Star–delta changeover: timer setting and interlock

The changeover, step by step

1. START: MC and SC pick up. The motor runs in star and the timer T starts counting.
2. T times out: its delayed NC contact 55–56 opens and SC drops out. Its delayed NO contact 67–68 closes only 50 ms later. Star–delta timer relays have this dead time built in: it lets the arc on SC’s opening contacts go out before delta closes.
3. DC picks up only when 67–68 has closed and SC is fully open, because SC’s NC contact sits in DC’s coil circuit. For well under a tenth of a second the motor is connected to nothing and coasts. Then it is in delta.
4. DC holds itself in with its own NO contact and drops out T with its NC contact. The start is finished.

The open transition

That gap is called an open transition. While the supply is off, the rotor field decays and drifts out of step with the supply. When delta closes, the difference causes a second surge. The faster the motor turns at that moment, the smaller the surge.

The simulator’s charts show only the slip current of that second surge. The short transient from the residual rotor voltage is left out, so real changeover peaks can be higher.

Setting the timer

The timer decides at what speed the motor changes over. A common rule is to change over at 80–90 % of synchronous speed, or once the star current has fallen back to about full-load current. On this pump the simulator asks for at least 86 %.

Too short, and the motor is still slow when delta closes. The second surge then comes close to a direct-on-line start, and the point of star–delta is lost.

Too long gains nothing. The motor has already reached the speed where star torque equals the pump load, and it carries on at a third of its torque with extra slip and extra heat in the windings.

The simulator charts the changeover surge against the timer setting for this pump with all four hydrants open, the heaviest load and the one its starter check uses.

SC and DC must never be closed together

SC joins W2, U2 and V2 into the star point. DC connects the same three terminals to L1, L2 and L3. With both closed at once, the three phases are tied together through the star bar: a bolted short circuit inside the starter.

The fault current runs to several kA, far above the MCCB’s instantaneous release, so the MCCB trips at once. Even so, the arc may already have welded the main contacts and damaged the starter.

Three things keep them apart: an NC auxiliary contact of each contactor in the other’s coil circuit (the interlock in rungs 3 and 4 of the control circuit), a mechanical interlock between the two contactors on many starters, and the dead time at the changeover.

Mirror contacts: a starter stuck in star

On most modern contactors the NC auxiliary contacts are mirror contacts (IEC 60947-4-1, Annex F): if the main contacts weld, the NC contact stays open. So a welded SC keeps DC out, and the motor simply stays in star and never changes over. A starter stuck in star is the clue. Without that NC contact in DC’s coil circuit, DC would close onto the welded star point.

Check yourself