---
title: "Why start shunt generator with output open"
subject: "Electrical Machines"
chapter: 3
key: "semester_4/electrical_machines:shunt-generator-start-open-circuit"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/shunt-generator-start-open-circuit
site: BCT Engineering
---

# Why start shunt generator with output open

Electrical Machines · Chapter 3: DC Generator

- Asked in: 2070 Bhadra, 4 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/shunt-generator-start-open-circuit
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-3

## Question

DC shunt generator shall be started keeping its output terminal open. Justify the statement.

## Short answer

The shunt generator is self-excited. Open terminals allow all armature current to flow into the field, building voltage from residual magnetism. With a load connected, the tiny terminal voltage cannot supply sufficient field current, and armature reaction further weakens flux, so voltage fails to rise.

## Answer

A shunt generator is **self-excited**: its field winding is connected across its own armature, so the field current comes from the voltage the machine itself produces.

**Voltage build-up:** residual flux produces a small emf $\to$ this drives a small field current through $R_{sh}$ $\to$ the field current strengthens the flux $\to$ the emf rises. This continues until the field-resistance line meets the open-circuit characteristic.

**Reasons for starting on open circuit:**
1. With a load connected, the load draws current from the small initial voltage, and the armature drop $I_aR_a$ reduces the terminal voltage further. The field current $I_{sh} = V/R_{sh}$ therefore stays too small to raise the flux.
2. A low-resistance load across the terminals effectively short-circuits the field, so the machine never builds up (the voltage collapses to the residual value).
3. The load current also causes armature reaction, which weakens the small flux even further.

With the output terminals open, the whole armature current goes into the field, so the voltage builds up quickly to its rated value. The load is switched on afterwards.

## Check yourself

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

1. Why is a shunt generator called self-excited?
   A. Its field is fed from an independent battery
   B. Its field winding is connected across its own armature and the field current comes from the generated voltage
   C. It has no field winding
   D. It uses a permanent magnet
   Answer: B. The field current comes from the voltage the machine produces, starting from residual flux. (Key point: Self-excitation)

2. With a load connected during start-up, the field current stays small because
   A. Ish = V/Rsh and terminal voltage is kept low by the load current and armature drop
   B. Rsh increases
   C. The load adds flux
   D. The speed is too high
   Answer: A. The load draws current from the small initial voltage, so V stays too low to raise the flux. (Key point: Effect of load during start-up)

3. With the terminals open, the voltage builds up quickly because
   A. The load absorbs the power
   B. The whole armature current goes into the field
   C. The armature resistance disappears
   D. The series field is added
   Answer: B. With no load, Ia = Ish, so the field is excited fully and the voltage rises until the field line meets the OCC. (Key point: Open-circuit start)

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