Efficiency of DC Generator

Efficiency is simply defined as the ratio of output power to the input power. Let R be the total resistance of the armature circuit (including the brush contact resistance, at series winding resistance, inter-pole winding resistance and compensating winding resistance).

The efficiency of the DC generator is explained below in the line diagram:

• I is the output current
• I_sh is the current through the shunt field
• I_a is the armature current = I + I_sh
• V is the terminal voltage.

Total copper loss in the armature circuit = I_a²R_at

Power loss in the shunt circuit = V_Ish (this includes the loss in the shunt regulating resistance).

Mechanical losses = friction loss of bearings + friction loss at a commutator + windage loss.

Core losses = hysteresis loss + eddy current loss

Stray loss = mechanical loss + core loss

The sum of the shunt field copper loss and stray losses may be considered as a combined fixed (constant) loss that does not vary with the load current I.

Therefore, the constant losses (in shunt and compound generators) = stray loss + shunt field copper losses.

Generator efficiency is given by the equation shown below:

If I_sh is small compared with I_r, then I_a = I

Therefore,

The efficiency Ƞ_G will be a maximum when the denominator D_r is a minimum.

Where,

D_r is minimum when

Also,

Since d²D_r/dI² is positive, the expression is given by the equation (1) is a condition for the maximum value of Dr and the condition for the maximum value of efficiency.

Equation (1) shows that the efficiency of a DC generator is maximum when those losses proportional to the square of the load current are equal to the constant losses of the DC generator.

This relation applies equally well to all rotating machines, regardless of the type of machine.

This relationship is sometimes incorrectly stated as maximum efficiency occurs when the variable losses are equal to the constant losses.

Load Corresponding to Maximum Efficiency

Let,

• I_fl be the full load current,
• I_M is the current at maximum efficiency

For maximum efficiency, therefore, the current at maximum efficiency is given by the equation shown below:
This is all about Efficiency of DC Generator.