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Induction Motor Over Voltages and the Effect of Power Factor Improvement Capacitors
For years it has been a standard practice in industry to utilize power factor correction capacitors on an induction motor. When disconnecting this combination from the source, self excitation can occur and damaging overvoltages may result for an oversized capacitor. This paper will discuss this problem using digital simulation techniques and compare the results with results obtained from experiments performed on a 1.5 HP, 208 V, 3-phase motor.
The use of a power factor improvement shunt capacitor of appropriate size at the induction motor terminal, to improve the steady-state performance, has been a common practice in the industry. Satisfactory compensation over the entire load range would necessitate slow speed switched capacitor banks. These capacitors act as a "VAR" generators and provide VARs required by the motor to varying degrees depending on the capacitor size. These capacitors while connected to the motor, however, may not perform optimally during motor starting or during fast and sudden load fluctuation periods. Therefore, design considerations of these banks during transient performance would be helpful. This paper reviews and analyzes the overvoltage problem due to fixed shunt compensation using digital simulation techniques and applies this to a present day design.
The concept of reference frame transformation is adopted in the model development. This reduces the time varying quantities, such as stator winding inductances in the "abc" windings which are dependent on the rotor position, to constant inductances in the "qdo" two-axes reference frame. The complete discussion is beyond the scope of this paper and has been discussed.
By comparing the saturated case with no saturation, it is observed that the effects of saturation tend to limit the post switching voltage transients. This is in close agreement with the results obtained by Walsh and deMello. Their results predicted that for capacitor values of 1, 2, and 3 per unit, peak voltages of approximately 0.95, 1.2 and l. 3 per unit would be seen at the motor terminals. Comparison of the no load vs. full load case indicates that the no load case is the worst case situation. The effect of motor loads is to dampen any post switching transients.
The indication provided by the computer plots and experimental results is that no problem is encountered with the correction up to 100% or less. The major hazardous effect of these capacitors at no load is that the terminal voltage is sustained for several seconds.
|Effect of Power Factor Improvement Capacitor on Induction Motor Over Voltages|