📘 Synchronous Motor – Working Principle & Power Factor Control
Synchronous motor runs at constant speed equal to synchronous speed. It is widely used for power factor correction in industries.
🔹 1️⃣ Construction
Two main parts:- Stator – 3 phase winding
- Rotor – DC excited field winding
🔹 2️⃣ Working Principle
3-phase supply → Rotating magnetic field produced in stator. Rotor excited by DC → Produces constant magnetic field. When rotor locks with stator magnetic field: Motor runs at synchronous speed.N = N_s = 120f / P
Very important: Synchronous motor speed is constant.🔹 3️⃣ Why Not Self Starting?
At start: Rotor at rest RMF rotating No average torque produced. Hence synchronous motor is not self-starting.🔹 4️⃣ Torque Angle (δ)
Torque depends on angle between stator and rotor magnetic fields.T ∝ sinδ
Where δ = torque angle. Maximum torque occurs at: δ = 90° Called pull-out torque.🔹 5️⃣ Power Factor Control (Most Important)
By changing field excitation: Under excited → Lagging PF Normally excited → Unity PF Over excited → Leading PF This is extremely important. Used as: Synchronous condenser (for PF correction).🔹 6️⃣ Example Concept Question
If field excitation increased: Motor operates at leading power factor. Used to improve system power factor.🔹 7️⃣ Important Observations
- Speed independent of load
- Not self-starting
- PF controllable
- Torque depends on δ
🎯 GATE Important Points
- Speed always synchronous
- Torque ∝ sinδ
- Over excitation → Leading PF
- Used for power factor correction
Synchronous Motor = Constant Speed + Power Factor Control
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