Protection arrangement for a brushless synchronous machine

In a protection arrangement for protecting a brushless synchronous machine against overvoltages at a field winding which is connected to a rotating excitation rectifier fed from an exciter machine which includes a solid state switching element for short circuiting the field winding if overvoltages occur, the improvement comprising:

• a. means for forming a first input voltage proportional, by a first proportionality constant, to the primary voltage of the exciter machine;
• b. means for forming a second input voltage proportional, by a second proportionality constant, to the primary current of the exciter machine, the two proportionality constants being chosen so that the absolute value of the second input voltage in normal operation of the synchronous machine is lower and, if the field winding is short circuited, is higher than the first input voltage;
• and
• c. voltage comparison means having said first and second voltages as input and providing at its output an indicating signal.