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

Answer: 3. Both 1 and 2

Explanation: 

The following two main requirements are to be met while connecting shunt generators in parallel.

(i) The positive terminals of all the machines must be connected to the positive bus bar and the negative terminals must be connected to the negative bus bar.

(ii) The terminal voltages of the machines being paralleled must be the same.

Answer: 4. All of the above

Explanation: 

In order to disconnect one of the two shunt generators operating in parallel, the following procedure is adopted

• The load of the generator to be disconnected is shifted over to the other by gradually reducing its field current and increasing the field current of the other generator.
• When the current output of the machine to be disconnected is reduced almost to zero, the main switch is opened.

Answer: 3. Starting from the same no-load point and above generator 1 and generator 2

Explanation: 

For a parallel operation of 2 DC shunt generators, we get net external characteristics from starting the same no-load point and above generator 1 and generator

The load sharing by parallel generators is determined by the addition of external characteristics of both generators. At no load, generators will have common point, on the y-axis so parallel characteristics will start from the same point and will lie above both generator 1 and generator 2.

Answer: 1. Small, Large

Explanation: 

In a power station, we have several small shunt generators in parallel instead of a single large shunt generator. This is due to the following reasons

1. The load supplied by a generating station is usually variable. As machines operate at the greatest efficiency at or near full load, it is possible with several machines to keep each machine either fully loaded or shut down entirely.
2. A generator that breaks down may be removed from the circuit for repairs without interrupting the power supply.
3. Generators may be added to the system as the power demand increases.

Answer: 4. Generator 2 will start running as motor

Explanation: 

• When two compound generators run in parallel, Because of the rising characteristics of the compounded generators, and in the absence of any corrective devices, the parallel operation of such generators is unstable.
• Let us suppose that, to begin with, each generator is taking its proper share of the load.
• Let us now assume that for some reason, generator No. 1 takes a slightly increased load. In that case, the current passing through its series winding increases which further strengthens its field and so raises its generated e.m.f. thus causing it to take still more load.
• Since the system load is assumed to be constant, generator No. 2 will drop some of its load, thereby weakening its series field which will result in its further dropping off its load. Since this effect is cumulative. Generator No. 1 will, therefore, tend to take the entire load and finally drive generator No. 2 as a motor.
• The circuit breaker of at least one of the two generators will open, thus stopping their parallel operation.

Answer: 4. All of the above

Explanation: 

The following reasons are given for parallefing of DC generator.

Reliability: The generators that are sources of power are primary safety items, and hence paralleled for reliability.

Continuity of service: Continuity of power supply is most important to satisfy the customers. If a single generator is used, it is difficult to give supply during its breakdown.

Efficiency: The load is not uniform. It is maximum during the day and minimum during the night. The generators must operate most efficiently during the full-load conditions. It is preferred to use a single generator during light load.

Maintenance and Repair: Inspection of generators is carried out frequently to check any possibility of future breakdown. This is possible only when the generator is at rest and other generators take care of the load. During the breakdown stage of a generator, it is repaired and other generators take over and meet the load requirements.

Answer: 4. Equalizer Ring

Explanation: 

For making the parallel operation of over-compound and level-compound generators stable, they are always used with an equalizer bar connected to the armature ends of the series coils of the generators.

• The equalizer bar is a conductor of low resistance and its operation is as follows:
• Suppose that generator No. 1 starts taking more than its proper share of the load. Its series field current is increased.
• But now this increased current passes partly through the series field coil of generator No. 1 and partly it flows via the equalizer bar through the series field winding of generator No. 2.
• Hence, the generators are affected in a similar manner with the result that generator No. 1 cannot take the entire load.

Answer: 1. Two armatures

Explanation: 

For making the parallel operation of over-compound and level-compound generators stable, they are always used with an equalizer bar connected to the armature ends of the series coils of the generators.

• The equalizer bar is a conductor of low resistance and its operation is as follows:
• Suppose that generator No. 1 starts taking more than its proper share of the load. Its series field current is increased.
• But now this increased current passes partly through the series field coil of generator No. 1 and partly it flows via the equalizer bar through the series field winding of generator No. 2.
• Hence, the generators are affected in a similar manner with the result that generator No. 1 cannot take the entire load.

Answer:2. Traction Purpose

Explanation: 

• DC Series generators are not usually operated in parallel. They are put in parallel for electric traction purposes to provide electric breaking of motors.
• If the load current shared by machine 1 is increased. the induced emf will also increase, which will lead machine 1 to share the greater load current and relieves the other machine.
• This causes the emf of machine 1 to increase and the emf of machine 2 to decrease continuously.
• When the emf of machine 2 is less than the terminal voltage. it receives current and its polarity gets reversed, which makes the EMFs of the two machines act in series around the load circuit of the generators having a very low resistance virtually acting as a short circuit on the two machines.

Answer:1. More than the normal speed

Explanation: 

When two DC series motors are connected in parallel then the resultant speed is more than the normal speed. If the voltage across each motor is V and the current through each motor in parallel connection is I/2.

Speed ∝ V/I

Hence when the current decreases the speed increases. So the resultant speed is more than the normal speed.