---
title: "200 V series motor speed with 5 ohm series resistance"
subject: "Electrical Machines"
chapter: 4
key: "semester_4/electrical_machines:dcm-series-200v-800rpm-5ohm"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dcm-series-200v-800rpm-5ohm
site: BCT Engineering
---

# 200 V series motor speed with 5 ohm series resistance

Electrical Machines · Chapter 4: DC Motor

- Asked in: 2075 Baisakh (old course), 8 marks
- Video: [speed control of Dc series mathod](https://www.youtube.com/watch?v=av_V0iXFdzo) — Micro Teach
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/dcm-series-200v-800rpm-5ohm
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4

## Question

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.

## 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.

## Answer

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 $475.7\ \text{rpm}$.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. The motor resistance used is:
   A. 0.6 ohm
   B. 0.4 ohm
   C. 1.0 ohm (Ra + Rse)
   D. 5 ohm
   Answer: C. Armature and series field resistances are in series: 0.6 + 0.4. (Key point: Total series resistance)

2. Eb1 at 15 A is:
   A. 185 V
   B. 200 V
   C. 191 V
   D. 170 V
   Answer: A. 200 - 15 x 1 = 185 V. (Key point: Eb1)

3. After adding 5 ohm (total 6 ohm), Eb2 is:
   A. 125 V
   B. 110 V
   C. 95 V
   D. 190 V
   Answer: B. 200 - 15 x 6 = 110 V. (Key point: Eb2)

4. The new speed is:
   A. 800 rpm
   B. 1345 rpm
   C. 475.7 rpm
   D. 550 rpm
   Answer: C. 800 x 110/185 = 475.7 rpm, because flux is unchanged at same current. (Key point: Speed)

## Related in Electrical Machines

- [Series motor speed with 5 Ω series resistance](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Aseries-motor-200v-800rpm-5ohm) — Electrical Machines I, chapter 4
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## Related in other subjects

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