Energy, Magnetism, Motors, Power systems

Asynchronous x synchronous generators

This post is about a comparison between asynchronous and synchronous generators. Showing main advantages and disadvantages of both.

Links for posts about operation of the two types of generators.

Asynchronous generatorClick here

Synchronous generatorClick here

Vantages and disadvantages of asynchronous generators (induction)

The advantages of this type of generator are:

  • If rotor is squirrel cage, construction is simpler and machine is more robust. Requires less maintenance due to absence of brushes and slip rings. Making it more economical.

Squirrel cage

  • Possess higher power/weight rate, usually are smaller than synchronous generators. Air gap is more narrow than the alternators.

While the disadvantages:

  • Needs to receive reactive power to generate active power. Can come from a capacitor bank or from grid through a power converter. Usually capacitor bank is in an electric panel like this one, with circuit breakers for protection.
Capacitor bank
Capacitor bank is at the bottom of panel.
  • Needs to turn on as induction motor to have necessary residual magnetic flux, to operation as generator.
  • Needs a power converter to stabilize voltage and frequency. In some cases, are used induction generator with wound rotor or doubly fed induction generators (DFIG), which require more maintenance.

Power converter in wind turbine

Vantages and disadvantages of synchronous generators (alternators)

The advantages in use alternator are:

  • The synchronous generator is more efficient than induction one in big scale. Because synchronous generators have better power factor and induction ones need capacitor bank to provide magnetization current, which in big scale, increases significantly the cost.
  • Higher stability of voltage and frequency due to synchronism of rotor speed and rotating magnetic field.

While the disadvantages:

  • Needs a source of direct current to supply the rotor. Unless it is a permanent magnet rotor, in this case, is used only for low power applications. Because there is scarcity of materials for construction of strong magnets to rotor.

Synchronous generators operation

  • Alternators with wound rotor requires more maintenance, due to brushes and slip rings weariness.


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