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Correct option is (a) SERIES MOTOR

Explanation: When load is MINIMUM, we will get very HIGH speed at shaft in DC series motor, but it is advisable that DC series motor should not be used on no-load application. For the application stated in question DC series motor is perfect option.

Right choice is (C) DIFFERENTIAL compound motors

Explanation: In differential compound motors FLUX will REDUCE as load is increased because series flux opposes shunt field flux. This will increase speed upon addition of load. THEREFORE, it has negative speed regulation means speed will increase due to increase in load.

Right answer is (c) Below

Easy EXPLANATION: At start, SPEED of DC cumulative motor is more than the speed of shunt motor because of series CHARACTERISTIC effect. Because of φse increasing with Ia beyond full load CURRENT, the speed of the compound motor falls much more SHARPLY than the shunt motor.

Correct answer is (d) DC differential compound MOTOR

Easiest explanation: Shunt motor speed though decreases by slight value as armature CURRENT increases, it is assumed to be constant. But series motor and cumulative motor speed decreases as armature current increases. Only differential motor FLUX reduces with increase in armature current thus, speed increases.

Right OPTION is (b) Above

The explanation is: Because of φse increasing with Ia, the SPEED of the compound motor falls much more SHARPLY than the shunt motor. Therefore, the n – armature current CHARACTERISTIC of the compound motor lies above that of the shunt motor for Ia < Ia ( fl) and lies below for Ia > Ia ( fl).

The correct answer is (b) False

The best explanation: A differentially compound motor has flux/pole φ = (φse – φf). It is SEEN that on over-load φ REDUCES sharply and so the motor ACTS like a series motor on no load. This is why the DIFFERENTIAL compound motor is not used in practice.

Right option is (d) DC series MOTOR

To explain: At higher values of current SPEED of the DC cumulative compound motor starts DECREASING DUE to effect of series field flux, which is also seen in DC series motor. Thus series motor and DC cumulative motor follow same characteristics.

Correct answer is (b) Reduces SHARPLY compare to SHUNT MOTOR

For explanation: As armature current increases, the back EMF decreases similar to shunt motor. By the same TIME series field resistance carries more armature current producing more amount of flux which is inversely proportional speed.

Correct option is (a) If (net)= If + (Nse/Nf) Ia

To elaborate: Net flux in a compound motor is DETERMINED by addition of shunt FIELD flux and series field flux. Thus, for calculating effective flux, net field current is FIND out. If is the constant value GIVEN for shunt field, while If net takes the presence of series field also.

Correct OPTION is (c) Compound motor

Best explanation: We can’t use series motor as our load is almost zero at some POINTS. Thus, we’ll use compound motor which can work on no load ALSO. Cumulative compound motor is PROVIDED with flywheel so that this machine can deal with peak VALUE.

The correct option is (b) Speed will increase

The explanation is: As the series field is short CIRCUITED now, current through series field becomes zero and MMF due to series field becomes zero. Hence, TOTAL MMF in MOTOR becomes LESS than previous case, which results in total decrease in the FLUX created and increase in speed.

Right option is (a) Motor will first STOP and then run in opposite direction as series motor

The best I can explain: If SHUNT FIELD is SUDDENLY opened, flux due to shunt field will become 0. As, net flux is equal to shunt field flux – series field flux, the direction of flux will be reversed and thus motor will run in opposite directions.

The correct choice is (c) Cumulatively compounded

The best I can explain: In case of cumulative compound DC motor, WHENEVER a mechanical load is SUBJECT to fluctuations for short durations, a flywheel is generally provided for CARRYING peaks. Flywheel will smooth out the load on the motor and will also reduce peaks on power SYSTEM.

Right answer is (a) True

Easiest explanation: CUMULATIVE motor takes best from shunt motor and SERIES motor as WELL. DUE to shunt field flux component, DC cumulative motor can be started on no load. No-load speed of DC cumulative motor is greater than that of DC shunt motor but it’s not equal to infinity like series motor.

The correct option is (d) DIFFERENTIAL compound motor

Easy explanation: In differential compound motor, series FIELD opposes shunt field. It has poor starting torque as the resultant flux is minimized by this OPPOSITION. The flux starts decreasing with increase in LOAD. The decrease in flux cause the starting torque to be less than any other DC motor.

Correct ANSWER is (d) Differential compound motor

To elaborate: A DIFFERENTIALLY compound DC motor, flux reduces so SHARPLY at small increase in load at higher VALUES of load. It is advisable that motor should not be used BEYOND some load value, as it may damage itself by self-loading.

Correct answer is (c) Cumulative compound motor

The explanation is: Cumulative compound DC motor provides high starting TORQUE which is REQUIRED at instant of starting in shears and punches. It also provides good SPEED regulation in ORDER to vary pressure. So, this motor suits well to shears and punches APPLICATION.

The correct CHOICE is (d) Cumulative compound motor

Explanation: Cumulative wound DC MOTORS give HIGH starting torque like a SERIES motor and reasonable good speed regulation at high speeds like a shunt dc motor. As this type of motor offers the best of both series and shunt motor, it is PRACTICALLY suitable for most common applications like elevators.

The correct choice is (a) Pulsating and unidirectional

The best explanation: In DC series motor, the AC CURRENTS through the field and armature WINDING will be MADE unidirectional by the function of COMMUTATOR and brushes. So, torque will be unidirectional but it will be pulsating ALSO due to AC.

The correct CHOICE is (a) True

Easiest explanation: At higher values of armature current or load current, saturation and demagnetization due to armature reaction BECOMES prominent, and DC machine shows its effect on torque. THUS, torque TENDS to remain at constant value.

