A separately excited D.C. motor operating from a single phase half controlled bridge rectifier as a speed of 1400 rpm has an input voltage of 330sin(314t) and aback emf of 80V. The SCR’s are fired symmetrically at α = 30° in every half cycle and the armature has a resistance of 4Ω. Calculate the average armature current and the motor torque.

Mumbai University > Electronics Engineering > Sem7 > Power Electronics 2

Marks: 10M

Year: Dec 2014

For a single phase semi converter feeding a separately. Excited motor,

$V_0=V_t=\frac{V_m}{\pi}(1+cos\alpha)=E_a+I_ar_a \\ \frac{330}{\pi}(1+cos30^0)=80+I_a.4 \\ 196.01=80+I_a.4 \\ $

Therefore, Average armature current,

Motor emf,

$I_a=\frac{196.01-80}{4}=23.003A \\ E_a=K_m.\omega_m=K_m\frac{2\pi \times 1400}{60} \\ K_m=\frac{80 \times 60}{2\pi \times 1400}=0.546V-s/rad \\ or \ \ 0.546Nm/A. $

Therefore, Motor torque

$T_e=K_mI_a=0.546 \times 29.003=15.836 Nm$