--- title: "Comparison of DC shunt and series generators" subject: "Electrical Machines I" chapter: 3 key: "semester_4/electrical_machines_i:shunt-vs-series-generator-comparison" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/shunt-vs-series-generator-comparison site: BCT Engineering --- # Comparison of DC shunt and series generators Electrical Machines I · Chapter 3: DC Generator - Asked in: 2071 Magh, 8 marks - Video: [Series generators | Shunt generators | Characteristics | DC machines | Lec-48](https://www.youtube.com/watch?v=xcCzQYO3puA) — Education 4u - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/shunt-vs-series-generator-comparison - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-3 ## Question Make a detail comparison between dc shunt generator and dc series generator with their circuit diagram, equations and characteristic curves. ## Short answer Shunt generator field is parallel with armature, has high resistance and gives nearly constant voltage with slight drop; series generator field is in series with load, low resistance, and voltage rises with load. Shunt is used for lighting/battery charging; series for boosters and series arc lighting. ## Answer A **shunt generator** has its field winding connected in parallel with the armature, while a **series generator** has its field winding connected in series with the armature and load. This one difference changes their field design, characteristics and uses. #### Circuit diagrams and equations **Shunt generator** ```text +---Ra---+------+ | | | (Eg) Rsh Load V | | | +--------+------+ ``` $$ I_{sh} = \frac{V}{R_{sh}}, \quad I_a = I_L + I_{sh}, \quad E_g = V + I_a R_a + V_b $$ **Series generator** ```text +---Ra---Rse---+ | | (Eg) Load V | | +--------------+ ``` $$ I_a = I_{se} = I_L, \quad E_g = V + I_a (R_a + R_{se}) + V_b $$ #### Characteristic curves **Open-circuit (magnetisation) characteristic:** $E_0$ vs $I_f$. For a shunt generator it is taken by exciting the field separately; its shape is like a B-H curve starting at the residual voltage. For a series generator it is also taken with separate excitation of the series field. **External characteristic** ($V$ vs $I_L$): ```text V | Shunt |----___ | ----____ | ---__ | \ (breakdown, | ___ \ curls back) | _/ \__ Series \ | _/ \_ | | _/ \ | |/________________\_____|_______ IL ``` - **Shunt:** $V$ drops slowly as load rises because of $I_a R_a$ drop, armature reaction, and the resulting fall in field current. Beyond a critical load the curve turns back and voltage collapses (short circuit current is small). - **Series:** at no load $V$ is only the residual voltage. As load current rises, field current (= load current) rises, so $V$ rises steeply; at very high load, saturation and armature reaction make $V$ fall again. #### Comparison | Point | Shunt generator | Series generator | |---|---|---| | Field connection | Parallel with armature | Series with armature and load | | Field winding | Many turns, thin wire | Few turns, thick wire | | Field resistance | High (tens to hundreds of Ω) | Very low (fraction of Ω) | | Field current | Small, $V/R_{sh}$ (2-5% of $I_a$) | Equal to the load current | | Current relation | $I_a = I_L + I_{sh}$ | $I_a = I_{se} = I_L$ | | Build-up condition | $R_{sh}$ < critical resistance | Load resistance < critical resistance (load needed) | | Voltage at no load | Full rated voltage | Only residual voltage | | Voltage vs load | Nearly constant, falls slightly | Rises sharply with load | | Regulation | Fairly good | Very poor (variable voltage) | | Uses | Lighting, battery charging, exciters | Boosters in DC feeders, series arc lighting | **Example:** a shunt generator charging a battery keeps nearly constant voltage; a series generator in a long DC feeder raises its voltage as load current rises, compensating line drop. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Which field winding suits a series generator? A. Many turns of thin wire B. Few turns of thick wire, very low resistance C. Many turns of thick wire D. Few turns of thin wire Answer: B. The series field carries full load current, so it needs thick wire with few turns. (Key point: Field winding design) 2. What is a series generator's voltage at no load? A. Rated voltage B. Zero exactly C. Only residual voltage D. Double the rated voltage Answer: C. At no load the field current is zero, so only residual flux gives voltage. (Key point: No-load voltage) 3. How does terminal voltage vary with load for the two types? A. Shunt rises sharply, series falls slightly B. Shunt falls slightly; series rises sharply with load C. Both rise D. Both stay constant Answer: B. Shunt voltage droops from I_a R_a, armature reaction and weaker field; series field current equals load current, so voltage rises. (Key point: Voltage vs load) 4. Which relation is right? A. Shunt: I_a = I_L; series: I_a = I_L + I_sh B. Shunt: I_a = I_L + I_sh; series: I_a = I_se = I_L C. Both: I_a = I_L + I_sh D. Shunt: I_a = I_L - I_sh Answer: B. The shunt field takes part of the armature current; the series field is in the load path. 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