ARMATURE REACTION IN DC GENERATOR

https://youtu.be/cySkKtj4nOI

First Let us Understand What is DC Generator.

Generator is an electrical machine (dynamic) which converts mechanical energy into electrical energy.

It works on principle of Faraday's Law of Electromagnetic Induction it states that when ever a flux changes through a conductor an EMF is Induced, polarity of EMF is governed by Lenz's law.

It says that "effect opposes its own cause".

Direction of current flowing in Armature conductor due to Induced emf can be found by Fleming's right hand rule,as shown in figure 1.

Figure 1. Fleming's Right hand rule 

All three fingers are held perpendicular to each other 

It is called as DC Generator because nature of voltage and current received at Terminal is DC in nature. Current is collected with the help of carbon brushes which are placed at Magnetic Neutral Axis (MNA). Initially MNA and GNA (Geometrical Neutral Axis) coincides with each other when either only field flux is present (armature flux = 0) or only armature flux is present (field flux =0) as shown in figure 2(a) and 2(b) respectively.

But if both armature flux and field flux is taken into account then waveform of main flux (field flux) is distorted due to cross magnetizing effect of armature reaction. As shown graphically in figure 3 below. Since MNA is perpendicular to axis of main flux now MNA is Shifted as shown in figure 3.

 Figure 3. Cross magnetizing effect 

Old MNA was axis of commutation as it was neutral point,but due to Armature reaction MNA shifted clockwise and axis of commutation has induced emf which causes problem for commutation.

EFFECTS of ARMATURE REACTION 

1. Demagnetising effect

2. cross Magnetizing effect.

3. Induce voltage at commutating zone that aggravates the commutation problem

youtube video link

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