Split-Winding Starting of 3-Phase Motors

In this paper there is presented a new method of normal voltage reduced current starting for induction motors. The scheme consists in connecting one phase of a 3-phase winding in series during the starting period only, the other phases having circuits in parallel. When full speed is attained, the series connected winding is opened momentarily, and then put back on the line in parallel. This gives the advantages of reduced current without any current surge on reclosing, no external impedance, and a low cost of control. It provides about half torque with two-thirds average current, as compared to full voltage starting; it gives a smooth torque curve on the Y connection, and only a small one-third speed torque dip on delta connection; and it allows about 50 per cent greater stalled time than on full voltage. Although this type of connection will draw unbalanced line currents, this appears unimportant. In most cases there will be other machines on the line to act as phase balancers and absorb the relatively small negative sequence current. Formulas are given for currents and torque with and without external impedances. Formulas are in terms of voltage and forward, backward, and zero phase sequence impedances.