Analysing the Types of Losses in DC Machines

The losses can be divided into three classes 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 to heat and therefore increase the machine’s temperature. Further, the efficiency of the device will reduce.

1. Copper Losses

This loss generally occurs due to current in the various windings on 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

There’s additional brush contact loss attributable to brush contact resistance (i.e., resistance in the middle of the surface of the 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’s 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 the 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 piece ab is underneath N-pole, the magnetic lines pass from a to b. Half a revolution well along, an identical piece of iron is underneath the S-pole, and magnetic lines pass from b to a to overturn magnetism within the iron. To constantly reverse the molecular magnets within the armature core, a particular quantity of power must be spent, named hysteresis loss. The Steinmetz formula gives it.

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

Suppose you want to cut back this loss in a d.c. The machine armature core is created of materials with a lesser value of Steinmetz hysteresis co-efficient, e.g., silicon steel.

Eddy’s current loss

In addition to the voltages evoked within the armature conductor, other voltages are produced 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 recent loss. This loss seems the machine’s temperature and efficiency will decrease as heat increases.

If a never-ending cast-iron core is employed, the resistance to the eddy current path is tiny attributable to the massive cross-sectional space of the body. Consequently, the magnitude of eddy current and recent eddy loss are huge. The volumes 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 thin, spherical iron sheets referred to as lamination, shown in Fig. The lamination is insulated from one another with a layer of varnish. The insulating layer features a high resistance; thus, only a small amount of current flows from one lamination to the opposite. Also, due to every lamination being extremely skinny, the resistance to recent passing over the lamination’s breadth is quite massive. Therefore laminating a core will increase the core resistance that drops the eddy current and, thus, 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

Constant (K_e) depends upon the resistance of the core and the system of unit used.

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

Mechanical Loss

These losses are attributable to friction and windage.

Friction loss occurs due to friction in bearings, brushes, etc.
Windage loss occurs due to the air friction of the rotating coil.

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

Constant and Variable Losses

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

Constant losses

Those losses in a d.c. Generators that stay constant at all loads are referred to as continuous losses. The ongoing losses in a very d.c. Generator is:

(a)iron losses

(b)mechanical losses

(c)shunt field losses

Variable losses

Those losses in a d.c. Generator differences with load are referred to as variable losses. The variable losses in a very d.c. Generator is:

Copper loss in armature winding (I²R_a)
Copper loss in the series field winding (I²_seR_se)