Parallel Operation of DC Series Generators

DC series generators are not usually operated in parallel. DC series generators are put in parallel for electric traction purposes to provide electric braking of motors in electric trains, cars etc. Below figure shows the Equalizer connection and Cross connection of two series generators put in parallel operations under normal working condition. In parallel operation of series generator, if the load current shared by generator 1 is increased, the induced emf will also increase, which will lead generator 1 to share the greater load current and it relieves the generator 2. This causes the electromotive force of generator 1 to increase and the electromotive force of generator 2 to decrease continuously. When the electromotive force of the generator 2 is less than the terminal voltage, it receives current and its polarity gets reversed, which makes the electromotive forces of two generators to act in series around the load circuit of the generators having a very low resistance virtually acting as a short circuit on the two generators.

Equalizer connection

If the field coils are connected in parallel using equalizing bus bars having negligible resistance, then this will make the current in the field windings of two generators equal. The increase in the current of generator 1 leads to an increase in current of generator 2, hence, the emf of generator 2 also increases preventing generator 1 to share more load. There is no tendency to relieve generator 2 from sharing its load.

Cross connection

If the field coils are cross-connected when the series motors are connected in parallel, then any increase in the current of the armature of generator 1, the increased current flows occurs through the field coil of generator 2. This increases the electromotive force of generator, which opposes the change in load sharing trying to stabilize the operation of the two generators at the original operating condition itself. Thus it will give more positive and better operation than equalizer connection.