PARALLEL RESONANT SINGLE PHASE MOTOR

Abstract: An improved electric motor is operated from a single phase alternating current power source (l0) and employs a start winding (l3) and a run winding (l4) connected in series. A capacitor (26) is connected in parallel with the start winding (l3); and the inductance of the start winding and the capacitance of the capacitor are selected to form an LC resonant circuit at the operating frequency, thereby displacing the phase of the alternating current in the motor winding by 90°. Some versions of the motor also have a starting capacitor connected in series with a starting switch in parallel with the first capacitor.

Description: Background Alternating current motors are widely used for a wide variety of different purposes. Such motors range in size from very small fractional horsepower motors on up to multiple horsepower sizes. Most large horsepower electric motors are three phase motors, while the majority of smaller fractional horsepower and low horsepower motors are operated from a source of single phase alternating current electricity. Single phase motors are particularly popular since most home and business alternating current supplies are in the form of single phase supplies. Electric motors typically include a stator, which is wound with start windings and run windings connected to the source of operating power. The stator windings surround a rotor which rotates a shaft to produce the motor output. Rotors are made in a number of different configurations, such as squirrel cage rotors, high resistance rotors, low resistance rotors, wound rotors or multiple winding high and low resistance rotors. All of these configurations along with various stator windings arrangements are well known in the electric motor industry. Conventional single phase motors frequently are constructed in a "capacitor-start" or "capacitor-start/capacitor-run" configuration. In conventional capacitor-start motors, the start winding is connected in series with a starting capacitor and a centrifugal or thermal switch across the input terminals. The run winding is connected in parallel with this series-connected starting circuit. In such capacitor start motors, the starting condition is such that the instantaneous locked rotor current is high and the motor starting current demand factor also is high. Such motors undergo relatively high operating temperatures and require some type of mechanical switch (such as a centrifugal or thermal switch) for disconnecting or opening the starting winding circuit after a pre-established rotational speed of the rotor is reached. The starting winding in such capacitor start motors typically is wound with relatively fine or small diameter wire compared to the wire used in the run winding. Because small, gauge fine wire is used in the starting winding, such motors have a relatively limited life due to burn-out of the start winding. In addition, the requirement for the cut-out switch in series with the start winding and start capacitor results in additional complexity, cost, and potential for failure of such motors. For example, if a centrifugal switch is employed, the switch contact necessarily must be built into the circuit, as well as the interconnections of the mechanical centrifugal switch parts with the shaft of the rotor. Such parts are subject to failure and in fact do fail. DE-C-214062 describes a known motor substantially in accordance with the pre-characterising part of claim 1. It is desirable to provide an electric motor operated from single phase operating current power which employs start windings and run windings electrically displaced in the stator by 90° without disconnecting the starting winding when the motor reaches its running condition of operation. Accordingly, it is an object of this invention to provide an improved alternating current motor. This object is achieved with a motor according to Claim 1. The parallel resonant LC circuit, utilizing a capacitor across the start winding of a single phase alternating current motor, produces electrical phase displacement between the start winding and run winding of such motor. A single phase electric motor in accordance with the invention is capable of using the same wire size for the start winding and run winding. A feature of this invention is the provision a single phase motor comprising two series-connected windings displaced electrically out of phase in the stator core by 90° a capacitor connected in parallel with one of the windings, whereupon the one winding and the capacitor have values selected to develop inductance and capacitance near or at parallel resonance at the operating frequency of the motor. In accordance with a preferred embodiment of this invention, an alternating current motor operated from a source of single phase alternating current power includes a start winding connected in parallel with a capacitor to form a variable motor inductance-capacitance LC resonant circuit at the frequency of the power supplied by a source. This parallel connection of the start winding which controls resultant motor current across the power supply terminals. The result is that the current through the two windings is at substantially near or at 90° phase displacement during both starting and running conditions of operation of the motor. For increased starting torque a second capacitor may be connected in parallel with the first may be connected in parallel with the first capacitor during start-up of the motor only. Brief Description of the Drawing Figure l is a schematic diagram of a prior art capacitor-start motor; Figure 2 is a schematic diagram of a prior art capacitor-start/capacitor-run single phase electric motor; Figure 3 is a schematic diagram of a preferred embodiment of the invention; Figures 4A through 4C are vector diagrams of operating conditions of the motor shown in Figure 3; Figure 5 is a curve illustrating the relative starting and running torques of the motor of Figure 3; Figure 6 through ll illustrated other embodiments of the invention; Figure l2 through l4 are vector diagrams of operating conditions of the motor of Figure ll; Figure l5 is a schematic diagram of a preferred embodiment of a motor according to the invention; Figures l6A through l6C are vector diagrams of operating conditions of the motor shown in Figure l5; Figure l7 is a curve illustrating the relative starting and running torque of the motor of Figure l5; Figures l8 through 2l are schematic diagrams of other embodiments of motors according to the invention; and Figures 22A through 22C are vector