Reversing pole winding

The reversing pole winding is a component in DC machines . It is responsible for sufficiently rapid commutation of the armature current and counteracts armature feedback. As a rule, reversing pole windings are only used in larger electrical machines from approx. 1 kW that are operated with different loads.

The main flux is provided via the main pole together with the excitation winding sitting on it. In the area between the main poles, the flux density changes sign, which is why the armature coils that are temporarily in this area have to change the direction of the current. To do this, two adjacent lamellas are short-circuited on the commutator. The short-circuited coil has an inductance that limits the rate of current change. The reversing pole winding is used to accelerate this change in current by compensating for the voltage that occurs via an additional flux linkage. Assuming linearity (no magnetic saturation effects) and taking into account the ohmic resistance, the inductance and the flux linkages, the following applies to the short-circuited coil:

${\displaystyle 0=R\cdot I_{\mathrm {Anker} }+L\cdot {\frac {dI_{\mathrm {Anker} }}{dt}}+{\frac {d\Psi _{\mathrm {Anker} }}{dt}}+{\frac {d\Psi _{\mathrm {Wendepol} }}{dt}}}$

First and foremost, the reversing pole winding extinguishes the armature transverse field via the generated field and thus reduces the armature reaction . The voltage generated by the resulting field counteracts the inductance and enables a higher current rise (or current fall) during the current reversal. Assuming ideal commutation time with linear current path, so the commutation as a function of the induced voltages and the current to be turned can be neglecting the ohmic resistance of to how to specify the following: ${\displaystyle I}$ ${\displaystyle -I}$

${\displaystyle \Delta t={\frac {2\cdot I_{\mathrm {Anker} }\cdot L}{U_{\mathrm {ind} }}}}$

The time remains constant when the induced voltage is proportional to the current. The current through the reversing pole winding should therefore be proportional to the armature current so that the reversal pole flow is also proportional to the armature current. The maximum time available for commutation results from the brush width and the peripheral speed ${\displaystyle U_{\mathrm {ind} }}$

${\displaystyle \Delta t_{\mathrm {max} }={\frac {d_{\mathrm {B{\ddot {u}}rsten} }}{v_{\mathrm {Umfang} }}}}$.

If the current is not reversed after this time , high voltages occur on the brushes, which lead to flashover. The spark from the brush fire corrodes both the commutator bars and the carbon brushes and also causes EMC problems. ${\displaystyle \Delta t_{\mathrm {max} }}$

literature

• Hans Günter Boy, Horst Flachmann, Otto Mai: The master's degree in electrical machines and control technology. 4th edition, Vogel Buchverlag, Würzburg, ISBN 3-8023-0725-9
• Günter Springer: Expertise in electrical engineering. 18th edition, Verlag Europa-Lehrmittel, Wuppertal, 1989, ISBN 3-8085-3018-9