Thermal relay is always adopted for phase break protection of three-phase AC induction motor. But in the engineering application, this paper finds that the thermal inertia of main bimetal heat expansion of thermal relay, and the inertia causes of action agencies sending signals will result in phase break protection to lack enough sensitivity and slow response to fault current. It consequently leads to motor misoperation or malprotection, resulting in motor to be burned. However, AC contactor itself lacks phase break protection. This paper uses AC contactor to redesign the line of phase break protection and cleverly uses the under-voltage and zero-voltage protection functions of the AC contactor to make the three-phase motors to realize the transient protection of power shortage phase.