Losses in DC Machine
The losses in dc machine can be divided into following parts are,
- Copper losses
- Iron or core losses
- Mechanical losses
- Stray losses
All these losses appear in form of heat and its raise the temperature of the machine. They also decrease the efficiency of the machine.
Copper loss :-
The copper losses are the losses taking place due to the current flowing in a winding. There are basically two windings in a d.c. machine namely armature winding and field winding. The copper losses are proportional to the square of the current flowing through these windings.
- Armature copper loss = Ia2Ra where , Ra is Armature resistance
- Shunt field copper loss = Ish2Rsh where , Rsh is Shunt field resistance
- Series field copper loss = Ise2Rse where , Rse is Series field resistance
In a compound d.c. machine, both shunt and series field copper losses are present. In addition to the copper losses, there exists brush contact resistance drop. But this drop is usually included in the armature copper loss.
Shunt field copper loss is constant for shunt compound wound DC machine. other all remaining loss are directly proportional to the square current its known as variable losses.
Iron or core losses :-
These losses are also called magnetic losses. this loss is subjected to the flux. it is occur in Armature teeth and core where the flux is changing.These losses include hysteresis loss and eddy current loss.
The hysteresis loss is proportional to the frequency and the maximum flux density in the air gap. 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.
Eddy current loss:-
Voltages induced in the armature conductors, there are also voltages induced in the armature core. These voltages produce circulating currents in the armature core. These are called eddy currents and power loss due to their flow is called eddy current loss.
By using thin laminated stamping steel we can minimize the Eddy current losses. The Thickness of lamination stamping is kept 0.35 to 0.50 mm.
Eddy current loss is given by,
Where K = Constant depending upon the electrical resistance of core
Bmax = Maximum flux density in core( Wb/m²)
t = Thickness of lamination in m
f = Frequency of magnetic reversals in Hz
V = Volume of core
- Friction loss occurs due to the friction in bearing, brushes etc.
- Windage loss occurs due to the air friction of rotating coil.
There are some losses can not determine easily, then only appear when machine is loaded. this losses are known as the stray losses. stray losses is the summation of iron losses and mechanical losses. Stray losses = Mechanical losses + Iron losses
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