Approaching the Truth in Induction Motor Efficiency

Abstract

The importance of motor efficiency in the motor market is increasing. Therefore the accuracy of the efficiency value given by the manufacturer is significant. The accuracy of the efficiency determination depends on the test method used and the precision of the loss determination by the test method. Determining methods can be grouped into two broad categories: direct measurement methods and indirect methods (loss segregation methods). In the following the effect of different methods in the given motor efficiency has been investigated in more detail. While testing low voltage, poly phase squirrel cage induction motors IEC publication 34-2 allows both a direct and an indirect method to be used, but according to NEMA the IEEE-112 test method B - Input-Output - with loss segregation is to be used. European manufacturers prefer to use the “summation of losses” according to the IEC 34-2. When comparing IEC 34-2 to IEEE 112 it can be noticed that the latter one defines stabilized operating temperatures for the measurements, more accurate measuring instruments and more precise procedures to determine all losses than the IEC 34-2. The IEC 34-2 assumes an operating temperature, assumes a value for stray load loss and permits measurements at any motor operating condition with less accurate instruments. On the other hand the IEC 34-2 tolerances for motor losses are smaller than in the IEEE 112. However, it is important to note that IEEE states that besides the nominal efficiency value also a guaranteed minimum efficiency shall be shown on the nameplate. Depending on the manufacturing tolerances this minimum value can be better than the worst value that the tolerance allows. The test results shown below were measured for a high efficiency 690/400 V, four pole, general purpose 5.5 kW motor using both the IEEE 112 method B and the IEC 34-2 indirect (summation of losses) and direct methods.