Question

a 230 v dc series motor takes 12a and runs 800rpm.at what speed it will run ,when 10 ohm resistance connected in series witt the armature the motor taking same current at same supply voltage?take ra and rse of motor as 0.5 ohm each

Answer 1

Solution;

Given;

V₁=230v

R₁=r_a+r_se=0.5+0.5=1ohm

N₁=800rpm

I₁=12A

In a dc series motor;

E=V-Ir

But ;

$E\propto N\phi$ and $I\propto \phi$

$\phi$ is flux if the motor.

Hence;

$\frac{E_1}{E_2}=\frac{N_1}{N_2}×\frac{I_2}{I_1}$

$E_1=230-(12×1)=218V$

r₂=1+10=11 ohms

Since;

V₁=V₂ and I₁=I₂

E₂=230-(11×12)=98

Therefore;

$N_2=\frac{N_1×E_2}{E_1}$ =$\frac{800×98}{218}=359.638$

Answer;

360rpm