🔗 Autotransformer

Autotransformer design check

First-cut check for an autotransformer: series and common are parts of one winding, with no galvanic isolation. Tender / estimation grade — verify with the manufacturer. Equivalent rating and winding currents, without an impedance split, are on the sizing page.

Method (step-down autotransformer): series voltage = HV−LV, common = LV; series:common turns = (HV−LV):LV. Common-winding current I_c = I_LV − I_HV. Self (transformer) rating = S·(HV−LV)/HV — the autotransformer only processes this fraction of throughput, which is why it is smaller. Short-circuit currents use the assumed %Z; the autotransformer's actual %Z is lower than an equivalent two-winding, and the exact IEC 60076-5 value needs the coupled-winding equivalent circuit — treat the impedance figures as first-cut.