Losses in DC Machine : DC generator & DC Motor

We know Energy neither can be created nor it can be destroyed, it can only be transferred from one form to another. A DC generator converts mechanical energy to Electrical energy and a DC Motor convert electrical energy mechanical energy. In DC generator input energy is in the form of mechanical energy and the output energy in the form of Electrical energy. On the other hand in DC Motor input energy is in the form of Electrical energy and the output energy in the form of Mechanical energy.

But in practical we not receive whole input energy in the form of output energy, because some energy lost in the conversion process. The difference between input power and output power are the losses.

(1) Copper Losses

• Armature Copper Loss
• Field Copper Loss
• Brush Contact Loss

(2) Iron losses:

• Hysteresis loss
• Eddy Current loss

(3) Mechanical Losses

• Winding loss
• Friction loss

(1)Copper Losses:

These are the losses that occur in the winding of the machine due to flow of electric current. These losses are called copper losses because armature and field windings are made of copper.

(i) Armature Copper Loss = I_a²R_a

Where, Ia = Armature Current

              Ra = Armature Resistance

                    

(ii) Field Copper Loss =

In Series field winding = I_se²R_se

Where, Ise = Series Field Current

              Rse = Series field Resistance

In Shunt field winding = I_sh²R_se

Where, Ish = Shunt field Current

              Rsh = Shunt field Resistance

(iii)Brush Contact Loss : This loss occurs due to brush contact resistance (between the surface of brush of brush and surface of the commutator). loss is included in the armature copper loss.

(2)Iron losses:

These take place in the armature due to rotation of its iron core in the magnetic field. These are also called magnetic or core losses and consist of Hysteresis loss and Eddy current loss which are elaborated below.

(i) Hysteresis loss: This loss occurs in the core due to magnetization, demagnetization and again magnetization of the core in the reverse direction. Hysteresis loss depends upon the frequency, flux density, volume and type of the core.

Wh = η B_max^1.6fV

Where,

 η = Hysteresis coefficient depending upon the material (constant)

B_max = Maximum value of flux density in webers/metre²

f = Number of magnetic reversals per second (i.e. frequency)

V = Volume of the magnetic core

Wh  = Power loss in watt/ metre² due to hysteresis

Note: 4% silicon steel is used to reduce the hysteresis loss.

(ii) Eddy Current loss : This loss take place in the armature core when it is rotated in the magnetic field Therefore, heavy currents flow through them which is due to the induced emf. To decrease the eddy current loss, the cores are made from thin sheets pressed together. Eddy currents loss is given as :

We = η B²_maxf²t²

Where,

B_max = Maximum value of flux density in webers/metre²

f = Number of magnetic reversals per second (i.e. frequency)

t = Thickness of lamination

We  = Eddy current loss in watts

(3) Mechanical Losses:

Mechanical losses are consist of air friction of rotating armature, bearing friction, brush friction.