# D.C. motor is to a drive a load which is almost nil for certain part of the load cycle...

D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select the?

D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select the compound motor.

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Description : D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the?

Answer : D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the series motor.

Answer : If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens this will make armature to take heavy current, possibly burning it.

Description : Why does the speed of a DC shunt motor decrease as the load increases?

Answer : Because if the load is increasing then the torque must also be increase. If not then the speed of motor will decrease.

Answer : Shunt motors,  Series motors, Compound motors

Description : Explain the necessity of starter for D.C. motor. State various types of D.C. motor starter.

Answer : Necessity of Starter for D.C. Motor: Armature current is given by equation Ia = (V−Eb)/Ra i) If the motor is at standstill or rest, back emf Eb is zero (as Eb= ZNP/(60A), at start speed N ... . Types of D.C. motor starters: i) Two point starter ii)Three point starter iii) Four point starter

Answer : Series motor has got maximum self relieving property.

Description : State at least one function and the material used for the following parts of DC Motor.

Answer : Part Function Material Yoke -Provides mechanical support for poles -Acts as protecting cover for machine -Provides path for magnetic flux Cast Iron OR Cast Steel Field Winding - ... Pole core  Provides mechanical support to field winding. Cast Iron OR Cast Steel

Description : A d.c. motor can be looked upon as d.c. generator with the power flow?

Answer : A d.c. motor can be looked upon as d.c. generator with the power flow reversed.

Description : The Eb/V ratio of a d.c. motor is an indication of its?

Answer : The Eb/V ratio of a d.c. motor is an indication of its efficiency.

Description : The normal value of the armature resistance of a d.c. motor is?

Answer : The normal value of the armature resistance of a d.c. motor is 0.5.

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.

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

Answer : Commutator provides unidirectional torque and pulsating dc

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.

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

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

Description : In DC motor iron losses are occurred in?

Answer : In DC motor iron losses are occurred in the armature.

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

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

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 : 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

Description : The no-load speed of a single-phase SCR bridge converter fed separately excited dc motor operating at a firing delay angle, α and flux, Φ (A) directly proportional to α and Φ (B) ... proportional to α and inversely proportional to Φ (D) directly proportional to Φ and inversely proportional to α

Answer : The no-load speed of a single-phase SCR bridge converter fed separately excited dc motor operating at a firing delay angle, α and flux, Φ directly proportional to α and inversely proportional to Φ

Description : Why dc series motor should not be started at no load?

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

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

Answer : Cumulative compound D.C. motor is preferred for elevators.

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

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

Answer : For starting a D.C. motor a starter is required because it limits the starting current to a safe value.

Answer : A D.C. series motor is that which has its field winding consisting of thick wire and less turns.

Description : The synchronous motor which works on leading power factor and does not drive any mechanical load is called?

Answer : The synchronous motor which works on leading power factor and does not drive any mechanical load is called synchronous condenser.

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

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.

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

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 : A.C. series motor as compared to D.C. series motor has smaller brush width, less number of field turns, more number of armature turns, less air gap.

Answer : When a D.C. series motor is connected to A.C. supply, the power factor will be low because of high inductance of field and armature circuits.

Description : Which type of starter is used to start the DC motor?

Description : Describe the reason of using dc series motor for electric trains.

Answer : Electric trains require an inverse torque speed characteristics for proper operation. That is a higher torque is required at start (when speed is nil or low) and as the train picks up ... torque - speed characteristics of DC series motors match, these motors are suitable for electric trains

Description : In a 4-pole wave winding connected dc motor, the cross-magnetizing AT/pole for a brush shift of θ radians (A) Armature amp-conductors × (θ/360o) (B) Armature amp-turns × (θ/360o) (C) Armature amp-conductors × (1/4 - θ/360o) (D) Armature amp-conductors × (1/8 - θ/360o)

Answer : In a 4-pole wave winding connected dc motor, the cross-magnetizing AT/pole for a brush shift of θ radians Armature amp-conductors × (1/8 - θ/360o)

Description : Why DC motor is used for traction?

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

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