The introduction of the electric energy in more and more diverse applications currently represents an evolution which is considered normal, taking into account the durable development concept, which are required among the decision makers, and also due to the fact that the oil and gas resources are not evenly distributed in the world, and those who do not have these resources look for solutions to provide them a certain independence over those who have resources. One of the technical solutions, available on Earth, is currently represented by the renewable energy sources. Their usage in the field of the electric automobile charging, is both an economic and technical challenge. Technical solutions are available, and they are developing, but their industrial application is related to an economic strategic approach and a systemic one, which are treated below.

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The main objectives of the heat transfer are: first, the determination or ensuring the amount of heat exchanged per time unit in fixed temperature conditions, and second, checking the compatibility of materials used with the regime of temperature faced by, in determining the temperature field. We can say that an exchange of heat device is an optimal solution from thermic, hydraulic, mechanic, economic and reliability points of view, mainly, the heat transfer being the determining factor. We should add to these, finding the enhancing methods and procedures or, in certain cases, heat transfer breaking. [5] Conduction, radiation and convection are distinct forms of heat transfer. As defined above, only conduction and radiation are heat exchange processes due solely to the difference in temperature, convection is a more complex process, necessarily involving the mass transfer. Because, the convection makes energy transfer from regions with higher temperature in regions with lower temperature, it was generally accepted "heat transfer by convection" as being the third mode of heat exchange. Basic sizes in heat transferThe temperature field. The status scalar parameter, the temperature, in any point of space M(x, y, z) depends on position and time, namely it is a function of the following form.( ) Calculation Methods for Heating and Ventilation System of Electrical Machines 29The temperature field is such, the total of the temperature t values throughout all space, at some time τ, the expression (1) representing the equation of this field.The temperature field may be constant (stationary or permanent) and transient (nonstationary or variable) as time τ is explicitly or nonexplicitly in equation (1) and namely:the temperature constant field has the equation:the temperature transient field is expressed by equation (1) Regarding the number of coordinates, the temperature field can be one, two or tridirectional. So, in equations (1) and (2), the temperature field is tridirectional in transient regime, respectively, constant. If the temperature is depending on two coordinates and time, the field is bidirectional transient, with the equation:and if it is expressed as a function of a coordinate and time, the field is onedirectional transient, with equation:The equation of temperature in the onedirectional constant field is the most simple:All points of space considered which have the same temperature t at a moment in time τ form isothermal surface. Because, a point in an object can not concomitant has two different temperature values, the result is that isothermal surfaces are continuous surfaces which do not intersect one another.[5]The temperature gradient is a measurement which expresses elementary increase of temperature to a point of a temperature field, at a given time τ.The temperature gradient is a normal vector to isothermal surface and it is numerically equal with the limit of the report between the temperature variation Δt between two isothermal surfaces and the distance Δn of these two, measured on normal to surfa...

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