Correct choice is (b) Higher

The explanation is: From SPEED current characteristics of a DC series MOTOR is easily seen that, initial value of speeds in both of the cases is same or RATHER infinite, but as a armature current or load current increases speed starts dropping down relatively more in the CASE of linear magnetization.

Right answer is (c) DIRECTLY proportional to square of ARMATURE current

For explanation: Torque in a DC SERIES motor, in the case of linear magnetization VARIES directly with armature current square. Practically at higher values of armature current it tends to settle at constant value.

Right choice is (a) [ Vt / kφ]- [ Rse +Ra / kφ] Ia

To explain I would SAY: Speed-torque characteristic equation is obtained by back emf = (Vt -IaRa-IaRse) and back emf= kφn. Thus, by combining these EQUATIONS and by assuming all CONSTANTS equal to k we get, the final equation shown above.

Correct answer is (a) DC Series motor

The best EXPLANATION: No-load speed of the DC motor is very high, it’ll create a dangerous situation and the centrifugal forces will destroy the ARMATURE and may HARM the personnel. As, flux is inversely proportional to speed, speed will be infinity by calculations.

The correct answer is (C) It provides high starting torque

For EXPLANATION I would say: DC series motor provides very high starting torque, and at higher TORQUES speed of the DC series motor is ALSO maintained to constant. Both of these properties are used in applications of DC series motor.

Correct option is (a) True

For explanation: Large starting TORQUE, INFINITE speed at no-load, LOW running torque for steady speed operation are called as Series characteristics. This are SHOWN by DC series MOTOR, so applications like cranes, traction, etc. use DC series motor.

Right choice is (c) 25% of the PREVIOUS VALUE

Explanation: TORQUE in the case of linear magnetization of DC series motor is directly proportional to SQUARE of the armature current. So, armature current is made 1/2th of the original value, then torque will be 1/4th of the original value.

The CORRECT choice is (d) Inversely proportional to the armature current

To elaborate: Speed of DC series motor is inversely proportional to the armature current because, as armature current INCREASES the flux produced also increases due to the series combination. Similarly if armature current is REDUCED flux is reduced which will INCREASE speed.

The correct option is (a) Starter for a car

The explanation is: Starter of the car simply implies START of operation at maximum load. THUS, series MOTOR can be used in this application. For HIGHER torques, since speed remains constant car starter is one of the best application of DC series motor.

The correct choice is (d) Has its field winding consisting of thick wire and LESS turns

To EXPLAIN: When field winding is connected in series it has to be of MADE by thick wire. When field winding is connected in PARALLEL it has to be of made by thin wire. So, thick wire is used with less number of turns while thin ONE is used with more number of turns.

Correct answer is (a) Series MOTOR

For EXPLANATION: A conveyor requires HIGH torque with steady speed, which provided by DC series motor. From torque speed characteristic of a DC series motor, it is easily STATED that speed of the series motor practically REMAINS constant at high torques.

The correct ANSWER is (b) Interchanging field terminals

Explanation: From speed current CHARACTERISTICS for a DC series motor, we get that speed is inversely proportional to FLUX. If flux direction is REVERSED, same flux will be available with negative sign in VECTORS, thus, speed will remain same but with the other direction.

The correct answer is (b) Series motor

Easy explanation: At no load, armature current TENDS to zero, flux φ tends to zero, where SPEED is inversely proportional to the flux, speed will tend to infinity. Thus, no load speed of DC series motor is HIGHEST.

The correct answer is (d) High STARTING speed

Explanation: DC series MOTORS are USED in those APPLICATIONS where high starting speed is REQUIRED at its initial phase. Since no load operation speed of DC series motor is very high, it is always operated on some non-zero load.

Right OPTION is (c) Not suitable for rapidly changing loads

For explanation: In DC shunt motor, due to shunt CHARACTERISTICS speed variation depends not only upon the controlling RESISTANCE but on the LOAD current also. This double dependence makes it impossible to keep the speed sensibly constant on rapidly changing load, which is in CONTRAST with characteristic.

Correct answer is (a) Series motor

The best I can explain: The mechanical torque T is directly PROPORTIONAL to the PRODUCT of flux per pole φ and armature current. In case of DC series motor, up to saturation point flux is proportional to field current because Ia = If. Now if small percentage increase in armature current (before saturation) will occur the same percentage of torque will increase. WHEREAS in DC series motor the torque is proportional to square of the armature current (before saturation).

The correct answer is (c) Starting from speed axis and decreasing

The best I can explain: Speed-TORQUE characteristic FALLS as torque increases, because armature reaction CAUSES decrease in flux WHOSE square is in INVERSE proportion with speed, causing decrease in speed rapidly compare to increase in torque.

Right answer is (b) Back emf

The best explanation: The armature CIRCUIT equation for DC shunt motor is given by EA = Vt -IARA. But back emf is also equal to kφn. Thus, kφn= Vt -IaRa, which further gives n = (Vt -IaRa)/ kφ. HENCE, speed is DIRECTLY proportional to back emf.

Right choice is (a) Will make armature to take HEAVY current, POSSIBLY burning it

To elaborate: At no load or lower loads, there is possibility of excess speed in such cases but here, when full load is GIVEN, armature CIRCUIT will DRAW very high current in order to maintain back emf. So, if fuses or circuit breakers are not used then, armature circuit may burn.

The correct OPTION is (b) Armature current only

Easy explanation: Torque of the DC shunt motor is directly proportional to the armature current. It’s almost a STRAIGHT line if effect of armature reaction is NEGLECTED. If armature reaction is taken in consideration then torque will increase but non-linearly with armature current.