diagrams of operating conditions of the motor shown in Figure 2l. Detailed Description Refernce now should be made to the drawings, where the same reference numbers are used in the different figures to designate the same or similar components. Figures l and 2 are schematic representations of conventional prior art single phase capacitor-start and capacitor-start/capacitor-run electric motors. The motor of Figure l is made to be connected with a suitable source of single phase alternating current power l0 (typically 60 Hz ll0 volt power) through a double-pole single-throw switch l2. The motor comprises a start winding l3 and a run winding l4 wound on a suitable stator frame to effect the rotation of a rotor 20 in a conventional manner. The start winding l3 is connected in series with a start capacitor l6 and a switch 2l across the terminals of the switch l2. The run winding l4 is connected across the switch l2 also, causing it to be in parallel with the series-connected circuit of the start winding l3, capacitor l6, and switch 2l. Typically, the switch 2l is a centrifugally operated switch, or the like; so that when the motor is in its stopped or non-running condition the switch 2l is closed. When the contacts l2 are closed to apply power from the source l0 to the motor, power initially flows through the capacitor l6 and the start winding l3 through the switch 2l to apply an out of phase starting current to the rotor 20 with respect to the run winding l4. The start winding l3 is made of relatively fine or small diameter wire compared to the wire used for the run winding l4. A relatively high starting voltage drop occurs across the winding l3, and relatively high capacitor current flows through the series-connected capacitor l6 and winding l3 during the start-up time. A fairly rapid temperature rise also takes place through the winding l3; so that if, for some reason, the rotor 20 does not attain operating speed quickly, damage to the starting winding l3 may occur. In extreme cases the motor may burn out. Under normal conditions of operation, however, the rotor 20 relatively quickly comes up to speed, causing the centrifugal switch 2l to open. When this occurs, all of the current for maintaining the running of the motor passes through the run winding l4, since the start winding l3 and the capacitor l6 are switched out of the circuit when the switch 2l opens. A variation of the prior art motor of Figure l is shown in Figure 2. In the motor of Figure 2, a pair of series-connected run windings l4A and l4B are connected across the terminals of the switch l2. In addition, a run capacitor 25 (having a lower capacitance than capacitor l6) is connected in shunt or parallel across the switch 2l and capacitor l6. The start-up conditions for the motor of Figure 2 are the same as for Figure l. Upon attainment of desired operating speed, however, when the switch 2l opens, current still flows through the series-connected start winding l3 and capacitor 25 during normal operating conditions of the motor. This current is out of phase with that flowing through the run windings l4A and l4B. The operation of the motor of Figure 2 is well known and need not be discussed further here. It should be noted, however, that the disadvantages of the starting conditons which exist with respect to the capacitor-start motor of Figure l still are present with the motor of Figure 2. The start winding l3 is typically wound with wire which is of smaller diameter than used in the run windings l4A and l4B. The disadvantages of the prior art motor of Figure l with respect to operating voltage, high temperature, and high current also are present with the prior art capacitor-start/capacitor-run motor shown in Figure 2. Figure 3 is a schematic diagram of a preferred embodiment two-phase capacitor motor adapted to be operated from a single phase alternating current source which has the advantages of a capacitor start motor, but without the necessity of the additional mechanically operated switch 2l and without the disadvantages of high voltage, high current, and high resistance which are present in the prior art motors of the type showing in Figure l and Figure 2. In place of connecting the start capacitor in series with the start winding, the start winding l3 is connected in parallel with a capacitor 26. This forms a parallel resonant circuit, the resonance of which is selected to be at or near the 60 Hz frequency of the power supply l0. Obviously, if power supplies of different frequencies, such as 50 Hz or l20 Hz are used, the resonance of the LC parallel resonant circuit, consisting of the winding l3 and capacitor 26, is selected to match the frequency of the source l0. The capacitor 26 is an alternating current non-polarized capacitor, and may be an electrolytic capacitor. The start winding l3 and the run winding l4 of the motor shown in Figure 3 are wound on the stator core (not shown) and are electrically displaced in phase by 90°. The run winding l4 is connected in series circuit with the parallel resonant circuit, consisting of the start winding l3 and the capacitor 26, directly across the terminals of the switch l2. The start winding l3 is dephased from the run winding l4 by 90° electrical phase displacement as a result of the capacitor 26. This causes the current through the winding l3 to be a lagging current while the current through the second phase winding l4 is controlled at or near unity power factor at the voltage across the winding l4. The parallel resonant circut, including the start winding l3, also permits the wire used in the start winding l3 to be of the same relatively large gauge or diameter as the wire used in the run winding l4. Consequently, the risk of burning out or overheating the start winding l3 which is present in prior art capacitor start motors, is not present with the motor shown in Figure 3. It also should be noted that the configuration illustrated in Figure 3 may be used with motors having rotors in various configurations. For example, the rotor 20 may be a squirrel cage rotor, a high resistance rotor, a low resistance rotor, a wound rotor, or a multiple two-winding high and low resistance rotor. The parallel resonant start winding in series with the run winding configuration for the stator which is shown in Figure 3 is capable of use with motors of a number of otherwise standard constructions. The motor in Figure 3 typically is used in applications which do not require a very high starting torque, such as pumps, blowers, machine tools and many commercial and domestic applicances. The motor of Figure 3 