ABSTRACT
This chapter deals with the supply voltage, that applies to a stationary induction motor by developing a full-load torque and draws a starting current to ten times its rated load current. This initial 'starting' or 'inrush' current falls as the machine accelerates towards the synchronous speed. Full-voltage starter or direct-on-line is applied to motors rated up to 4kW because beyond this rating the resulting high inrush current may overheat the machine's field windings. The main component of a direct-on-line starter system is a heavy-duty electromagnetically operated switch, called contactor. The chapter discusses the methods used to limit the starting current in order to avoid overheating the stator windings. One of them is the 'star-delta' starter that normally operates with the motor running with its field windings connected in delta. An important feature in all motor-control circuits is the 'latching' or 'hold-in' feature, which keeps the contactor's operating coil energised after the 'start' pushbutton switch is released.
