# Losses in DC Machine ( DC Generator and Motor )

- Link: https://studyelectrical.com/2014/05/losses-in-dc-machine-dc-generator-and.html
- Published: 2014-05-17T13:51:00+00:00
- Author: 

The **losses in a dc machine (generator or motor) **may be divided into three classes
viz 

 1. **Copper losses **
 2. **Iron or core losses and **
 3. **Mechanical losses. **

![Untitled 2](https://1.bp.blogspot.com/-RckGE1FByy4/U3dayYkrzWI/AAAAAAAAA3s/1a0g8twhCiU/
s385/Untitled-2.jpg)

 All these losses appear as heat and thus raise the temperature of the machine.
They also lower the efficiency of the machine.

Read: [Construction of DC machines](https://studyelectrical.com/2014/06/construction-dc-motor-dc-motor-construction.html)

## Copper losses

These losses occur due to currents in the various windings of the machine.

 * **Armature copper loss = I_(a)^(2)R_(a)**
 * **Shunt field copper loss = I_(sh)^(2)R_(sh)**
 * **Series field copper loss = I_(se)^(2)R_(se)**

**Note**. There is also brush contact loss due to brush contact resistance (i.e.,
the resistance between the surface of brush and surface of commutator). This loss
is generally included in armature copper loss.

## Iron or Core losses

These losses occur in the [armature of a d.c. machine](https://studyelectrical.com/2014/06/construction-dc-motor-dc-motor-construction.html)
and are due to the rotation of armature in the magnetic field of the poles. They
are of two types viz., 

 1. **hysteresis loss **
 2. **eddy current loss.**

### (i) Hysteresis loss

Hysteresis loss occurs in the armature of the d.c. machine since any given part 
of the armature is subjected to magnetic field reversals as it passes under successive
poles.

![Hysterisis Loss](https://2.bp.blogspot.com/-iQvKkGntViU/U3devXp0HoI/AAAAAAAAA34/
nEy1AvNwh2k/s1600/Untitled-3.jpg "Hysterisis Loss")

Hysteresis Loss

 

Figure shows an armature rotating in a two-pole machine. Consider a small piece 
ab of the armature. When the piece ab is under N-pole, the magnetic lines pass from
a to b.

Half a revolution later, the same piece of iron is under S-pole and magnetic lines
pass from b to a so that magnetism in the iron is reversed. 

In order to reverse continuously the molecular magnets in the armature core, some
amount of power has to be spent which is called hysteresis loss. It is given by **
Steinmetz formula**. This formula is

**Hysteresis loss P_(h) = η B_(max)^(1.6 )f V** 

B_(max) = Maximum flux density in armature
 f  = Frequency of magnetic reversals
= NP/120 (where N is in r.p.m.) V = Volume of armature in m^(3)  h = Steinmetz hysteresis
co-efficient

In order to reduce this loss in a d.c. machine, armature core is made of such materials
which have a low value of Steinmetz hysteresis co-efficient e.g.,silicon steel.

### (ii) Eddy current loss

In addition to the voltages induced in the armature conductors, there are also voltages
induced in the armature core. These voltages produce circulating currents in the
armature core as shown in Fig. These are called eddy currents and power loss due
to their flow is called eddy current loss. 
  The eddy current loss appears as heat
which raises the temperature of the machine and lowers its efficiency.

![Eddy current loss](https://1.bp.blogspot.com/-AgBi9p6eFSc/U3dfsEPyYII/AAAAAAAAA4A/
b7vPOvPbKRQ/s1600/Untitled-4.jpg "Eddy current loss")

Eddy current loss

If a continuous solid iron core is used, the resistance to eddy current path will
be small due to large cross-sectional area of the core. Consequently, the magnitude
of eddy current and hence eddy current loss will be large. 
  The magnitude of eddy
current can be reduced by making core resistance as high as practical.   The core
resistance can be greatly increased by constructing the core of thin, round iron
sheets called laminations. The laminations are insulated from each other with a 
coating of varnish. The insulating coating has a high resistance, so very little
current flows from one lamination to the other.   Also, because each lamination 
is very thin, the resistance to current flowing through the width of a lamination
is also quite large. Thus laminating a core increases the core resistance which 
decreases the eddy current and hence the eddy current loss.

**Eddy current loss P_(e) = K_(e )B_(max)^( )f^(2) t^(2) V  **

where,

Ke = Constant depending upon the electrical resistance of core and system of units
used
 Bmax = Maximum flux density in Wb/m_(2) f = Frequency of magnetic reversals
in Hz t = Thickness of lamination in m V = Volume of core in m³

It may be noted that eddy current loss depends upon the square of lamination thickness.
For this reason, lamination thickness should be kept as small as possible.

### Mechanical losses

These losses are due to friction and windage.

 1. **friction loss e.g., bearing friction, brush friction etc.**
 2. **windage loss i.e., air friction of rotating armature.**

These losses depend upon the speed of the machine. But for a given speed, they are
practically constant.
  **Note.** Iron losses and mechanical losses together are
called stray losses

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