# Multiple choice question for engineering

## Set 1

1. In which mode machine is operating, given that conductor current is in the same direction of conductor emf?

a) Motoring

b) Generating

c) Can’t be determined using directions

d) In both modes for different cycles

### View Answer

2. Nature of the flux density wave in the air gap is__________ (for armature current equal to 0)

a) Flat topped with quarter wave symmetry

b) Point topped with quarter wave symmetry

c) Flat topped with half wave symmetry

d) Point topped with half wave symmetry

### View Answer

3. In a DC machine, average energy stored in the magnetic field remains constant independent of the armature rotation.

a) True

b) False

### View Answer

4. Emf produced by DC machine, for zero armature current (E1) and non-zero armature current (E2) can be related as__________

a) E1 = E2

b) E1 > E2

c) E1 < E2

d) Can’t be determined

### View Answer

5. Average coil emf for 20 coil turns (E1) and 40 coil turns (E2), will have ratio E1/E2=____ (assuming all other parameters same for both machines).

a) 1/2

b) 2/1

c) 1/4

d) 4/1

### View Answer

6. What is the average coil emf generated in a 4-pole DC machine having flux/pole equal to 0.1 wb rotating at 1500 rpm? (No. of coil sides = 100)

a) 19 kV

b) 1.9 kV

c) 190 V

d) 19 V

### View Answer

7. Emf and torque produced in a DC machine are proportional to ________ and _________ respectively.

a) Armature speed and armature emf

b) Armature emf and armature speed

c) Armature current and armature emf

d) Armature speed and armature current

### View Answer

8. What is the value of Np in an average coil emf equation, for 10 armature conductors with 2 parallel paths?

a) 2

b) 3

c) 2.5

d) 4

### View Answer

9.What is the torque equation in terms of B, Ic, l, Zr (r= mean air gap radius)?

a) Bav*Ic*l*Zr

b) Bav*Ic*l/Zr

c) Bav*Ic*Zr/l

d) Can’t be expressed

### View Answer

10. What is the value of pole pitch (in SI unit) for mean air gap radius= 0.5mm and P=4?

a) 0.785* 10^{-6}

b) 0.785* 10^{-3}

c) 0.785* 10^{-2}

d) 0.785* 10^{-4}

### View Answer

^{-3}/ 4= 0.785* 10

^{-3}m.

