Question

A 220 V, 970 rpm, 100 A dc separately excited motor has an armature resistance of 0.05 $\Omega$. It is braked by plugging from an initial speed of 1000 rpm. The resistance to be placed in armature circuit to limit braking current to twice the full load value is

• A. $2.158\Omega$
• B. $1.160\Omega$
• C. $4.146\Omega$
• D. $1.71\Omega$

Answer 1

The correct option is A $2.158\Omega$

At 970 rpm, back emf,
$E_b=220-0.05\times 100=215V$

At 1000 rpm, back emf, $E_b=\frac{1000}{970}\times 215=221.65V$

For plugging operation
$R_B+R_a=\frac{E+V}{I_{a\left(b\right)}}=\frac{221.65+220}{200}=2.21\Omega$

The external resistance,
$R_B=2.21-0.05=2.158\Omega$