--- title: "Self-excited vs separately excited DC generators" subject: "Electrical Machines I" chapter: 3 key: "semester_4/electrical_machines_i:self-vs-separately-excited-classification" source: https://www.bctengineer.com/q/semester_4/electrical_machines_i/self-vs-separately-excited-classification site: BCT Engineering --- # Self-excited vs separately excited DC generators Electrical Machines I · Chapter 3: DC Generator - Asked in: 2075 Bhadra, 8 marks - Video: [Types of DC Generators - Separately & Self Excited DC Generator | Shunt, Series & Compound Generator](https://www.youtube.com/watch?v=xi0jaPAl2II) — Electrical Deck - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines_i/self-vs-separately-excited-classification - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines_i/chapter-3 ## Question Distinguish between self-excited and separately excited d.c generators. How are self-excited d.c. generators classified? Give their circuit diagrams and respective equations. ## Short answer Separately excited: field from external source; self-excited: field from own armature, needs residual flux. Self-excited types: shunt (Ia=IL+Ish, Eg=V+IaRa), series (Ia=IL, Eg=V+Ia(Ra+Rse)), compound (long-shunt: Ish=V/Rsh, Ia=IL+Ish; short-shunt: Ish=(V+ILRse)/Rsh, Ia=IL+Ish). ## Answer A DC generator needs a magnetic field from its field winding. If the field current comes from an **external DC source**, the generator is **separately excited**. If the field current comes from the **generator's own armature**, it is **self-excited**; it builds up voltage from the residual magnetism of the poles. #### Self-excited vs separately excited | Point | Separately excited | Self-excited | |---|---|---| | Field supply | External battery or DC source | Own armature output | | Residual magnetism | Not needed | Essential for voltage build-up | | Voltage build-up | Always, as soon as field is fed | Only if field resistance < critical value and polarity is correct | | Voltage regulation | Better (field current fixed) | Poorer in shunt type (field current falls with V) | | Cost and setup | Needs extra source, costly | Simple, no extra source | | Uses | Lab tests, Ward-Leonard, wide voltage control | General supply, battery charging, welding, boosters | #### Classification of self-excited generators Depending on how the field winding is connected to the armature: 1. **Shunt generator** 2. **Series generator** 3. **Compound generator** – (a) long shunt, (b) short shunt; each can be cumulative or differential. #### 1. Shunt generator Field winding (many turns of thin wire, high resistance) is connected in parallel with the armature. ```text +---Ra---+------+ | | | (Eg) Rsh Load V | | | +--------+------+ ``` $$ I_a = I_L + I_{sh}, \quad I_{sh} = \frac{V}{R_{sh}}, \quad E_g = V + I_a R_a + V_b $$ #### 2. Series generator Field winding (few turns of thick wire, low resistance) is in series with the armature and load. ```text +---Ra---Rse---+ | | (Eg) Load V | | +--------------+ ``` $$ I_a = I_{se} = I_L, \quad E_g = V + I_a (R_a + R_{se}) + V_b $$ #### 3. Compound generator Has both shunt and series fields. **Long shunt:** shunt field across the series combination of armature and series field. ```text +--Ra--Rse--+------+ | | | (Eg) Rsh Load | | | +-----------+------+ ``` $$ I_{sh} = \frac{V}{R_{sh}}, \quad I_a = I_{se} = I_L + I_{sh}, \quad E_g = V + I_a (R_a + R_{se}) + V_b $$ **Short shunt:** shunt field directly across the armature only. ```text +--Ra--+----Rse----+ | | | (Eg) Rsh Load | | | +------+-----------+ ``` $$ I_{se} = I_L, \quad I_{sh} = \frac{V + I_L R_{se}}{R_{sh}}, \quad I_a = I_L + I_{sh}, \quad E_g = V + I_L R_{se} + I_a R_a + V_b $$ In a **cumulative** compound generator the series flux aids the shunt flux; in a **differential** one it opposes it. Here $V_b$ is the total brush drop. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What distinguishes a self-excited from a separately excited generator? A. Self-excited uses an external battery B. Self-excited takes field current from its own armature; separately excited uses an external DC source C. Separately excited has no field winding D. Self-excited has no residual magnetism Answer: B. The only difference is the field supply: own armature versus an external DC source. (Key point: Source of field current) 2. Which statement about residual magnetism is correct? A. It is essential for voltage build-up in self-excited machines only B. It is essential in separately excited machines only C. It is needed by both equally D. It is not needed in any generator Answer: A. A self-excited machine starts from residual flux; a separately excited one gets flux from the external source. (Key point: Residual magnetism) 3. How are self-excited generators classified? A. Lap, wave and duplex B. Shunt, series and compound (long or short shunt) C. Cumulative, differential and flat D. Separate, shunt and series Answer: B. By field connection: shunt, series, and compound (long shunt or short shunt, cumulative or differential). (Key point: Classification) 4. Which set matches a short-shunt compound generator? A. I_sh = V/R_sh, I_a = I_L + I_sh B. I_a = I_L, E_g = V + I_a(R_a + R_se) C. I_a = I_L + I_sh, E_g = V + I_a(R_a + R_se) + V_b D. I_sh = (V + I_L R_se)/R_sh, E_g = V + I_L R_se + I_a R_a + V_b Answer: D. In short shunt the shunt field sits across the armature, so it sees V + I_L R_se and the series field carries I_L. 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