1. Field of the Invention
This invention pertains to the field of electric power delivery systems. More particularly, this invention concerns polyfield driven devices and more specifically to devices to convert single-phase alternating current to three-phase alternating current.
2. Description of the Prior Art
Three-phase electric motors are generally considered creatures of heavy industry necessitating three separate electrical input phases in the form of heavy power lines. Single-phase electric motors are generally found in light industry and most commercial activity, for example, dry cleaning shops and auto repair stations and the like, where single-phase electric power may be brought in by lighter gauge power lines. While historically three-phase electric motors are more powerful, on a pound-for-pound basis, than their single-phase counterparts, modern technology has made them even more desirable with the advent of lighter gauge construction and higher torque output. A problem remains, however, in that the cost of installing three-phase electrical power lines into a business served by existing single-phase is so costly that it far outweighs the benefits of three-phase motors.
In the three-phase electric motor, there are three separate field or stator windings equally spaced about the centerline of the rotor and connected at the center to form the Y-wind. In three-phase power, the phase angles between the alternating current in each winding is equally spaced apart from that of the neighbor winding by 120.degree.; this provides maximum torque to the motor at start and provides balanced power during running. Should single-phase power be connected to two of the windings of a three-phase motor, there would be no power developed in the third winding and the motor would either not run at all or run so slowly that heat build-up would eventually burn the windings. However, three-phase motors are very efficient and deliver more power per watt than single-phase motors so that, whenever it is economically feasible to create a third phase, the use of three-phase motors becomes exceedingly desirable.
The prior art has recognized that single-phase power can be split into three, phase-differentiated phase angled currents to effectively power three-phase motors. This operation makes three-phase motors useful in light industrial applications. Devices that develop three-phase alternating current from a single-phase source are of two general types: The static phase converter, a device comprising transformers and other related electronic components, see U.S. Pat. No. 2,832,925, and the rotary phase converter, a device comprising an induction motor powered by single-phase and producing three-phase current, see U.S. Pat. Nos. 2,593,987; 2,922,942; 3,122,693; 3,271,646; 3,387,202; 3,566,226; 3,670,238; 3,673,480; 3,809,980; 4,079,446; 4,249,237; and, 4,484,125. This invention concerns the latter type.
The prior art rotary phase converter generally comprises a three-phase induction motor where a single-phase of 240 volts is connected across the ends of two of the Y-windings leaving the third winding unpowered. The third winding is usually connected to one of the powered windings through a plurality of capacitors that shift the phase angle of the third winding into a three-phase balance. The three-phase motors or loads used in the industrial application are then connected in parallel with this "converter" motor. They draw most of their power from the single phase current fed to them and the converter motor from commercial power lines. The third phase is properly phase-shifted by the converter motor to balance out their operation. The converter motor runs under no external load, its sole function is to generate a third phase from a single phase.
In this type of converter, the motor must be started and brought up to speed from the single-phase input current. Since one phase does not provide a proper phase angle in the third winding, the motor starts slowly, if at all, gathers speed very slowly and generally runs sluggishly and overheats. The prior art has discovered that by connecting a large bank of capacitors from one powered winding to the third unpowered winding, sufficient phase shifting could be generated in the third winding, along with the increased voltage provided by capacitors in an alternating current system, to "kick" over the third winding and cause faster starting.
The problem has not been with the use of capacitors but with the amount of capacitance in the capacitors. Large capacitors such as electrolytic capacitors provide good starting power, but, due to their construction, cannot remain in the circuit for more than a few seconds less they overheat and disintegrate. Oil-filled metal film capacitors, on the other hand, provide good long term power for running but usually are insufficient in Farad capacity to provide good starting power. The prior art has therefore just used large quantities of oil-filled metal film capacitors to start and run the three-phase converter motor. While a large enough number of these capacitors will start the motor, they contain too much capacitance to remain in the circuit during motor running. The phase angle is over-shifted in the third winding resulting in poor running characteristics, undesirable heat build-up and the voltage build-up in the third winding is too great for economical running. The prior art has sought to solve this problem using a plurality of electronic devices involving complicated circuitry, see the United States Patents cited, supra.
This invention solves the aforesaid problems and provides a quick-starting converter motor that runs economically and without heat build-up; it uses much less power than the prior art devices and far less complicated circuitry. The invention comprises the utilization of a bank of electrolytic capacitors to provide starting power and a switching circuit to switch them out of use during converter motor running. The oil-filled metal film capacitors are only used in running the motor and thus can be sized to provide proper phase angle at running speed without producing problems during starting. The two sets of capacitors are separated to different powered windings so that they do not over shift any phase angle in any of the windings. The overall unit of this invention is small, compact and provides three-phase alternating current to start three-phase phase units connected in parallel thereto of the same gross horsepower if started simultaneously and will support motors having twice the amount of converter motor rated horsepower if started consecutively. The converter of this invention uses only two relays in addition to the starting capacitors and the runing capacitors thus eliminating the need for expensive transformers, resistors, diodes and transistors.