Unbalance voltage can occur due to the presence of larger single-phase consumers or asymmetrical capacitor banks with damage or capacitors that are switched off due to the fuse burning only in one phase. Nowadays, the presence of higher harmonics in the supply voltage is ever more frequent due to the growth of consumers who are supplied through the rectifiers and inverters: regulated actuators, electrothermical consumers and consumers alike. The effect of a variation in supply voltage, wave-form or frequency on the motor's efficiency and power factor characteristics depends on the individual motor design. Typical variations of current, speed, power factor and efficiency with voltage for constant output power are given in Fink (1983). The usual characteristics of induction motors within the ±10% voltage band (Un±10%) are well known. These are included in corresponding table for typical 30-100 kW, 1500 or 1800 1/min motors (Linders, 1972; Fink, 1983), but the effect of saturation has been largely neglected in these tables. However, it is the author's intent to show a correlation between motor characteristics and voltage level. This proposed has three parts: 1. Study of the effect of voltage magnitude on motor losses and motor reactive loads, 2. analysis of the effect of unbalanced voltages, and 3. analysis of the effect of non sinusoidal voltages to the efficient energy use. 2. Effect of voltage magnitude on motor power losses and motor reactive loads Voltage magnitude has a significant and different influence on motor loads and electric energy consumption, depending on types, nominal powers and load level (p=PPN) of motors. Data found in classic references (Fink, 1983) are almost identical to data from older references (Linders, 1972;), although they are not accurate enough for motors made after 1970. The main reason for disagreement is higher values of no-load current and more significant dependency of core losses on voltage magnitude for newer motors (U or T shape of magnetization curve). Data for loads less than 50% are not available in Fink (1983) and Hamer et al. (1997), although more than half of motors operate in these load regimes.
It is established that there exists a significant possibility for energy savings, on the basis of application of induction motors with YY/D connection change proposed in this paper, especially for power to 30 kW. In connection D (0.866Un), total losses and reactive loads are reduced to loads up to 75-85%. Benefits of such motors would be significant because more than 80% induction motors are light loaded (mean value ? 70%). In such manner one motor with two characteristics is attained, but that is better than to offer to the market two different motors, because only in exploitation it is possible to accurately select the one which is working with greater efficiency values in given conditions. However, at the motor with YY/D winding it is always possible, by selection of corresponding connection for (mean value) load measured in operation. Even, with connection change it is possible to adjust to load changes in future. Moreover, at loads ? 30%, energy saving by induction motors with YY/D connection is greater than the one using induction motors with thyristor controllers. Induction motors YY/D connection application may be economical in cases where it is not recommended to use higher costs of energy efficiency of motors.
The parameters of the equivalent Г-circuit, which is proposed for this improvement method, may be derived by data from the no-load test and locked-rotor test at reduced frequency, but direct calculations, i.e. simpler than parameters of the conventional T -circuit. And as for the simpler Г-circuit, the operating characteristics (losses, output power, efficiency and power factor) are calculated with no loss of accuracy. The accuracy of the proposed procedure was verified on the basis of results of comparative calculations for motor given in the literature.
Motors with efficiency high values can became inefficient then standard motors when they are lightly loaded. The calculation procedure for the estimation of any part load efficiency when two load points are given is presented in paper. Efficiency dependence is expressed in the form of ? =?N ?k(1-?N), where is magnitude kP = f1(P0,P?N,P/PN), so that changes-deviations of efficiency values compared to rated value (?N) would be explicitly expressed. Based on those deviations, energy efficiency class changes of observed motors are directly identified with load change. Efficiency curve for several motors, with typical shapes, are considered, especially for loads P/PN = 0.50-1.00. These results are shown justification claim that efficiency values at 50% and 75% load shall be stated in the documentation. The users can then select the motor best suited for their application.
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