consumes less electrical energy, has a lower starting current, and a lower motor running current than the motors of the prior art shown in Figures l and 2. As the capacitance of the capacitor 26 is increased, the power and starting and running torque of the motor is increased, until the flux density l3 and l4 of the stator core and windings nears saturations. In addition, the motor of Figure 3 operates at very high efficiency near unity power factor (or substantially at unity power factor). The motor also may be adapted for different motor speeds such as two-pole 3450 rpm, four-pole l750 rpm, six-pole ll00 rpm, et cetera. Figures 4A through 4C illustrate respectively the vector diagrams for the motor of Figure 3 at start, full load, and no load conditions of operation. In Figure 4A, the motor starting condition, the vector voltage from 0 to V is a single phase line reference voltage. The voltage from 0 to V₁ is the first phase voltage, (the voltage across winding l3), and the second phase voltage (that across the winding l4) is from vector Vl to V. The capacitor current through the capacitor 26 leads the voltage V by approximately 90°. This is apparent from an examination of the lower portion of the vector diagram of Figure 4A. The current through the first phase (the start winding l3) is illustrated from 0 to A. The capacitor current vector is from A to B (leading the voltage vector, 0 to V, by approximately 90°). Consequently, the resultant current vector is from 0 to B, which is the current through the run winding l4 or second phase, constituting the vector sum total current drawn from the single phase power source at the motor starting conditions. As is apparent from a subsequent comparison of Figure 4A with Figures 4B and 4C, this starting current is higher than the full load and no load current, but still is a relatively low current in comparison with prior art motors of the type shown in Figures l and 2. It may be seen in Figure 4A that the current vectors of 0 to A and A to B, develop the resultant current vector B to 0. The phase displacement of this vector (Angle D) is the dephased amperes of the motor at the instant starting condition. The Angle D may be in the vicinity of 5° to 90° for dephasing the current at motor starting condition. Figure 4B illustrates the vectors (voltage and current) for the motor of Figure 3 operating at full load condition. The vector from 0′ to V′ is the voltage reference vector single phase power available from the source l0. The voltage vector from 0′ to V₁′ is the first phase voltage of the start winding l3, and the second phase voltage of the run winding l4 is from V₁′ to V′. The current vector under full load conditions of operation (Figure 4B) are shown in the resultant vector from 0′ to B′. This is the resultant of the vector sum current of the first phase vector 0′ to A′ and the capacitor current vector A′ to B′. The Angle D′ is the current phase displacement of the motor full load running condition. This angle, at full load operating conditions, is approximately 90°. Figure 4C shows the voltage and current vectors of the motor of Figure 3 operating at no load. Under this condition of operation, parallel resonance of the winding l3 and capacitor 26 takes place, where the capacitor current and the winding current through the winding l3 is relatively high, while the current through the winding l4, the run winding, is comparatively low. The voltage vectors 0″ to V₁″ through the winding l3, and the voltage vector V₁″ to V″ of the second phase through the run winding l4, create the resultant vector 0″ to V″. A considerable difference in the current vector from the full load condition shown in Figure 4B is apparent. The current vector of the winding l3, 0″ to A″ and the current vector of the capacitor 26 A″ to B″ are approximately equal; but because of the parallel resonance of the winding l3 and the capacitor 26, the current vector 0″ to B″ through the second phase run winding l4 is quite short. An actual motor, modified to have the winding configuration illustrated in Figure 3 and from which the information providing the basis for the vector diagrams of Figures 4A through 4C, was constructed. The motor was a standard General Electric motor, Frame l82, Model No. 5KSl82AC287. This was a one horsepower motor, l7l5 rpm, 208 volts, 7.2 amperes. The stator was rewound in accordance with the configuration of Figure 3 with a #l6 wire for both windings l3 and l4, and a parallel resonant circuit was produced by adding the capacitor 26; so that the electrical configuration of the stator was in accordance with the embodiment of Figure 3. The run winding had a greater turn ratio with respect to the start winding. The table reproduced below indicates measurements which were taken at start, full load, and no load conditions of operation (allowing approximately 2% to 5% ± meter accuracy readings). From the measured currents of the motors shown in the above table, the Angle D (Figure 4A) for the dephasing of the starting current equals approximately l9°. For the run condition the Angle D′ of Figure 4B is approximately 86°, while the Angle D″ (Figure 4C) at the no load running condition is approximately 38°. Because of the possibility of instrument readings which varied approximately 2% to 5%, these angles also vary somewhat. The relative angles, however are as shown in Figure 4A, B, and C, and as indicated above. At starting conditions, the above motor developed approximately 3.0 ft.lbs. of starting torque. At l740 rpm and 5 amperes power input, the motor pulling or breakdown torque reached 8.75 ft.lbs. As noted from the above table, the motor operating at full load and at l740 rpm produced a torque of 3 ft.lbs. on 5 amperes of current for the full one horsepower motor output. The motor as originally designed draws 7.2 amperes for its full one horsepower output, so that the modified winding configuration of Figure 3 results in significantly higher efficiency. Figure 5 is an output curve of the increasing rpm from zero to maximum (l740 rpm) versus torque of the motor shown in Figure 3 and having the characteristic illustrated in the above table. The "X" on Figure 5 indicates the relative torque at full load operating conditions. The maximum rpm is shown for the no load condition. Figures 6 through l0 illustrate variations in start and run winding configurations which employ the same principles of the embodiment shown in Figure 3. For example, Figure 6 illustrates the use of parallel start windings 33 and 34, having corresponding parallel capacitors 35 and 36 connected across