Series Booster - Construction, Diagram & Working Principle

Construction And Working: 

When a series generator is operated at low flux densities so that its terminal voltage rises in direct proportion to the load current (E α Φ, where speed N is a constant) and is used to compensate the line drop in a transmission system it is called a series booster.

The wiring diagram of a series booster is shown in fig. 

The series booster is driven by a shunt motor connected to the bus bars. 

Since the booster generator is to compensate the line drop only, its output, therefore, need not be as much as that of the load connected to the feeder in spite of the fact that it has to carry the feeder total current.

In operation, such a motor-generator set must be provided protection against the possibility of getting the driving motor accidentally disconnected from the main busbars. 

Should this happen, the the series generator would immediately start working as a series motor without load, reversing its direction of rotation and attempt to race. 

How it happens are explained below.

When the machine is working as a generator, booster voltage Ebb adds to V₁, to give V₂; brush A is then positive and brush B is negative. 

However, when the booster acts as a motor because of losing its motive power, brush B becomes positive and brush A negative. 

Finally, since the field current has not changed direction, the booster rotates in reversed direction and tends to run away as an unloaded series motor.