Star-Delta starter

The sizing rules: main and delta contactors both carry winding current, 57.7 percent of the line figure, so they are sized at 0.58 times full-load current. The star contactor conducts only during starting and is conventionally sized at 33 percent. The overload relay setting here assumes it sits in the windings, where it reads phase current of 0.58 times nameplate; a relay ahead of the starter reads line current and needs the full nameplate figure. In star, each winding sees V over √3, so starting current and starting torque are exactly one third of the direct-on-line values. Star-delta suits loads that start light; a compressor under load or a conveyor under belt tension may never reach changeover speed in star.