---
title: "DC series motor should not start on no-load"
subject: "Electrical Machines"
chapter: 4
key: "semester_4/electrical_machines:dc-series-motor-no-load-start"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-series-motor-no-load-start
site: BCT Engineering
---

# DC series motor should not start on no-load

Electrical Machines · Chapter 4: DC Motor

- Asked in: 2067 Mangsir (old course), 4 marks
- Video: [Why DC series Motor Never started on No Load?](https://www.youtube.com/watch?v=HTAMEEFSyTI) — Vishal Mahajan
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-series-motor-no-load-start
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4

## Question

Justify the statement: DC series motor should never be started on no-load.

## Short answer

At no load, armature current and series field flux are very small; since speed N ∝ 1/Φ, the motor races to dangerously high speed, risking mechanical damage from centrifugal forces. Therefore it must always start with a coupled load.

## Answer

In a series motor the field winding carries the armature current, so the flux depends on the load: $\Phi\propto I_a$ (below saturation). The speed is

$$
N \propto \frac{E_b}{\Phi} \approx \frac{V - I_a(R_a + R_{se})}{\Phi}
$$

- At no load $I_a$ becomes very small, so the flux $\Phi$ becomes very small (only the residual value).
- The numerator $E_b\approx V$ stays nearly constant, so the speed $N\propto 1/\Phi$ rises to a **dangerously high value** (several times rated speed).
- This is called **racing**. The centrifugal forces can burst the armature, throw the winding out of the slots and damage the commutator and bearings.
- The torque $T\propto\Phi I_a\propto I_a^2$ is small at no load, so the motor has no load to hold the speed down.

Hence a series motor must always be started and run with a load connected (preferably directly coupled or geared, never by a belt that may slip or break).

## Check yourself

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

1. Why does the flux of a series motor become very small at no load?
   A. The field winding is across the supply
   B. The field carries armature current, which is very small at no load
   C. The brushes lift off
   D. The back emf becomes zero
   Answer: B. Phi is proportional to Ia in a series motor, and Ia is tiny at no load. (Key point: Series flux depends on load)

2. At no load Eb is nearly V and Phi is very small. What happens to speed N proportional to Eb/Phi?
   A. It falls to zero
   B. It stays at rated value
   C. It becomes constant at half rated value
   D. It rises to a dangerously high value
   Answer: D. With Eb nearly constant and Phi tiny, N proportional to 1/Phi becomes very large. (Key point: Speed relation)

3. What is the danger of the high no-load speed (racing)?
   A. Centrifugal force can burst the armature and damage winding, commutator and bearings
   B. The supply fuse becomes too small
   C. The field winding overheats from high current
   D. The motor runs in reverse
   Answer: A. Excessive centrifugal forces mechanically damage the rotating parts. (Key point: Racing and damage)

4. How should a series motor be coupled to its load?
   A. By a belt for flexibility
   B. Through a clutch that can be released
   C. Directly or by gears, never by a belt that may slip or break
   D. It needs no coupling
   Answer: C. A broken belt would suddenly remove the load and cause racing. (Key point: Coupling choice)

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