📘 DC Motor – Starting Methods (3-Point & 4-Point Starter)
DC motors cannot be started directly at rated voltage. At starting, back EMF is zero → very high current flows. Starter is used to limit this current.
🔹 1. Why Starter is Required?
At starting:E = 0
Therefore:I_a = V / R_a
Since R_a is very small → Starting current extremely high. Can damage:- Armature winding
- Commutator
- Brushes
🔹 2. Principle of Starter
Add external resistance in series with armature. As speed increases → Back EMF increases → Reduce resistance gradually.🔹 3. 3-Point Starter
Terminals:- L – Line
- A – Armature
- F – Field
- Starting resistance
- Overload release (OLR)
- No-voltage coil (NVC)
Working:
• Handle moved gradually • Resistance reduced step by step • At full speed, no external resistance🔹 4. Limitation of 3-Point Starter
If field current reduced for speed control: NVC weakens → Motor may stop accidentally.🔹 5. 4-Point Starter
Terminals:- L – Line
- A – Armature
- F – Field
- N – Separate supply for NVC
🔹 6. Example Concept Question
Why is DC motor not self-starting at rated voltage?Because back EMF is zero at starting → Excessive armature current flows.
🔹 7. Starting Current Example
Given: V = 220 V R_a = 0.5 Ω Starting current:I = 220 / 0.5 = 440 A
Very dangerous.🎯 GATE Important Points
- Starter limits initial current
- 3-point vs 4-point difference
- Back EMF zero at start
- Starting current = V / R_a
Starter = Protection + Controlled Acceleration
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