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تعریف کommenزیشن
کommenزیشن فرآیند معکوس کردن جریان در سیم پیچ برای حفظ کارایی بالای موتور است.
سه روش اصلی برای بهبود کommenزیشن وجود دارد.
کommenزیشن مقاومتی
کommenزیشن با E.M.F
سیم‌پیچ‌های جبران‌کننده
کommenزیشن مقاومتی
در این روش کommenزیشن، ازブラシの抵抗が高い電気抵抗ブラシを使用して、火花のない換流を実現します。これは、低抵抗の銅製ブラシを高抵抗の炭素ブラシに置き換えることで得られます。
図から明らかなように、コイルCからの電流ICは、換流期間中に2つの経路でブラシに到達する可能性があります。1つ目の経路は、コムミュテータセグメントbを通って直接ブラシに到達する経路で、2つ目の経路はまずショートサーキットコイルBを通ってから、コムミュテータセグメントaを通ってブラシに到達する経路です。ブラシの抵抗が低い場合、コイルCからの電流ICは比較的抵抗が低い最初の経路を選択します。
高抵抗ブラシを使用すると、ブラシがコムミュテータセグメントに近づくにつれて、ブラシとセグメントbの接触面積が減少し、セグメントaとの接触面積が増加します。電気抵抗は接触面積に反比例するため、ブラシが動くにつれて抵抗Rbが増加し、Raが減少します。そのため、電流は2つ目の経路を選んでブラシに到達します。
この方法により、電流が望まれる方向に迅速に逆転し、換流が改善されます。
ρは導体の抵抗率です。
lは導体の長さです。
Aは導体の断面積（ここでは接触面積として使用されています）です。
E.M.F. Commutation
The main reason of the delay of the current reversing time in the short circuit coil during commutation period is the inductive property of the coil. In this type of commutation, the reactance voltage produced by the coil due to its inductive property, is neutralized by producing a reversing emf in the short circuit coil during commutation period.
Reactance Voltage
The voltage rise in the short circuit coil due to inductive property of the coil, which opposes the current reversal in it during the commutation period, is called the reactance voltage.
We can produce reversing emf in two ways
By brush shifting.
By using inter-poles or commutating poles.
Brush Shifting Method of Commutation
In this method of improving commutation the brushes are shifted forward direction for the DC generator and in backward direction for the motor for producing the sufficient reversing emf for eliminating the reactance voltage. When the brushes are given the forward or backward lead then it brings the short circuit coil under the influence of the next pole which is of the opposite polarity. Then the sides of the coil will cut the necessary flux form the main poles of opposite polarity for producing the sufficient reversing emf. This method is rarely used because for best result, with every variation of load, the brushes have to be shifted.
Method of Using Inter-Pole
In this method, small poles called inter-poles are fixed to the yoke and placed between the main poles. For generators, their polarity matches the adjacent main poles, and for motors, it matches the preceding main poles. The inter-poles induce an emf in the short circuit coil during the commutation period, opposing the reactance voltage and ensuring spark-less commutation.
Compensating Windings
This is the most effective mean of eliminating the problem of armature reaction and flash over by balancing the armature mmf. Compensating windings are placed in slots provided in pole faces parallel to the rotor (armature) conductors.
The major drawback of compensating windings is their high cost. They are mainly used in large machines subject to heavy overloads or plugging and in small motors requiring sudden reversal and high acceleration.
