A Transient Analysis of Field Aligned Starting at Mains Connection

Field aligned starting (FAS) is a technique for starting 3-phase cage induction motors on single-phase supply lines under varying load conditions with minimal inrush currents. The technique starts the motor independent of the mains supply with a high positive transient torque before connecting the spinning motor to the mains supply to operate in a standard Steinmetz configuration. It is imperative to the success of an FAS start-up that the spinning motor is connected to the mains at the correct moment. Incorrectly timed mains connection leads to the generation of transient torques of such a severity as to slow, or possibly stall the already started motor with very high inrush currents. This paper analyses key aspects related to the timing of the transition to mains supply, through the use of experimental results recorded under various starting conditions, and simulations based on a dynamic model of the FAS system. A simple yet practical method, which ensures that the timing of the mains connection is correct, is also described.