Electric motors are employed in a variety of applications. For applications requiring less than one horsepower, single phase motors provide satisfactory performance. However, as power requirements increase into the integral horsepower range and beyond, single phase motors have lower performance, higher cost, and much greater failure rates than three phase motors. Consequently, where three phase power is available, for applications requiring all but the smallest drive motors, three phase motors are typically used.
Much of the demand for motors in the fifteen to one hundred horsepower range is in fluid handling applications, primarily irrigation pumping and oil well pumping, air handling applications, and more demanding applications such as grain handling, conveyers, saw mills, etc. Many of these applications are performed in remote rural areas where only single phase AC power is available. Thus, in order for a three phase motor to be used to perform these applications, a converter for converting the available single phase AC power to three phase power is required.
Various solutions have been proposed for converting single phase power to three phase power to allow three phase motors to be utilized where only single phase power is available. Some such single phase to three phase converters have been available commercially. The most commonly employed solutions include rotary and reactive phase converters. Rotary phase converters use motor-generator sets to provide single phase to three phase power conversion. Rotary phase converters include traditional phase shifted motor solutions, as well as more unconventional solutions. However, all such solutions are generally limited by high start-up current levels, require minutes to spin up to speed, have very poor efficiency, are very bulky, and are relatively very expensive. Reactive phase converters employ passive components, such as transformers and capacitors, and relays to provide single phase to three phase power conversion. Reactive phase converters thus also tend to be heavy and bulky. Reactive phase converters tend to be more efficient than rotary phase converters, but require extensive tuning to operate with any given load. Furthermore, reactive phase converters cannot be used in applications where loading conditions vary substantially. Significant three phase motor unbalance can occur in such situations.
An alternative solution which may be employed in many cases is to use a three phase inverter, derated to operate from a single phase line, to convert single phase power to three phase power. Such a solution may take advantage of the variable speed operation, high torque across the speed range, and limited inrush current drawn from the input line which are features of many conventional three phase to three phase inverter designs. However, when a three phase to three phase inverter is employed for single phase to three phase power conversion, the input diode bridge, input capacitors, input line inductors, and any other input side switch gear and fusing employed in the inverter must be derated. The RMS current on a single phase input line would be approximately 73% higher than for the three phase input case, and the input side diodes would see approximately 80% more current stress than in the three phase input case under rated load for which the inverter was designed. This suggests that a user would need to select an inverter which was rated at least 50% higher than the name plate rating for three phase inputs for the single phase to three phase application. It is also likely that the DC bus voltage would droop to unacceptable levels in the three phase to three phase inverter, unless the DC bus capacitance is substantially increased, for single to three phase power conversion applications. Finally, with the single phase input bridge, the overall input power factor of a three phase to three phase converter is likely to be around 0.65-0.7, unless a large input DC reactor is used. All of this suggests that a standard three phase to three phase inverter may not provide a cost effective solution for operation of a three phase motor load from a single phase input supply, unless the capability of torque control or variable speed operation is required for the end use application.
To overcome some of the limitations of the methods for converting single phase to three phase power just discussed, single phase to three phase power converters employing power switching devices have been developed. To minimize costs, it is desirable that the number of power switching devices in a single phase to three phase converter be minimized. Some prior converters, which allow for variable speed drive of induction motors, utilize a full rectification of the single phase input power, with at least six switching devices in a full bridge inverter providing the three phase output power. While such systems effectively provide three phase power to drive motors at variable speeds, they are costly because of the number of switching devices required, which makes such systems economically impractical for many applications. In U.S. Pat. No. 5,272,616, to Divan, et al., a single phase to three phase converter which utilizes as few as two inverter switching devices, is disclosed. In this single phase to three phase converter design, two single phase input lines are directly connected to two inputs of a three phase motor. The input lines are also connected to the two-switching device inverter, the output of which is controlled to provide the third phase input to the three phase motor. To provide sufficient start-up torque, large capacitors are switched into the converter circuit, between the input and output lines, during motor start-up. The capacitors are switched out of the circuit after start-up. As described in U.S. Pat. No. 5,402,053, also to Divan, et al., this basic single phase to three phase converter design may be modified to eliminate the need for large starting capacitors by connecting a controllable bi-directional switch, such as a triac or back-to-back parallel thyristors, in one of the converter output lines connected to the three phase motor input. The bi-directional switch is operated as a single element variable frequency cyclo-converter and the inverter is operated as a variable frequency power supply to supply power to the motor at varying frequencies, lower than the single phase input power frequency, to start the motor and bring the motor up to speed. This single phase to three phase converter topology is also capable of being controlled to provide variable speed motor operation.