What are the types of Cooling methods in Transformers ???

                    No transformer is truly an 'ideal transformer' and hence each will incur some losses, most of which get converted into heat. If this heat is not dissipated properly, the excess temperature in transformer may cause serious problems like insulation failure. It is obvious that transformer needs a cooling system. Transformers can be divided in two types as (i) dry type transformers and (ii) oil immersed transformers.

Different cooling methods of transformers are - 

§  For dry type transformers

§  Air Natural (AN)

§  Air Blast

§  For oil immersed tranformers

§  Oil Natural Air Natural (ONAN)

§  Oil Natural Air Forced (ONAF)

§  Oil Forced Air Forced (OFAF)

§  Oil Forced Water Forced  (OFWF)

Cooling Methods For Dry Type Transformers:

Air Natural Or Self Air Cooled Transformer:

              This method of transformer cooling is generally used in small transformers (upto 3 MVA). In this method the transformer is allowed to cool by natural air flow surrounding it.

Air Blast:

              For transformers rated more than 3 MVA, cooling by natural air method is inadequate. In this method, air is forced on the core and windings with the help of fans or blowers. The air supply must be filtered to prevent the accumulation of dust particles in ventilation ducts. This method can be used for transformers upto 15 MVA.

Cooling Methods For Oil Immersed Transformers:

Oil Natural Air Natural (ONAN):

                This method is used for oil immersed transformers. In this method, the heat generated in the core and winding is transferred to the oil. According to the principle of convection, the heated oil flows in the upward direction and then in the radiator. The vacant place is filled up by cooled oil from the radiator. The heat from the oil will dissipate in the atmosphere due to the natural air flow around the transformer. In this way, the oil in transformer keeps circulating due to natural convection and dissipating heat in atmosphere due to natural conduction. This method can be used for transformers upto about 30 MVA.

Oil Natural Air Forced (ONAF):

 The heat dissipation can be improved further by applying forced air on the dissipating surface. Forced air provides faster heat dissipation than natural air flow. In this method, fans are mounted near the radiator and may be provided with an automatic starting arrangement, which turns on when temperature increases beyond certain value. This transformer cooling method is generally used for large transformers upto about 60 MVA.

Oil Forced Air Forced (OFAF):

               In this method, oil is circulated with the help of a pump. The oil circulation is forced through the heat exchangers. Then compressed air is forced to flow on the heat exchanger with the help of fans. The heat exchangers may be mounted separately from the transformer tank and connected through pipes at top and bottom as shown in the figure. This type of cooling is provided for higher rating transformers at substations or power stations.

Oil Forced Water Forced (OFWF):

                  This method is similar to OFAF method, but here forced water flow is used to dissipate hear from the heat exchangers. The oil is forced to flow through the heat exchanger with the help of a pump, where the heat is dissipated in the water which is also forced to flow. The heated water is taken away to cool in separate coolers. This type of cooling is used in very large transformers having rating of several hundreds MVA.

Source : Circuits Globe.

What are Random wound and Form wound Coils in Electric Motor ??? How they influence the Motor ???

                 A typical low-voltage generator is built with multi turn stator coils, ranging from one to 16 turns per coil. These coils can either be form wound (where the wire is square or rectangular and the turns are systematically arranged) or random wound (where the wire is round and the arrangement between the turns is not defi nite).With units less than 1,500 kW, the size of the stator and the minimum wire thickness usually do not allow form-wound coils. However, in some cases, either form-wound or random-wound coils can be used. Generators with random-wound coils can be made at a reduced cost, and their capability to withstand severe environmental conditions can be enhanced through the use of vacuum-pressure impregnation (VPI). Still, performance, capability and endurance to the environment make units with form-wound coils superior.

FORM WOUND COILS:

                A form winding uses square or rectangular magnet wire. The wire insulation is designed to handle operating turnto-turn voltages as well as maximum surge or impulse voltages The coil winding process begins with skeining (looping) of the magnet wire. Because of the difficulty to form wire of high width-to-thickness ratio, several wires in parallel may make one turn. Individual turns are arranged in precise location with respect to each other. That is, turn one is always next to turn two is always next to turn three, and so on.
ADVANTAGES:

               The main advantage is as there is uniform turn-to-turn voltage stress due to uniform arrangement of coils.Form  windings have uniform temperature distribution as well as resin build up, which minimize the chance of localized hot spots. Insulation systems are categorized by temperature limits of the materials. For example, a class F system is designed to withstand an overall temperature of 155° C. This temperature is made up of ambient air plus average temperature of the windings with allowance for hot spots.

             With form wound machines the hot spot is always in a predictable location at the slot portion of the laminations. The hot spot temperature can be measured and duplicated from unit to unit. However, for random windings, because of uneven resin build up, the hot spot location can vary in duplicate units. In continuous duty applications, premature winding failures of identical units having random coils is a prime example of localized excessive hot spot.

            The end windings of form coils have large openings for air circulation, which prevent contamination build up. Therefore, the likelihood of tracking-type failures is minimized. In addition, the insulating tape on the coils provides additional environmental protection. The inherent rigidness of the rectangular wire coupled with the lacing of the end turns and VPI ensures a rigid insulation system. A rigid coil structure is very important to minimize movement at the coil-lamination interface when there are high surge currents, such as motor starting loads or short circuit faults. These can exert extreme forces between the coils.
DISADVANTAGES:

              Magnet wire is rectangular or square with double dacron glass cover or mica turn tape over 200° C heavy film. The wire is more costly and inventory costs are increased because many different sized wires are used. Round wire with 200° C heavy fi lm is used. Fewer sizes need to be kept on hand, and the wire is more economically priced.
RANDOM WOUND COILS:

            When the arrangement between turns is not definite during coil skeining or insertion, the windings are called “random.” That is, for example, turn one can be touching turn four. Random windings are used on lower kW machines where it is impractical to use form coils. Random windings are made from a lower cost magnet wire that is fi lm coated and round. With random windings, the mechanization of manufacturing is also increased to add to a lower cost.
ADVANTAGES:

               The primary advantage for manufacturing generators with random windings is economics: lower cost wire and mechanized construction. However, without vacuum-pressure impregnation (VPI), the life expectancy of random windings is drastically reduced under severe environmental conditions or with applications involving nonlinear loads, such as silicon-controlled-rectifi ed (SCR)-type loads. SCR type loads induce high turn-to-turn surge voltages into the windings, as high as twice the normal operating voltage.
DISADVANTAGES:

               In the case of random windings, the ability to withstand operating turn-to-turn voltages, which can be marginal for normal operation, will cause failures within a few hours under SCR type loads. For example, a four-pole generator rated 600 kVA and 600 volts may have four turns per coil and peak turn-to-turn voltage of 38 volts. For a random-wound design if turn one and turn four are touching, this voltage can be as high as 114 volts (38 volts x a three-turn difference), and can be amplifi ed by SCR loads. That is why random coils may prematurely fail. A similar form-coil design would have a peak voltage of only 38 volts because turns one and four would never be next to each other.

How Insulation fails in Electric Motor ???

                Moisture and contaminants promote tracking and deterioration that can lead to failure of majority of motors. Tracking failures are caused by small partial discharge currents, which develop localized heating and cause chemical decomposition of any weak insulation barrier. 

                  Arcs of relatively high currents develop additional heating, which will carbonize (track) the resin surface. This is the start of most winding failures.