Losses in DC Machine


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.

Hysteresis 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.

Hysteresis loss

It’s given by Steinmetz formula, The steinmetz formula is
     Hysteresis loss Ph= ηB16max fV watts
            η = Steinmetz hysteresis co-efficient
            Bmax = Maximum flux Density in armature winding
            F = Frequency of magnetic reversals
               = NP/120 (N is in RPM)
           V = Volume of armature in m3
By using core material of silicon steel which has low Hysteresis coefficient Hysteresis loss can be reduced.

Eddy current loss:-

This loss is basically due to reversal of magnetization of the armature core.

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.

The eddy current loss exists due to eddy currents. When armature core rotates, it cuts the magnetic flux and e.m.f. gets induced in the core. This induced e.m.f. sets up eddy currents which cause the power 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,

        Eddy current loss Pe = K Bmax f2 t2 V 

           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                                                                                                     

Mechanical Loss:-

These losses consist of friction and windage losses. Some power is required to overcome mechanical friction and wind resistance at the shaft. This loss is nothing but the friction and windage loss. The mechanical losses are also constant for a d.c. machine.These losses depend upon the speed of the machine, they are practically constant.
  • Friction loss occurs due to the friction in bearing, brushes etc.
  • Windage loss occurs due to the air friction of rotating coil.

 Stray losses:-

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

If any query or suggestion about Losses in DC Machine please comment below or Email on sohal@electricalidea.com.





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