Description : The slight curvature at the lower end of the O.C.C. of a self-excited dc generator is due to 1. residual pole flux 2. high armature speed 3.magnetic inertia 4. high field circuit resistance

**Answer** : The slight curvature at the lower end of the O.C.C. of a self-excited dc generator is due to magnetic inertia

Description : separately excited dc motor controlled by chopper with a frequency of 500hz and source voltage of 230v and rated value of motor is 220v 24A 100 rpm find its duty ratio at 1.2 times rated torgue and 500rpm

Description : A separately excited dc motor operating in first quadrant is fed from 3-phase semi -converter free -wheeling diode is open circuited, then the motor : (A) can be operated in first quadrant (B) ... in first & second quadrants (C) can be operated in first & fourth quadrants (D) cannot be operated

**Answer** : A separately excited dc motor operating in first quadrant is fed from 3-phase semi -converter free -wheeling diode is open circuited, then the motor : can be operated in first & fourth quadrants

Description : A separately excited dc motor is required to be controlled, from a 3-phase supply, for its operation in the first quadrant only. The most preferred converter would be (a) 3-phase fully controlled ... converter with free wheel diode (c) 3-phase dual converter (d) 3-phase half wave converter

**Answer** : A

Description : A single-phase half-controlled rectifier is driving a separately excited dc motor. The dc motor has a back emf constant of 0.5 V/rpm. The armature current is 5 A without any ripple. The armature resistance is 2 Ohm. The ... of the motor will be (A) 339 rpm (B) 359 rpm (C) 366 rpm (D) 386 rpm

**Answer** : 359

Description : A DC chopper is used for regenerative braking of a separately excited DC motor. The DC supply voltage is 400V. The armature current during regenerative braking is kept constant at 300 A with negligible ripple. For a duty cycle of 60% ... to the DC supply is (a) 78 kW (b) 58 kW (c) 38 kW (d) 48 kW

**Answer** : A DC chopper is used for regenerative braking of a separately excited DC motor. The DC supply voltage is 400V. The armature current during regenerative braking is kept constant at 300 A with negligible ripple. For a duty cycle of 60% the power return to the DC supply is 48 kW

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 : Separately Excited DC motor

**Answer** : Separately Excited DC motor

Description : The supply both to field and armature circuits are disconnected simultaneously in a separately excited dc motor and it comes to a standstill in 5 sec. If the armature circuit of this motor is disconnected from supply with field ... rest in (1) 5 sec (2) 7 sec (3) 4 sec (4) A very long time

**Answer** : 5 sec only.standstill time doesn't depend upon field excitation

Description : A separately excited dc generator is running at rated speed and at no load. If its field winding is suddenly connected to a dc source then rise in armature generated voltage is governed by (1) Armature time constant (2) Field time constant (3) Both (a) and (b) (4) Mechanical time constant

Description : If a self excited d.c. generator after being installed, fails to build up voltage on its first trial run, the first thing to do is (a) reverse the field winding connections (b) increase the field resistance (c) decrease the speed of prime mover (d) increase the speed of prime mover

Description : for the voltage built-up a self-excited D.C. generator, which of the following is not an essential condition. 1.there must be some residual flux 2.field winding mmf must aid the residual ... 3.total field circuit resistance must be less than t critical value 4.armature speed must be very high

**Answer** : for the voltage built-up a self-excited D.C. generator, which of the following is not an essential condition. 1.there must be some residual flux 2.field winding mmf must aid the ... .total field circuit resistance must be less than t critical value 4.armature speed must be very high

**Answer** : A separately excited generator as compared to a self-excited generator is amenable to better voltage control, more stable, has exciting current independent of load current.

