Question

A 250 V, self excited shunt motor has an armature resistance of $0.5\Omega$ and field resistance of $200\Omega$. At no load and rated voltage the speed is 1200 rpm and armature current is 3 A. At full load and rated voltage, the line current is 47 A and because of armature reaction, the flux is 4% less than its no load value. The full load speed is

• A. 1080 rpm
• B. 1142 rpm
• C. 1314 rpm
• D. 996 rpm

Answer 1

The correct option is B 1142 rpm

$\text{At no load,}{I}_f=\frac{250}{200}=1.25A$

$I_{a0}=3A$

$E_{a0}=250-3\times 0.5=248.5V$

$\text{At full load,}{\varphi }_{fl}=0.96{\varphi }_0$

$I_{a,fl}=47-1.25=45.75$

$E_{a,fl}=250-45.75\times 0.5=227.1V$

$N_{fl}=\left(1200\right)\times \left(\frac{227.1}{248.5}\right)\times \left(\frac{1}{0.96}\right)$

$=1142rpm$