📘 DC Generator – Characteristics (OCC, Internal & External)
DC generator characteristics describe the relationship between voltage and load current. Understanding these curves is very important for GATE and interviews.
🔹 1. Types of Characteristics
There are three important characteristics:- Open Circuit Characteristic (OCC)
- Internal Characteristic
- External Characteristic
🔹 2. Open Circuit Characteristic (OCC)
OCC is the graph between:Generated EMF (E) vs Field Current (I_f)
At constant speed and no-load condition. Important features:- Initially linear
- Then saturation occurs
- Due to B-H curve non-linearity
🔹 3. Internal Characteristic
Graph between:Generated EMF (E) vs Armature Current (I_a)
Takes into account armature reaction. Voltage slightly decreases as load increases.🔹 4. External Characteristic
Graph between:Terminal Voltage (V) vs Load Current (I_L)
Includes:- Armature reaction
- Voltage drop due to armature resistance
🔹 5. Example – Voltage Drop
Given: Generated EMF E = 250 V Armature current I_a = 40 A Armature resistance R_a = 0.5 Ω Find terminal voltage. Solution: Voltage drop = I_a R_a = 40 × 0.5 = 20 VTerminal voltage V = 250 − 20 = 230 V
🔹 6. Important Observations
- OCC shows magnetic saturation
- External characteristic most practical
- Voltage drop increases with load
- Armature reaction weakens main flux
🎯 GATE Important Points
- OCC drawn at constant speed
- Voltage decreases with load
- Know difference between E and V
- Armature reaction reduces flux
OCC → Magnetic Behavior External → Practical Performance
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