## 1 Introduction  
During the operation of an electric motor, heat is dissipated during start-up, speed control, and motor running at constant speed. The conversion of electrical and mechanical energy into heat takes place in both the rotor and stator and in the bearings, resulting in a reduction of the energy efficiency of the machine [1]. Energy converted into heat leads to an increase of the temperature in motor components; their temperature sensivity may lead to emergency situations, such as motor overheating or excessive thermal stresses.  
Finned motor housing can be applied in order to prevent emergency situations, such as failure of the rotor, which can occur while the motor is overloaded. The fins provide an increase of heat transfer surface, thus improving the cooling efficiency and enhancing heat dissipation. This also contributes to more uniform temperature distribution in motor parts, and to better working conditions of the motor. Temperature distribution in the finned housing and the associated heat losses can be evaluated by using an approximate analytical solution, which in typical engineering applications is sufficiently accurate under the condition of large temperature differences between the motor housing and the environment [2]. At smaller temperature differences, the determination of heat losses is a complex problem that can be solved by using numerical methods [3], among which, the Finite Element Method (FEM) is widely used. It should be noticed that the Computational Fluid Dynamics (CFD), which includes FEM, is used to determine the temperature distribution in machines [4]. FEM is a method of approximation of partial differential equations, which constitute the mathematical model of the studied object. It should be noted that the results that were obtained with the FEM have an error, which can be neglected while research are properly conducted. Examples of application of the FEM in the field of heat exchange can be found in [5,6]. Validation of the results can be carried out by using experimental data [7]. In general, after validation, the accuracy of the obtained results can be considered as satisfied when the complexity of the geometrical model is taken into account [8]. Unrealistic assumptions are applied in many studies where steady-state heat transfer in motor is modelled. These assumptions strongly affect the accuracy of the models [9].  
The aim of this article is to present a method of evaluating the heat loss by determining the coefficient of convective heat transfer from the housing to the environment, and determining the conduction heat flux that is transferred through the motor housing. Empirical relationships, thermographical measurements, and numerical simulations were applied. Experimental tests were carried out on a single-phase induction electric motor, which is a component of a vacuum pump unit. The studied phenomena included the conduction of heat through the finned motor housing and heat exchange by forced convection between the housing and the environment. An examination of the conditions of heat and energy losses is particularly important in the efforts to reduce energy consumption by machines and devices [10]. Efficient use of energy in production is key to ensuring the healthy functioning of the world economies [11]. This study presents a method for the determination of overall heat losses during the functioning of appliance, which is equipped in finned housing. Which is why it broadens state-of-the-art and make it possible to quickly estimate heat transfer conditions.