## Set 2

1. The condition for maximum efficiency for a DC generator is __________

a) Eddy current losses = stray losses

b) Hysteresis losses = eddy current losses

c) Copper losses = 0

d) Variable losses = constant losses

### View Answer

2. DC generators are normally designed for maximum efficiency at or near ____________

a) Full-load

b) Minimum load

c) Rated voltage

d) At all loads

### View Answer

3. In a DC generator, the iron losses mainly take place in ____________

a) Yoke

b) Commutator

c) Armature conductors

d) Armature rotor

### View Answer

4. If DC generators are located near load centres, which losses can be minimised?

a) Iron losses

b) Eddy current losses

c) Line losses

d) Corona losses

### View Answer

5. Nature of efficiency curve of DC machine is _________________

a) First decreases then increases

b) First constant then decreases

c) First constant then increases

d) First increases then decreases

### View Answer

6. Why retardation test is carried on DC machine?

a) To find stray losses

b) To find eddy current losses

c) To find field copper losses

d) To find windage losses

### View Answer

7. In the DC motor the iron losses occur in __________

a) Field

b) Rotor

c) Brushes

d) Commutator

### View Answer

8. Which of the following losses is not under constant losses?

a) Friction and windage losses

b) No load core losses

c) Shunt field losses

d) Hysteresis losses

### View Answer

9. Variable losses are proportional to ________

a) Armature current

b) Square of armature current

c) Inverse of armature current

d) Inverse of square of armature current

### View Answer

## Set 3

1. Axis undergo shifting as a result of armature reaction, can be balanced by ______

a) Increase in armature current

b) Decrease in armature current

c) Introducing interpoles

d) Removing interpoles

### View Answer

2. The choice of average coil voltage determines the minimum number of commutator segments for its design.

a) True

b) False

### View Answer

3. Compensating winding will provide incomplete neutralization ____________

a) Under pole region

b) In interpolar region

c) Everywhere

d) Complete neutralization

### View Answer

4. Cross-magnetizing effect of armature reaction can be reduced by __________

a) Removing saturation in teeth and pole-shoe

b) Making smooth pole shoes

c) Introducing saturation in teeth and pole-shoe

d) Cannot be determined

### View Answer

5. Cross-magnetizing effect of armature reaction can be reduced by __________

a) Removing saturation in teeth and pole-shoe

b) Making smooth pole shoes

c) Chamfering the pole shoes

d) Cannot be determined

### View Answer

6. To counter the effect of shift in MNA due to armature reaction, which of the following component can be shifted?

a) Poles

b) Commutator

c) Brushes

d) Cannot be determined

### View Answer

7. Calculate the number of conductors on each pole piece required in a compensating winding for a 6-pole lap-wound dc armature containing 286 conductors. The compensating winding carries full armature current. Assume ratio of pole arc/ pole pitch = 0.7.

a) 6

b) 8

c) 9

d) 7

### View Answer

_{cw}/pole = AT

_{a}(peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). N

_{cw}/pole = (Z/2AP) * (pole arc/pole pitch) = [286 / (2*6*6)] 0.7 = 2.78. Compensating conductors/pole = 2 * 2.78 = 6 (nearest integer).

8. A compensating winding with ampere-turns greater than peak ampere turns is required in order to neutralize the effect of armature reaction because _____________

a) Pole arc = Pole pitch

b) Pole arc > Pole pitch

c) Pole arc < Pole pitch

d) Can’t be determined using pole arc, pole pitch

### View Answer

_{cw}/pole = AT

_{a}(peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are more then, pole arc is definitely greater than pole pitch.

9. If pole arc is less than pole pitch, a compensating winding will have ampere-turns _________ (compare to peak ampere turns).

a) Less

b) Equal

c) More

d) Can’t be specified

### View Answer

_{cw}/pole = AT

_{a}(peak) *(pole arc/pole pitch) = [IaZ/(AP2)] * (pole arc/pole pitch). Thus, if compensating winding ampere turns are less then, pole arc is smaller than pole pitch and vice-versa.

10. What is the pole arc/pitch ratio, if 360 AT compensating winding is used where 1960AT is peak value?

a) 0.7

b) 0.8

c) 0.9

d) 0.6

### View Answer

_{cw}/pole = AT

_{a}(peak) *(pole arc/pole pitch). If compensating winding of 360AT is used the, 360/1960 will give ratio of pole pitch /pole arc, equal to 0.6.

11. Only drawback of compensating winding is _______

a) Cost

b) Unavailability of material

c) Construction

d) Not a single drawback

### View Answer

## Set 4

1. A 4-pole Dc wound machine is lap wound with 400 conductors. The pole shore is 20 cm long and average flux density over one-pole pitch is 0.4 T, the armature diameter being 30 cm. What is the value of flux/pole?

a) 0.188 Wb

b) 18.88 Wb

c) 0.0188 Wb

d) 1.888 Wb

### View Answer

2. A 4-pole Dc wound machine is lap wound with 400 conductors. The pole shore is 20 cm long and average flux density over one-pole pitch is 0.4 T, the armature diameter being 30 cm. What is the value of induced emf?

a) 188 V

b) 276 V

c) 94 V

d) 188 mV

### View Answer

3. Coil torque for 20 kA armature current (T1) and 40 mA armature current (T2), will have ratio T1/T2=____ (assuming all other parameters same for both machines).

