Operation of induction machines in the presence of unbalanced supplies

Master Thesis


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University of Cape Town

Three-phase induction machines are widely used in South Africa for industrial, commercial and residential applications because of their simple construction and low maintenance. The manufacturers design these motors to provide their nameplate ratings when supplied with balanced sinusoidal voltages. However, the power system experiences unbalanced voltages, overvoltages, undervoltages, etc, which are seen at the motor terminals. The South African Electricity Supply Utility (Eskom) supplies the commercial and industrial systems with a line voltage range of 400V +- 10%. An Induction motor rated at 380V or 400V will therefore experience overvoltages or undervoltages depending on the location of the motor from the supply. Besides overvoltages and undervoltages, unbalanced voltages exist on the power system. The tolerable level of voltage unbalance on the South African three-phase networks is 2%, and may be up to 3% in the rural areas. In practice, induction motors experience a combination of overvoltages or undervoltages with unbalanced voltages. This affects motor life. NEMA has done research on induction motors operating under unbalanced voltages. They defined voltage unbalance and produced a derating curve, assuming the average voltage applied to the motor is equal to the rated voltage. The EEC has another definition of voltage unbalance based on the positive and negative sequence voltages. The IEC definition is referred to as the true definition. Two formulas that approximate the true definition also exist. An analysis was done in order to understand the implications of using these definitions of voltage unbalance in the South African power system. It was found that the NEMA definition and the true definition do not differ significantly below a 5% unbalance. in order to estimate motor life when a motor is supplied with unbalanced voltages, in combination with over or undervoltages, electrical and thermal models were developed to predict the temperature. The electrical model was based on the positive and negative sequence equivalent circuits. The thermal model used, separated stator and rotor thermal circuits. The thermal parameters were obtained from simple tests rather than from motor design data. The predicted temperature values agree closely to the measured values. The interaction between the electrical model, thermal model and the thermal ageing equation were used to estimate motor life when the motor is supplied with unbalanced voltages, in combination with over or undervoltages. Motor life is reduced when the operating temperature exceeds the rated. In order to restore motor life, new derating curves were developed since the NEMA derating curve is limited to rated average voltage. Motor life can be restored by applying the new derating curves developed here.

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