Question

A separately excited dc motor is supplied from a 60 V dc source through a fixed frequency chopper. The rated speed is 900 rpm and rated current is 30 A. Armature circuit resistance is 0.35 $\Omega$. What is the value of duty ratio of the copper at rated motor torque for a speed of 300 rpm ignoring current pulsations.

• A. 0.9
• B. 0.3
• C. 0.25
• D. 0.45

Answer 1

The correct option is D 0.45

(b)
In case of motor,
$V=E_b+I_a{R}_a$

$E_b=V-I_a{R}_a$

At 900 rpm

$E_b=60-(30\times 0.35)=49.5V$

$E_b\propto N\varphi$
$E_{b2}=E_{b1}\times \frac{N_2}{N_1}=49.5\times \frac{300}{900}$

$E_{b2}=16.5V$

For chopper, $V=E_{b2}+I_a{R}_a$

$=16.5+(30\times 0.35)=27V$

$V_0=\alpha V$

$27=\alpha \left(60\right)$

$\alpha =0.45$