them. The parallel winding/capacitor configuration then is connected in series with a pair of parallel-connected run windings 38 and 39. The performance and operating characteristrics of the motor shown in Figure 6 are similar to those of the motor of Figure 3, but the motor of Figure 6 is capable of operating at lower voltage than the motor of Figure 3. Figure 7 illustrates another variation of stator windings in which the stator core is wound in what is termed a "consequent pole" arrangement. In this arrangement, the first phase or start winding constitutes a pair of series-connected windings 40 and 4l wound to develop the north pole connections internally in the stator all facing the same direction. The capacitor 26 is connected across the series-connected windings 40 and 4l and produces the desired parallel resonance at motor no load running conditions. Similarly, the run winding is made up of four series-connected windings 44, 45, 46, and 47, which are wound to produce alternate magnetic polarities as indicated in Figure 7. Consequent pole motors are known; and the invention, illustrated in its basic form in Figure 3, may be applied to these motors as well as to the basic motor of Figure 3. The wire sizes or wire gauge of the motor winding 40, 4l, 44, 45, 46 and 47 are the same; and the return ratios may be equal or unequal with respect to the start windings 40, 4l, and the run windings 44, 45, 46, and 47. Figure 8 illustrates another arrangement with which the invention may be used. The first or start winding comprises of four series-connected windings 50, 5l, 52, and 53, with the connection between the windings 5l and 52 comprising a center tap to which four series-connected run windings 57, 58, 59, and 60 are connected. The capacitor 26 forms a parallel resonant circuit across all four of the start windings 50 through 53. The primary object of the circuit arrangement of Figure 8 is to increase the capacitor voltage and to use a smaller capacitance for the capacitor 26. The motor also produces a higher voltage and increased starting torque, increased pull-in torque, and increased motor full-load operating torque. The version of Figure 8 is used for applications where a higher starting torque is required and where the starting torque of the motor configuration of Figure 3 would be insufficient. Figure 9 illustrates a multiple speed motor in which the run winding comprises four series-connected windings 65 through 68, the junctures between each of which are connected to one terminal of a rotary switch 70, the other terminal of which is connected to the lower terminal of the switch l2. The moving contact of the switch 70 then may be connected to any one of the four windings to connect any number of them (from one to four) in series with one another and with the power supply supplied through the switch l2. The start winding l3 and the parallel-connected capacitor 26 are the same for the motor shown in Figure 9 as for the motor described previously in conjunction with Figure 3. By making the connection through the switch 70 at any one of the four different positions, the motor may be operated at any one of four different speeds. The windings 65, 66, 67, and 68 typically are wound with different size of gauges of wire, each with the same polarity. A motor of the type shown in Figure 9 typically is used for a fan air conditioning motor, such as a ll5 volt six-pole ll00 rpm motor which, by changing the position of the switch 70, increases impedance to cause it to operate as low as 600 rpm. Figure l0 is a variation of the motor shown in Figure 3 which illustrated the manner in which the motor can be operated to instantly reverse direction of rotation of the rotor 20. To accomplish this, a double-pole, double-throw switch 80 is used to interconnect the lower terminal of the switch l2 and the junction of the parallel-connected start winding l3 and capacitor 26 with the ends of the run winding l4. In one switch position, the connections are the same as shown in Figure 3. In the opposite switch position (to the right in Figure l0) that the currentflow through the winding l4 is reversed l80° from that of Figure 3; and the rotor 20 rotates in the opposite direction. Since there is no centrifugal switch or other mechanical apparatus used in the motor of Figure l0, instant reversal or quick reversal of the direction of the rotor 20 may be effected. The switch 80 may be thrown from one position to the other, even while the motor is energized and operating at its maximum rpm. The motor will instantly reverse direction under either no load or full load conditions. This is to be contrasted with a conventional capacitor start motor in which the centrifugal switch (such as the switch 2l of Figure l) is employed. In such conventional motors, the motor must be brought to a full stop before reversing the direction of its operation. That is not necessary with the motor shown in figure l0. Figure ll illustrates another embodiment which permits a reduction in the size of the capacitor 26 from the embodiments. In Figure ll, the start winding is split into two parts to form the primary winding 8l and the secondary winding 82, respectively, of a transformer 80. The capacitor 26 forms a parallel resonant circuit across both the primary and secondary windings 8l and 82. So far as the capacitor 26 is concerned, the windings 8l and 82 are connected in series. They are wound, however, with two wires or two separate insulated coils coinciding with one another in the stator core in the same slots. A center tap between the windings 8l and 82 is connected to one end of the run winding l4, the other end of which is connected to the opposite pole of the switch l2 in a manner similar to the connections of the windings of the embodiment of Figure 8. In the operation of the circuit shown in Figure ll, a maximum Counter-Electro Magnetic Force (CEMF) is generated in the transformer windings 8l and 82 as an additive or cumulative voltage. As a consequence the voltage across the capacitor 26 is a higher voltage than for the embodiments described above in conjunction with Figure 3, 6, and 7 through l0. The voltage across the capacitor appears to be l80° out of phase with the voltage across the windings 8l and 82 of the transformer 80. Since a higher voltage appears across the capacitor 26, lower cost, standard, high voltage capacitors may be used for the capacitor 26. It is desirable to have an equal number of stator slots for distribution of the two windings 8l and 82, so that the voltages are balanced. The motor stator slots in many