The importance of energy savings began to be emphasized about 20 years ago. It was proposed to take into account not only the price of the electric motor, but also that of the energy consumed during the whole lifetime of the machine. It was stressed then how important regarding energy saving is the high efficiency of the motor [1, 2]. For many years, the motor manufacturer's interest was focused only on the maximum benefit and consequently on the material cost minimization. Therefore a high efficiency motor manufacture is required by standards. It is estimated that between 30 and 40 percent of all the energy produced worldwide is consumed by industrial electrical motors. At present the induction machine is one of the most useful machine. Recent researches emphasize that around one-third of the energy saving potential corresponds to the motors rated in the range of 0.75 to 4 kW, and another one-third to the motors in the range 4 to 30 kW. The remaining energy saving potential corresponds to the motors rated from 30 to 500kW.

The new requirement for the high efficiency, low voltage induction motor is a serious challenge for the motor designers and for the new technologies. The first step to improve the motor efficiency was made by trying all the possibilities to change the motor data without any tools cost increasing – the strategy “no tools cost”. Obviously, it is the right “urgent” solution, but it is limited to a very small domain. The purpose this paper is to remind but also to emphasize that one way for a significant efficiency increasing implies using copper as material for the squirrel cage. The paper investigates the influence of the squirrel cage losses upon the machine efficiency. The cost of the saved energy for the whole machine lifetime and the material cost increase were considered. The required efficiency for the motors with 2, 4 and 6 poles was assumed according to the standard IEC 60034-30:2008 Rotating electrical machines- Part 30 Efficiency class of single speed, three phase cage induction motors. For the motors with 8 poles the assumed efficiency values are those defined by the manufacturer, according to the existing national standard.

The paper analyzes only industrial induction motors with squirrel cage rotor, with 2, 4, 6 or 8 poles, designed for duty type S1 (continuous duty) or S3 (intermittent periodic duty) with a rated cyclic duration factor of 80% or higher. The paper deals not with the motors for variable-frequency drive or with the motors integrated into a machine (pump, fan, compressor) which cannot be tested apart from the machine.

Although the usual material for the squirrel cage is aluminium, mostly because of its price (lower in comparison with that of copper)and it is very convenient for the actual technological solutions, some recent research remind the benefits of copper [1, 2, 3, 4]. As it is known, that copper's resistivity being less than that of aluminium, the copper squirrel cage losses decrease with the ratio Cu/Al. But the actual technology cannot provide applicable solutions for low voltage motors. Obviously, the most important development is necessary in the technological area[3, 4, 5, 6]. The paper analyzes the efficiency improvement for a series of industrial induction machines taking into account only the influence of the rotor losses decreasing, the material cost increase and the estimated saved energy for the whole machine life.

The considered industrial motors (IP44, IP54) are the existing ones, and have the rated power between 0.55 and 45 kW. All other motor data (geometrical dimensions, winding data, steel lamination quality), and the technological conditions are not modified. Only the material of the squirrel cage was modified. The results are presented in Table 1-4 for the 2-pole, 4-pole, 6-pole and 8-pole machine respectively. The index “Al” indicates the computed efficiency for the existing motors, having the squirrel cage from aluminium. These solutions satisfy the existing standard requirements. The efficiency in the case of the squirrel cage from copper, (the index “Cu”) is also computed .

The paper doesn’t deal with the efficiency computing method accuracy, but emphasizes the influence of the copper squirrel cage upon the machine efficiency. The saved energy corresponding to the entire machine life was estimated for an operating time of 8 hours daily, 300 days per year and 10 years of use. The price of copper is greater in comparison with that of aluminium; therefore the material cost increase was also computed. The assumed prices are as follows: 25 RON/kg for Cu, 7.5 RON/kg for Al, and 40$ for 1MW. The considered exchange rate is 2.8RON / 1$.

