DC machine

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When the coil is powered, a magnetic field is generated around the armature. The left side of the armature is pushed away from the left magnet and drawn toward the right, causing rotation.

A DC motor converts electrical energy in mechanical energy (or vice versa) and is operated with direct current. It consists of a stationray stator and a moveable rotor. Advantages are the good start-up behaviour and controllability.

The armature continues to rotate.

When the armature becomes horizontally aligned, the commutator reverses the direction of current through the coil, reversing the magnetic field. The process then repeats.

Streamlines within a DC machine

As long as the differently adjusted D.C. machines are at a constant voltage power grid U_N, they do not differ in their operational behaviour. In the shunt-wound machine, only the size of the excitation voltage differs from the machine voltage.

In permanently excited D.C. machines, permanent magnets are used in the stator with distinctive north and south poles for the excitation. On the rotor, several rotor windings are accommodated in slots, which are connected with the commutator. The input and output of the rotor current takes place through carbon brush conductors, which have a sliding contact to the commutator. The commutator exists of from each other isolated contact areas. It causes an angle-dependent adjustment of the rotor winding. At a correspondingly high number of rotor windings, a roughly constant torque can be produced. The rotation direction of the motor can be inverted via the polarity of the applied voltage.

The mechanical sliding contact to the commutator leads to a wear out of the carbon brush conductors, so that these must be renewed in regular intervals. The use of permanently excited D.C. motor requires thus unavoidable maintenance costs, which can be substantially reduced, however, by the usage of brushless machines. The brushless direct current motor is characterized by the fact that, in comparison to conventional permanent-excited D.C. engines, stator and rotor exchange their places. The rotor is thus implemented by a permanent magnet, while the original rotor windings are shifted onto the stator. In this way, no more carbon brush conductors are needed, as the rotor does not have to be supplied with power anymore. Thus, the engine is maintenance-free in the ideal case.

The magnetic field is formed according to the current flow within the rotor windings. It drives or brakes the machine. With the arrangement above, the permanent magnet corresponds to the stator and the conducting circuit represents the rotor. An increase on the number of rotors leads to an increase of the number of charge carriers in the magnetic field and subsequently leads, exactly as with a power increase, to a reinforcement of the Lorenz strength, which altogether affects the rotor.

References

• H. Wallentowitz
  Alternative Vehicle Propulsion Systems
  August 2003
• [1]