This set of DC Machines Questions and Answers for Freshers focuses on “EMF and Torque Production – 2”.

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

Explanation: Flux is defined as flux density for a given surface area. Here, Surface area can be calculated and multiplied with B to give the value of flux. Flux= 2πr*l*B. now, for calculating flux per pole, divide it by P=4. So, Flux per pole after substituting all values is equal to 0.0188 Wb.

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

Explanation: Torque produced in a DC machine is directly proportional to number of coil turns, Flux per pole, number of poles and armature current. Thus, ratio T1/T2= 20/40 (assuming all other parameters same for both machines).

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

Explanation: For A DC machine when all other parameters are fixed average coil emf generated is proportional directly to the speed of the dc motor. Voltage regulation is defined as ratio of difference of no load voltage and full load voltage to the full load voltage. VR= (1300-1100/1100)*100%.

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

Explanation: Induced emf in a DC machine is equal to,

From the emf equation we get speed of the shaft i.e. n α E/Z, when all other parameters are kept constant. So, when E is doubled n becomes twice the original, halving flux on reduced emf will quadrupled the speed of a DC machine.

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

Explanation: DC Machine torque equation: T = ka*∅*Ia. Here, ka= ZP/(2πA), Z= total armature conductors, P= No. of poles, A= No. of parallel paths. For a wave winding A=2. So, substituting all the values in the torque equation we get torque equal to 34.37 N-m.

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

Explanation: In a construction of a DC machine poles are located in magnetically neutral region. The magnetic field at the pole terminals in a DC machine will be equal to 0. Thus, cross product with the current flowing through the armature yields zero.

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

Explanation: When the field current is reduced, the field produced by the field winding also reduces. Thus, the term Φ from the emf equation also decreases. For all other parameters kept constant speed of the DC machine is inversely proportional to field. Hence, speed of DC motor will increase.

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

Explanation: In an ideal case, Commutation does not reduce the terminal voltage of a dc machine. In a non-ideal case, commutation takes place improperly at desired timings, thus losses contribute to the terminal voltage reduction. Armature reaction, ohmic losses due to winding resistance contribute to the losses in the terminal voltage.

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