Shri Navsari Paschim Vibhag Koli Samal Kalyankari Trust Sanchalit MAHATMA GANDHI INSTITUTE OF TECHNICAL EDUCATION & RESEARCH CENTER NAVSARI NPE Campus, Bhanunagar, Eru-Aat Road, Po.Bhutsad, Tal.Jalalpore, Dist. Navsari

ELECTRONICSAND COMMUNICATION ENGINEERING B.E Ⅱ (SEM Ⅲ )

ELECTRICAL MACHINES GROUP NO : 15 NAME : Enrolment No : 1.PATEL VISHAL H PATEL SHIVANG G PATEL NIRAV S ADHYARU DHAVAL N SUBJECT CODE :

ARMATURE REACTION AND METHODS OF IMPROVING COMMUTATION

Armature Reaction : When the armature winding of a dc machine carries current the armature wining produces its own mmf known as Armature Reaction. Note that in dc generator the armature current flows due to the induction whereas in dc motor it flows due to conduction. The air gap between armature and field is now acted upon by the resultant of two mmfs namely the field ampere turns and the armature ampere turns. Therefore the air gap flux density will get distorted. Cross-magnetizing effect : Due to armature reaction, the flux in the air gap gets strengthened at one end of each main pole and weakness at the other end. This is called as “cross magnetizing effect of armature reaction”.

Demagnetizing effect : In across magnetizing, the increase in flux strength at one end of the pole is equal to decrease in flux at the other end. So there is no flux/pole. But if the main pole excitation is such that iron is in the saturated region which is normally the case foe practical machines. Then the increase in flux density at one end due to armature reaction is not same as reduction in flux density at the other end. In fact, amount of increase in flux density is less than the amount of decrease at the other end. Hence, there is a net reduction in flux / pole which is called as a demagnetizing effect.

Summary of armature reaction : In case of dc machine cross magnetizing takes place due to armature reaction. It results in distorted flux density distribution but no change in flux/pole, if the iron core is unsaturated. But across magnetizing results in reduction of flux/pole if the iron core is in saturation. This effect is called demagnetizing effect. The distortion of flux density has an adverse effect on commutation. This effect can be overcome by interpoles. Remedy to cross magnetizing effects : The cross magnetizing effects of armature reaction can be reduced by make the field winding ampere turns higher than the armature winding ampere turns. Due to this the effects of field mmf will be more than that of armature mmf.

This can be practically achieved by taking the following measure : 1.Introduce saturation in teeth and pole shoes. 2.Champehor the pole shoes to increase the air gap at the pole tips. 3.Use of compensating winding compensate the armature reaction mmf Concept of Reactance Voltage : We have discussed that the current induced in armature conductors of a dc generator are alternating. We need to use the commutator in order to make the current flow unidirectional in the external circuit. During the commutation interval Tc the particle coil in the armature which is undergoing a reversal of current is short circuited. If the current reversal from +I to –I is completed by the end of the short circuit or commutation period then the commutation is said to be ideal.

If the current reversal is not completed by that time, then sparking is produced between the brushes and commutator and both can be get damaged. Thus sparking at the brushes are due to poor commutation which is due to the inability current in short circuited coil to reverse completely by the end of short circuited period. The main reason which makes the current reversal during the commutation interval impossible is the production of self induced emf in the coil undergoing the commutation. This self induced emf is called as Reactance Voltage. It is a small voltage but possible current through the short circuited coil (during commutation interval). Value of reactance voltage : Reactance voltage = Self inductance of coil × Rate of change of current = L × 2I / T c Where, 2I = Change in coil current from +I to –I. Tc= Commutation interval.

Methods of Improving Commutation : An improved commutation means “sudden reversal of current during the commutation interval and hence no sparking takes place at the brushes.” There are two practical methods of improving the commutation. They are as follow : 1.Resistance commutation. 2.Emf commutation. Resistance Commutation : In this method, the low resistance copper brushes are replaced by comparatively high resistance carbon brushes. Refer Fig. (a) to understand resistance commutation. When current I through coil C reaches the commutator segment b, it can take two parallel path in resistance commutation.

A B C coils I X I I + X I – X Commutator a b c segments r 2 r 1 Brush I + X 2I I - X Fig. (a) Resistance commutation

The first is from bar b to the brush. The current through this path is (I – X) and the other path is through coil short circuited B to bar a of commutator and to brush. This is I + X. With the low resistance copper brushes used, the current would not take the second i.e. the longer path. It would probably follow the first path only. But if the large resistance carbon brushes are used then the current I coming from coil C the current I will prefer to pass through the second path because the resistance r1 of the first path will increase due to reducing area of contact of bar b with the brushes and because resistance r2 of the second path will decrease due to increasing area of bar a with the brushes. Hence the current through coil C will first take the first path and then gradual gets shifted over to path number two. This will making the commutation fast and sparkless. However the main cause of sparking commutation is the self induced voltage (reactance voltage). Hence, change of brushes alone will not give a sparkless commutation.

Advantages of carbon brushes : 1.They improve commutation. 2.They are to some degree are self lubricating and polish the commutator. 3.In the event of sparking they damage the commutator less than copper brushes. Disadvantages of carbon brushes : 1.Due to their high contact resistance a drop of about 2V is caused. So they are not suitable for small machines. 2.The size of commutator needs to be made larger than the one used with copper brushes. 3.Due to their low current density, larger holders are required to be used.

EMF Commutation : EMF commutation is a technique used to neutralize the reactance voltage by producing a reversing emf in the short circuited coil under commutation. The reversing emf is the emf in opposition to the reactance voltage and if both these voltage are equal, then the effect of reactance voltage can be completely wiped off. We can then achieve the ideal commutation. Following are the two methods of developing the reversing emf : 1.By giving the brushes a forward lead. 2.By using the interpoles. Out of these, the first methods is not used due to many disadvantages.

Interpoles of Compoles : Interpoles are small poles fixed to the yoke. They are spaced in between the main poles. The windings on the interpoles are of heavy gauge wire and small number of turns. These are connected in series with the armature so that they carry the full armature current. Function of interpoles : The functions of interpoles are as follows : 1.They induce emf in the coil under commutation which helps to reverse the currents. The emf induced by the interpoles is called Commutating or Reversing emf. This emf neutralizes the reactance emf and makes the commutation sparkless. As interpoles carry armature current, the commutating emf is proportional to armature current.

2. This will ensure an automatic neutralization of reactance voltage which also is proportional to armature current. Interpoles To loads Main field poles Fig. (b) Interpoles s N N S

N S N S A B F O C 3. The interpoles also neutralize the cross magnetism effect of armature reaction. So it is not necessary to shift the brushes from their original positions. This is shown in fig. (c) where OF represents the mmf due to main poles, OA represents the cross magnetising mmf due to armature. BC is the mmf due to interpoles. It is in opposition to OA, hence cancel each other. Fig. (c)Neutralization of cross magnetism effect using interpoles

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