Losses in DC Machines

In DC machines (generator or motor), the losses may be classified into three categories namely,

  • Copper losses

  • Iron or core losses

  • Mechanical losses

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

Copper Losses

In dc machines, the losses that occur due to resistance of the various windings of the machine are called copper losses. The copper losses are also known as I2R losses because these losses occur due to current flowing through the resistance of the windings.

The major copper losses that occur in dc machines are as,

$$\mathrm{\mathrm{Armature\:copper\:loss}\:=\:\mathit{I_{a}^{\mathrm{2}}R_{a}}}$$

$$\mathrm{\mathrm{Series\:field\:copper\:loss}\:=\:\mathit{I_{se}^{\mathrm{2}}R_{se}}}$$

$$\mathrm{\mathrm{Shunt\:field\:copper\:loss}\:=\:\mathit{I_{sh}^{\mathrm{2}}R_{sh}}}$$

In dc machines, there is also a brush contact loss due to brush contact resistance. In practical calculation, this loss is generally included in armature copper loss.

Iron Losses

The iron losses occur in core of the armature of a DC machine due to rotation of the armature in the magnetic field. Because these losses occur in core of the armature, these are also called core losses.

There are two types iron or core losses namely hysteresis loss and eddy current loss.

Hysteresis Loss

The core loss that occurs in core of the armature of a dc machine due to magnetic field reversal in the armature core when it passes under the successive magnetic poles of different polarity is called hysteresis loss. The hysteresis loss is given by the following empirical formula,

$$\mathrm{\mathrm{Hysteresis\:loss,}\mathit{P_{h}}\:=\:\mathit{k_{h}B_{max}^{\mathrm{1.6}}fV}}$$

Where, $\mathit{k_{h}}$ is the Steinmetz’s hysteresis coefficient, $\mathit{B_{max}}$ the maximum flux density,f is the frequency of magnetic reversal, and V is the volume of armature core.

The hysteresis loss in dc machines can be reduced by making the armature core of such materials that have a low value of Steinmetz’s hysteresis coefficient like silicon steel.

Eddy Current Loss

When the armature of a DC machine rotates in the magnetic field of the poles, an EMF is induced in core of the armature which circulates eddy currents in it. The power loss due to these eddy currents is known as eddy current loss. The eddy current loss is given by,

$$\mathrm{\mathrm{Eddy\:current\:loss,}\mathit{P_{e}}\:=\:\mathit{k_{e}B_{max}^{\mathrm{2}}f^{\mathrm{2}}t^{\mathrm{2}}V}}$$

Where,$\mathit{K_{e}}$ is a constant of proportionality, and tis the thickness of lamination.

From the expression for eddy current loss it is clear that the eddy current loss depends upon the square of thickness of lamination. Therefore, to reduce this loss, the armature core is built up of thin laminations that are insulated from each other by a thin layer of varnish.

Mechanical Losses

The power losses due to friction and windage in a dc machine are known as mechanical losses. In a dc machine, the friction loss occurs in form of bearing friction, brush friction, etc. while the windage loss occurs due to air friction of rotating armature.

The mechanical losses depend upon the speed of the machine. But these losses are practically constant for a given speed.

Note− Iron or core losses and mechanical losses together are known as stray losses.

Constant and Variable Losses

In DC machines, we may group the above discussed losses in the following two categories −

  • Constant Losses

  • Variable Losses

Those losses in a DC machine that remain constant at all loads are called constant losses. These losses include − iron losses, shunt field copper loss, and mechanical losses.

Those losses in a DC machine that vary with load are known as variable losses. The variable losses in a DC machine are − armature copper loss and series field copper loss.

Total losses in a DC machine = Constant losses + Variable losses
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