motors, however, are unbalanced in a ratio which causes the distribution also to be unbalanced. As a consequence, with such motors, unbalanced voltages occur in the windings 8l and 82. Even so, the advantages of the transformer-coupled embodiment of Figure ll, permitting the use of standard high-voltage capacitors for the capacitor 26 still are realized. A motor having the configuration shown in Figure ll employs a reduced size of capacitor for the capacitor 26 and is smaller in size than a standard motor producing the same amount of power. The configuration of Figure ll produces increased efficiency, and the motor develops moderate starting torque at low starting and running current. Such a motor develops what may be termed a "soft" motor starting torque. As a consequence, a motor configured in accordance with the embodiment shown in Figure ll is ideal for applications where the source of power is a portable alternator or portable alternating current generator where a limited amount of motor starting current is desirable. Motors can be operated from a small output AC generator if the slow starting and running current is required. Conventional capacitor start motors cannot be used with such small portable generators because such conventional motors require high motor starting current. Reference now should be made to Figures l2 through l4 which illustrate, respectively, the vector diagrams for the motor of Figure ll at start, full load, and no load conditions of operation. In Figure l2, the motor starting condition, the vector voltage from 0 to V, line 85, is the line voltage. Vector 86 and 90 represent respectively the voltage vectors for the primary winding 8l and the secondary winding 82. The vector 87 is the voltage across the run winding l4. The voltage across the secondary winding 82, and therefore across the capacitor 26, is l80° out of phase with the voltage across the primary winding 8l, as indicated by the voltage vector 86 for the winding 8l. The current through the winding l is shown by the vector 93. The capacitor current vector is illustrated in Figure l2 by the vector 95; and the resultant current vector 92 is the current through the run winding l4, constituting the vector sum total current drawn from the single phase power source at the motor starting conditions. As is apparent from a comparison of the starting conditions shown in Figure l2 with the run and the no load conditions illustrated in Figures l3 and l4, the starting current shown by the vector 92 is higher than the full load and no load currents illustrated by the vectors 92′ and 92″, but still is a relatively low current in comparison with prior art motors of the type shown in Figures l and 2. The vectors shown in Figure l3 for the full load condition are provided with the same numbers primed (′) and the vectors for the motor of Figure ll operated at no load conditions are indicated by double primed (″) indications for comparison of the various vectors of Figures l2, l3, and l4. An actual motor, modified to have the winding configuration illustrated in Figure ll and from which the information providing the basis for the vector diagrams of Figure l2, l3, and l4 was obtained, has been constructed. The motor was a standard Leland Faraday motor, Frame l45T, Serial No. 38J50-7ll0. The motor specification were l.5 horsepower, l725 rpm, ll5/230 volts, 9.5 amperes as manufactured. The stator was rewound in accordance with the configuration of Figure ll to cause the primary winding 8l and the secondary winding 82 to be placed in the same stator core slots. The wire size was selected to provide a sufficient safety factor for the current drawn through each winding as indicated in TABLE II below. In addition to the modification to form the transformer 80, a capacitor 26 was added and interconnected as illustrated in Figure ll. TABLE II indicates the actual measurements which were taken at start, full load, and no load conditions of operations (Figures l2, l3, and l4 respectively), allowing approximately 2% to 5% ± meter accuracy readings: This motor, with the operating characteristics shown above in TABLE II, developed a starting torque of 3.0 ft.lbs., a full load torque of 4.5 ft.lbs., and a breakdown torque of approximately 8.0 ft.lbs. or slightly greater. As is apparent from TABLE II the motor starting and running current was low. The running current of 7.5 amperes at full load, l740 rpm, is signficantly lower than the 9.5 amperes for the same motor wound in a conventional manner. Consequently, the modified winding configuration of Figure ll results in significantly higher efficiency for this motor. Figure l5 is a schematic diagram of a two-phase capacitor motor, adapted to be operated from a single phase alternating current source and which incorporates the principles of another embodiment of the invention. The motor shown in Figure l5 employs an LC resonant circuit of the type disclosed in Figure 3. The motor of Figure l5, however, also functions to develop substantially higher starting torque than the motors of Figures 3 through l4. In order to increase the starting torque, a second capacitor ll8 (a non-polarized AC electrolytic type) is connected in parallel with the capacitor ll7 through the contacts of a starting switch l20. The results of the second capacitor ll8 in parallel circuit with the capacitor ll7 is to cause the phase displacement of the currents through the windings l3 and l4 to be in the vicinity of 90° phase displacement during motor starting conditions. In addition, the overall current demand of the motor at start-up is greater than that with the motor of my co-pending application. Once the motor attains or approaches its normal operating speed, a centrifugal switch l2l coupled to the rotor ll6 opens the contacts l20 and removes the capacitor ll8 from the circuit. An energy dissipating resistor ll9 is connected across the capacitor ll8 for the purpose of dissipating any energy stored in the capacitor ll8 at the time the switch l20 opens. Figures l6A through l6C illustrate, respectively, the vector diagrams for the motor of Figure l5 at start, full load, and no load conditions of operation. In figure l6A, the motor starting condition, the vector voltage from zero to V is a single phase line reference voltage. The voltage from zero to V′ is the first phase voltage (the voltage in winding l3), and the second phase voltage (the voltage across the winding l4) is from the vector V′ to V. The current vectors, at motor start-up are dephased or out-of-phase by approximately 90° to produce a high starting torque. This dephasing