Lesson Notes By Weeks and Term v3 - Senior Secondary 2
D.C Generator Field System
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
D.C Generator Field System
Download the Lessonotes Mobile Nigeria 2025 app for faster lesson access on Android and iPhone.
Subject: Basic Electricity
Class: Senior Secondary 2
Term: 1st Term
Week: 3
Theme: Electrical Machine
This page supports the lesson note with a companion video and a short classroom-ready summary.
For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.
Identify types ofd.c. generators. Differentiatesbetween seriesand shunt and compound d.cgenerators. State the uses ofd.c. generator.
(R_sh is large).
Characteristics: Field current (I_sh = V_t / R_sh) is relatively constant, assuming terminal voltage is stable. Provides a relatively constant voltage output over a wide range of loads, although voltage drops slightly with increasing load due to armature reaction and internal resistance drop. Good voltage regulation compared to series generators.
Circuit Diagram: ``` Load | ---(+)---Armature---(-)--- | | | | | [ R_sh ] | | ------------------------- ``` Uses: Most common type for general D.C. power supply where a constant voltage is required. Applications include battery charging (e.g., for vehicles, solar power systems in rural homes), electrolysis, electroplating (e.g., for plating jewelry or industrial components in Aba), and as exciters for large A.C. alternators. c. Compound Wound D.
C. Generator: Field Winding Connection: This generator has both a series field winding and a shunt field winding. The combination of these two windings gives it characteristics that can be tailored for specific applications.
Types of Compound Generators: Long Shunt Compound: The shunt field winding is connected in parallel with the series field winding and the armature.
Short Shunt Compound: The shunt field winding is connected in parallel only with the armature.
Types based on Field Flux Direction: Cumulative Compound: The magnetic flux produced by the series field aids (adds to) the magnetic flux produced by the shunt field.
Characteristics: Provides excellent voltage regulation over a wide load range.
Can be designed as: Over-compounded: Terminal voltage rises with increasing load. Used to compensate for voltage drops in long feeders (e.g., for power supply to a distant village).
Flat-compounded: Terminal voltage remains nearly constant from no-load to full-load. Ideal for general D.C. power supply.
Under-compounded: Terminal voltage drops slightly with increasing load (less than a shunt generator).
Uses: General D.C. power supply, industrial applications requiring steady voltage, driving D.C. motors, and applications where voltage needs to remain stable over varying loads.
Differential Compound: The magnetic flux produced by the series field opposes (subtracts from) the magnetic flux produced by the shunt field.
Characteristics: Terminal voltage drops sharply with increasing load.
Uses: Seldom used for normal power supply due to poor voltage regulation. Primarily used in arc welding (similar to series generators but with some shunt characteristics for stability) and sometimes for special applications requiring a large current limit, where large current draw must be prevented (e.g., to protect against short circuits). Circuit Diagram (Long Shunt Cumulative Compound): ``` Load | ----[ R_se ]---- | | | -----(+)- Armature -(-)-- | | | | | [ R_sh ] | | ------------------------- ``` Circuit Diagram (Short Shunt Cumulative Compound): ``` Load | ----[ R_se ]---- | | | -----(+)- Armature -(-)-- | | | | | [ R_sh ] | | ------------------------- ``` Differentiation between Shunt and Compound D.
C. Generators: | Feature | Shunt D.
C. Generator | Compound D.
C. Generator | | :-------------------- | :---------------------------------------------------- | :----------------------------------------------------- | | Field Winding | One field winding (shunt field) | Two field windings (series and shunt fields) | | Connection | Shunt field connected in parallel with armature. | Both series and shunt fields present. Shunt field can be long or short shunt. Series field always in series with armature/load. | | Turns/Wire Size | Shunt field: Many turns, thin wire (high resistance). | Shunt field: Many turns, thin wire.
Series field: Few turns, thick wire (low resistance). | | Field Current | Shunt field current (I_sh) depends on terminal voltage. | Shunt field current (I_sh) depends on terminal voltage; Series field current (I_se) is load current or armature current. | | Voltage Regulation| Relatively good, voltage drops slightly with load. | Excellent, can be designed for constant (flat), rising (over), or slightly dropping (under) voltage with load. | | Complexity | Simpler construction and wiring. | More complex construction and wiring due to two field windings. | | Applications | Battery charging, electroplating, general D.C. supply. | General D.C. supply, industrial motors, applications needing very stable voltage over varying loads, arc welding (differential). | Uses of D.
