---
title: "DC series motor started at no load: true/false"
subject: "Electrical Machines"
chapter: 4
key: "semester_4/electrical_machines:dc-series-no-load-true-false"
source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-series-no-load-true-false
site: BCT Engineering
---

# DC series motor started at no load: true/false

Electrical Machines · 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_machines/dc-series-no-load-true-false
- Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-4

## Question

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

## Short answer

False. A series motor must never be started without load because at no load the field current and flux are negligible, causing speed to become excessively high (racing), which can damage the armature mechanically.

## Answer

**False.** A DC series motor must never be started (or run) at no load.

**Justification:**
- The series field carries the armature current, so $\Phi\propto I_a$. At no load $I_a$ is very small, so the flux is very small.
- Speed $N\propto E_b/\Phi$, and $E_b\approx V$, so as $\Phi\to$ residual value the speed rises to a very high, dangerous value (racing).
- The excessive centrifugal force can damage the armature winding, commutator and bearings.
- Torque $T\propto I_a^2$ is small at no load, so there is no load to restrain the speed.

So the series motor should always be started with a load coupled directly or through gears, never with a belt (the belt may slip or break).

## Check yourself

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

1. The statement 'DC series motor should always be started at no load' is:
   A. True, because the starting current is lower
   B. False, because it must never be started at no load
   C. True, because torque is highest at no load
   D. False, but only for large motors
   Answer: B. A series motor races at no load, so it must be started with load. (Key point: True or false verdict)

2. Why is the flux very small at no load in a series motor?
   A. Field winding resistance is large
   B. Supply voltage drops
   C. Residual magnetism disappears
   D. The series field carries the small armature current
   Answer: D. Phi is proportional to Ia, which is very small at no load. (Key point: Flux at no load)

3. Since N is proportional to Eb/Phi and Eb is about V, what happens as Phi falls to residual value?
   A. Speed rises very high (racing)
   B. Speed falls
   C. Speed stays the same
   D. Torque rises sharply
   Answer: A. Dividing by a tiny flux makes speed dangerously large. (Key point: Speed rise)

4. Why is there nothing to restrain the speed at no load?
   A. Back emf is zero
   B. Torque T proportional to Ia^2 is small and there is no load torque to balance
   C. The brakes engage automatically
   D. The field is open
   Answer: B. Torque is tiny and no load opposes it, so the motor accelerates until very high speed. (Key point: Why nothing restrains speed)

## Related in Electrical Machines

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