📘 Induction Motor – Circle Diagram (Step-by-Step Explanation)
Circle Diagram is a graphical method to determine current, power factor, torque, losses and efficiency of induction motor. Very important for conceptual clarity.
🔹 1️⃣ Why Circle Diagram?
Instead of solving many equations repeatedly, we draw one circle and read:- Input current
- Power factor
- Output power
- Torque
- Efficiency
🔹 2️⃣ Basic Principle
From equivalent circuit: Current equation shows that stator current locus forms a circle. When slip changes → Rotor resistance changes → Current changes. That variation traces a circle.🔹 3️⃣ How Circle is Constructed?
Step 1: Perform No-Load Test → Get I₀ and power factor. Step 2: Perform Blocked Rotor Test → Get I_sc and power factor. Step 3: Draw current phasor diagram. Step 4: Construct circle using geometrical method.🔹 4️⃣ Important Points on Circle
• Leftmost point → No load • Rightmost point → Blocked rotor • Top point → Maximum output • Vertical line → Power line🔹 5️⃣ Power Calculation Using Circle
From diagram: Horizontal projection → Active power Vertical projection → Reactive power Air gap power and mechanical power are obtained using segments.🔹 6️⃣ Torque from Circle Diagram
Torque ∝ Rotor current component. Maximum torque occurs at specific point on circle. Matches condition:R₂'/s = X₂'
Same as analytical method.🔹 7️⃣ Important Observations
- Circle diagram valid at constant voltage
- Gives complete performance picture
- Slip variation shifts operating point on circle
- Used to calculate efficiency graphically
🔹 8️⃣ Example Concept Question
If slip increases, operating point moves towards blocked rotor point. Therefore: Current increases Power factor reduces Rotor copper loss increases🎯 GATE Important Points
- Know construction steps clearly
- Understand power projection method
- Know maximum torque point location
- Slip controls position on circle
Circle Diagram = Graphical Performance Calculator
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