Abstract
To achieve optimal voltage regulation and energy savings of an AC induction motor operating under constant-torque and variable-working-load conditions, and consider a motor's T-type and Γ-type equivalent circuit, the copper and iron losses of the stator were regarded as invariable losses that were only related to the stator voltage, and the copper losses of the rotor were regarded as variable losses that varied with the load torque. The total electrical loss formula and the optimal voltage regulation formula for the motor were deduced. The formulas revealed that the total electrical loss of the motor was affected by the stator voltage and the rotor load torque, and the optimal voltage regulation changed exponentially with the load torque of the motor. The calculation results of an engineering example showed that when s =0.01-0.03 and the motor operated stably, the motor load torque error was not more than 1.8 N·m based on the exact and approximate solutions of the slip s. The total electrical loss showed almost no calculation error when the working voltage of the motor was higher than 230 V. The optimal voltage regulation and its error increased with the increase in the motor load torque, but within the working voltage range of the motor, the optimal voltage regulation error did not exceed 6 V. The total electrical loss with the optimal voltage regulation mode and a variable load torque was less than the total electrical loss of a 380- or 220-V constant driving mode. With the increase in the load rate, the total electrical loss increased. When the motor's load did not exceed the medium load and the motor operated with optimal voltage regulation, the energy-saving effect was significant. In particular, when the motor operated without a load, the total electrical loss was almost 0.
Highlights
If the load torque driven by the AC induction motor is unknown and has a wide load-torque range, when selecting the motor, the rated power of the motor is generally determined according to the actual maximum load torque driven [1], [2]
When the load torque TL applied to the rotor is constant, there must be a minimum stator voltage to minimize the electrical total loss
Because an induction motor drive system operates with constant-torque and variable-working loads, the energysaving effect based on speed regulation is not significant
Summary
If the load torque driven by the AC induction motor (hereinafter referred to as motor) is unknown and has a wide load-torque range, when selecting the motor, the rated power of the motor is generally determined according to the actual maximum load torque driven [1], [2]. When the motor drives a constant-torque load, if the principle above is applied to establish the objective function of the total electrical loss and determine the optimal voltage, the calculation process cannot be simplified. By comparing the energy consumption of the crane motor with that of the constant-voltage power supply, it was shown that a motor operating with the optimal regulation voltage based on a changing load torque exhibited a significant energy-saving effect. Based on the analysis above, the rotor current depends on the load of the motor, and the excitation current mainly depends on the stator voltage, i.e., the stator copper consumption has an invariable loss caused by the excitation current component that changes with the load, which can be expressed as follows: PCu1 = m1(I0 − I2)2R1. Compared with (2), the objective function of the total electrical loss given by (12) has clearer electrical and physical interpretations, which can facilitate the direct analysis and optimization of the motor’s voltage regulation and energy savings
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