Commutator (electric)

In electrical engineering, is the commutator (from Latin commutare - swap ), collector or commutator means for reversing the polarity ( commutation ) referred to in electrical machines.

History

Already in 1834 designed Moritz Hermann von Jacobi first commercially useful forerunner of today's commutator for his motor, with which he on the Neva River in Saint Petersburg drive a ship on 13 September 1838.

Function and structure

Generating a commutation from winding to winding of the rotor and changing migratory stream and is necessary for direct current machines, in order to produce torque at rotational movement. Since opposite poles attract, the Rev. changes to bring the motor to rotate. Commutators consist of a classic sliding contact between the blades of the collector, and two or more brushes ( → DC machine )

The blades are made of copper and form during operation, a gray-brown layer. You are stuck on an insulating cylinder or circular area and have an air - gap - The abrasion of the brush can not fix and therefore does not affect the insulation. The brushes are made of graphite, partly mixed with copper powder, or - in some small engines - all metal.

Operation

Excessive sparking at the commutator must be avoided because the resulting heat would cause wear. These commutating poles are used on large motors, which ensure that the excitation field is always aligned with changing loads of the motor, that the commutation is less tension between the commutator bars in the area. This is necessary because the brush is always applied at the same time at more than one lamella and during commutation adjacent slats shorting together. In the ideal case, the voltage between the protruding blades of the brush zero. One therefore used as many fins and armature windings and brushes as narrow as possible. To achieve a high current carrying capacity, widen to the collector and the brushes. The brushes wear in operation and are shorter. Therefore you get them often in metallic guides below, in which she expresses a spring on the collector. For better contact in the brush to the power connector, this often has a pressed- copper wire or she is directly soldered to a bronze leaf spring.

Carbon brushes are wear parts and therefore can be substituted for most motors. The commutator wear out, but much slower than the carbon brush. For a given lamella thickness stress can be used to increase the service life of the motor.

Avoid sparking

At a relatively slow running engines occur during operation with 50 Hz alternating current to multiple power direction change in each of the brush -fed armature winding of the coils. Here, during the change of the actual current flowing through the collector segments carried by the rotation when passing through the brushes usually no or only a random coordination between the moment of the disk change and the change in polarity with the lower voltage minimum of the AC wave. The result are spark discharges that cause particularly at high motor ratings considerable wear. For this reason, the frequency of the traction current was found to low values ​​15 to 16 ⅔ Hz catenary voltages between 10 and 15 kV for a low cost remote transmission an operational portable compromise for electrically operated standard gauge railways in the early 20th century.

The brushless or electronic commutation wear-free. In so-called brushless DC motor such as Hall sensors are used for rotor position detection, which drive the coils via power driver ( transistors, thyristors, triac ).