📘 DC Motor – Speed Equation & Speed Control
Speed control of DC motor is one of the most important applications. Understanding speed equation is fundamental.
🔹 1. Speed Equation
We know:Back EMF E = V − I_a R_a
Also,E ∝ Φ N
Therefore:N ∝ (V − I_a R_a) / Φ
This is the most important formula.🔹 2. Methods of Speed Control
Three main methods:- Field Control
- Armature Resistance Control
- Armature Voltage Control
🔹 3. Field Control Method
Speed inversely proportional to flux.N ∝ 1 / Φ
Reducing field current → Reduces flux → Speed increases. Used for: Speed above rated value.🔹 4. Armature Resistance Control
Increase R_a → Voltage drop increases → Speed decreases.N ∝ (V − I_a R_a) / Φ
Used for: Speed below rated value. Disadvantage: Power loss in external resistor.🔹 5. Armature Voltage Control
Vary supply voltage directly. Most efficient method. Used in: Thyristor-based drives.🔹 6. Example
Given: V = 220 V I_a = 20 A R_a = 0.5 Ω Φ constant Find back EMF. Solution:E = 220 − (20 × 0.5)
E = 210 V
Since speed proportional to E, speed reduces slightly.🔹 7. Important Observations
- Field control → Speed above rated
- Armature control → Speed below rated
- Series motor speed control different
- Voltage control most efficient
🎯 GATE Important Points
- Remember speed formula clearly
- Know which method gives high/low speed
- Field control economical
- Armature control wastes power
Speed ∝ (Voltage Drop Corrected) / Flux
.png)
No comments:
Post a Comment