Loss-of-field (LOF) phenomenon in synchronous generators may cause equipment damage and voltage drop in the power system, which can be terminated to a blackout. Conventional schemes, which detect LOF on the basis of the measured impedance from the generator terminal viewpoint, have some major drawbacks such as low speed, complicated setting procedure, and unsecure performance in the face of similar phenomena, e. g., stable power swing. In this paper, different electrical quantities of the generator including terminal voltage (V), current (I), active (P) and reactive power (Q), and power angle (Omega) variations are studied to achieve a secure technique for LOF protection in synchronous generators. Comprehensive studies show that a simple criterion based on polarities of V, Q, and d variations is accessible to achieve this goal, which does not need a complicated procedure to set it. To evaluate the performance of the proposed algorithm, some cases are simulated on sample power systems under various operating conditions. Also, superiority of the proposed algorithm in comparison with the other schemes is proven by the obtained results, as well. Moreover, the proposed technique is experimentally tested by using some 2-kVA laboratory generators to detect LOF.