Fluid coupling is a kind of non-rigid coupling using fluid as its working medium, the pump wheel and turbine of fluid coupling which are installed on the input or output shaft with radical blades form a sealed working chamber, allowing fluid circulation inside. When electric motor runs it drives the case and pump wheel of fluid coupling together, then the fluid oil inside the blades of pump wheel is thrown out by the centrifugal wheel. This high speed fluid enters turbine to push it to spin with increasing speed and power. At last the fluid returns to the pump wheel before repeating the circulation again and again. The input and output shaft of fluid coupling which has no rigid connection between working pieces connect to each other with fluid. If stall occurs due to excessive loading, the input shaft still can run to protect the engine from damaging. When loading reduces the output shaft will speed up until matches the speed of the input shaft, resulting in almost zero transmission torque.