Armature Reaction
What is armature reaction?
The effect of magnetic field set up by the armature current on the distribution of flux under the main poles of a dc machine (a generator or a motor) is known as armature reaction.
What are the effects of armature reaction?
The armature reaction produces demagnetizing and cross-magnetizing effects.
The demagnetizing effect of armature reaction reduces the total flux per pole from its no-load value due to magnetic saturation and the reduction in flux depends upon the degree of magnetic saturation.
The decrease in main flux on load due to armature reaction may be about 10%. The decrease in flux reduces the magnitude of generated emf in case of a generator and electromagnetic torque developed in case of a motor.
The cross-magnetizing effect of armature reaction distorts the field in the air gap.
Due to this effect increased iron losses, poor commutation or even sparking at the commutator surface occur.
What are the remedial measures of armature reaction?
The remedial measure of armature reaction are,
1. By flattening of the pole faces slightly so that the air gap is longer at the pole tips than it is at the centre of the pole. This results in increase in reluctance at the pole tips and so in the magnitude of armature cross-flux thereby reducing the distortion of the resultant flux density wave.
2. By making the trailing horn of the pole piece longer than the advancing horn and cutting farther from the surface of the armature so as to equalise the distribution of magnetic flux.
3. The air-gap length may be increased. By doing so the reluctance increases and more ampere-turns are required in the field winding. The field distortion, however, will not be so great as it would be if the magnetic field of the machine were weakened.
4. By reducing the cross section of the pole-pieces, it becomes highly saturated and offers large reluctance to the cross-field.
5. By providing interpoles between the main poles and/or compensating windings if necessary. This will be described in detail later on.
What is commutation in dc Generator ? What do you mean by good commutation? How commutation can be improved?
Ans:
The reversal of current in the armature coil by means of brush and commutator bars, is called the commutation process.
And the period during which the coil remains short circuited is called the commutation period.
Good commutation means no sparking at the brushes and with commutator surface remaining unaffected during continuous operation of the dc machine.
Commutation can be improved by
(i) using high resistance brushes
(ii) by shifting brushes
(iii) by using interpoles or commutating poles
and (iv) by using compensating winding.
How interpoles and compensating winding improve commutation?
Commutating or interpoles improve commutation by neutralizing the reactance voltage and cross-magnetization.
Compensating winding neutralizes the effect of armature reaction outside the influence of the interpoles and maintains a uniform flux distribution under the faces of the mainpoles.
What is reactance voltage?
Ans: The self induced EMF in coil undergoing commutation is called the reactance voltage
Short Answer Type question answer:
Armature reaction::
1. What is the nature (shape) of the de armature mmf waveform?
Ans. Triangular.
Q. 2. What is the nature (shape) of air gap flux distribution in space at no load in de machines?
Ans. Flat topped.
Q. 3. What is armature reaction?
Ans. Armature reaction is the effect of the magnetic field set up by the armature current on the distribution of flux under the main poles of a de machine.
Q. 4. What are the two unwanted effects of armature reaction?
Ans. Demagnetization or weakening of main field and cross-magnetization or distortion of field.
Q. 5. Differentiate between geometrical neutral axis and magnetic neutral axis.
Ans. Geometrical neutral axis is the axis between the magnetic poles and magnetic neutral axis is along zero magnetic field.
Q. 6. Define magnetic neutral plane (mnp)?
Ans. The plane through the axis, along which no emf is induced in the armature conductors, is known as magnetic neutral plane.
Q. 7. In a de machine, along which one of the following axes the armature mmf is directed?
(a) Polar axis
(b) Brush axis
(c) Interpolar axis.
Ans. Brush axis.
Q. 8. When the brushes in a de machine are placed at the interpolar axis, is the armature mmf fully demagnetising, fully cross-magnetising or partly demagnetising and partly cross-magnetising?
Ans. Fully cross-magnetising.
Q. 9. How the effect of armature réaction can be minimized in a de machine?
Q. 10. Why in de machines sometimes brushes are shifted from neutral axis by a small angle?
Ans. So as to lie along mnp to provide sparkless commutation.
Q. 11. What is meant by reactance voltage?
Ans: . The self-induced emf in the coil undergoing commutation is called the reactance voltage.
Q. 12. In a de machine without interpole to get improved commutation, whether the brush shift should be varied with change in load or brush shift should be fixed?
Ans. Brush shift should be varied with the change in load in a de machine so as to lie along mnp to provide improved commutation.
Q. 13. Define commutation?
Ans. The process of reversal of current in the armature coils by means of brushes and commutator bars, is called the commutation process.
Q. 14. Explain bad commutation.
Ans. A machine is said to have poor or bad commutation if there is sparking at the brushes and commutator surface. The bad commutation may be caused by mechanical or electrical conditions. The mechanical conditions include uneven commutator surface, non-uniform brush pressure, vibration of brushes in the brush holders etc. The electrical conditions include an increase in voltage between the commutator segments, an increase in the current density at the trailing edge of the brush etc.
Q. 15. What is linear commutation?
Ans. Linear commutation is an ideal commutation and causes change of current in a coil undergoing commutation from + Ito - I uniformly.
Q. 16. Name the factor that opposes full reversal of current in a coil undergoing commutation.
Ans. Self-inductance of the coil undergoing commutation.
Q17. What is the location of interpole?
Ans. Commutating poles are located midway between the main poles.
Q. 18. Why are the interpoles of a de machine tapered?
Ans. Interpoles are tapered in order to ensure that there is no saturation at the root of the pole at heavy overloads.
Q. 19. Is the interpole AT required for a compensated dc machine more or less than of an uncompensated one?
Ans. Interpole AT required for a compensated de machine are less than of an uncompensated one.
Q. 20. Why are commutating poles provided in the construction of a large dc machine?
Ans. In a large de machine armature reaction effect becomes very large and the commutation problem increases owing to large armature currents. Commutating poles are, therefore, provided to overcome these problems.
Q. 21. What is compensating winding?
Ans. A compensating winding is an auxiliary winding embedded in slots located in the faces of the main poles.
Q. 22. What is the purpose of compensating winding in a dc machine?
Ans. The purpose of compensating winding is to neutralize the effect of armature reaction outside the influence of the interpoles and particularly to maintain uniform flux distribution under the faces of the main poles.
Q. 23. In what way compensating winding is connected to the armature?
Ans. The compensating winding is connected in series with the armature in a manner so that the direction of current through the compensating winding conductors in any pole face will be opposite to the direction of current through the adjacent armature conductors.
Q. 24 Where are compensating windings placed in a de machine?
Ans. Compensating windings are placed in slots located in the faces of the main poles.
Q. 25. Air-gap lengths under the main poles of a de machine are less than those under the interpoles. Why?
Ans. The gaps under the interpoles are kept longer than that under the main poles (1.5 to 1.7 times the main gap) in order to avoid saturation in the interpoles
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