A star-connected 12.5 kW, 208 V line, 3-phase, 60 Hz squirrel-cage motor has per-phase stator-referred parameters R1=R2=0.3 Ω and X1=X2=0.41 Ω. Neglect the shunt branch. Find starting current in amperes on an 80 V line, 20 Hz supply.
At starting, slip is one, so the series resistance is 0.3+0.3=0.6 Ω. Reactance scales with frequency: total X at 20 Hz is $(0.41+0.41)(20/60)=0.273333$ Ω.
The star phase voltage is 80/√3 V and line current equals phase current. Thus $$I_{start}=\frac{80/\sqrt3}{\sqrt{0.6^2+0.273333^2}}.$$ This gives **70.06 A**, in the 69–71 A range.
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