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Losses in DC Machine – Iron Loss, Copper Loss and Mechanical Losses
The losses in DC machine (generator or motor) may be divided into three categories as −
- Iron or Core Losses
- Copper Losses
- Mechanical Losses
The losses in DC machines appear in the form of heat and hence increases the temperature of the machine. Also, the losses reduces the efficiency of the machine.
Iron or Core Losses
The iron losses occur in the armature core of the DC machine since the armature core is subjected to the magnetic field reversal i.e. changing magnetic field. The core losses are of two types viz.
- Hysteresis Loss
- Eddy Current Loss
Hysteresis Loss
The armature core of DC machine is subjected to magnetic field reversal when it passes under successive poles. An energy loss takes place due to molecular friction in the material of the armature core i.e. the domains of the core material resist being turned first in one direction and then in the other. Thus, some energy is expanded in the material of the armature core in overcoming this opposition and appears in the form of heat. This loss is known as hysteresis loss. The empirical formula of hysteresis loss is,
$$\mathrm{Hysteresis\:Loss,\:P_{h}\:=\:\eta B_{max}^{1.6}fV\:\:Watts}$$
Where,
- η = Steinmetz hysteresis co-efficient,
- Bmax = Maximum flux density in the core,
- f = frequency of magnetic reversal,
- V = volume of the core.
In order to reduce the hysteresis loss, the armature core is made of materials having narrower B-H curve such as silicon steel.
Eddy Current Loss
When the armature of DC machine rotates in the magnetic field produced by the poles, an emf is induced in the core and due to this induced emf, eddy currents are circulated in the armature core. The power loss due to these eddy currents is termed as eddy current loss. The eddy current loss is given by,
$$\mathrm{Eddy\:current\:loss,\:P_{e}\:=\:K_{e} B_{max}^{2}f^2t^2V\:\:Watts}$$
In order to reduce the eddy current loss, the armature core is built up of thin laminations which insulated from each with a coating of varnish.
Copper Losses
The copper losses in a DC machine occur in the various windings of the machine due to currents flowing in them. In a typical DC machine, following copper losses occur
$$\mathrm{Armature\:Copper\:Loss\:=\:I_{a}^{2}R_{a}}$$
$$\mathrm{Series\:Field\:Copper\:Loss\:=\:I_{se}^{2}R_{se}}$$
$$\mathrm{Shunt\:Field\:Copper\:Loss\:=\:I_{sh}^{2}R_{sh}}$$
Important – There is also brush contact drop due to the resistance between the contact surface of brush and commutator. In general, this loss is included in armature copper loss.
Mechanical Losses
Mechanical losses occur in the moving parts of the DC machine due to friction and windage. These losses are also called rotational losses. The mechanical losses depend upon the speed of the machine. In a DC machine, two types mechanical losses occur, which are as follows
- Friction Loss – such as bearing friction, brush friction, etc.
- Windage Loss – This loss is the air friction of rotating armature.
Important – The iron losses and mechanical losses together are known as stray losses, i.e.,
$$\mathrm{Stray\:Losses \:= \:Iron Losses \:+\: Mechanical Losses}$$
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