Lesson 15 · Motor Control · Advanced

Reduced-Voltage Starting

Big motors started straight across the line draw enormous inrush. Reduced-voltage starting brings them up gently, then switches to full power.

Why start at reduced voltage?

A large motor started straight across the line draws a huge inrush current — often six to eight times its running current — for the first moment. On a big motor that surge can dim lights, trip breakers, stress the power system, and snap couplings with sudden torque. Reduced-voltage starting brings the motor up gently, then switches to full power once it is moving.

The common methods

Wye-Delta (Star-Delta): start the motor with its windings connected in wye (which puts less voltage across each winding, reducing inrush), then after it spins up, switch the windings to delta for full-voltage running. Uses three contactors and a timer.
Part-Winding: energize only part of the motor’s windings to start, then bring in the rest. Simpler, cheaper, used on motors built for it.
Autotransformer: feed the motor through taps of an autotransformer at reduced voltage to start, then bypass it for full voltage. Good for high inertia loads.
Soft starter (solid-state): the modern way — electronics ramp the voltage up smoothly. Fewer contactors, adjustable, but it is electronics rather than pure relay logic.

The control idea (wye-delta example)

The control circuit is a timed sequence: press Start → the wye contactor and the main contactor pull in, motor starts softly in wye. A timer runs. When it times out, the control drops the wye contactor and pulls in the delta contactor, switching the motor to full-voltage delta running. Interlocking is critical here — the wye and delta contactors must never be closed at the same time, so they are interlocked exactly like a forward/reverse pair.

It is everything you have learned, sequenced by a timer: contactors doing the switching, interlocks keeping them apart, and a timed transition from start mode to run mode. The new ingredient is the timer relay that drives the changeover.

When you use it

Reduced-voltage starting shows up on large motors — big pumps, fans, compressors, crushers — especially on weaker supplies where a full-voltage start would cause problems, or on driven equipment that cannot take the mechanical shock of an across-the-line start.

What to take away

Reduced-voltage starting limits inrush by starting the motor at lower voltage, then switching to full voltage once it is up to speed. Wye-delta, part-winding, autotransformer, and soft starters are the common methods. In relay form it is contactors + interlocks + a timer driving the transition.

Note: this is a concept lesson. Reduced-voltage starting needs timed contactor switching that the current Sandbox doesn't model yet — so there's no build exercise for it. Focus on understanding the why and the sequence; the building blocks (contactors, interlocks) are ones you've already practiced.