a) 1/2

b) 2/1

c) 1/4

d) 4/1

### View Answer

4. If the no load speed of DC motor is 1300 rpm and full load speed is 1100 rpm, then its voltage regulation is ____________

a. 12.56%

b. 18.18 %

c. 17.39%

d. 18.39%

### View Answer

5. If the average coil emf of a DC motor is doubled and flux is halved (keeping other parameters constant) then its shaft speed will become ___________

a. Twice of the original speed

b. Square of the original speed

c. Four times of the original speed

d. Half of the original speed

### View Answer

6. A 4-pole wave wound DC motor drawing an armature current of 20 A has provided with 360 armature conductors. If the flux per pole is 0.015 Wb then the torque developed by the armature of motor is _______

a. 10.23 N-m

b. 34.37 N-m

c. 17.17 N-m

d. 19.08 N-m

### View Answer

7. In a DC machine, what is the torque induced beyond the pole shoes?

a) 0

b) 2/π *∅*i

c) π *∅*i/2

d) Can’t be calculated

### View Answer

8. For a constant emf, if field current is reduced then the speed of the DC motor will_____

a) Remains same

b) Increases

c) Decreases

d) Can’t say

### View Answer

9. For an ideal DC machine, which phenomenon will reduce the terminal voltage?

a) Armature reaction

b) Commutation

c) Armature ohmic losses

d) All will contribute in reducing the terminal voltage

### View Answer

## Set 5

1. No load point of DC generator is __________

a) Intersection of OCC and Rf line

b) Point on the Y axis at rated field current

c) Point on the X axis at rated terminal voltage

d) Can’t find through graphical interpretation

### View Answer

2. Rf line intersects with OCC in ________

a) 1^{st} quadrant

b) 2^{nd} quadrant

c) 3^{rd} quadrant

d) 4^{th} quadrant

### View Answer

3. How armature resistance effect is shown graphically?

a) By adding I_{a}R_{a} product horizontally with R_{f} line

b) By subtracting I_{a}R_{a} product vertically with R_{f} line

c) By adding I_{a}R_{a} product vertically with R_{f} line

d) By subtracting I_{a}R_{a} product horizontally with R_{f} line

### View Answer

_{a}R

_{a}vertically with R

_{f}line at minimum 2 points and draw line parallel to R

_{f}to get line with V + I

_{a}R

_{a}= constant.

4. For determining IaRa maximum _____________

a) Distance between R_{f} line and v+ I_{a}R_{a} line is taken

b) Distance between OCC and v+ I_{a}R_{a} line is taken

c) Maximum Distance between OCC and v+ I_{a}R_{a} line is taken

d) Can’t calculated graphically

### View Answer

_{a}R

_{a}line is taken, to get effective frop in armature.

5. How demagnetization effect of armature reaction is shown graphically?

a) By shifting origin towards +ve y axis

b) By shifting origin towards +ve x axis

c) By shifting origin towards -ve y axis

d) By shifting origin towards -ve x axis

### View Answer

6. External characteristics of DC shunt motor lies in ______________

a) 1^{st} quadrant

b) 2^{nd} quadrant

c) 1^{st} and 2^{nd} quadrant

d) 4^{th} quadrant

### View Answer

^{nd}quadrant as well.

7. From magnetization characteristic at I_{f} = 7.1 A, E_{a} = 225 V at 1000 rpm. What will be the terminal voltage at speed 950 rpm?

a) 225 V

b) 235 V

c) 214 V

d) 220 V

### View Answer

_{a}= 225 V at 1000 rpm E

_{a}(950 rpm) = 225 x 950 / 1000, this will give speed of given DC machine at 950 rpm. Upon calculations we get, E

_{a}= 213.7 V= 214 V approx.

8. For a given compound DC machine, Net field current obtained from characteristic is equal to 7.5 A, where shunt field current is equal to 5 A, armature current is 505 A, demagnetizing current equal to 0.95 A and shunt field winding of 1000 turns at rated speed of 1000 rpm. What will be the series field turns?

a) 7

b) 8

c) 5

d) 9

### View Answer

_{f}+ [N

_{se}/N

_{f}] I

_{a}– I

_{fd}=I

_{f}(net). 5 + 505(N

_{se}/1000) – 0.95 = If (net). Calculating for N

_{se}by substituting If (net), we get N

_{se}= 6.8 that is 7 turns.