C. Generators (General):
1. Battery Charging: Shunt generators Recall: A D.C. generator operates on the principle of electromagnetic induction (Faraday's Law), where a conductor cutting through magnetic flux lines generates an electromotive force (EMF). The two main parts of a D.C. generator are the armature (where EMF is induced) and the field system (which produces the magnetic flux). This lesson focuses on the various methods used to establish and maintain this magnetic field within the generator. The D.
C. Generator Field System: The field system of a D.C. generator is responsible for producing the magnetic flux that the armature conductors cut to generate voltage. This magnetic field is typically produced by electromagnets, which consist of field coils wound around pole pieces. The way these field coils are supplied with current (excited) determines the type and characteristics of the D.C. generator. Methods of Excitation for D.
C. Generators: D.C. generators are primarily classified based on how their field windings are excited:
1. Separately Excited D.
C. Generators: Explanation: In this type, the field winding is energized by an independent external D.C. power source (e.g., a battery or a separate D.C. power supply). The field current is independent of the armature current.
Characteristics: Provides excellent control over the field current and thus the generated voltage.
Uses: Used in laboratories for experimental purposes, for precise speed control of D.C. motors, and in special industrial applications where a wide range of voltage control is required.
Limitation: Requires an additional, separate D.C. source.
2. Self-Excited D.
C. Generators: Explanation: In these generators, the field winding is supplied with current by the generator's own generated EMF. This is possible due to residual magnetism present in the pole pieces. When the armature starts rotating, it cuts this residual flux, generating a small EMF. This small EMF then drives a small current through the field winding, strengthening the magnetic field, which in turn increases the generated EMF, and the process continues until the rated voltage is reached.
Types of Self-Excited Generators: a. Series Wound D.
C. Generator: Field Winding Connection: The field winding (series field, R_se) is connected in series with the armature winding (R_a) and the load. This means the entire armature current (I_a) flows through the series field winding.
Construction: The series field winding consists of a few turns of thick wire, making its resistance very low (R_se is small).
Characteristics: Generated voltage is directly proportional to the load current. As load current increases, field current increases, strengthening the magnetic field, and thus increasing the generated voltage. Poor voltage regulation, as output voltage varies significantly with load. Can build up high currents rapidly under load.
Circuit Diagram: ``` Load | ----[ R_se ]---- | | | -----(+)- Armature -(-)-- | | ------------------------- ``` (Where R_se is series field resistance, R_a is armature resistance)
Uses: Due to its rising voltage characteristic with increasing load, it is not suitable for supplying constant voltage to general loads. It is primarily used as a booster in distribution systems (to compensate for voltage drop) and, more importantly, in D.C. arc welding machines (e.g., those used by fabricators and welders in industrial estates like Nnewi or Lagos) where a high, varying current is needed for arc stability. Also used in D.C. locomotives (older designs). b. Shunt Wound D.
C. Generator: Field Winding Connection: The field winding (shunt field, R_sh) is connected in parallel (shunt) with the armature winding (R_a) and the load. The terminal voltage (V_t) is applied across the shunt field winding.
Construction: The shunt field winding consists of many turns of thin wire, making its resistance very high (R_sh is large).
Characteristics: Field current (I_sh = V_t / R_sh) is relatively constant, assuming terminal voltage is stable. Provides a relatively constant voltage output over a wide range of loads, although voltage drops slightly with increasing load due to armature reaction and internal resistance drop. Good voltage regulation compared to series generators.
Circuit Diagram: ``` Load | ---(+)---Armature---(-)--- | | | | | [ R_sh ] | | ------------------------- ``` Uses: Most common type for general D.C. power supply where a constant voltage is required. (I_se) is load current or armature current. | | Voltage Regulation| Relatively good, voltage drops slightly with load. | Excellent, can be designed for constant (flat), rising (over), or slightly dropping (under) voltage with load. | | Complexity | Simpler construction and wiring. | More complex construction and wiring due to two field windings. | | Applications | Battery charging, electroplating, general D.C. supply. | General D.C. supply, industrial motors, applications needing very stable voltage over varying loads, arc welding (differential). | Uses of D.
C. Generators (General):
1. Battery Charging: Shunt generators are widely used for charging D.C. batteries (e.g., car batteries, solar battery banks in homes and businesses across Nigeria).