is a result of the combined capacitance of capacitors ll7 and ll8. The relative capacitance of the capacitors ll7 and ll8 is such that the capacitance of the start capacitor ll8 is considerably higher than the capacitance of the capacitor ll7. In a typical motor of the type shown in Figure l5, the capacitor ll7 may have a capacitance of l00 microfarads and the capacitor ll8 may have a capacitance of 400 microfarads. The composite effect of this capacitance upon the starting current is illustrated in Figure l6A. The starting current through the first phase winding (the starting winding l3) is illustrated in Figure l6A as from zero to A. The composite capacitor current vector at start, which is the result of the capacitances of both capacitors ll7 and ll8, is from A to B (leading the voltage vector zero to V′ by approximately 90°). Therefore, the resultant current vector through the second phase winding (the run winding l4) is from zero to B. This resultant current vector constitutes the sum total of the current drawn from the single phase power source l0 during the motor starting conditions. This phase displacement of the current vectors develops high motor starting torque. As is apparent from a subsequent comparison of Figure l6A with Figures l6B and l6C, this starting current is higher than the full load and no load currents, but it still is a relatively low current in comparison with prior art motors not employing the parallel resonant circuit formed by the winding l3 and the capacitor ll7. Figure l6B illustrates the voltage and current vectors for the motor of Figure l5 operating at full load conditions. As described previously, under full load conditions, the switch l20 is opened and the capacitor ll8 no longer is connected in the circuit. Consequently, the capacitor ll7 is the only capacitor in parallel with the first phase winding l3 to form the parallel resonant LC circuit during normal full load operating conditions. The vector from O′ to V is the voltage reference vector single phase power available from the source l0. The voltage vector from O′ to V″ is the first phase voltage of the winding l3, and the second phase voltage of the winding l4 is from V″ to V. The current vectors under full load conditions of operation, as illustrated in Figure l6B, are shown in the resultant vector from O′ to B″. This is a resultant of the vector sum current of the first phase current vector O′ to A′ and the capacitor current vector A′ to B′. It may be seen that the vector 0′ to B′ is relatively short (low current) and is at or near unity power factor, resulting in high efficiency. The angle D (90°) is the phase displacement of the current vectors during motor full load running condition. As is apparent from an examination of Figures l6A and l6B, the phase displacement of the current vector is substantially 90° both during starting and during full load operating conditions. Under full load operating conditions, with the capacitor ll8 disconnected, the operation of the motor of Figure l5 essentially is the same as the motor of Figure 3. Figure l6C shows the voltage and current vectors of the motor of Figure l5 operating at no load. Under this condition of operation, the switch l20 is open and the capacitor ll8 is not in the circuit. Parallel resonance of the winding l3 and the capacitor ll7 takes place; so that the capacitor current and the winding current through the winding l3 is relatively high, while the current through the winding l4, which may be considered the run winding, is relatively low. The voltage vectors O‴ to V‴ through the winding l3, and the voltage vector V‴ to V through the second phase winding l4 create the resultant vector O″ to V. A considerable difference in the current vectors from the full load condition shown in Figure l6B is apparent. The current vector of the winding l3, O″ to A″ and the current vector of the capacitor ll7 A″ to B″ are fairly large; but because of the parallel resonance of the winding l3 and the capacitor ll7, the current vector 0″ to B″, through the second phase or run winding l4, is quite short. An actual motor modified to have the winding configuration illustrated in Figure l5 and from which the information providing the basis for the vector diagrams of Figures l6A through l6C was constructed. The motor was a standard General Electric motor, Frame l82, Model No. 5KSl82AC287. This was a one horsepower motor, l7l5 rpm, 208 volts, 7.2 amps. The stator was rewound in accordance with the configuration of Figure l5, with No. l6 wire for both windings l3 and l4. In addition, the winding which was used as the run winding in the original motor was employed as the start winding l3 and the rewound start winding was used as the run winding l4. Since the windings are of the same gauge wire, this can be accomplished. The run winding l4 had a greater turns ratio with respect to the start winding l3. The table reproduced below indicates measurements which were taken at start, full load, and no load conditions of operations (allowing approximately 2% to 5% +/- accuracy readings): From the measured currents of the motor shown in the above table, the Angle D (Figure l6A) for the dephasing of the starting currents equals approximately 90°. This phase displacement develops a very high motor starting torque for the motor. In addition, for the run condition (full load) it may be seen that the Angle D also is approximately 90°. The result is that the resultant current vector O′ to B′ is relatively low and that the current is at or near unity power factor as drawn from the single phase source of power. This results in high efficiency of operation. At starting conditions, the motor of the above table developed ll ft.lbs. of starting torque. At full load or running condition, the motor developed one horsepower at 3 ft.lbs. of torque at 5.5 amperes of current at l740 rpm. The breakdown torque from the motor full load to stalling is in the vicinity of 8.5 ft.lbs. The motor, as originally designed, draws 7.2 amperes for its l horsepower output; so that the modified winding configuration employing the parallel resonance combination of the capacitor ll7 and the winding l3 results in significantly higher efficiency during the full load or running condition of operation of the motor. Figure l7 is an output curve of the increasing rpm from 0 to maximum (l740 rpm) versus torque of the motor shown in Figure l5 and having the characteristics illustrated in the above table. The "X" on figure l7 indicates the relative torque at full load operating conditions. The maximum rpm is shown for the no