Question

(a) What are factors on which the speed of a motor depends? Discuss them for series and shunt motors. (6)

(b) A shunt motor supplied at 230 V runs at 900 rpm. When the armature current is 30 A, the resistance of the armature circuit is 0.4 Ω, calculate the resistance required in series with the armature circuit to reduce the speed to 500 rpm. Assume that the armature current is 25 Amps. (10)

Answer 1

(b) given:

$$V=230V$$

$$N_1=900rpm$$

$$I_{a1}=30A$$

$$R_{a1}=0.4\Omega$$

$$N_2=500rpm$$

$$I_{a2}=25A$$

$$R_{a2}=?$$

$$E_{b1}=V-I_{a}R_{a1}$$

$$=230-30\times0.4$$

$$E_{b1}=218V$$

$$\frac{E_{b1}}{E_{b2}}=\frac{N_1}{N_2}$$

$$E_{b2}=\frac{E_{b1}\times N_2}{N_1}$$

$$E_{b2}=\frac{218\times500}{900}$$

$$E_{b2}=121.11V$$

$$E_{b2}=V-I_{a2}R_{a2}$$

$$R_{a2}=\:\frac{V-E_{b2}}{I_{a2}}$$

$$R_{a2}=\frac{230-121.11}{25}$$

$$R_{a2}=4.35\Omega$$

$$R\:to\:be\:added=4.35-0.4=3.95\Omega$$

$$R_{a2}=3.95\Omega$$

✓ Verified Model Answer (Text Solution)