- 2068 Bhadra · 1+3 marks
True/false: series motor 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.
State whether the following statement is true or false and justify it: DC series motor should always be started at no load.
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
- In a series motor the field winding carries the armature current, so the flux Φ ∝ I_a.
- Speed N ∝ E_b / Φ ≈ (V − I_a R) / Φ.
- 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).
- 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.
✓ Answer checked by Claude Opus 5.5 (high effort)
Related questions
- DC series motor started at no load: true/false · Electrical Machines, chapter 4
- DC series motor should not start on no-load · Electrical Machines, chapter 4
- True/false: derivative controllers always used with others · Control Systems, chapter 7 · asked 3×