Losses in a D.C. Machine

Losses in a D.C. Machine

AU: Dec.-06, May-15

• The various losses in a d.c. machine whether it is a motor or a generator are classified into three groups as:

1. Copper losses 2. Iron or core losses 3. Mechanical losses.

1. Copper Losses

• 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. Thus the various copper losses can be given by,

Armature copper loss = I_a² R_a

where R_a = Armature winding resistance

and I_a= Armature current

Shunt field copper loss = I_sh² R_sh

where R_sh = Shunt field winding resistance

and I_sh= Shunt field current

Series field copper loss = I_Se² R_se

where R_se = Series field winding resistance

and I_se  = Series field current.

• 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.

• The power loss at the brush is obtained as the product of the voltage drop at the brush and the current passing through the brush. Hence this loss is proportional to the armature current in a d.c. machine.

Brush contact loss ∝ I_a

2. Iron or Core Losses

• These losses are also called magnetic losses. These losses include hysteresis loss and eddy current loss.

• The hysteresis loss is proportional to the frequency and the maximum flux density B_m in the air gap and is given by,

Hysteresis loss =  η B^1.6_m f v watts

Where

 η = Steinmetz hysteresis coefficient

v = Volume of core in m³,

f = Frequency of magnetic reversals.

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

• 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. This loss is given by,

Eddy current loss = K B_m2 f² t² v watts

Where K = Constant,

t = Thickness of each lamination

v = Volume of core,

f = Frequency of magnetic reversals.

• The hysteresis loss is minimized by selecting the core material having low hysteresis coefficient. While eddy current loss is minimized by selecting the laminated construction for the core.

• These losses are almost constant for the d.c. machines.

3. Mechanical Losses

• 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.

• The magnetic and mechanical losses together are called stray losses. For the shunt and compound d.c. machines where field current is constant, field copper losses are also constant. Thus stray losses along with constant field copper losses are called constant losses. While the armature current is dependent on the load and thus armature copper losses are called variable losses.

Thus for a d.c. machine,

Total losses = Constant losses + Variable losses.

• The power flow and energy transformation diagrams at various stages, which takes place in a d.c. machine are represented diagrammatically in Fig. 5.2.1 (a) and (b).

Ex. 5.2.1 A 4 pole DC shunt generator with lap connected armature supplies 5 kilowatt at 230 Volts. The armature and field copper losses are 360 Watts and 200 Watts respectively. Calculate the armature current and generated EMF? AU: May-15, Marks 10

Sol. :

Review Question

1. Explain the various losses which takes place in a d.c. machine.AU: Dec.-06, Marks 8