Note that until now, the required values of the machine efficiency were established only for the 2-pole, 4-pole and 6-pole machines, having the rated power over 0.75 kW, IE1 corresponding to the lowest efficiency values required according to the standard IE 600034 30 (2008).

Table 1. The Results for the two-pole Machines

The standard has no required efficiency value for the two pole machine 1, having the rated power of 0.55kW. For the most part of the machines presented in Table 1 the copper squirrel cage means an efficiency improvement about one percent or more. Only for three motors (12, 13, 14) the efficiency increase is about 0.6%. One emphasizes that, with the copper cage as unique design change, for six two-pole machines (8-11,13,14) the efficiency is greater than the maximum required values and for two motors (7,12) the obtained efficiency is approximately equal with that required one (corresponding to the level IE1). For all the motors, the efficiency is under the required values corresponding to the IE2 level. For all the cases, the cost of the saved energy for the whole machine life (ΔC_w ) is greater than the material cost increase (ΔC_m ).

The standard has no required efficiency value for the four pole machine 1, having the rated power 0.55kW. The most part of the machines presented in Table 2 obtained with the copper squirrel cage is a significant efficiency improvement: more than 2% for two machines (1,2), more than 1.2 % for 8 machines (3-10) and between 0.82 and 0.94 percent for the other 5 machines (11-15). For all the analyzed four-pole machines, with the copper cage as unique design change the efficiency is greater than the maximum required values (IE1), but only one machine (13) satisfies also the requirements of the IE2 level. For all these cases, the cost of the saved energy for the whole machine life (ΔCW) is greater than the material cost increase (ΔCm). Moreover, for a third part of the four pole machines, the cost of the saved energy is two times greater than the material cost increase.

The most part of the machines presented in Table 3 obtained with the copper squirrel cage is a significant efficiency improvement: around 2% for two machines (1,3), more than 1 % for 9 machines (2, 4-10 and 12) and around one percent for two machines (11,13). All these motors with copper squirrel cage obtained an efficiency corresponding to the required values IE1, and only two machines (6,10) have the efficiency corresponding also to the required values of the IE2 level.

For 10 of these cases, the cost of the saved energy for the whole machine life (ΔCW) is greater than the material cost increase (ΔCm). For 3 six-pole machines (6-8), the cost of the saved energy is two times greater than the material cost increase. For the machines 2,3 and 4 the material cost increase (due to the use of copper for the squirrel cage) is greater than the saved energy cost for the whole machine life. It is another reason to emphasize how important is the redesign of the induction motor for performance improvement.

Table 2. The Results for the four-pole Machines

Table 3. The Results for the six-pole Machine

The efficiency improvement for the eight-pole machines with copper squirrel cage is around one percent for 3 cases (8-10), between 1.38% and 1.86% for other 6 (1-6) and 2.43% for the case7. For 8 of these cases (3-10), the cost of the saved energy corresponding to the whole machine life (ΔCW) is greater than the material cost increase (ΔCm). For the machines 1 and 2, the material cost increase (due to the copper use for the squirrel cage) is greater than the saved energy cost in the machine life. Obviously, the rotor losses, even with an aluminium squirrel cage, have low values. Therefore, a significant increase of the machine efficiency requires the machine redesign. In comparison with the four–pole machines, the efficiency increase of the eight pole machines is not so significant, but the saved energy is not negligible.

Table 4. The Results for the Eight-pole Machine

As it was expected, using copper for the squirrel cage of the induction machine, one obtains a significant efficiency increase for most part of range of the analyzed industrial machines. The change of a single motor data has theoretical value giving some useful information, but the optimization problem can be successfully solved only as a function depending on multiple quantities. Although the motor manufacturers avoid change of motor data because of the possible increase of the technological issues and costs, the high efficiency machine is very different in comparison with the existing one. It is a necessary solution, but only this one cannot provide the required values for the machine efficiency according to IE1 and IE2 level. Although the complete motor redesign – including the new tools and new technologies – requires significant financial efforts, it is necessary in order to satisfy the new motor standards.