In a dc machine, the given input energy will be not equal to output energy i.e., the output energy will be always less than the input energy, thereby reducing the efficiency. The whole electrical input is not available at the mechanical output in the case of dc motor and the whole mechanical input is not available at the electrical output in the case of dc generator. This is mainly because there is some wastage of energy in form of various losses during the conversion of electrical energy to mechanical energy ( in case of dc motor ) and mechanical to electrical energy conversion ( in case of dc generator ). Let us see various losses occurring in a dc machine.
Copper Loss or Winding Loss :
Copper loss is those which occur in the windings of the dc machine. Generally, there are field and armature windings in a dc machine, and losses are named armature winding loss and field winding loss. Since the conductor used is copper, hence the losses are known as copper loss.
Armature Copper Loss :
The armature winding of a dc machine consists of coils made up of insulated copper conductors and are housed in slots on the armature core. We know every material even it is a conductor it posses some ohmic resistance ( opposition to the flow of current ). Due to this resistance when current flows in the armature winding considerable amount of power is lost in the winding.
If Ra is the resistance of armature winding, and Ia is the current flowing through armature winding, the copper loss is expressed as Ia2 Ra i.e., proportional to the square of the current. The copper loss will be one-fourth when the current is halved.
Field Copper Loss :
The electrical output of dc generator and mechanical output of dc motor cannot be obtained without the presents of field flux produced by field winding. Similar to the armature winding the field winding consists of coils of copper wire wound on the pole cores.
Depending upon the type of generator the field copper loss is given as- In dc shunt generator, if Ish is the shunt field current and Rsh is the shunt field resistance then,
Shunt field copper loss = Ish2 Rsh
- In dc series generator, if Ise is the series field current and Rse is the series field resistance then,
Series field copper loss = Ise2 Rse
- In dc compound generator a part of field is connected in series and a part in parallel, hence there exists both shunt and series field copper losses given as,
Compound field copper loss = Ish2 Rsh + Ise2 Rse
In addition to field and armature windings, there are other windings ( compensating winding and interpole winding ) placed in a dc machine to overcome the armature reaction effect. The copper losses of these windings are also proportional to the square of the armature current.
Since armature current varies with the variation in load, the armature copper also varies as the square of the load current. Therefore, armature copper loss is also called 'Variable Loss'. While in dc machine operation, the field current is almost kept constant, and thus field copper loss remains constant throughout the operation.
Shunt field copper loss = Ish2 Rsh
Series field copper loss = Ise2 Rse
Compound field copper loss = Ish2 Rsh + Ise2 Rse