Back to Back Startup


The back to back start-up of synchronous machines is a very suitable procedure to start smoothly a synchronous motor in pump operating mode with the help of a second synchronous generator driven by a turbine. The generator works like a variable frequency voltage supply. Both machines have to be connected through a transmission line. Of course, SIMSEN can be used to simulate a direct asynchronous start-up.


Both machines are excited at standstill with a specified field current depending on the operating mode (generator or motor).

The generator is then accelerated by the mechanical torque of the turbine.

The voltage increases as well as the frequency at the terminals of the generator.

The excited rotor of the motor follows the rotating field and synchronizes after some oscillations due to its initial position relative to the rotor.


The above curves represent the speed of both synchronous machines.

The pump friction torque has also been taken into account.

The mechanical driven torque of the turbine is kept constant.


The 2 last figures represent the air-gap torque of both synchronous machines as well as their field currents.

Depending on the rotor positions and field currents, the back-to-back start-up may fail.