load condition. The lower curve in Figure l7 illustrates the effect on the rpm versus torque which is contributed by the parallel resonant circuit of the capacitor ll7 and the winding l3. The upper curve is the composite effect which is provided by the addition of the starting capacitor ll8. As is readily apparent, the upper or composite curve provides a very high starting torque at low rpms. To achieve the efficiency of continuous operation from the LC resonant circuit, once motor start-up has been attained, the capacitor ll8 is disconnected or removed from the circuit; and the remainder of the curve of Figure l7 then is resonant circuit comprising the winding l3 and the capacitor ll7. Figure l8 through 2l illustrate other circuit configurations which employ the same principles of the embodiment of the invention shown in Figure l5, but which employ different circuitry or which add other characteristics to the embodiment which has been described. For example, Figure l8 adds an additional feature of power relay l28 to the circuit of Figure l5 for the purpose of eliminating the effect of the capacitor voltage discharge CEMF (Counter Electromotive Force) induction from the motor upon turn-off of the motor when the switch l2 is opened. The relay l28 is connected across the terminals of the switch l2 and has a pair of contacts l30, a different one of which is connected in series with the circuit of the capacitors ll7 and ll8. When the switch l2 is initially closed, current flow through the operating coil l29 of the relay l28 to close the contacts l30. This provides operating current to both of the capacitors ll7 and ll8 (for the capacitor ll8 through the closed contacts of the switch l20). The contacts l30 remain closed during the entire motor operation. As the motor of Figure l8 is deenergized by opening the switch l2, operating current through the relay coil l29 ceases; and the contacts l30 open. This causes the circuits for both of the capacitors ll7 and ll8 to be opened. This prevents the surge voltage resulting from the discharge of the capacitor ll7 from being applied to the motor windings l3 and l4. As a result, unnecessary vibration of the motor at disconect, as well as the noise created by such vibration, is eliminated. In addition, the relay l28 causes elimination of the dynamic braking of the motor when the capacitors ll7 and ll8 are discharged at the instant of motor disconnection. As a consequence, the motor decelerates to a stop silently and without vibration. This increases the life of the motor, especially for large horsepower integral motors. Figure l9 is another variation of the embodiment of Figure l5. In this arrangement, a voltage sensitive relay coil l35 operates to control contacts l36 for the purpose of connecting and disconnecting the starting capacitor ll8 from the circuit. This operation is controlled by the voltage of the motor at its starting and running conditions. The voltage sensitive relay, comprising the coil l35 and contacts l36, is a small size relay; and the contacts l36 normally are closed when the motor is in its off condition of operation and during start-up. The contacts l36 are connected in series with the operating coil l37 of the power relay across the input terminals controlled by the switch l2. Consequently, when the contacts l36 are closed, operating current for the coil l37 flows through those contacts from the source l0. This closes contacts l38 to connect the start capacitor ll8 into the circuit. The arrangement of Figure l9 is very practical for large integral horsepower motors. As the motor attains its run voltage, the voltage across the relay coil l35 increases; and it operates to open the contact l36. This in turn removes operating current from the coil l37; so that the contacts l38 are opened. The capacitor ll8 then is disconnected from the circuit, and the motor continues operation in its run mode of operation in the same manner as described above in conjunction with Figure l5. The advantage of the circuit of Figure l9 is that the circuit suppresses the capacitor voltage discharge upon disconnection of the operating current to the motor by opening up the switch l2. No mechanical contacts of the type required for the switch l20/l2l are necessary, so that increase life of the motor and the starting circuit results. The power relay l37/l38 has contacts l38 capable of handling heavy high current from the capacitor ll8 as it is switched into and out of the circuit. Another embodiment which does not require a mechanical switch is illustrated in Figure 20. Only a single capacitor ll8 is used in this embodiment, and the capacitor ll7 has been eliminated. In Figure 20, the centrifugal switch l20/l2l has been replaced with a solid state motor starting control circuit of the type disclosed in United States Patent No. 3,9l6,274. This circuit comprises a triac l40 connected in series with the starting capacitor ll8 to function as the on/off switch in that circuit. The gate of the triac l40 is connected to a pair of parallel back-to-back diodes in a circuit l4l. These diodes control the gate voltage for the triac l40 in accordance with the voltage drop across a variable inductor l43. The circuit of Figure 20 operates to connect the capacitor ll8 in the circuit during the relatively high current starting conditions and operates to disconnect the capacitor ll8 from the circuit during normal full load or no load operating conditions. The capacitor ll8 in the embodiment of Figure 20 is selected to form an LC parallel resonant circuit with the winding l3 during the starting phase of operation of the motor shown in Figure 20. Consequently, during motor start-up, parallel resonance continues to be employed. At the time the capacitor ll8 is switched out of the circuit, operating current for the motor continues to flow through both windings l3 and l4. The windings are connected in series with the source of alternating current power l0 through the switch l2 at all times during the operation of the circuit. There is no "start-winding" which is switched out of the circuit as is common with prior art capacitor start motors. The first winding l3 of the motor of Figure 20 has a greater number of turns than the second winding l4, with both windings using the same large wire size of low impedance. The winding l4 has about one-third or one-fourth the turn ratio of the winding l3. The motor of Figure 20 develops the same horsepower output during its running condition, with a much lower temperature rise, than comparable prior art capacitor start motors. There is no starting winding