| Working Principle |
Rotor rotates at synchronous speed (same as stator field) |
Rotor rotates at less than synchronous speed |
| Slip |
Zero slip (S = 0) |
Slip required (S > 0) |
| Speed |
Constant speed regardless of load |
Varies slightly with load |
| Starting |
Not self-starting (needs damper winding/VFD) |
Self-starting |
| Rotor Excitation |
Requires DC excitation |
No external excitation (induced current) |
| Construction |
Complex and costly |
Simple and rugged |
| Power Factor |
Can operate at lagging, leading, or unity |
Mostly lagging power factor |
| Efficiency |
Higher efficiency (especially at large ratings) |
Slightly lower than synchronous |
| Maintenance |
More maintenance (brushes, excitation system) |
Less maintenance |
| Applications |
Power factor correction, compressors, large drives |
Pumps, fans, conveyors, general loads |
| Cost |
Higher |
Lower |
| Torque |
No starting torque (without auxiliary means) |
Good starting torque |
| Speed Control |
Difficult without VFD |
Easier with VFD |
