECHANICALLY at brakes to REDUCE the SPEED of the MOTOR. Thus, eddy current is not an electrical brake, it is mechanical one.

The correct choice is (B) Reversal of armature connections

The best explanation: Plugging is METHOD where connections are REVERSED at a GIVEN instant. Because of the problem of interrupting highly inductive field current and the time needed for the field current to build up in opposite direction, it is a common practice to reverse armature connections.

The correct option is (b) False

The explanation: Dc motor is used in TRACTIONS because of its excellent braking characteristics and ABILITY of smooth transition from the motor to the GENERATOR mode and vice versa. ALSO, characteristics suit perfectly for traction application.

The CORRECT option is (d) Electric braking

For explanation: Braking methods based on friction, electromechanical ACTION, eddy-currents, ETC. are independent of the motor but sometimes electric braking is better JUSTIFIED owing to its GREATER economy and absence of brake wear.

Right answer is (d) High cost, high MAINTENANCE and low efficiency

To EXPLAIN I would SAY: Ward Leonard speed control method requires large number of building blocks like GENERATORS, motors. Thus, installing cost and maintenance cost of the whole unit is very high. Apart from cost, it gives low efficiency at very high loads.

The CORRECT CHOICE is (a) VOLTAGE control method

To elaborate: Ward Leonard SPEED control method is combination of rheostatic series control method and shunted armature control method with field control as well. Thus, it can be called as voltage control method also.

The correct choice is (a) In both directions

Best explanation: Speed CONTROL by Ward LEONARD method, gives uniform speed VARIATION in both the directions and above and below of normal speed as well. Speed REVERSAL is carried out smoothly by this control method.

The correct option is (a) WARD Leonard control

The best EXPLANATION: Ward Leonard METHOD of speed control is most EFFICIENT method of speed control in all aspects. We can get constant torque OPERATION and constant power operations as well, with this method.

Correct ANSWER is (d) Is absent

Easiest explanation: No special starting gear is required in Ward Leonard method of speed control. As the induced VOLTAGE by generator is gradually raised from zero, the MOTOR STARTS up smoothly. Speed reversal is smoothly CARRIED out.

The correct option is (a) True

For explanation: Absence of external resistance increases efficiency. ALSO, when the generator emf becomes less than the BACK emf of the motor, electrical power flows back from motor to generator, is converted to mechanical form and is RETURNED to the mains VIA the driving ac motor. This aspect makes Ward Leonard method perfect for given APPLICATION.

The correct option is (c) Higher than rheostatic control method and shunted field control method

Best EXPLANATION: UNLIKE all other methods, external RESISTANCE is not added in the circuit of control system. Thus, efficiency of Ward Leonard control method is always highest at various different SPEEDS.

Correct option is (a) Will start from ORIGIN

Easy explanation: Speed power CHARACTERISTIC of DC MOTOR is plotted when, Ward Leonard speed control method is employed. For speed equal to zero, which is less than base speed, we get constant torque but variable power operation. THUS, power will start increasing from origin.

The CORRECT choice is (B) Constant torque speed control

Explanation: As SEEN from speed torque characteristics, reducing FIELD CURRENT will increase the speed of the motor above base value but power will remain same. Thus, reducing armature voltage will give constant power speed control, with variable torque.

Correct OPTION is (b) Constant torque SPEED control

For EXPLANATION: As seen from speed torque characteristics, reducing armature voltage will reduce the speed of the motor below base VALUE but torque will remain same. Thus, reducing armature voltage will give constant torque speed control.

The correct option is (d) AC generator

Easy explanation: Whole UNIT of Ward Leonard speed CONTROL unit consists of various units like DC generator, DC motor, AC motor, exciter CIRCUIT and various pots which are USED for carrying out smooth operation.

The correct choice is (a) Reduce armature voltage

The BEST EXPLANATION: In Ward Leonard speed control METHOD, speed can be reduced under BASE value by reducing armature voltage. By increasing field current speed can be reduced but this is not EMPLOYED in Ward Leonard method.

The correct choice is (c) Combination of armature control method and field control method

For EXPLANATION I would say: Ward LEONARD method is the combination of armature control method and field control method, which can also be called as VOLTAGE control method. This is the most efficient method of SPEED control over wide range.

Right option is (c) Series parallel control method

For explanation: Series-parallel control method is SUPERIOR to the rheostatic control INSOFAR as efficiency is concerned. However, it is limited to two SPEED steps. The method is commonly employed for speed control of series traction MOTORS.

The correct choice is (C) 2

Easiest explanation: Here two identical motors are coupled TOGETHER mechanically to a COMMON LOAD. Two speeds at constant torque are possible in this method—one by connecting the motors ARMATURES in series and the other by connecting them in parallel.

Correct choice is (C) Variable resistor is added in parallel with armature and another variable resistor is added in series

Best explanation: In SHUNTED armature speed CONTROL METHOD we add one variable resistance in series with armature and one variable resistance in parallel with armature, so by varying the ratio of this resistances we can get various different speeds.

Correct choice is (C) Poor

Easiest explanation: The speed regulation of the method is poor as for a FIXED value of series armature resistance, the speed VARIES directly with load, being dependent upon the resistance voltage drop. In general, rheostatic control is economically feasible only for very small motors (FRACTIONAL KW) or for short-time, intermittent show-downs for medium-sized motors.

The CORRECT option is (a) Speed-torque CHARACTERISTIC of both will start from same point

The best explanation: Speed-torque characteristic will start from same point because at zero torque presence of series resistance is not taken into account. Afterwards speed torque characteristic with higher series resistance VALUE will lie below the other.

Correct choice is (c) Speed below RATED speed

To explain: The drawback of ARMATURE speed CONTROL method is we get only SPEEDS which are below the rated speed. As the value of series resistance increases, back emf DECREASES which result in lowering of the speed.

Correct option is (b) False

To explain: Unlike FIELD control method, the main field ampere-turns are maintained at a LARGE VALUE, flux density DISTORTION caused by armature reaction is limited. Unlike field control scheme, SPEED reversal can be easily implemented here.

Correct answer is (a) Drum type CONTROLLER is used

The best I can EXPLAIN: Drum controllers are used when an operator is controlling the MOTOR directly/manually. The drum controller is used to start, stop, reverse, and vary the speed of a motor, by its mechanism. This type of controller is used on crane MOTORS, elevators, machine TOOLS, and other heavy applications.

Right option is (c) Back emf

For explanation I would SAY: According to terminology used in DC MACHINES, emf INDUCED in a machine when armature rotates is CALLED as back emf. This back emf value is directly proportional to the SPEED of the motor.

Right OPTION is (b) Armature voltage control

Easiest explanation: Armature control method provides a constant-torque DRIVE. In the shunt MOTOR case by keeping the field current at maximum VALUE full motor torque can be obtained at full-load armature current at all SPEEDS.