to burn out, and the motor draws a relatively low starting current. Figure 2l illustrates another embodiment which is similar to the one of Figure l5, but in which the first phase winding l3 comprises a center tapped winding including two sections l3A and l3B. The connection between the sections l3A and l3B comprises the upper connection to the winding l4 of the motor. The capacitors ll7 and ll8 are connected in parallel with both sections l3A and l3B of the start winding to form a parallel resonant circuit with both winding sections. The nature of the current flow through the windings l3A and l3B and the capacitors ll7 and ll8 is such that the capacitors ll7 and ll8 also are connected in electrical series with the winding section l3B and the winding l4. This assists in utilizing the capacitor leading current for increased starting torque in conjunction with the winding section l3B. The operation of the circuit of Figure 2l, is otherwise the same as in Figure l5. The circuit of Figure 2l, however, produces an even higher starting torque and motor pull-in torque than the motor shown in Figure l5. The same motor and the same frame used in the example of Figure l were wound with the configuration of Figure 2l. The capacitor ll7 and a 70 microfarad capacitor and the capacitor ll8 was a 500 microfarad capacitor. The motor developed a starting torque of l4.5 ft.lbs. at 38 amperes, and the motor pull-in torque was 8.5 at l3 amperes. At full load, the motor produced 3 ft.lbs. of torque at l750 rpm, and the breakdown torque was 9 ft.lbs. up to l600 rpm. Figure 22A through 22C illustrate the vector diagrams for the motor of Figure 2l, at start, full load, and no load conditions of operation respectively. The individual voltage vectors have not been illustrated, but the resultant reference line voltage vector V is shown in each of these three figures. For the most efficient operating conditions of the motor of Figure 2l, it is desirable to have all of the current vectors equal in length to produce an equiangular or equilateral triangle. For the motor of Figure 2l, this is readily apparent from a reference to the vectors of Figures 22A and 22B. Figure 22A shows the current conditions at motor start and Figure 22B shows the conditions at normal full load operation at l750 rpm. At no load conditions of operation, the equilateral triangle effect is not present, as is apparent from an examination of Figure 22C. Only a limited number of winding configurations and starting switch circuits have been illustrated. Others may be employed. It also is possible to provide a reversing switch for the motor to permit it to instantly reverse direction under either no load or full load conditions.

Claims: An alternating current motor with a stator core for operation from a source of single phase alternating current power, and a rotor (20), with a start winding (13) and a run winding (14) connected in series with a source of single phase alternating current power (10, via 12) on the stator core for inducing rotation of the rotor (20), and a capacitor (26) connected in parallel with the start winding (13), characterised in that the start winding (13) and capacitor (26) form an LC resonant circuit at the frequency of alternating current power from the source of power (10) throughout the starting and running operation of the motor. A motor according to Claim 1 further characterized in that the winding is a plurality of start windings (33, 34) with the capacitor (35,36) connected in parallel with all of the start windings. A motor according to Claims l or Claim 2 further characterized in that the run winding comprises a plurality of run windings (33,39 or 44 to 47). A motor according to Claim 3 further characterized by a switch (70) to selectively connect different numbers of run windings (65,66,67 or 68) in series circuit with the start winding (l3). A motor according to any preceding claim further characterized in that the winding (l3) and the run winding (l4) are formed of wire of substantially the same guage. A motor according to any preceding claim further characterized in that the start winding (50 to 53 or 8l, 82) has a center tap and , said run winding (57 to 60 or l4) is connected to said center tap on said start winding. A motor according to any preceding claim further characterized in that the first and second phase windings (l3 and l4) are wound on the stator core in an electrical phase displacement of 90° from each other. A motor according to any preceding claim further characterized in that the capacitor (26) is an alternating current non-polarized capacitor. A motor according to Claim l further characterized by a second capacitor (ll8) and switch (l2) connected in series with one another and in parallel across the capacitor (ll7) with a device (l2l) for closing the switch (l20) during starting of the motor for opening the switch (l20) during normal load conditions of operation of the motor. A motor according to Claim 9 further characterized in that the switch is an AC bi-directional semiconductor switch (l40) which is conductive during motor starting and is non-conductive during normal load conditions of operation of the motor. A motor according to Claim 9 or Claim l0 further characterized in that the capacitance of the second capacitor (ll8) is substantially greater than the capacitance of the first capacitor (ll7). A motor according to Claim 9 further characterized in that the switch (l20) is a normally closed centrifugal switch coupled (via l2l) with the rotor (ll6) of the motor; so that upon attainment of a predetermined rotational speed of the rotor (ll6) the switch (l20) is opened to disconnect the second capacitor (ll8) from the circuit. A motor according to any one of claims 9 to l2 further characterized in that the device is power relay (l28) with contacts (l30) connected in series circuit with each of the first and second capacitors (ll7, ll8), and the power relay (l28) closes the contacts (l30) whenever alternating current power is applied to the first and second windings (l3, l4). A motor according to any one of Claims 9 to l2 further characterized in that the device is a voltage sensitive relay (l35, l36) connected in parallel with the second winding (l4) to operating the switch (l38), such that the sensitive relay (l35, l36) closes the switch (l38) during starting of the motor and opens the switch (l38) during normal load conditions of operation of the motor.

Inventors: LEWUS, ALEXANDER J
Applicants: LEWUS, ALEXANDER J
Application year: 1987
Main classification: H02P1
IPC codes: H02K17, H02P1, H02K57