A real-time realistic framework for synchronous generator loss of field (LOF) protection studies is suggested in this paper by using the real-time digital simulator, which allows the user to realistically model the generator field supply circuit. By using such framework, the performance of LOF protection algorithms can be comprehensively evaluated in the face of the various LOF events including the short circuit and two types of the open circuit events, while the open circuit occurrence cannot be studied in the conventional simulation programs. The obtained results demonstrate that the generator dynamics are significantly different in the face of various LOF failures and then all of them must be regarded to develop the new LOF protection algorithms. By using the suggested framework, the performance of some recently proposed schemes along with the conventional impedance and the admittance relays are comprehensively evaluated. The obtained results from applying the various LOF failures show that, against the conventional relays, the new schemes cannot detect all types of LOF failures and so, they do not have enough reliability.