Written by the following Authors:
SAYYED ASGHAR GHOLAMIAN
Department of Electrical and Computer Engineering
Babol University of Technology
Because of various techno-economic benefits, three-phase induction motors are used extensively in industry, commercial and residential applications and most of them are connected to electric power distribution system directly, thus they will be affected by voltage quality problems. An important voltage quality problem in power systems is voltage unbalance.
Therefore, it is very important to study performance of these motors under unbalanced voltages. In this paper, Two-Dimensional Finite Element Method is employed to analyze the performance of a three-phase squirrel cage induction motor under voltage unbalance, focusing on electromagnetic torque.
Voltage unbalance combined with over- or under- voltage is a voltage quality problem. In three-phase power systems, the generated voltages are sinusoidal and balanced but they will be unbalanced commonly at the distribution end and the point of utilization for several reasons. Some causes of voltage unbalance are the uneven distribution of single-phase loads in three-phase power systems, asymmetrical transformer winding impedances, open-Y, open-Δ transformer banks, incomplete transposition of transmission lines, blown fuse on three-phase capacitor banks and etc[1-8].
Note that, between mentioned causes of voltage unbalance, rule of the uneven distribution of single-phase loads is significant clearly. For more about, the rural electric power systems with long distribution lines and large urban power system with heavy single-phase demands are examples for problem areas that the single-phase loads are not uniformly spread among the three phases [1, 2].
According to the above description performance analysis of equipments in power systems under voltage unbalance condition is very important. Three-phase induction motor is one of the most widely used equipment in industrial, commercial and residential applications for energy conversion purposes. Based on U.S.
Department of energy, industrial motors consume seventy percent of electricity, and induction motors consists eighty percent of the loads in a typical industry . Because of various techno-economic benefits, the three phase induction motors are used more than ever before.
However, most of them are connected directly to the electric power distribution system and they are exposed to unbalanced voltages unfortunately. Supplying a three-phase induction motor with unbalanced voltages has many undesirable effects on its performance. In theoretical point of view, the unbalanced voltages induce negative sequence current and mentioned current produces a backward rotating field in addition to the forward rotating field produced by the positive sequence one. The interaction of these fields produces pulsating electromagnetic torque and ripple in speed [11, 12].
Such condition has severe negative effects on the performance of an induction motor. The inﬂuence of
unbalance on the efficiency , derating in the machine , increase of losses, and the undesirable effects on the insulation life , and life reduction due to temperature rise [16,17], are some contributions in this area.
This paper attempts to study performance of a three-phase squirrel cage induction motor focusing on the electromagnetic torque under voltage unbalance using finite element method simulation. For this purpose, a 2.2 kW, 380V induction motor has been simulated and it is supplied by eight types of unbalanced voltages with same voltage unbalance factor and also seven numbers of unbalanced voltages with the same positive sequence phase voltage component and its performance analyzed under mentioned conditions.
DEFINITIONS OF VOLTAGE UNBALANCE
Three general definitions for measuring the voltage unbalance are founded in standards. The first definition is Phase Voltage Unbalance Rate or PVUR as defined by IEEE Std 141, the ratio of maximum voltage deviation from Average phase voltage magnitude to the average phase voltage magnitude: Read the complete document here...