What is the Difference Between Asynchronous and Synchronous Motors?

Core Concept in One Sentence

Induction Motor: Rotor chases the magnetic field, always slightly behind (has slip).

Synchronous Motor: Rotor locks with the magnetic field, turning at exactly the same speed (synchronous).

Five Key Differences at a Glance

Aspect	Induction (Asynchronous) Motor	Synchronous Motor
1. Working Principle	Electromagnetic Induction: Rotating field induces current in rotor, creating its own magnetic field to produce torque.	Magnetic Locking: Rotor has its own fixed magnetic field (from magnets or DC current) that locks onto the stator's rotating field.
2. Speed	n < n₀ (Has slip) • Speed drops as load increases.	n = n₀ (Absolutely synchronous) • Speed remains constant regardless of load (within limits).
3. Rotor Construction	Simple & Robust • Squirrel-cage (copper/aluminum bars) • Wound rotor	More Complex • Permanent Magnet (PM) • Wound Field (DC excited) • Reluctance (salient pole)
4. Starting	Self-starting • Direct-on-line (small power) • Reduced voltage/soft starter (large power)	Not self-starting • Needs a damper/amortisseur winding to start as an induction motor first. • Or uses a VFD to start from zero to synchronous speed.
5. Power Factor	Always consumes reactive power • Operates at lagging PF (typically 0.7-0.9) • PF is very low at light loads.	Adjustable Power Factor (for wound field type) • Can be set to lagging, unity, or leading. • Can supply reactive power to the grid, acting as a compensator.