--- title: "Justify: series motor power; shunt motor for traction" subject: "Utilization of Electrical Energy" chapter: 4 key: "semester_7/utilization_of_electrical_energy:justify-series-motor-power-shunt-traction" source: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/justify-series-motor-power-shunt-traction site: BCT Engineering --- # Justify: series motor power; shunt motor for traction Utilization of Electrical Energy · Chapter 4: Electric Traction - Asked in: 2079 Baisakh, 8 marks - Video: [TRACTION MOTOR AND THERE CONTROL || VARIOUS CONTROL METHOD APPLIED TO TRACTION MOTOR || SUBHAM SIR](https://www.youtube.com/watch?v=oSvnbb39wkA) — Study Apne Liye - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_7/utilization_of_electrical_energy/justify-series-motor-power-shunt-traction - Whole chapter: https://www.bctengineer.com/md/semester_7/utilization_of_electrical_energy/chapter-4 ## Question Justify the statements: a) Power drawn from supply mains varies as the square root of the load torque in case of dc series motors. b) Shunt motor is not suitable for traction purposes. ## Short answer For a series motor, flux is proportional to current, so torque ∝ I^2, hence I ∝ √T and power P=VI ∝ √T. Shunt motors maintain nearly constant speed and have torque ∝ I, requiring high power for heavy loads, poor load sharing, and voltage surge sensitivity, making them unsuitable for traction. ## Answer #### a) Power drawn by a dc series motor varies as $\sqrt{T}$ In a series motor the field current is the armature current, so (below saturation) $\phi \propto I$: $$ T \propto \phi I \propto I^2 \Rightarrow I \propto \sqrt{T} $$ The supply voltage $V$ is constant, so the power drawn is $$ P = VI \propto I \propto \sqrt{T} $$ Example: if the load torque rises four times (steep gradient), the current and power drawn only double; speed falls automatically since $N \propto E/\phi \propto 1/I$. For a shunt motor, $\phi$ is constant, so $T \propto I$ and $P \propto T$: a four-fold torque needs four-fold power. Hence a series motor puts much less strain on the supply and substation during heavy loads such as starting and climbing. This is a main reason dc series motors are used for traction. #### b) Shunt motor is not suitable for traction 1. **Power demand ∝ torque**: since $T \propto I$ at constant flux and speed is nearly constant, heavy starting and gradient torques draw very large current and power from the supply (series motor: only $\sqrt T$). 2. **Constant speed characteristic**: traction needs a speed that falls on up-gradients and rises on light loads (falling characteristic). A shunt motor tries to keep the same speed, so power peaks are high. 3. **Unequal load sharing**: locomotives run several motors in parallel. Wheel diameters differ slightly by wear; with flat speed–torque curves, a small speed difference makes one shunt motor take a very large share of load, overloading it. Series motors with drooping curves share load nearly equally. 4. **Supply voltage fluctuations**: traction line voltage varies widely. In a shunt motor flux follows voltage; a sudden voltage rise or dip causes large current surges, since back emf cannot change instantly with speed. In a series motor the field current is the armature current, so surges are small. 5. **Low starting torque per ampere**: series motor torque rises with $I^2$, giving high starting torque with moderate current; a shunt motor gives only $T \propto I$. 6. Interruption of supply (e.g. pantograph bounce) is dangerous with shunt motors due to heavy current on reconnection. | Feature | Series motor | Shunt motor | |---|---|---| | Torque | $\propto I^2$ | $\propto I$ | | Power vs torque | $\propto \sqrt T$ | $\propto T$ | | Speed-load | Falls with load | Nearly constant | | Load sharing | Good | Poor | | Voltage surge effect | Small | Large | (The shunt motor's only merit, easy regenerative braking, is outweighed by these drawbacks.) ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Why does power drawn by a series motor vary as sqrt(T)? A. Flux is constant so T is proportional to I B. Flux is proportional to I, so T is proportional to I^2, I to sqrt(T) and P = VI C. Voltage varies as sqrt(T) D. Speed is constant Answer: B. With phi proportional to I, T is proportional to I^2, so I and P (V constant) go as sqrt(T). (Key point: Series motor power) 2. If load torque rises four times, what happens to power in a series motor and in a shunt motor? A. Series doubles; shunt rises four times B. Both rise four times C. Series four times; shunt doubles D. Both double Answer: A. Series: P proportional to sqrt(T) gives 2x. Shunt: P proportional to T gives 4x. (Key point: Four-fold torque) 3. Why is a shunt motor poor for several motors in parallel on a locomotive? A. It has too much starting torque B. It cannot be reversed C. It has drooping characteristic D. Its speed-torque curve is flat, so small wheel diameter differences make one motor overloaded Answer: D. Flat characteristics mean a slight speed difference causes big load imbalance; series motors share well. (Key point: Load sharing) 4. Why are line voltage surges worse for shunt motors? A. They have no armature B. Their field current equals armature current C. Flux follows voltage and back emf cannot change instantly, so large current surges occur D. They run only on AC Answer: C. In a series motor field current equals armature current, so surges are small. 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