Question

A 220 V, dc shunt motor takes no load armature current of 3 A and runs at 1500 rpm. If the full load armature current is 45 A then what is the speed, assuming that flux remains constant and armature resistance is 0.5 $\Omega$?

• A. 1356 rpm
• B. 1326 rpm
• C. 1345 rpm
• D. 1659.5 rpm

Answer 1

The correct option is A 1356 rpm

$\text{At no load;}$
$\text{Back emf,}{E}_{b0}=V_t-I_{a0}\left(R_a\right)$
$E_{b0}=220-3\left(0.5\right)$
$E_{b0}=218.5V$
$\text{At full load;}$
$\text{Back emf,}{E}_{b,fl}=V_t-I_{afl}\left(R_a\right)\text{]}$
$=220-45\left(0.5\right)$
$E_{b,fl}=197.5V$
$\text{As flux is given constant;}$
$\text{then, we can write;}$
$E_b\propto N$

or, $\frac{E_{b,fl}}{E_{b0}}=\frac{N_{fl}}{N_0}$

$N_{fl}=\left(\frac{197.5}{218.5}\right)\times 1500$ $=1355.83\approx 1356rpm$