PURPOSE: To prevent operational fault such as burning of field winding by detecting the field current and opening the field winding circuit mounted on an AC exciter if thus detected field current is higher than a predetermined limit value.

CONSTITUTION: Rated field current of an AC exciter 5 is added with an allowable overload factor to obtain a limit value of field current which is then set in an overexcitation relay 17. The limit value of field current is then subjected to current/voltage conversion through a field current shunt 16 for a field winding 6 and amplified. When a detected value exceeds over the limit value, the overexcitation relay 17 functions to open the circuit of the field winding 6 thus actuating an alarm or stop protective circuit(not shown) and protecting the field winding 3 of an AC generator 2. Consequently, an inexpensive protective unit can be obtained and mounted easily, and even an unskilled operator can operate the generator with no operational burden.