---
title: "Shunt motor speed at 30 A armature current"
subject: "Electrical Machine"
chapter: 4
key: "semester_4/electrical_machine:shunt-motor-60a-230v-speed"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/shunt-motor-60a-230v-speed
site: BCT Engineering
---

# Shunt motor speed at 30 A armature current

Electrical Machine · Chapter 4: DC Motor

- Asked in: 2071 Magh, 6 marks
- Video: [DC Shunt Machine Numerical: Calculate Speed as Motor and Generator](https://www.youtube.com/watch?v=EYpBSBGbBNY) — 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_machine/shunt-motor-60a-230v-speed
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4

## Question

A dc shunt motor runs at 600 RPM taking 60 A from a 230 V supply. Armature resistance is 0.2 Ω and field resistance is 115 Ω. Find the speed when the current through the armature is 30 A.

## Short answer

For a shunt motor, flux is constant, so speed is proportional to back emf. With armature current 30 A, Eb = 224 V, giving speed = 600 × 224/218.4 ≈ 615 rpm.

## Answer

In a shunt motor the flux is constant (field current depends only on V and R_f), so N ∝ E_b.

#### Equivalent circuit
```text
 +--[Ra=0.2]--(Eb)--+
 |       Ia          |
 +--- Rf = 115 ohm --+--- V = 230 V
```

#### First condition
- I_sh = 230 / 115 = 2 A
- I_a1 = 60 − 2 = 58 A
- E_b1 = 230 − 58 × 0.2 = 230 − 11.6 = 218.4 V
- N₁ = 600 rpm

#### Second condition (I_a = 30 A)
E_b2 = 230 − 30 × 0.2 = 224 V

#### Speed
N₂ = N₁ × (E_b2 / E_b1) = 600 × 224 / 218.4 = **615.4 rpm**

(The speed rises slightly because the armature drop is smaller at the lighter load.)

**Answer: speed ≈ 615 rpm.**

## Check yourself

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

1. Why is N ∝ E_b for this shunt motor?
   A. Flux is constant because field current depends only on V and R_f
   B. Armature current is constant
   C. Voltage changes
   D. Torque is constant
   Answer: A. N = E_b/(KΦ) and Φ is constant. (Key point: Speed relation)

2. What is I_a1?
   A. 2 A
   B. 58 A
   C. 60 A
   D. 62 A
   Answer: B. I_a1 = 60 - 2 = 58 A. (Key point: Initial armature current)

3. What is E_b1?
   A. 211.6 V
   B. 230 V
   C. 218.4 V
   D. 224 V
   Answer: C. 230 - 58 × 0.2 = 218.4 V. (Key point: Initial back emf)

4. What is the speed at I_a = 30 A?
   A. 300 rpm
   B. 600 rpm
   C. 584.8 rpm
   D. 615.4 rpm
   Answer: D. N2 = 600 × 224/218.4 = 615.4 rpm; speed rises slightly with smaller armature drop. (Key point: Final speed)

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