---
title: "Shunt motor speed from no load to load"
subject: "Electrical Machines I"
chapter: 4
key: "semester_4/electrical_machines_i:shunt-motor-250v-light-5a-25a"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/shunt-motor-250v-light-5a-25a
site: BCT Engineering
---

# Shunt motor speed from no load to load

Electrical Machines I · Chapter 4: DC Motor

- Asked in: 2073 Magh, 8 marks
- Video: [DC Shunt Motor No-Load and Full-Load Speed Calculation | Lap Connected 4-Pole Motor](https://www.youtube.com/watch?v=ix5iJ_L1H_o) — 5 Minute Electrical 
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/shunt-motor-250v-light-5a-25a
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-4

## Question

A 250 V dc shunt motor has the armature and field winding resistance of 1 Ω and 125 Ω respectively. When running light, it takes a current of 5 A and the speed is 1500 rpm. When the load is added on the shaft, the motor draws a current of 25 Amp. Calculate the speed at this load.

## Short answer

Field current is constant 2 A, so flux remains unchanged. No-load back emf is 247 V, on-load 227 V; speed = 1500 × 227/247 ≈ 1379 rpm.

## Answer

With constant supply voltage, the shunt field current and flux stay constant, so $N \propto E_b$.

**Given:** $V = 250$ V, $R_a = 1\ \Omega$, $R_{sh} = 125\ \Omega$; no load: $I_{L0} = 5$ A, $N_0 = 1500$ rpm; load: $I_L = 25$ A.

#### Field current
$$
I_{sh} = \frac{250}{125} = 2\ \text{A}
$$

#### No-load (running light)
$$
\begin{aligned}
I_{a0} &= 5 - 2 = 3\ \text{A} \\
E_{b0} &= 250 - 3 \times 1 = 247\ \text{V}
\end{aligned}
$$

#### On load
$$
\begin{aligned}
I_a &= 25 - 2 = 23\ \text{A} \\
E_b &= 250 - 23 \times 1 = 227\ \text{V}
\end{aligned}
$$

#### Speed on load
$$
N = N_0 \times \frac{E_b}{E_{b0}} = 1500 \times \frac{227}{247} = 1378.5\ \text{rpm}
$$

(Armature reaction is neglected, so flux is taken as unchanged.)

**Answer:** Speed at 25 A $\approx 1378.5$ rpm.

## Check yourself

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

1. Why N ∝ Eb here?
   A. Constant voltage keeps field current and flux constant
   B. Torque is constant
   C. Current is constant
   D. Ra is zero
   Answer: A. Armature reaction is neglected. (Key point: Flux)

2. What is Eb0?
   A. 250 V
   B. 247 V
   C. 245 V
   D. 242 V
   Answer: B. 250 - 3×1. (Key point: No-load Eb)

3. What is Eb on load?
   A. 225 V
   B. 250 V
   C. 227 V
   D. 247 V
   Answer: C. Ia = 23 A. (Key point: Load Eb)

4. What is the speed?
   A. About 1500 rpm
   B. About 1630 rpm
   C. About 1242 rpm
   D. About 1378.5 rpm
   Answer: D. 1500×227/247. (Key point: Speed)

## Related in Electrical Machines I

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## Related in other subjects

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- [Shunt motor speed with series armature resistance](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adcm-shunt-half-full-load-torque) — Electrical Machines, chapter 4