Electrical World

The various effects armature reaction can be summarised as, 1) The armature reaction always results in reduction of generated e.m.f. due to decrease in value of flux per pole. 2) The iron losses in the teeth and pole shoes are determined by the maximum value of flux density at which they work. Due to distortion in main field flux the maximum density at load increase above no load. Thus iron losses are observed to be more on load than on no load. 3) Due to the armature reaction the maximum value of gap flux density increases. This will increase the maximum voltage between adjacent commutator segments at load. If this voltage exceeds beyond 30 V the sparking may take place between adjacent commutator segments. 4) The armature reaction shifts brush axis from GNA. Thus flux density in the interpolar axis is not zero but having some value. Thus there will be an induced e.m.f. in the coil undergoing commutation which will try to maintain the current in original direction. This will...

The conductors which are responsible for producing demagnetizing and distortion effects are shown in the Fig.1. Fig. 1        The brushes are lying along the new position of MNA which is at angle θ  from GNA. The conductors in the region AOC = BOD = 2θ  at the top and bottom of the armature are carrying current in such a direction as to send the flux in armature from right to left. Thus these conductors are in direct opposition to main field and called demagnetizing armature conductors.         The remaining armature conductors which are lying in the region AOD and BOC carry current in such a direction as to send the flux pointing vertically downwards i.e. at right angles to the main field flux. Hence these conductors are called cross magnetizing armature conductors which will cause distortion in main field flux.        These conductors are shown in the Fig. 2 Fig. 2  ...

To understand the concept of armature reaction, consider a two pole d.c.generator. For simplicity we will assume that the brushes are touching the armature conductors directly, although, they commutator segments in actual practice.        Assuming that the generator is not driving and load. So that there is no current in the armature conductors. The distribution of main field flux under this case is as shown in the Fig. 1. The flux is distributed symmetrically with respect to axis called polar axis which is line joining the centres of N and S poles. The direction of flux produced by field coil is from left to right through an armature core and lines of force through the core are parallel to field axis.        The axis along which there is no e.m.f. induced in the armature conductors is called Magnetic Neutral Axis (MNA). From the Fig. 1 it can be seen that magnetic neutral axis (MNA) and Geometric Neutral Axis (GN...

Now we will consider the definitions of different types of winding pitches, required in the design of the armature winding. 1.1 Back Pitch         The distance which is measured interns of armature conductors i.e. between top and bottom coil sides of a coil measured around the back of the armature i.e. away from the commutator is called back pitch and is denoted by . It is measured interms of coil sides. Note : Since it is difference between odd and even number, it is always odd number.        The size of the coil is decided by the back pitch. The back pitch is normally equal to coilsides per pole. It is shown for lap and wave type of winding in the Fig. 1. Fig. 1        The back pitch for the winding shown in the Fig. 1 is y b = 14 - 1 = 13 as coilsides 1and 14 are of coil number 1. 1.2 Front Pitch        It is defined as the distance measured between two c...

For the operation of any d.c. machine, presence of magnetic flux is essential. Whenever current flows through a field coil, it produces a flux which is called as field flux. Now suppose the d.c. machine is functioning as a generator, an e.m.f. will be induced in the armature when it is driven by a prime mover. If the generator is driving a load, the induced e.m.f. in the armature will cause current to flow through the load. Thus current will start flowing through the armature conductors. Now every current carrying conductor will set up its own magnetic field. All these armature conductors combine together to produce a flux which can be called armature flux. Note : Thus the armature current  will set up its own magnetic field. The effect of this armature flux on the distribution of main flux is called armature reaction.        The armature reaction will try to make the main field flux to be weakened. Also it will distort the main flux. Note...

The armature winding used in d.c. machines are closed and double layer. Each of the coil has its upper coil side placed at the top of one slot and lower coilside placed at the bottom of another slot which is approximately one pole pitch away.       Normally the shape of the coils is like a diamond. These coils are made in special forming machines. The back and front ends are constructed in such a way that on coilside is on a circumferentially higher level than the other. Due to this, higher coilsides are plcaed in the top of a slot while lower coilsides can be placed in the bottom of a slot.        If each slot contains two coil sides then the number of coils equal the number of slots. One coilside is there in each of top and bottom layer. But in case of low speed and high voltage machines, there are large number of coils and hence the coilsides per slot may be 4, 6, 8 or more which means 2, 3, 4 or more coilsides per l...

  For every generator which is used as a self excited generator there must exist some residual magnetic flux. When armature rotates, conductors cut this small residual flux to produce the e.m.f. E r . This e.m.f. drives small current through field winding. Thus field current I f now produces more flux which is greater than residual flux. Hence more e.m.f. gets induced. This further drives more current through field to produce more flux.which is greater than residual flux. Hence more e.m.f. gets induced. This further drives more current through field to produce more flux. This process is cumulative and continues till rated voltage gets build up. This is shown in the Fig. 1. Fig. 1  Voltage building in self excited generator 1.1 Causes of Failure to Excite Self Excited Generators        The causes of failure, the method of detection and the corresponding remedy are given in table 1.