Losses

 

Losses in Electrical Equipments

Loss is the energy spent in internal activity of the equipment and not contributing in its output. However it is unavoidable. Various types of losses are as under.

 1.       Equipments have internal circuit made of conductor for current path. Current flows in the circuit when connected to supply. Current flowing in the circuit face opposition. Part of the energy supplied to equipment for its function is spent in overcoming this opposition to make the way for current. The extent of oppose and energy spent on this account depends upon the type of the material used as conductor in the circuit, its length, cross section and temperature. This opposing effect of conductor is identified as Electrical Resistance. As the energy spent on this account is loss.

       Material used as conductor is mostly copper. Therefore loss occurring in it is known as Copper Loss. However the circuit may of any conducting material like Gold, Silver, Copper, Aluminum, Iron, etc. But in any case it is known as Copper Loss.

        This loss is not fixed but varies with the load on equipment and therefore it is also called Variable Loss. It is also called I²R Loss as quantum of energy lost is proportional to product of square of load current and resistance of the circuit. This loss is load dependant. So it is insignificant at no load or open circuit condition but rated full load loss is termed as Load Loss. Occasionally other expressions are also used for this loss. Resistive Loss as the loss occurring in the resistance of the circuit, Short Circuit Loss as it is estimated by short circuit (or blocked rotor) test.  This loss is related to load on equipment and is represented as series element with the load in equivalent circuit of equipment. So some time consider as Series Loss.

       This loss can be controlled by adopting low current density in coil winding, i.e. using thick winding wire of higher cross section or using conductor material of low resistivity.

 2.     Almost all equipments have built in current circuit. Similarly most of equipments have built in magnetic circuit

 A.    Magnetic circuit known as core is of high permeability material. Magnetizing force is produced by flow of current in the coil around the core to establish magnetic flux in it. Some energy is spent in aligning the dipoles in core to get the magnetic effect. When DC supply is applied, phenomenon occurs for short while only. Once dipoles are aligned, no more energy is required. But in case of AC supply, dipoles have to realign forward and backward repeatedly.  So continuously energy is consumed. Energy spent on this activity is loss as it don’t contribute directly in output.

       Magnetic property persists even after removal of magnetizing force. This magnetic memory of core is known as residual magnetism due to hysteresis effect. Therefore the loss is known as Hysteresis Loss. Level of residual magnetism after magnetizing force is removed following saturation is called retentivity of material. Retentivity of the core material is correlated to hysteresis loss.

 B.  Alternating current in the coil produce continuously altering magnetic field in the core. This altering field produces electro motive force within various points in the core itself due to electromagnetic induction. This give rise to circulating current within the core material and ultimately energy is wasted in the process. As this is due to whirling of the current, it is known as Eddy Current Loss.

 AB. Both the above losses are occurring within core. So known as Core Loss and core is of iron so also known as Iron Loss. These losses are due to magnetizing phenomenon and therefore known as Magnetizing Loss. Magnetizing phenomenon is due to current flowing in the coils of the winding. So it is also identified as Winding Loss. It is also referred as No Load Loss or Open Circuit Loss as it is determine by performing no load (or open circuit) test. This loss is fixed for particular equipment and does not vary with load so it is called Fix Loss or Constant Loss. This loss is proportional to the square of applied voltage. During short circuit (or blocked rotor) test, applied voltage is kept very low to limit short circuit current to full load current. Therefore this loss is insignificant in short circuit (or blocked rotor) test and is ignored. As this loss is fixed, it is represented as shunt element in equivalent circuit of equipment. So some time consider as Shunt Loss.

       This loss can be controlled by using core material of lower retentivity. Also loss can be controlled by adopting low flux density in core i.e. using core of higher cross section. Core material with high permeability is also useful. Thinner core stampings offer higher resistance to eddy current and reduce eddy current and losses.

 3.     Current is meant to flow through the conducting circuit in the equipment. Dielectric material known as insulation is provided around conducting path to block the diversion of the current. However part of the current break the queue and make the way through insulation. This is known as leakage current. This is very small current through insulation having very high resistance. This loss is proportional to square of leakage current. Obviously is known as Leakage Current Loss or Dielectric Loss or Ohmic Loss in insulation.

       This loss depends on type, thickness and contact area of the insulation. So it can be control by using better quality material and more thickness of insulation.  This loss increases with ageing, moisture ingress etc. Rather than absolute value of the loss, its increasing trend is serious. Periodic checking is done for ratio of leakage current to capacitive current in insulation as health check up of equipment. This is known as tan delta test or dissipation factor test.

 4.       Similarly magnetic flux meant to flow through high permittivity path known as core. But part of the flux escape the path and wander through surroundings, particularly body of the machine. This will interact with metallic surrounding and produce heat wasting energy. Energy lost on this account is Stray Loss or Leakage Flux Loss. This diversion of flux can be control by shielding core with material of high reluctivity.

 5.       When object slide in contact with the other, there is force opposing the movement. Therefore some energy is spent in overcoming this is opposition and make the way. Energy so spent is waste and consider as loss because it does not directly contribute in output of the equipment.

A.     When objects on both sides are solid, the energy spent against such opposition it is known as Friction Loss. It occurs in bushing, bearing, brushes with commutator or slip rings.

 B.        Frictional loss between solid and air is known as Windage Loss.

         Windage loss in rotating machine is due to friction of air with fan blade, uneven rotor   surface on account of slots, uneven surface of coils on both sides, etc.

         These friction and windage losses are also known as Mechanical Losses.

    Friction and windage loss occurs in moving (rotating) equipments like motor, generators, synchronous condenser etc. But no such losses in static device like transformer, rectifiers, inductors, capacitors, etc. Large power transformers have fans on radiators to force air for faster cooling, particularly at higher load. Energy consumed by these fans cannot be considered as windage loss but it is auxiliary consumption.

      Losses have adverse effect on performance of the equipment. It is wastage of energy and hence uneconomical. Losses do not contribute in output of the equipment. So it reduces the efficiency of equipment. But the more severe effect is the heat produced by the losses. It is a task to manage heat disposal at appropriately so that temperature rise is within permissible limit. Excessive temperature will damage insulation system and ultimately failure of equipment. Therefore all equipments capacity ratings are linked to allowable temperature rise.

       Equipments with reduced losses are available known as high efficiency equipments. Of course, they are costlier because of more input expense for material quality, quantity, new item and workmanship. But saving on energy may pay of the additional cost. However it all depends on working hours of equipment, rate of energy utilized and rent on capital. So decision may be based on life cost.