Field weakening

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In rotating electrical machines, field weakening is a reduction in the magnetic flux of the excitation winding , which as a result causes a change in speed . In the series machine and the single-phase series motor , the field weakening is specifically used by means of switchable excitation windings as one of several methods for speed control. In the case of a shunt machine that runs as a generator on a rigid DC voltage network , a field weakening occurs when the DC network voltage drops. If the value falls below a limit value, this can cause a mostly undesirable transition from generator operation to motor operation and, in extreme cases, damage the shunt machine due to an increase in speed. In principle, there is no field weakening when an electrical machine is permanently excited.

Effects

Series motor with switchable excitation winding at D1 for speed change by means of field weakening

The effects of field weakening differ depending on the machine type:

Shunt machine

A shunt machine that runs as a generator on a rigid DC voltage network feeds a smaller current into the network when the field is weakened. If the field is weakened further, the machine switches from generator operation to motor operation and draws a current from the network. The torque changes accordingly: the generator torque decreases, the engine torque increases. The result can be a higher speed; if the field is weakened too much, a large machine can be destroyed by the centrifugal forces that occur .

If the excitation flow disappears completely, for example if there is a line break in the excitation circuit, the current in the armature rises to values ​​that correspond to an electrical short circuit , since only the relatively small ohmic resistance of the armature acts as a limitation. For this reason, the stator field (excitation) must first be built up in a shunt machine before switching on, and only then may the armature be switched on.

Inline machine

The series motor , which also includes the single-phase series motor (universal motor ), as it is often found in small household appliances, cannot run as a generator on a rigid supply network. Generator operation would only be possible with additional devices such as the DC chopper. When the field is weakened, the series motor draws a higher current from the network; this is achieved by additional taps and switchovers on the field winding, and delivers a higher torque. In a series motor, a reduction in the external load torque means a field weakening, which accelerates the motor.

literature

  • Th. Bödefeld, H. Sequence: Electrical machines . Springer, Vienna 1972.
  • Rolf Fischer: Electrical machines . 14th edition. Hanser Verlag, 2009, ISBN 978-3-446-41754-0 .