Induction vs Synchronous Generator Working Principle Differences
AC machines are split into two primary types: synchronous machines and asynchronous machines. Asynchronous machines are commonly known as induction machines. There are clear distinctions between these two machine types. Below we cover the major differences between induction and synchronous generators.
Differences in construction
Synchronous generators: The stator features axial slots fitted with stator windings configured for a set pole count. Most designs adopt a salient-pole rotor carrying rotor windings. DC excitation power is fed to rotor windings through slip rings. Permanent magnet rotors are also available for synchronous generator designs.
Induction generators: Their stator windings are wound for a defined number of poles. Two rotor configurations are available: squirrel-cage rotor and wound rotor. For squirrel-cage rotors, rotor conductors are permanently short-circuited by end rings. Wound rotor designs have rotor windings connected to slip rings.
Differences in operation
Synchronous generators: When three-phase power is applied to the stator, stator magnetic poles revolve at synchronous speed. The rotor receives DC excitation. For startup, the rotor must be accelerated to a speed close to synchronous speed. Once this condition is achieved, rotor magnetic poles lock with the rotating stator magnetic field, making the rotor turn at synchronous speed.
Induction generators: When two-phase or three-phase AC power energizes the stator, a rotating magnetic field forms. Relative motion between the stator rotating magnetic field and the rotor induces current inside rotor conductors. This rotor current creates rotor magnetic flux. Per Lenz’s law, the induced current works to counteract its source, which is the speed difference between the stator field and rotor. As a result, the rotor chases the rotating magnetic field to minimize this relative speed.
Other key differences
Synchronous generators need an extra DC power source to excite rotor windings. Induction generators run without any separate excitation power supply.
Synchronous generators require slip rings and brushes, unlike induction generators.
Synchronous generators rely on auxiliary starting equipment to bring the rotor up to near-synchronous speed. Induction generators do not need dedicated starting mechanisms.
By adjusting excitation current, synchronous generators can operate at lagging, unity or leading power factors. Induction generators always operate with lagging power factor.
Synchronous generators generally deliver higher efficiency compared with induction generators.
Synchronous generators come with higher procurement costs.
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