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Thursday, 26 December 2019

Losses in DC Generators

Losses in DC Generators:

The total losses in DC generators fall under the following three categories:

(a) Copper losses
(b) Magnetic losses
(c ) Mechanical losses

The difference between the power input and power output is called power loss, Ii is essential to consider the power loss for the following reasons

(a) The temperature rise of the generator is due to the heat developed within it and is proportional to the losses. Greater the heat developed, greater is the temperature rise and greater is the loss.

(b) The efficiency of the machine is dependent on the quantum of losses and the cost of operation depends upon the efficiency of the machine.

(c) Break down and machine failure mostly depends upon the losses and least on mechanical factors.

At the time of designing the machine the operating temperature is fixed and the design finalized, if this is not done, then the losses would dominate and the temperature rise beyond the operating value damage the insulation which ultimately leads to break down of machine. The different types of losses are given in Figure.

1. Copper Losses : Copper losses are due to the resistance in the winding wire and the brush contact losses. The armature copper losses amounts to Ia2Ra which is normally 30 to 40% of full load losses. The field copper losses are

(a) Ish2Rsh ---------- For shunt field generator
(b) Ise2Rse ---------- For series filed generator

The field copper loss amounts to 20 to 30% of full load losses. The brush contact losses are usually included in the armature copper loss.

2. Magnetic Losses : These losses are also known as iron losses or core losses and are of two types :

(a) Hysteresis Loss (Wh) :

This is due to the reversal of magnetism in the armature core. The core undergoes one complete cycle of reversal after passing through a pair of poles. If P is the number of poles and N the armature speed in r.p.m. then the frequency of reversals is given by

f = PN/120

The loss is dependent upon the grade of iron, its volume, maximum flux density Bmax and the frequency of magnetic reversals and is given by

Wh = ηB1.6max f V watts

where η = Steinmetz hysteresis coefficient
V = Volume of the core in m3


The value of η for different material is given in Table.

Material
η(J/m3)
Silicon Steel
191
Sheet Steel
502
Hard Cast Steel
7040
Cast Steel
750 - 3000
Cast Iron
2700 - 4000

(b) Eddy Current Losses : 

When the armature core rotates, it also cuts the magnetic field due to which a small amount of e.m.f. is induced in the core. This sets up a large current in the body of the core and is called eddy current. If it is a solid core, the loss will be considerable and to reduce it, the core is built up of thin laminations which are stacked and riveted at If right angles to the path of eddy current. Each lamination from each other and hence the loss is considerably reduced, In short

(a) A solid core of large volume offers least resistance resulting, in large eddy current.

(b) A laminated core of much lesser core section offers large resistance reducing the eddy current to least.

Eddy current loss is given by

We = KB2max f2 t2 V2 watts

Where Bmax is maximum flux density
f is the frequency of magnetic reversal
t is the thickness of each lamination
V is the volume of armature core.

3. Mechanical Losses :

These losses are due to friction and are of two types

(a) air frictional losses which are also called windage loss of rotating armature.
(b) frictional loss at hearings and commutator.

The mechanical loss amounts to about 10 to 20% of full load losses. Magnetic and mechanical loss are collectively called stray losses. Since for a given machine stray losses and copper losses are constant, the total constant loss collectively amounts to the sum of stray losses and copper losses. 

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