Types of Losses in a DC Machines

The losses can be divided into three types in a dc machine (Generator or Motor). They are

1. Copper losses

2. Iron or core losses and

3. Mechanical losses.

All these losses seem as heat and therefore increase the temperature of the machine. Further the efficiency of the machine will reduce.

1. Copper Losses:

This loss generally occurs due to current in the various windings on of the machine. The different winding losses are;

Armature copper loss = I²_a R_a

Shunt field copper loss = I²_shR_sh

Series field copper loss = I²_se R_se

Note: There’s additionally brush contact loss attributable to brush contact resistance (i.e., resistance in the middle of the surface of brush and commutator). This loss is mostly enclosed in armature copper loss.

2. Iron Losses

This loss occurs within the armature of a d.c. machine and are attributable to the rotation of armature within the magnetic field of the poles. They’re of 2 sorts viz., 

(i) Hysteresis loss 

(ii) eddy current loss.

Hysteresis loss:

Hysteresis loss happens in the armature winding of the d.c. machine since any given part of the armature is exposed to magnetic field of reverses as it passes underneath sequence poles. The above fig shows the 2 pole DC machine of rotating armature. Consider a tiny low piece ab of the armature winding. Once the piece ab is underneath N-pole, the magnetic lines pass from a to b. Half a revolution well along, identical piece of iron is underneath S-pole and magnetic lines pass from b to a in order that magnetism within the iron is overturned. So as to reverse constantly the molecular magnets within the armature core, particular quantity of power must be spent that is named hysteresis loss. It’s given by Steinmetz formula.

The steinmetz formula is

Hysteresis loss P_h= ηB¹⁶_max fV watts

Where,

            η = Steinmetz hysteresis co-efficient

            B_max = Maximum flux Density in armature winding

            F = Frequency of magnetic reversals

               = NP/120 (N is in RPM)

           V = Volume of armature in m³

If you want to cut back this loss in a d.c. machine, armature core is created of such materials that have an lesser value of Steinmetz hysteresis co-efficient e.g., silicon steel.

Eddy current loss:

In addition to the voltages evoked within the armature conductors, some of other voltages evoked within the armature core. These voltages turn out current currents within the coil core as shown in Fig. These are referred to as eddy currents and power loss attributable to their flow is named eddy current loss. This loss seems as heat that increases the temperature of the machine and efficiency will decrease.

If never-ending cast-iron core is employed, the resistance to eddy current path is tiny attributable to massive cross-sectional space of the core. Consequently, the magnitude of eddy current and therefore eddy current loss are massive. The magnitudes of eddy current are often decreased by creating core resistance as high as sensible. The core resistances are often greatly exaggerated by making the core of skinny, spherical iron sheets referred to as lamination's shown in the fig. The lamination's are insulated from one another with a layer of varnish. The insulating layer features a high resistance, thus only small amount of current flows from one lamination to the opposite. Also, as a result of every lamination is extremely skinny, the resistance to current passing over the breadth of a lamination is additionally quite massive. Therefore laminating a core will increase the core resistance that drops the eddy current and therefore the eddy current loss.

Eddy Current loss P_e=K_eB²_maxf²t²V Watts

Where,   k_e = constant

              B_max = Maximum flux density in wb/m²

              T = Thickness of lamination in m

              V = Volume of core in m³

Note: Constant (K_e) depend upon the resistance of core and system of unit used.

It may well be noted that eddy current loss be subject to upon the sq. of lamination thickness. For this reason, lamination thickness ought to be unbroken as tiny as potential.

3.Mechanical Loss

These losses are attributable to friction and windage.

• Friction loss occurs due to the friction in bearing, brushes etc.
• windage loss occurs due to the air friction of rotating coil.

These losses rely on the speed of the machine. Except for a given speed, they're much constant.

Constant and Variable Losses

The losses in a d.c. machine is also further classified into (i) constant losses (ii) variable losses.

Constant losses

Those losses in a d.c. generator that stay constant at all loads are referred to as constant losses. The constant losses in a very d.c. generator are:
(a)iron losses 

(b)mechanical losses 

(c)shunt field losses 

Variable losses

Those losses in a d.c. generator that differ with load are referred to as variable losses. The variable losses in a very d.c. generator are:

Copper loss in armature winding (I²R_a)

Copper loss in series field winding (I²_seR_se)

Total losses = Constant losses + Variable losses.

Generally this copper loss is constant for shunt and compound generators.

CSV Nov 23, 2014

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