Star-Delta Transition Timer Calculator
Enter inertia, speed and accelerating torque to get the star-to-delta timer and the changeover dwell.
Step-by-step
- Enter valid values to begin.
t = J × ΔN / (9.55 × Taccel). Taccel is the average motor torque in star minus the load torque, not the nameplate torque. Star gives roughly one third of delta torque, so check the load can actually reach the switching speed. Dwell times per frame are typical practice values, not a standard: follow your starter manufacturer’s data.
Star-Delta Transition Timer: Switching at the Right Moment
Core Engineering Principles
A star-delta starter starts the motor on a reduced winding voltage, which is the line voltage divided by √3, and then reconfigures the windings into delta. In star, line current and starting torque both fall to a third. The timer’s job is to hold star long enough for the rotor to reach about 90% of rated speed and then switch. Switch too early and the delta contactor closes on a motor still far below speed, and you get a current spike close to full direct-on-line starting current. Switch too late and you waste time, heat the winding in star at low torque, and risk stalling.
The acceleration time follows from rotational dynamics: t = J × ΔN / (9.55 × Taccel), where J is the total inertia referred to the motor shaft, ΔN is the speed gain and Taccel is the average torque left over after the load’s own torque is subtracted. The 9.55 constant is 60 / 2π. The changeover itself is an open transition: star contactor out, a short dwell, delta contactor in. The dwell has to be long enough for the star arc to clear and short enough that the rotor doesn’t slow noticeably. Typical values run 50 to 100 ms.
taccel = J × ΔN / (9.55 × Taccel) • Ttimer = taccel rounded up
ΔNdip = 9.55 × Tload × tdwell / J
NEC & Standard References
IEC 60947-4-1 covers contactors and motor starters, including the star-delta starter and its utilisation categories, with AC-3 for starting squirrel-cage motors. NEMA ICS 2 is the North American counterpart. NEC 430.52 and 430.32 still govern branch protection and overload protection for the whole starter, and the motor needs six leads brought out. Contactor dwell times and the 90% switching point are typical industry practice rather than a clause in these standards. Verify against your starter manufacturer’s data.1. ΔN = 0.90 × 1470 = 1323 rpm.
2. t = 3.2 × 1323 / (9.55 × 60) = 7.39 s.
3. Timer = rounded up to the next whole second = 8 s.
4. Dwell for a medium contactor = 80 ms.
5. Speed loss in the dwell = 9.55 × 20 × 0.08 / 3.2 = 4.8 rpm, or 0.36% of the switching speed, which is negligible.
- Don’t set the timer by guesswork. A star timer of 3 s on a pump that needs 7 s makes the delta contactor close on a half-speed rotor and trips the breaker.
- Check the star torque. Star gives one third of the direct-on-line torque. If the load torque at standstill exceeds that, the motor will never reach 90%.
- Interlock the contactors. The star and delta contactors closed together is a phase-to-phase short circuit. Use a mechanical interlock as well as the electrical one.
- Respect the open transition. There will always be a short transient on closing. For loads where it matters, use a closed-transition starter or a soft starter.
- Watch the long start. Over about 15 s in star, the winding heats and the thermal relay may trip.