Description : A 0.5 Hp, 6 pole wound rotor induction motor is excited by a 3 phase 60 Hz source. Calculate the frequency of the rotor current at standstill. (A) 50 Hz (B) 30 Hz (C) 60 Hz (D) 120 Hz

Description : The stator of a 415V, 50 Hz, 4-pole slip ring Induction Motor is excited by rated frequency with slip rings open circuited. The rotor is made to rotate at a speed of 600 rpm in the opposite direction to the ... frequency of induced voltage across slip ring is A) 30 Hz B) 70 Hz C) 20 Hz D) 35 Hz

**Answer** : The stator of a 415V, 50 Hz, 4-pole slip ring Induction Motor is excited by rated frequency with slip rings open circuited. The rotor is made to rotate at a speed of 600 rpm in the opposite direction to the normal direction, The frequency of induced voltage across slip ring is 70 Hz

Description : A separately excited d.c. motor fed from single phase full converter with firing angle 60° runs at 1000 r.p.m. If motor is connected to single phase semiconverter with the same firing angle of 60°, it would run at (a) 2000 rpm (b) 1850 rpm (c) 1500 rpm (d) 1000 rpm

**Answer** : Ans =1500

Description : A single-phase induction motor with only the main winding excited would exhibit the following response at synchronous speed (A) Rotor current is zero (B) Rotor current is non-zero and is ... and backward rotating fields are equal (D) Forward rotating field is more than the backward rotating field

**Answer** : A single-phase induction motor with only the main winding excited would exhibit the following response at synchronous speed (A) Rotor current is zero (B) Rotor current is non-zero and is ... backward rotating fields are equal (D) Forward rotating field is more than the backward rotating field

**Answer** : An over excited synchronous motor on noload is known as synchronous condenser.

Description : If the load of the normally excited synchronous motor is increased the power factor will?

**Answer** : If the load of the normally excited synchronous motor is increased the power factor will become lagging.

Description : If the field of synchronous motor is under excited, power factor will be?

**Answer** : If the field of synchronous motor is under excited, power factor will be lagging.

Description : what is separately excited motor ?

**Answer** : the field winding have different source power supply and armature have different power supply source

Description : Why a single phase motor is not a self starting motor

**Answer** : The motor mechanical degree 360 degree this must covered revolving field so single phase to starting need starting winding with 90 degree to main winding

Description : Application of DC Series Motor

**Answer** : 1. Electric traction 2. Cranes, 3. Passenger elevators, 4. Continuous conveyors, 5. Grinders, 6. Polishers,

Description : If field current is decreased in shunt dc motor, the speed of the motor (a) remains same. (b) increases. (c) decreases. (d) none of the above.

**Answer** : increases

Description : What would happen if a DC motor was supplied with AC ?

**Answer** : With three phase induction motor, humming and not rotate, but if single phase universal motor like air blower, home mixture, motor run on constant speed..speed variation not possible...

Description : State the function of following parts of DC motor : i) Yoke ii) Poles iii) Field winding iv) Commutator.

**Answer** : Function : i) Yoke: The yoke serves the following two purposes. i) It supports the other components such as poles and provides mechanical protection for whole machine. ii) It forms a part of ... one pole to another and thus to help the motor to develop a continuous and unidirectional torque

Description : DC series motor should never be started at no-load. Justify.

**Answer** : At no load the field current (which is also the armature current) is very small and hence the useful air-gap field flux. Also the torque is very small. As N ∝ 1/Ф the speed rises to ... on the rotating parts they may get damaged. Hence DC series motor should never be started on no-load.

Description : With the help of neat diagrams,explain the following methods of speed control in DC series motor. i) Field diverter method. ii) Armature diverter method.

**Answer** : (i) Field diverter method: - Resistance connected in parallel with field winding -By adjusting this resistance current can by diverted from field winding -Thus field current decreases and the speed can be ... decreased, the flux must increase and hence speed is decreased than the rated speed.

**Answer** : Back emf: When the armature of DC motor rotates under the fixed magnetic field and cuts it, by Faraday's law an emf is induced in them. The induced emf acts in opposite direction to the ... the flow of armature current i.e. it automatically changes the armature current to meet load requirements.

**Answer** : Principle of operation of a dc motor: When a current carrying conductor is placed in a magnetic field, the conductor experiences the mechanical force. The magnitude of force is given by F = BIL newton where F – Force , B – Maximum flux density , I – Current , L – Active length of conductor

Description : Give the two functions of yoke in DC motor.

**Answer** : Functions of yoke in DC motor: 1) Provides mechanical support for poles. 2) Acts as protecting cover for machine. 3) Provides path for magnetic flux.

Description : State any two applications of brushless DC motor.

**Answer** : 1) Synthetic fibre drawing works. 2) Turn table drives in record players. 3) Spindle drives in hard disk circuits. 4) Computer peripheral equipments as plotters etc. 5) Gyroscope motors. 6) Biomedical machines as heart pumps.

