PER UNIT RESISTANCE, LEAKAGE REACTANCE AND IMPEDANCE VOLTAGE DROP

Full-load voltage of a transformer can be expressed as a fraction of the full-load terminal voltage.

Let I_1fl be the full-load primary current, I_2fl be the full-load secondary current, V₁ be the rated pri-mary voltage and V₂ be the rated secondary voltage.

Per unit resistance drop of a transformer

∴

Per unit reactance drop of a transformer

Per unit reactance drop of a transformer is called per unit reactance and it is given by

Per unit impedance drop of a transformer is called per unit impedance and it is given by

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Practical Commutation

The e.m.f. induced in each coil of armature is alternating in nature. If load is connected, the current flowing will also be alternating. But the flow of current in a d.c. generator must be undirectional. This can be achieved by the use of commutator. When the armature conductors are under the influence of one pole they carry current in one direction whereas the current is reversed when the conductors are under the influence of other pole. This reversal of current takes place along the magnetic neutral axis. Fig. 1 Note : The reversal of current is likely to take place in short interval when a coil is short circuited by a brush so that transfer of current from one direction to other is carried out without any sparking. This process is called commutation. Thus a process by which current in the short circuited coil is reversed while it crosses the MNA is called commutation. The time during which the coil remains short circuited is known as...

Demagnetising and Cross Magnetizing Conductors

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 ...

Winding Pitches

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...

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