Part-winding or "step-starting" of polyphase induction motors for holding down the initial starting current inrush is on the increase. The conventional scheme provides two half-current starting contactors, each with three poles. The motors have dual-voltage 9-lead windings, arranged in two independent Y's, separately connected to the two contactors. The two contactors are closed in sequence with a few seconds' delay between, giving about 60 per cent (%) current on the first step. The delay must be very short, because the accelerating torque on half-winding is usually low, the motor may be noisy, and the rate of heating is high.
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Alger et al. (1955) studied this question.
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