I'd like to know if there is some magnet in the rotor or in the stator of the generator? Or neither of those (rotor & stator) need a magnet for the current to flow?

  • \$\begingroup\$ All motors and generators use magnetism, some use permanent magnets and others use electromagnets. \$\endgroup\$ – Andy aka Oct 6 '14 at 7:34

Depends on the type of electrical machine. All electrical machines can operate as Generators & Motors.

An electrical machine is an electrical <-> kinetic energy converter which utilises magnetic fields. The real question is by what means does the stator & rotor generate its magnetic field.

Types of electrical machines

Brushed DC

  • Stator: Wound or permanent magnet
  • Rotor: wound

Induction Machine

  • Stator: Wound
  • Rotor: Wound

Synchronous machine

  • Stator: Wound
  • Rotor: Wound or permanent magnet

Brushless DC or AC (these are sync machines)

  • Stator: Wound
  • Rotor: Permanent magnet

Switched Reluctance

  • Stator: Wound
  • Rotor: neither

This is ignoring inside out machines.

For higher power generators however, national grid type machines, the topologies of these machines are wound rotor synchronous machines

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All electro-magnetic generators need magnetic field to induce electric current. This is called excitation.

Some generators use permanent magnets to create magnetic field.

  • usually small and low power
  • simple to build
  • simple to use
  • no voltage/power control (only by changing applied speed/torque)

Some generators use field coils to create magnetic field.

  • they have disadvantage - you cannot create magnetic field without flowing current. Not excited generator will not take mechanical energy (except loss on friction etc). You have to deliver a little bit power to generator for startup (It's like lighting a candle, it will burn after you remove source of fire). Later it creates electricity to power field coils. Note that this is not perpetuum mobile - when magnetic field appear - mechanical energy will be taken (rotor will resist). Very large generators in power plants are connected to grid, and almost never stop. Small generators without magnets usually rely on remanent magnetization. More: link

  • high power - field coil can produce stronger field than a magnet, so generator with field coils can be smaller than generator with magnets. This is one of reasons why there are no generators based on permanent magnets in power plants (they probably would be much bigger)

  • good control: voltage (and power) doesn't have to be proportional to speed because excitation current can be adjusted to archieve same output voltage or power at diffrent speeds.

Read also JonSB answer above/below to see types of generators. I didn't listed them because he did it and I'm afraid my english is not good enough for that kind of answer.

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