---
title: "True/false: series motor started at no load"
subject: "Electrical Machine"
chapter: 4
key: "semester_4/electrical_machine:tf-dc-series-motor-start-noload"
source: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-dc-series-motor-start-noload
site: BCT Engineering
---

# True/false: series motor started at no load

Electrical Machine · Chapter 4: DC Motor

- Asked in: 2068 Bhadra, 1+3 marks
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machine/tf-dc-series-motor-start-noload
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machine/chapter-4

## Question

State whether the following statement is true or false and justify it: DC series motor should always be started at no load.

## Short answer

False. A DC series motor must never start at no load because flux is proportional to armature current; at no load, current and flux are very small, causing dangerously high speed (racing) that can destroy the armature.

## Answer

**The statement is FALSE.** A DC series motor must never be started (or run) at no load; it should always be started with some load coupled to the shaft.

#### Justification
1. In a series motor the field winding carries the armature current, so the flux Φ ∝ I_a.
2. Speed N ∝ E_b / Φ ≈ (V − I_a R) / Φ.
3. At no load, the armature current becomes very small, so the flux Φ becomes very small. The speed then rises to a dangerously high value, since N ∝ 1/Φ, and in principle it tends to infinity (limited only by the residual flux and losses).
4. This is called **racing**. The centrifugal forces can burst the armature, the commutator segments can fly off, and the bearings and windings may be damaged.

Therefore the series motor is always coupled directly to the load (never through a belt, which may slip or break) and is started with a load. In contrast, a shunt motor has constant flux, and is safe at no load.

## Check yourself

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

1. Is the statement 'a DC series motor should always be started at no load' true or false?
   A. False; it must always be started with load
   B. True; load reduces the starting current
   C. True; no load is safest
   D. False; it must be started on a belt drive
   Answer: A. At no load a series motor races to a dangerous speed, so it needs a load. (Key point: True or false)

2. Why does the flux become very small at no load in a series motor?
   A. The field is disconnected
   B. The field carries the armature current, so Φ ∝ I_a and I_a is small at no load
   C. The armature resistance rises
   D. The supply voltage falls
   Answer: B. The series field is in series with the armature, so low I_a means low flux. (Key point: Flux in series motor)

3. With N ∝ E_b/Φ, what happens to speed when Φ becomes very small?
   A. Speed falls to zero
   B. Speed oscillates at supply frequency
   C. Speed rises to a dangerously high value (racing)
   D. Speed stays the same
   Answer: C. N is inversely proportional to flux, so speed tends to run away. (Key point: Speed relation)

4. How should a series motor be coupled to its load?
   A. Through a belt
   B. Through a loose clutch
   C. It needs no coupling
   D. Directly, never by a belt which may slip or break
   Answer: D. A belt failure would remove the load and cause racing. (Key point: Coupling)

## Related in Electrical Machine

- [DC series motor started at no load: true/false](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-no-load-true-false) — Electrical Machines, chapter 4
- [DC series motor should not start on no-load](https://www.bctengineer.com/question.md?key=semester_4%2Felectrical_machines%3Adc-series-motor-no-load-start) — Electrical Machines, chapter 4

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