Star Delta Starter Calculator
See how much a star delta starter reduces starting current and torque compared with direct on line starting, and the currents for sizing contactors and the overload relay.
- DOL current = 27 × 7 = 189 A
- Star connection gives 1/√3 voltage per winding, so line current and torque fall to 1/3: 63 A
- In delta each winding carries FLC ÷ √3 = 15.59 A
Star Delta Starting
Tstart,star = Tstart,DOL ÷ 3
Iwinding = FLC ÷ √3
Worked example
A motor with 27 A FLC and 7 × locked rotor current draws 189 A DOL but only 63 A in star. Starting torque falls from 200 % to 67 % of rated.
Frequently Asked Questions
Why does star delta reduce starting current to one third?
Each winding gets 1/√3 of the line voltage, so its current falls by √3; and line current in star equals winding current instead of √3 times it. Together that gives one third.
What should the overload relay be set to in a star delta starter?
When it sits in the winding circuit (after the main contactor), set it to FLC ÷ √3, about 58 % of the nameplate current.
When should star delta not be used?
For loads that need high starting torque, such as loaded conveyors or positive displacement compressors. A soft starter or VFD is better there.
Learn more on the blog
Motor starting methods compared
A practical, field focused guide on instrumentationblog.in that explains the theory behind this calculator and how engineers apply it on real plants.
Read Full Article