Dynamo machines

In most Dynamo-machines to create the required magnetic field is not permanent and the electromagnet. However, the core of the electromagnet is magnetized a little, and his force field enough the car began to generate electricity at power up. Therefore, the portion of the produced current goes through the electromagnet winding reinforcing its magnetic field and the increase of electricity.
Some alternators (such as a car) produce direct current with built-in rectifiers - devices that allow current flow in only one direction.
In most alternators - battery vehicles, which are used for recharging, to huge machines generating electricity for the mains, the coils are on the rotor and on the stator, that the rotor generates a magnetic field. Relatively weak current passes through the excitation winding on the rotor by brushes and slip rings, and a stronger current, which is produced, comes directly from the stator. This avoids power loss and arcing, which can occur during weaning produced a strong current of rotor by means of rings and brushes.