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 the 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.
Core Losses or Iron Losses :
The core of the dc machine when subjected to pulsation magnetic flux, some power is lost in the magnetic core know as iron or core losses. There are two types of core losses in dc machine,
Hysteresis Loss :
The armature winding is placed on the armature core made up of steel. When this armature core rotates in the magnetic field produced by the field winding. The armature core is continuously subjected to varying magnetic flux i.e., under the north and south poles.
Due to this, the tiny domains which behave as magnets on the core material will frequently change their direction due to the change in flux direction alternatively. In the process, an appreciable amount of power is wasted in the armature core in the form of heat, and can be expressed as,
Where,- K = Core material constant
- Bm = Maximum flux density
- f = Frequency of flux variation or supply frequency
- Vc = Volume of core material in m3.
Eddy Current Loss :
When the armature core is rotated in field flux, similar to emf induced in armature winding (in case of dc generator) or back emf (in case of dc motor) some amount of emf will also be induced in the armature core. This induced emf circulates currents throughout the core called eddy currents.
These unwanted currents produce a power loss due to the resistance of the core material. By increasing the core resistance the eddy currents can be reduced which is done by using thin steel sheets insulated from one another. The expression for eddy current loss is given as,
Where,- K = Core material constant
- t = thickness of core sheets.
The hysteresis and eddy current losses depend upon flux density and frequency of flux variation in the machine. Since the machine is operated at constant frequency and field current, by maintaining the same flux density, the core losses also remain constant and also called constant losses.
Mechanical Losses :
The mechanical losses of the dc machine include frictional and windage losses. While the rotation of shaft due to friction at the bearings and at the brush some power is wasted to overcome this bearing-friction and brush-friction called as 'Frictional Losses'.
In the same way, the armature experience some sort of friction when it rotated in the air due to air friction called 'Windage Loss'. The mechanical losses are proportional to the speed of the armature.
Stray Losses :
There are some uncountable losses in a dc machine that are difficult to measure are called 'Stray Losses'. The loss due to sparking between the brushes and commutator segments, loss due to the armature reaction effect that distorts the flux distribution, and many more are considered as stray losses. Since they are difficult to measure, stray losses are considered as 1% of the total output power.
