--- title: "Methods of excitation and types of DC generator" subject: "Electrical Machines" chapter: 3 key: "semester_4/electrical_machines:dc-generator-excitation-types" source: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-generator-excitation-types site: BCT Engineering --- # Methods of excitation and types of DC generator Electrical Machines · Chapter 3: DC Generator - Asked in: 2075 Baisakh (old course), 8 marks - Video: [What are Different Types of Excitation for DC machines? | Classification of DC Machines](https://www.youtube.com/watch?v=MrHfEOE47m4) — EE-Lectures - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/electrical_machines/dc-generator-excitation-types - Whole chapter: https://www.bctengineer.com/md/semester_4/electrical_machines/chapter-3 ## Question Describe the method of excitation and types of D.C. Generator. ## Short answer Excitation supplies field current. DC generators are separately excited (field fed from an external source) or self-excited. Self-excited types include shunt (field across armature), series (field in series), and compound (both shunt and series fields, either cumulative or differential). ## Answer **Excitation** means supplying current to the field winding to produce the main flux. On this basis DC generators are divided into two groups. #### 1. Separately excited generator The field winding is fed from an independent DC source (battery or another generator). ```text +----[Rheo]---[ Field ] Armature | Ext. DC (E, Ra)----> Load ``` - Field current is independent of the armature load, so flux is nearly constant. - Terminal voltage falls only slightly with load (armature drop and armature reaction). - Used where wide voltage control is needed (e.g. Ward-Leonard system, testing). #### 2. Self-excited generator The field current is taken from the generator's own armature. Residual magnetism is required to start. **(a) Shunt generator:** the field winding (many turns of thin wire, high resistance) is connected across the armature. $$ I_a = I_L + I_{sh}, \qquad V = E - I_aR_a $$ It gives nearly constant voltage with moderate drooping; used for battery charging and lighting. **(b) Series generator:** the field winding (few turns of thick wire) is in series with the armature and load. $$ I_a = I_{se} = I_L, \qquad V = E - I_a(R_a + R_{se}) $$ The voltage rises with load current. Used as a booster and in arc lamps. **(c) Compound generator:** has both shunt and series fields. - **Short shunt:** the shunt field is across the armature only; the series field is in series with the load. - **Long shunt:** the shunt field is across the armature and series field combination. - **Cumulative compound:** the series field aids the shunt field (flat, over- or under-compounded). - **Differential compound:** the series field opposes the shunt field (drooping voltage; used in welding). ```text Short shunt Long shunt +--[Ra]--+--[Rse]---o +--[Ra]--[Rse]--+---o | | | | | [Rsh] Load | [Rsh] Load +--------+------------o +---------------+---o ``` ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. Excitation of a DC generator means A. Supplying current to the field winding to produce the main flux B. Rotating the armature C. Connecting the load D. Adding brushes Answer: A. Generators are classed as separately excited or self-excited by where the field current comes from. (Key point: Meaning of excitation) 2. A separately excited generator has A. Its field fed from its own armature B. Its field fed from an independent DC source, so field current is independent of load C. Only a series field D. No field winding Answer: B. Flux is nearly constant, so the terminal voltage falls only slightly with load; it is used where wide voltage control is needed. (Key point: Separately excited generator) 3. For a series generator, the terminal voltage as load current increases A. Falls steadily B. Rises (until saturation) C. Stays exactly constant D. Becomes zero Answer: B. Field current equals load current, so more load raises flux and emf: V = E - Ia(Ra + Rse). (Key point: Series generator) 4. A differential compound generator is one in which the series field A. Aids the shunt field B. Opposes the shunt field, giving a drooping voltage (used in welding) C. Is absent D. Is across the armature only Answer: B. In cumulative compounding the series field aids the shunt field; in differential compounding it opposes it. 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