Description : With the help of necessary sketch, explain in brief the working of brushless DC motor.

**Answer** : Constructional Diagram of Brushless DC Motor: Working of brushless DC motor: The brushless DC motor is the combination of a permanent excited synchronous motor and a frequency ... Suitable PWM techniques allow regenerative breaking, which increases dynamic and efficiency of the drive.

Description : List the various losses on a DC Motor.

**Answer** : The various losses on a DC Motor: 1) Winding losses (Copper losses): i) Armature winding losses ii) Shunt field losses iii) Series field losses 2) Core losses (iron losses): 3) Brush contact losses: 4) Windage and frictional (Mechanical losses):

Description : In a DC motor, it is important to place the brushes along ______ A) Magnetic Neutral Axis B) Normal Geometric Axis C) Nominal Geometric Axis D) None of these

**Answer** : In a DC motor, it is important to place the brushes along Magnetic Neutral Axis

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 : In a differentially compounded dc motor (A) the speed is constant as the torque increases (B) the speed falls non-linearly as the torque increases (C) the speed increases non-linearly as the torque increases (D) the flux increases as the load increases

**Answer** : In a differentially compounded dc motor the speed increases non-linearly as the torque increases

Description : A large dc motor is required to control the speed of the blower from a 3 -phase ac source. The suitable ac to dc converter is, 3 -phase (a) Fully controlled bridge converter ( ... bridge converter with freewheeling diode (c) Half controlled bridge converter (d) Converter pair in sequence control

**Answer** : A large dc motor is required to control the speed of the blower from a 3 -phase ac source. The suitable ac to dc converter is, 3 -phase Half controlled bridge converter

Description : A DC shunt motor having unsaturated magnetic circuit runs at 1000 rpm with rated voltage If the applied voltage is reduced to half of the rated voltage, the motor will run at (A) 2000 rpm (B) 1000 rpm (C) 750 rpm (D) 500 rpm

Description : To enable dc series motor work satisfactory with an AC supply, the following modifications should be done (1) The yoke and poles should be completely laminated (2) The poles should be made of ... (3) The air gap between stator and rotor be reduced (4) Compensating poles should be introduced

Description : A dc series motor when connected across an AC supply will (1) Develop torque in same direction (2) Not develop any torque (3) Draw dangerously high current (4) Develop a pulsating torque

Description : A dc shunt motor runs at 500 r.p.m at 220 V. A resistance of 4.5 Ω is added in series with the armature for speed control. The armature resistance is 0.5 ohms. The current to stall the motor will be (1) 44 A (2) 50 A (3) 44.4 A (4) 60 A

Description : The dc motor used for intermittent high torque loads is (1) dc series motor (2) dc shunt motor (3) differential compound motor (4) cumulative compound motor

**Answer** : 4 cumulative compound motor

Description : A cumulative dc compound motor runs at 1500 rpm. on full load. If its series field is short-circuited, its speed (1) becomes zero (2) remains unchanged (3) increases (4) decreases

**Answer** : Increases. As it's cumulative compound motor, both series and shunt field both assist each other. If series field is short circuited, the overall field strength will decrease and atop of that ... speed is inversely proportional to field strength. So, the speed will increase overall.

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

Description : The method of speed control used for increasing the speed of a dc shunt motor above its rated speed is 1) armature resistance control 2) field control (3) armature voltage control (4) series parallel control

Description : Variation in dc excitation of a synchronous motor causes variation in (a) Speed of motor (b) Power factor (c) Armature current (d) Both armature current and power factor

Description : Armature shunting method of speed control of dc shunt motor is preferred over annature resistance method due to: (1) Better speed regulation (2) Less loss in external resistance (3) Simplicity in control circuit (4) Reduced cost of controller

**Answer** : Armature shunting method of speed control of dc shunt motor is preferred over annature resistance method due to: Better speed regulation.

Description : A 4 pole lap wound dc shunt motor rotates at the speed of 1500 rpm, having a flux of 0.4 mWb and total number of conductors 1000. The value of e.m.f. will be: (A) 100 Volts (B) 0.1 Volts (C) 1 Volts (D) 10 Volts

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