Parallel Operation of DC Generators MCQ || Parallel Operation of DC Generators Questions and Answers

DC Generator, MCQ / April 17, 2022

1. For connecting two generators in parallel, they should have _________

  1. Same current rating
  2. Same ohmic rating
  3. Same voltage rating
  4. All ratings must be same

Answer: 4. All ratings must be same

Explanation:

The following conditions must be satisfied for the paralleling of DC generators:

  • For successful paralleling of DC generators, the voltage of each parallel unit must be equal.
  • The polarities of the generators must be identical.
  • The terminal voltages must be the same.
  • The change of voltage with the change of load must be of the same character.
  • The prime movers driving the armatures of the generators must have similar and stable rotational characteristics.

 

2. Two DC shunt generator is connected in parallel to deliver a load current of 240 A. The ratings of both the generators are being given as 80 kW, 640 V, 8% regulation and 120 kW, 640 V, 4 % regulation. What is the terminal voltage at which the generators are operating?

  1. 592.74 V
  2. 600.25 V
  3. 615.4 V
  4. 621.7 V

Answer: 3. 615.4 V

Explanation: 

For Ist generator,

Drop in voltage at full load = 640 × 0.08

= 51.2 V

Full load current = 80000/240 =125 A

Voltage drop per ampere = 51.2/125 = 0.4096 V/A

For IInd generator, drop in voltage at full load = 640 × 0.04 = 25.6 V

Full load current = (120 × 103)/640 = 187.5 A

Voltage drop per ampere = 25.6/187.5 = 0.13653 V/A

Now, the terminal voltage at which the generators are operating remains same.

Let it be V.

∴ V = 640 – 0.4096 I1 = 640 – 0.13653 I2

I2 = 3I1

Also, I1 + I2 = 240

⇒ 4I1 = 240I1 = 60A

I2 = 180 A

V = 640 – (0.409 × 60)

= 615.424 V

 

3. While connecting two DC generators in parallel, which of the following is not a desirable condition?

  1. Same voltage rating
  2. Same percentage voltage regulation
  3. Same percentage speed regulation of the prime movers
  4. Same current rating

Answer: 4. Same current rating

Explanation: 

Essential and desirable conditions of parallel operation of DC generators are:

  • Same drooping voltage characteristics
  • Same voltage rating
  • Same percentage voltage regulation
  • Same percentage speed regulation of the prime movers

The same current rating is not a desirable condition for connecting two generators in parallel.

 

4. __________ type of DC generator does not need equalisers for satisfactory parallel operation.

  1. Over-compound
  2. Series
  3. Flat-compound
  4. Under-compound

Answer: 4. Under-compound

Explanation: 

For satisfactory parallel operation of dc series generators and over the compound generator, it is necessary to connect the armatures of the two machines through a heavy copper bar, called the equalizing bar.

An equalizer bar has a low resistance conductor wire, which connects together the points in the armature winding which should be at the same potential.

​An under-compound DC generator does not need equalizers for satisfactory parallel operation.

 

5. Two DC generator are connected in parallel to feed a common load. Both have the armature emf of 180 V and 200 V respectively while the armature resistance of 0.4 Ω and 0.6 Ω respectively. What is the value of terminal voltage if the load suddenly gets open-circuited?

  1. 188 V
  2. 198 V
  3. 158 V
  4. 165 V

Answer: 1. 188 V

Explanation: 

Circulating current, ICR = (200 − 180)/(0.6 + 0.4)

ICR = 20 A

The current is flowing from 200 V to 180 V generator

∴ Terminal voltage = 180 + (20 × 0.4) = 188 V

 

6. We connect two generators in parallel for ______

  1. Large DC load
  2. Small DC load
  3. Any DC load
  4. Any AC or DC load

Answer: 3. Large DC load

Explanation: 

We connect two generators in parallel for Large DC Load. There are several reasons for operating generators in parallel. The number of generators used may be selected in accordance with the load demand. By operating each generator as nearly as possible to its rated capacity, maximum efficiency is achieved.

 

7. Which of the following generator will be preferred if they are required to be run in parallel?

  1. Series generators
  2. Shunt generators
  3. Shunt and series generators
  4. Compound generators

Answer: 2. Shunt generators

Explanation: 

  • For stable parallel operation, the most suitable type of DC generator is a shunt generator as it has slightly drooping characteristics.
  • If there is any tendency for a generator to supply more or less than its proper share of load it changes system voltage which certainly opposes this tendency.
  • This restores the original division of load. Thus the shunt generators automatically remain in parallel, once they are paralleled.

 

8. Two 250 V dc shunt generators are connected in parallel to feed a common load. Both are having linear external characteristics. Generator 1 and Generator 2 have no load terminal voltages of 300 V and 320 V respectively. Also, generators 1 & 2 supply 50 A and 60 A respectively at a terminal voltage of 250 V respectively. In parallel connection when the total current is 90 A, what is the output current of generator 1 in Ampere?

  1. 45.48 A
  2. 35.55 A
  3. 39.24 A
  4. 29.36 A

Answer: 3. 39.24 A

Explanation: 

For generator 1, voltage drop for

50 A = 300 – 250 = 50 V

∴ Voltage drop /ampere for generator

50/50 = 1 V/A

for generator 2, voltage drop for 60A

= 320 – 250 =70 V

∴ Voltage drop / ampere for generator 2

= 70/60 = 1.167 V/A

Now, if the total load current is 90A,  the current supply share of generator 1.

Let I1 & I2 he the current supplied by generators 1 & 2 respectively

we have I1+ I2 = 90

Also, terminal voltage is same in parallel connections

∴ V = 300 – I1 = 320 – 1.167 I2

1.167 I2 – I1 = 20

∴ 2.167 I2 = 110

I2 = 50.76 A

I1 = 39.24 A

 

9. Load sharing of two generators connected in parallel is determined by __________

  1. Internal characteristics
  2. External characteristics
  3. Both internal and external characteristics
  4. It doesn’t depend on load sharing

Answer:2. External characteristics

Explanation: 

Load sharing of two generators connected in parallel is determined by external characteristics. The external characteristics of the two generators with their field currents are adjusted for the no-load voltage.

The summation of current carried by both generators is equal to the load current.

I1 + I2 = IL

The adjustments of the field currents modify the external characteristic changing the load sharing.

 

10. Two 300 V dc generators are operating in parallel. Generator 1 has a no-load terminal voltage of 360 V and supplies 50 A at 300 V. generator 2 has a no-load terminal voltage of 375 V and supplies 60 A at 300 V. if both generators are having linear external characteristics, what is the output current of the generator 2 in amperes when they are is parallel supplying a total current of 90 A?

  1. 50.2 A
  2. 30.3 A
  3. 45.5 A
  4. 40. 6 A

Answer: 1. 50.2 A

Explanation: 

For generator 1, voltage drop for 50 A = 360 – 300 = 60 V

Voltage drop/ampere  = 60/50 = 1.2 V/A

For generator 2, voltages drop for 60 A = 375 – 300 = 75 V

Voltage drop per ampere = 75/60 = 1.25 V/A

When supplying ta a load of 90 A,

Their currents be I1 & I2 respectively

I1 + I2 = 90

Also V = 360 – 1.2 I1 = 375 – 1.25 I2

1.25 I2 – 1.2 I1 = 15

1.2 I2 + 1.2 I1 = 108

2.45 I2 = 123

I2 = 50.2 A

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