---
title: "Compound generator EMF in long and short shunt"
subject: "Electrical Machines I"
chapter: 3
key: "semester_4/electrical_machines_i:compound-gen-120a-120v-long-short"
source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/compound-gen-120a-120v-long-short
site: BCT Engineering
---

# Compound generator EMF in long and short shunt

Electrical Machines I · Chapter 3: DC Generator

- Asked in: 2078 Baisakh, 8 marks; 2069 Poush, 8 marks (asked 2 times)
- Video: [long shunt compound generators](https://www.youtube.com/watch?v=CWJjNBdG8ow) — Stephen Mutale
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/compound-gen-120a-120v-long-short
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-3

## Question

A dc compound generator has to supply a current of 120 A to the load at 120 V. The shunt field, series field and armature winding resistances are 30 ohm, 0.05 ohm and 0.1 ohm respectively. Calculate the emf generated by the armature in the following two cases: (i) long shunt connection (ii) short shunt connection

## Short answer

Long shunt generated EMF = 138.6 V (Ish = 4 A, Ia = 124 A, EMF = 120 + 124×(0.1+0.05)). Short shunt generated EMF = 138.42 V (Vsh = 120 + 120×0.05 = 126 V, Ish = 4.2 A, Ia = 124.2 A, EMF = 126 + 124.2×0.1).

## Answer

Given: $I_L = 120$ A, $V = 120$ V, $R_{sh} = 30\ \Omega$, $R_{se} = 0.05\ \Omega$, $R_a = 0.1\ \Omega$. Brush drop is neglected.

#### (i) Long-shunt connection
The series field is in series with the armature; the shunt field is across the load terminals.
```text
       Ia    Rse              IL = 120 A
   +--->---/\/\/--+------+--->---o +
   |              |      |
 ( E )          [Rsh]  load     V = 120 V
   Ra             |      |
   +--------------+------+-------o -
```
$$
\begin{aligned}
I_{sh} &= \frac{V}{R_{sh}} = \frac{120}{30} = 4\ \text{A} \\
I_a &= I_L + I_{sh} = 120 + 4 = 124\ \text{A} \\
E_g &= V + I_a(R_a + R_{se}) = 120 + 124 \times (0.1 + 0.05) \\
&= 120 + 18.6 = 138.6\ \text{V}
\end{aligned}
$$

#### (ii) Short-shunt connection
The shunt field is across the armature; the series field carries the load current.
```text
       Ia            Rse       IL = 120 A
   +--->---+-------/\/\/---+--->---o +
   |       |               |
 ( E )   [Rsh]           load    V = 120 V
   Ra      |               |
   +-------+---------------+-------o -
```
$$
\begin{aligned}
V_{sh} &= V + I_LR_{se} = 120 + 120 \times 0.05 = 126\ \text{V} \\
I_{sh} &= \frac{126}{30} = 4.2\ \text{A} \\
I_a &= I_L + I_{sh} = 120 + 4.2 = 124.2\ \text{A} \\
E_g &= V_{sh} + I_aR_a = 126 + 124.2 \times 0.1 \\
&= 126 + 12.42 = 138.42\ \text{V}
\end{aligned}
$$

| Quantity | Long shunt | Short shunt |
|---|---|---|
| Shunt field current | 4 A | 4.2 A |
| Armature current | 124 A | 124.2 A |
| Generated EMF | 138.6 V | 138.42 V |

**Answer:** (i) Long shunt: $E_g =$ **138.6 V**; (ii) Short shunt: $E_g =$ **138.42 V**.

## Check yourself

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

1. In the long-shunt connection the shunt field current is:
   A. V/Rsh = 4 A
   B. (V + IL Rse)/Rsh = 4.2 A
   C. IL/Rsh
   D. V/Ra
   Answer: A. Shunt field is across the load terminals so Ish = 120/30 = 4 A. (Key point: Long shunt field voltage)

2. EMF for long shunt is:
   A. 120 + 124 x 0.15 = 138.6 V
   B. 120 + 120 x 0.15
   C. 126 + 12.4
   D. 120 + 124 x 0.1
   Answer: A. Series field is in series with armature: Eg = V + Ia(Ra + Rse) with Ia = 124 A. (Key point: Long shunt EMF)

3. For short shunt, Eg is:
   A. 138.6 V
   B. 138.42 V
   C. 126 V
   D. 132 V
   Answer: B. Vsh = 126 V, Ish = 4.2 A, Ia = 124.2 A, Eg = 126 + 12.42 = 138.42 V. (Key point: Short shunt EMF)

## Related in Electrical Machines I

- [EMF of long shunt compound generator](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Along-shunt-compound-40a-200v) — Electrical Machines I, chapter 3, asked 2×
- [EMF of short shunt compound generator (0.5 Ω armature)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashort-shunt-compound-50a-200v-0p5ohm) — Electrical Machines I, chapter 3, asked 2×
- [Short shunt vs long shunt generated voltage (150 A, 230 V)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashort-shunt-150a-230v-long-shunt) — Electrical Machines I, chapter 3
- [EMF of short shunt compound generator (50 A, 220 V)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashort-shunt-50a-220v-emf) — Electrical Machines I, chapter 3
- [EMF and efficiency of short shunt compound generator (50 A, 200 V)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashort-shunt-compound-50a-200v) — Electrical Machines I, chapter 3, asked 2×
- [Short shunt to long shunt compound generator (10 kW, 220 V)](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines_i%3Ashort-shunt-cumulative-10kw-220v-long) — Electrical Machines I, chapter 3

## Related in other subjects

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- [Long shunt compound generator emf, 40 A at 200 V](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Agen-long-shunt-40a-200v) — Electrical Machines, chapter 3
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- [Short shunt compound generator emf, 80 A at 220 V](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Agen-short-shunt-80a-220v) — Electrical Machines, chapter 3, asked 2×