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Power losses and Efficiency in DC Machines

Power losses and Efficiency


Electromechanical energy conversion is necessary accompanied by a certain amount of irreversible conversion of energy to heat in the conversion device or Machine.


DC Motors Rotor


These energy losses arise because or Circuits resistances ( Copper or Electrical losses ), because of the exitance of alternating or Fluctuating Magnetic Fields
( Core or iron losses ), and from Mechanical factors (friction and windage losses).

Although they play essentially no basic role in the energy conversion process, losses are nevertheless important factors in the practical application of Machines.

Treatment of Machines losses is important for three reasons below :



  • Power losses determine the Efficiency of the machine and appreciably influence its operating cost. For example, a machine with low Efficiency has more power losses and, therefore, increased operating cost.




  • Power losses determine the heating of the machine and hence fix the rating of power Output that can be obtained without deterioration of the insulation because of overheating.




  • Voltage drops are associated with ohmic or Electrical losses while Current components, like core loss Current, pertains to the iron loss in Electrical Machines.


Obviously, the losses associated with Voltage drops or Current components must be properly accounted for in a machine Representation so that tha analysis of the Electrical Machine can be carried out as desired.

The Efficiency of an electrical machine, like that of any other apparatus, is defined as the ratio of useful power Output to the input power, the two being measured in the some units ( eighter in watts or kw ).

                                                      O/P
i.e., Machines Efficiency, η  = ------------
                                                                  I/P

                              O/P

                 =  ----------------------
                       O/P + losses


                       I/P - losses                 losses

                 =  ----------------------  =  1 -  -------------
                               I/P                           I/P

Rotating Electrical Machines in general operate

Efficiently at full loads. The full load Efficiency of average Motors, for example, is about 74 per cent for 0.75 KW size, 89 per cent for 37 KW, 93 KW per cent for 375 KW, and 97 per cent for 3,750 KW.

The Efficiency of slow - speed motors is usually lower than that of high - speed motors, the total being 3 or 4 per cent.

Considerations of Machines parts in which power losses can occur will show that the following list includes all the possibilities within the Machine itself.


Copper or Electrical (I2 R) losses


The losses occur in the rotor and Stator
(Armature and field) windings.

In synchronous and DC shunt Machines
( Expect where variable speed is obtained by control of shunt Field Current), the field copper loss is constant because such Machines are normally operated with constant Feild Current.

In series and Induction Machines, the losses in both winding vary as the square of the line Current.


Core or Iron losses


These losses consist of Eddy Current and Hysteresis losses. In synchronous and Induction Machines, these losses are confined essentially to the Stator iron, and in DC Machines essentially to the rotor (or armature) iron, although in both cases a small core loss will be present in the other member because of small flux variations caused by the slots.

The iron in the offending member is laminated to reduce the Eddy Current loss. In all except series Machines, variable - speed shunt motors, and to a lesser degree, Compound Motors, the air - gap flux and hence the core losses are sensibly constant regardless of Load.


Friction and windage losses 


These losses are constant unless the speed varies appreciably. The sum of the friction and windage and core losses is called the rotational losses.


Stray - load losses


These are additional Hysteresis and Eddy-current losses arising from any distortion in flux Distribution caused by the load Current.

These losses are difficult to measure and are usually about 1 per cent of the machine output.


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