Year

1997

Degree Name

Doctor of Philosophy

Department

School of Electrical, Computer and Telecommunications Engineering

Abstract

An original contribution of this project is the design and construction of a new type calorimeter, a double chamber calorimeter (DCC), to directly and accurately estimate total losses of a 7.5 kW induction motor. The DCC is utilised to investigate the additional losses due to the presence of time harmonics in the supply of mains-connected induction motors. The main advantage of using the DCC is that it enables estimation of machine losses independent of the level of the supply distortion and motor loading conditions.

The DCC is made of class VH expanded polystyrene insulation material, large enough to house the test motor and a reference heater for balance type of loss measurement. A variable speed fan is used to maintain the air with sufficient flow rate through the calorimeter to remove the generated heat within the calorimeter chambers. Motor losses are estimated as a function of reference heater input power and the air temperature rise across the calorimeter chambers after thermal equilibrium has been achieved.

One-dimensional conducted heat leakage through the calorimeter walls is estimated by developing a simple loss model for the calorimeter and validated using experimental tests. The model utilises conduction shape factors to evaluate the heat leakage through the calorimeter walls, edges and corners. Dynamic operation of the DCC is examined by performing substantial experimental tests using two identical heaters. Accordingly, limits for the air flow rate through the calorimeter, air temperature rise inside and across each chamber and heater input power are derived. Experimental results confirmed that motor losses up to 1 kW can be estimated using the DCC with a resolution of 10 W and an accuracy of 4%.

Motor line-line voltages and line currents are measured by developing voltage and current measurement circuits. For data collection a PC-based data acquisition (DA) system in conjunction with a computer software package is employed in this project. The DA system is also used for absolute temperature measurement using RTDs and relative temperature measurement using thermopiles.

A 10 kVA inverter capable of producing harmonically distorted waveforms (up to 1 kHz) is employed to conduct harmonic tests on the test induction motor. Experimental tests are performed under non-distorted (nominal) and various distorted supply conditions and with the motor operating under no load, half load and full load. Experimental results confirmed that a distorted voltage containing low order harmonic causes more losses in a motor when compared with a distorted voltage having a higher order harmonic. A weighted THD (WTHD) is defined to specify the limits for additional losses in a motor supplied by distorted voltages. In terms of loading effect, the additional losses significantly increase with load mainly due to the increased resistance with temperature. Therefore, one can conclude that the harmonic losses are load independent and are constant for a known voltage distortion level except for the temperature effect.

The variation of test motor parameters with harmonic order as well as the variation of additional losses with WTHD has led to establishment of derating factor (DF) for induction motors. Depending upon the supply WTHD, a DF can be determined which suggests the fraction of machine loading under which the additional losses due to the distorted supply can be safely tolerated by the machine. This figure has been calculated using the data for several machines with various power ratings from 3.7 kW to 1.6 MW. The results confirmed that a higher WTHD can be applied to the larger machines as compared with smaller machines. In other words, larger machines are more capable of handling additional losses due to the supply distortion. It has also been demonstrated that most induction motors can afford a WTHD up to 8% if a service factor of 1.15 is applied. The 8% figure corresponds to an average THD of about 15% which is much larger than the commonly used 5% limit for THD in utility power networks as specified by standards.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.