Part-winding starting is a reduced-inrush starting method that energizes only part of a specially wound motor's stator windings first, then connects the remaining part after a short delay, so the motor draws less current at the initial instant than it would if all windings were energized at once. It works only on motors built with two parallel winding sections that are brought out to separate terminals, and it reduces inrush without any external transformer, resistor, or winding reconfiguration, just a second contactor and a timer. Because it energizes half the copper first, it also develops reduced torque during the first step, which is its main limitation. This page explains which motors support it, how the two-step sequence works, and the time-delay parameter that automation sets.
Part-Winding Starting in one line: Part-winding starting energizes one half of a dual-winding motor's stator windings first, drawing reduced inrush, then closes a second contactor after a short time delay to bring in the remaining winding for full running. It requires a motor specifically wound with two parallel sections brought out to separate leads, needs no transformer or resistor, and uses only a second contactor plus a timer. It reduces starting current at the cost of reduced starting torque during the first step, so it suits loads that start easily.
A part-winding motor is built with its stator winding split into two parallel halves, each brought out to its own set of terminals in the motor's connection box. In a part-winding start, the first contactor closes and energizes only one half of the winding. With half the copper carrying the load, the initial inrush the motor draws is lower than if the whole winding were energized at once, so the starting current step is reduced. The motor begins to accelerate on that single winding half.
After a short, fixed delay, a second contactor closes and connects the remaining winding half in parallel with the first, so the full winding is now energized and the motor runs on its complete stator as a normal motor. The whole sequence is just two steps, first half then both, separated by a brief timed interval. Because the two winding halves are simply paralleled, there is no reconfiguration of the connection as in wye-delta and no external component in the circuit as in autotransformer or resistor starting; the reduced inrush comes purely from energizing less copper at the start.
The method is inherently a closed transition in the sense that the first winding is never disconnected when the second comes in; the second is simply added alongside it. There is no open gap and no out-of-phase reconnection of the running winding, though bringing in the second winding does produce its own smaller current step as the additional half energizes. The simplicity of the scheme, one extra contactor and one timer with no transformer or resistor bank, is a large part of why part-winding starting has long been popular where it is applicable.
Part-winding starting is only possible on a motor specifically designed and wound for it, with two parallel winding sections and enough leads brought out to energize them separately. Many dual-voltage motors, and motors wound with six or nine leads in the appropriate configuration, are suitable, but the motor nameplate or manufacturer data must confirm that part-winding start is supported and specify how the leads are to be connected. Wiring a motor for part-winding start when it is not designed for it, or connecting the wrong leads, can leave one winding carrying unbalanced current and overheat, so the connection is not something to improvise; it follows the manufacturer's stated arrangement.
The core limitation is torque. Starting on half the winding develops considerably reduced starting torque compared with energizing the whole winding, so part-winding starting is suited to loads that begin turning easily and do not demand high breakaway torque, such as lightly loaded pumps and fans. A load that needs strong torque to break away may not accelerate at all on the first winding half, stalling until the second winding comes in, which defeats the gentle start. As with the other reduced-voltage methods, the load characteristic dictates whether the method is viable.
There is also a thermal consideration during the first step. Because one winding half is doing all the work while the motor draws heavy starting current, that half heats quickly, so the first step is meant to be brief. This ties the method's viability to keeping the delay short enough that the single energized winding is not overstressed, which in turn depends on the motor being able to accelerate meaningfully on half its winding within that short window. Where it works, part-winding start is a cheap and simple way to knock the peak off the inrush; where the load is too heavy for the first step, another method is chosen.
The one setting that automation controls in a part-winding start is the delay between energizing the first winding and closing the second contactor. It is a short interval, chosen so the motor has begun to accelerate on the first winding but the single energized half is not left carrying heavy starting current for long. Set the delay too long and the first winding overheats and the motor may stall at reduced torque; set it too short and the second winding comes in while the motor is still drawing its full initial inrush, reducing the benefit of the staged start. The timer that governs this interval is the parameter a PLC or dedicated starter module sets.
The current signature a controller logs is a two-step profile distinct from the other starting methods. Current rises at the first step as the initial winding energizes, then there is a second, smaller step as the remaining winding comes in and the motor completes its acceleration on the full stator, after which the current settles to running level. This staged, additive pattern differs from the reduced-then-full plateau of an autotransformer start, the reconfiguration step of a wye-delta start, and the smooth ramp of a soft start, which makes the starting method identifiable from the current trace alone.
For remote and unmanned sites, a cloud SCADA system that captures the start current profile lets an operator confirm the two-step sequence is happening on schedule and that the second contactor is pulling in on time. A start where the second step is missing points to a failed second contactor, leaving the motor running on half its winding and overheating, which is exactly the kind of fault that would otherwise go unnoticed at a site nobody attends until the motor trips. Trending successive starts across a fleet also flags a motor whose acceleration on the first winding is drifting, an early sign the driven load has grown heavier than the part-winding start was set up for.
No. Part-winding starting only works on a motor specifically wound with two parallel winding sections brought out to separate leads, such as certain dual-voltage or six- and nine-lead motors that the manufacturer confirms support it. The correct lead connection must follow the manufacturer's stated arrangement, because wiring a non-suitable motor for part-winding start, or connecting the wrong leads, can leave a winding carrying unbalanced current and overheat.
Part-winding energizes one half of the stator winding first, then adds the second half in parallel after a delay, with no reconfiguration of the connection. Wye-delta reconnects the whole winding from a wye configuration to a delta configuration during the changeover. Part-winding needs a motor built with two parallel winding sections, while wye-delta needs a six-lead motor whose winding connection can be changed. Part-winding is inherently additive and closed, whereas open-transition wye-delta briefly disconnects the motor at the switch.
During the first step only half the stator winding is energized, so the motor develops considerably less torque than it would with the whole winding carrying current. That is why part-winding starting suits loads that begin turning easily, such as lightly loaded pumps and fans. A load needing high breakaway torque may not accelerate at all on the first winding half and could stall until the second winding comes in, defeating the purpose of the staged start.
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