International standards for the induction motor efficiency evaluation: a critical analysis of the stray-load loss determination

Motor efficiency has to be measured or calculated in accordance with international standards. The most important standards are the IEEE 112-B, IEC 34-2, and JEC 3 . In this paper, a comparison of the measurement procedures defined by these international standards is reported, together with some comments on the prescribed methodologies. The comparison is based on experimental results obtained by tests on four general-purpose three-phase induction motors. The stray-load loss measurement represents a critical key for the correct evaluation of the motor efficiency. For this reason, a critical analysis of this type of losses has been performed. In particular, in order to understand which are the most critical quantities that influence their evaluation, the stray-load loss sensitivity to the measurement errors is analyzed. In the final part of the paper the temperature influence, on the conventional iron losses, is experimentally analyzed. The performed tests show that the temperature difference between the no-load test and the motor real operative conditions is not negligible.

I. Introduction

IN THE ACTUAL electric energy market the energy saving policies are more and more important. In fact, cost and availability of the electric energy can vary in a complex manner. For these reasons, electric energy consumers are interested in using apparatuses with high efficiencies in order to reduce their electric consumption. It is important to observe that the induction motors can be considered as larger users of electrical energy. In the European Union, the electric motors used in the industrial field typically consume 60%–70% of the total absorbed electrical energy. TABLE I European CEMEP Efficiency Lower Limits

Rated power	I 4 kW I	7.5 kWI	11 kW	I 15 kW
Effl lower limit	88.3	90.1	91.0	91.8
Eff2 lower limit	84.2	87.0	88.4	89.4
Eff3 typical value	80.0	83.0	85.0	87.0

In the commercial sector, this percentage is up to 35%. More information can be found in [1]. For these reasons, laws or protocols (such as the EPAC in the U.S. and the European Union and the CEMEP committee agreement in Europe [2], [3]) have been promulgated in order to define the efficiency class of the motors. For the industrial induction motors the European CEMEP protocol defines the following efficiency classes: •

Eff1 class: “high efficiency” motors;

•

Eff2 class: “energy efficiency” motors;

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Eff3 class: “standard efficiency” motors (motors now in production).