Question 10.7: Find the ratio of starting to full load current for 10 kW in...

Input power, P = 10  kW = 1000  W

Supply voltage (line value), V_{L} = 415  V

Power factor, \cos \phi = 0.8

Short circuit current at 210  V = 40  A

Short-circuit current at 415 V, I_{sc} = \frac{415 \times 40}{210} = 79  A

Full load current, I_{fl} = \frac{P}{\sqrt{3} V_{L} \cos \phi } = \frac{10000}{\sqrt{3} \times 415 \times 0.8} = 17.39  A

In case of star-delta starter;

Starting current, I_{st} = \frac{I_{sc}}{3} = \frac{79}{3} = 26.33  A

Ratio of starting to full load current, \frac{I_{st}}{I_{fl}} = \frac{26.33}{17.39} = \mathbf{1.514}