2075 Baisakh (old course) · 8 marks

200 V series motor speed with 5 ohm series resistance

Short answer

Flux is constant because current is the same. Initial Eb = 185 V at 800 rpm; with series resistance, Eb = 110 V. Speed = 475.7 rpm.

A 200 V d.c. series motor runs at 800 rpm when taking a line current of 15 A. The armature and field resistances are $0.6\ \Omega$ and $0.4\ \Omega$ respectively. Find the speed at which it will run when connected in series with a $5\ \Omega$ resistance and taking the same current at the same voltage.

The current is unchanged, so the flux is unchanged ( $\Phi\propto I$ ) and $N\propto E_b$ .

Total motor resistance: $R = R_a + R_{se} = 0.6 + 0.4 = 1.0\ \Omega$ .

Case 1: 800 rpm, $I = 15$ A.

E_{b1} = 200 - 15(1.0) = 185\ \text{V}

Case 2: 5 Ω added in series, so the total resistance is $1.0 + 5 = 6.0\ \Omega$ and $I = 15$ A.

E_{b2} = 200 - 15(6.0) = 110\ \text{V}

N_2 = 800\times\frac{110}{185} = 475.7\ \text{rpm}

Answer: the motor runs at $475.7\ \text{rpm}$ .

✓ Answer checked by Claude Opus 5.5 (high effort)

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