Question

A separately excited dc generator running at a speed of 1500 rpm delivers 150 A at 400 V to constant resistive load. The armature resistance of generator is $0.12\Omega$.If the current is reduced to 100 A and the armature reaction to be neglected, the speed of the generator will be nearly.

• A. 2000 rpm
• B. 1001 rpm
• C. 1500 rpm
• D. 650 rpm

Answer 1

The correct option is B 1001 rpm

$V_{t1}=400V;I_{a1}=150A$
$R_a=0.12\Omega ;N_1=1500rpm$
The value pf load resistance,
$R_L=\frac{400}{150}=2.67\Omega$
Load current 100 A, the terminal voltage,
$V_{t2}=100\times R_L=100\times 2.67=267V$
$I_{a2}=100A;R_a=0.12\Omega$
$E_g=\frac{\varphi NZ}{60}\times \left(\frac{P}{A}\right)\Rightarrow E_g\alpha N$
$\frac{E_{g1}}{E_{g2}}=\frac{N_1}{N_2}=\frac{V_{t1}+I_{a1}{R}_a}{V_{t2}+I_{a2}{R}_a}$
$\Rightarrow$ $\frac{400+150\times 0.12}{267+100\times 0.12}=\frac{1500}{N_2}$
$N_2=1001.19rpm$