In the previous articles we discussed about DC generator working, Voltage equation of dc generator. Efficiency is a very
important specification of any type of electrical machine. When we talk about
efficiency, losses comes into the picture. DC generator efficiency can be
calculated by finding the total losses in it. There are various losses in DC
generator. We classified

**DC generator losses**into 3 types.###
__Total Loss in a D.C. Generator__

*(a)*Copper Losses

*(b)*Magnetic Losses

*(c)*Mechanical Losses

The above 3 losses are primary losses in any type of electrical machine
except in transformer. In transformer there are no rotating parts so no
mechanical losses.

Let us have a brief
discussion on each and every loss in dc generator.

*(a)*Copper Losses

Copper losses occur
in dc generator when current passes through conductors of armature and field.
Due the resistive property of conductors some amount of power wasted in the
form of heat. Most of the time we neglect copper losses of dc generator filed,
because the amount of current through the field is too low [Copper losses=I²R, I²
will be negligible if I is too small].

**Note :**

*E*

*g*

*I*

*a*

*is the power output from armature.*

If we consider all
losses,

Total Copper
Losses=I²Ra

Where

*Ra*= resistance of armature and interpoles and series field winding etc.
This loss is about 30
to 40% of full-load losses.

**(**Field copper loss. In the case of shunt generators, it is practically constant and

*i*)*I*

*sh*

*²*

*R*

*sh*

*(or*

*VI*

*sh*). In the case of series generator, it is =

*I*

*se*²

*R*

*se*

*where*

*R*

*se*

*is resistance of the series field winding.*

*This loss is about 20 to 30% of F.L. losses.*

**(**The loss due to brush contact resistance. It is usually included in the armature copper loss.

*ii*)
Armature copper loss = I2a Ra

Shunt field copper loss = I2shRsh

Series field copper loss = I2se Rse

Note: There’s additionally brush contact loss attributable to brush contact resistance (i.e., resistance in the middle of the surface of brush and commutator). This loss is mostly enclosed in armature copper loss.

*(b)*

**Magnetic Losses**(also known as iron or core losses),

Hysteresis losses or
Magnetic losses occur due to demagnetization of armature core. This losses are
constant unless until frequency changes. Eddy current losses are due to
circular currents in the armature core.

We know armature core is
also a conductor, when magnetic flux cuts it, EMF will induce in the core, due
to its closed path currents will flow. This currents causes eddy current
losses.

**hysteresis loss,**

*(i)**Wh*∝

*B*1.6 max

*f*and

**(**eddy current loss,

*ii*)*We*∝

*B²*max

*f ²*

These losses are practically constant
for shunt and compound-wound generators, because in their case, field current
is approximately constant.

Both these losses total up to about 20
to 30% of F.L. losses.

**(**

*c*) Mechanical Losses.
We know generator is a rotating machine
it consist of friction loss at bearings and commutator and air-friction or windage
loss of rotating armature. We can’t neglect this losses because they always
present ,

**These are about 10 to 20% of F.L. Losses.**
The total losses in a d.c. generator are
summarized below :

**Stray Losses**

Usually, magnetic and mechanical losses are collectively known as

*Stray Losses***.**These are also known as rotational losses for obvious reasons.**Total loss = armature copper loss +**

*Wc*=*Ia²**Ra*+*Wc*= (*I*+*Ish*)²*Ra*+*Wc*.**Armature Cu loss**

*Ia²**Ra*is known as variable loss because it varies with the load current.**Total loss = variable loss + constant losses**

*Wc*