2. Electroplating and Electrolysis: Processes requiring a steady D.C. supply for chemical reactions, like chrome plating or producing chemicals, are powered by D.C. generators.
3. Arc Welding: Series or differentially compound generators provide the high, variable current necessary for arc welding operations, common in fabrication workshops and construction sites.
4. Excitation of Alternators: D.C. generators (often shunt-wound) are used as exciters to provide the D.C. field current for larger A.C. alternators in power stations.
5. D.
C. Motor Drives: Providing power to D.C. motors, especially in older industrial applications (e.g., some printing presses, textile mills) and specialized D.C. railway traction systems where precise speed control is needed.
6. Remote or Backup Power: In areas without grid electricity, small D.C. generators can be used to power D.C. loads directly, or charge batteries for later use.
Teacher Activities: Introduction (10 minutes): Begin by reviewing the basic principle of a D.C. generator and the function of the field system from previous lessons. Engage students by asking questions about where D.C. power might be needed in Nigeria (e.g., "How do vulcanizers charge car batteries if NEPA goes off?"). Introduce the concept that how the magnetic field is created (excited) determines the generator's characteristics.
Types of Excitation (15 minutes): Explain "Separately Excited" and "Self-Excited" concepts, emphasizing the role of residual magnetism for self-excitation. Draw and explain the circuit diagram for a separately excited generator on the board. Self-Excited Generators - Series (20 minutes): Draw the circuit diagram for a series-wound D.C. generator clearly on the board. Explain the connection (series with armature and load), winding characteristics (few turns, thick wire, low resistance). Discuss the voltage-load characteristic (rising voltage) and why it's suitable for arc welding, giving examples of welders in Nigerian markets. Self-Excited Generators - Shunt (20 minutes): Draw the circuit diagram for a shunt-wound D.C. generator. Explain the connection (parallel with armature and load), winding characteristics (many turns, thin wire, high resistance). Discuss the voltage-load characteristic (relatively constant voltage) and its suitability for battery charging, electroplating, using Nigerian examples. Self-Excited Generators - Compound (25 minutes): Introduce compound generators as a combination of series and shunt windings. Explain "Long Shunt" and "Short Shunt" connections with clear diagrams. Explain "Cumulative" and "Differential" compounding by discussing how the magnetic fluxes interact. Focus on Cumulative's better voltage regulation (over, flat, under) and Differential's use in welding/current limiting. Emphasize the advantages of compound generators for various industrial applications.
Differentiation and Uses (15 minutes): Lead a class discussion to collaboratively build a comparison table (similar to the one in "Key Concepts") for series, shunt, and compound generators on the board. Solicit student input for various uses of D.C. generators in Nigeria based on the characteristics discussed.
Summary and Q&A (10 minutes): Summarize the key points of the lesson. Address any student questions or misconceptions.
Student Activities: Note-Taking: Students will actively take notes during explanations and diagramming.
Observation: Observe and analyze the circuit diagrams drawn on the board.
Participation: Actively participate in discussions, answer questions, and contribute ideas for generator applications.
Diagramming: Students will be encouraged to sketch the circuit diagrams of each generator type in their notebooks.
Comparison Task: In pairs or small groups, students will work on completing a comparison table for the different generator types, using information from the lesson.
Application Brainstorm: Brainstorm and list various real-world applications of D.C. generators in Nigeria.
Rural Electrification and Small Businesses: In many parts of Nigeria, particularly remote villages or small towns, D.C. generators are used to charge batteries that power homes, small businesses (e.g., barber shops, phone charging kiosks), and communication systems. For example, a shunt generator might be employed at a community charging hub to ensure a stable voltage output for various devices, directly addressing a common power challenge in the country.
Industrial Fabrication and Repairs: The ubiquity of arc welding in Nigeria, from roadside mechanics to large-scale construction projects and oil & gas facilities, highlights a crucial application. Series-wound or differentially compound D.C. generators are favored for welding machines due to their ability to provide the high, variable current necessary for stable arc formation and penetration, crucial for fabricating gates, vehicles, and structural components.
Specialized Industrial Processes: Industries like metal plating (e.g., galvanizing, chrome plating for car parts in local markets like Ladipo), or even small-scale chemical production often require a continuous, stable D.C. power supply. Shunt or compound D.C. generators are instrumental in these processes, ensuring the precise current and voltage needed for quality control and efficient operation. This integrates the concept with Nigerian entrepreneurial ventures and industrial practices.