Question

A 3-phase squirrel cage induction motor has starting current 6 times the full load current and full load slip of 5%. If an autotransformer is used for reduced voltage starting to provide 1.5 p.u starting torque, the auto transformer ratio will be

• A. 0.9128
• B. 0.7825
• C. 0.5
• D. 0.625

Answer 1

The correct option is A 0.9128

$\text{Starting torque,}$ $T_{st}=1.5T_{fl}$

$\text{Now,}$ $\frac{T_{st}}{T_{fl}}=x^2{\left(\frac{T_{st}}{T_{fl}}\right)}^2\times s_{fl}$

$1.5=x^2\times 6^2\times 0.05$

$\Rightarrow x^2=0.833$

$\text{Auto-transformer ratio,}$ $x=0.9128$