HK1131843B - Cycloconverter power control system - Google Patents

Description

Cycle converter power control system

Technical Field

The present invention relates generally to ac power distribution systems and, more particularly, to an ac power controller system that controls the application of ac operating power to an ac induction motor.

Background

Spurious noise signals are generated in the ac distribution line, including harmonic currents, background noise, and spike noise. Such noise signals may originate from power sources, distribution networks, local and remote loads connected to the network, lightning strikes, and distribution equipment failures. The ac supply current delivered by a utility is not a pure sine wave and contains harmonics that interfere with the proper operation of the connected equipment. In addition, noise and transient over-voltages may be introduced from the payload. For example, if an electronic dimmer and lamp were loaded on one leg, the dimmer would "chop" the 60 hz ac power waveform at a high frequency, thereby reducing the light intensity. This introduces harmonics and high frequency noise on the power distribution line.

This noise is not constant with respect to time and it varies from place to place in the distribution network. In addition, a typical ac power line network distributes power to various electrical load devices. Each load may reverse significant noise levels and harmonic currents back to the power line, causing distortion in the power waveform. Different loads and control equipment produce different types and degrees of distortion that may interfere with the operation of the equipment and machinery being supplied by the power distribution grid.

The amount of power used by the machine, as well as the machine itself, may be affected by waveform distortions present in the power distribution system. Eliminating or controlling distortion can provide significant savings in power consumption costs as well as repair or replacement costs due to machine failure. Thus, reducing and mitigating harmonic distortion in an ac power distribution system can provide significant energy cost savings to an industry user.

In an ac power distribution system environment, a linear electrical load is a load device that operates in a steady state, presenting a substantially constant impedance to the power source during the period of applied voltage. An example of a linear load is an ac induction motor, which applies torque to a constant (non-time varying) mechanical load. A non-linear load is a load with discontinuous current or whose impedance varies over the period of the input ac sine wave. Examples of non-linear loads in industrial power distribution systems include arc lighting, welding machines, variable frequency drive inverter power supplies, switch mode power supplies and induction motors, all of which apply torque to mechanical loads that vary with time.

Harmonic current produced by nonlinear loads in the power distribution system flows from the nonlinear power source and to the power distribution system power source. Injection of harmonic currents into the power distribution system can cause overheating of the transformer and high neutral currents in a three-phase grounded four-wire system. When harmonic currents flow through the distribution system, voltage drops are generated for each individual harmonic, causing the applied voltage waveform to be distorted and applied to all loads connected to the distribution bus.

Harmonic distortion of the voltage waveform causes harmonic flux in the motor magnetic circuit, affecting the performance of the ac induction motor. These harmonic fluxes cause heat accumulation and additional losses in the motor core, reducing power conversion efficiency. The induction heating effect generally increases in proportion to the square of the harmonic current. If the supply voltage is distorted, the induction motor may be destroyed by harmonic current heating or performance may be affected. The reverse order harmonic currents cause a reduction in motor torque output. These effects combine to reduce power conversion efficiency and may lead to motor overheating and burnout.

The harmonic flux in the motor windings is either positive, negative or zero sequence, depending on the amount or sequence of harmonic distortion that produces the harmonic flux. The positive sequence harmonic magnetic field (flux) will rotate in the synchronous field direction. The negative sequence harmonic flux will rotate in opposition to the synchronous field, thus reducing torque and increasing overall current demand. The zero sequence harmonic flux does not produce a rotating field, but will still cause additional heat in the stator windings as it flows through the motor magnetic circuit.

Industrial power distribution systems provide ac operating power to connected machines and devices that generate some harmonic distortion of the ac voltage waveform. Each fundamental harmonic, depending on whether it is positive, negative or zero sequence, and its percentage can have adverse effects on motor performance and temperature rise, as well as increasing the energy costs of the electrical service borne by the utility provider. Whether or not the customer is using the current efficiently, the utility must produce sufficient service capacity to meet the expected peak demand, kVA (kilovolt amperes apparent power). The ratio of kilowatts (real active power) to kVA (apparent power) is referred to as the load power factor. When the customer total load power factor is low, most utility providers are penalized.

When a non-linear load is present, the apparent power may be greater than the actual power. The nonlinear load produces harmonic currents that circulate back to the branch distribution transformer and into the distribution network. The harmonic current adds to the Root Mean Square (RMS) value of the fundamental current supplied to the load, but does not provide any real power. Using the definition of the total power factor, the real active power kW is essentially only the fundamental (60 hz) ac waveform, while the root mean square value of the apparent active power kVA is larger due to the presence of harmonic current components.

The low kW/kVA power factor rate may be a large phase difference between the motor load terminal voltage and current, and may also be caused by higher harmonic content or distorted/discontinuous current waveforms. Unacceptable load current phase angle differences may occur due to the high inductive reactance of the induction motor stator windings. The distorted current waveform will also be caused by the induction motor applying a torque to the non-linear load. When the induction motor is operated under discontinuous load conditions, or when the load is non-linear, higher harmonic currents will cause motor performance degradation and power factor reduction.

Conventional controllers for ac induction motors use power factor measurements to generate feedback signals for controlling the amount of power delivered to the motor. In order to maintain a sufficient rotor slip to operate with a relatively high power factor and good power conversion efficiency, the control signals are adjusted over time to reduce the average power applied to the motor during light loads.

Various problems arise when operating conventional controllers, particularly when controlling power to a non-linear load. For example, there is a complex power control factor for the operation of an ac induction motor that drives a pumping unit (a linkage pumping unit) used to draw fluid from an underground structure. Such pumping units are alternately loaded by the weight of the sucker rod, the structural fluid column, and provide opposing counterweights twice during each pumping cycle. In addition, the reverse load is balanced twice per pumping cycle, so the motor is unloaded twice per cycle. The constantly changing mechanical load between the minimum peak and the maximum peak creates serious control difficulties for power factor control systems that must continuously regulate the output power in order to maintain optimal motor efficiency and economy.

Currently, thyristor switches are used in conventional controllers for controlling the ac power supplied to an ac induction motor. Due to the fast switching on and off action (fast dv/dt) of the thyristor on high peak voltages and high switching frequencies, high frequency switching transient distortion occurs in the input current at the feed side of the power controller, which results in an increase in the harmonic components in the ac power supply output to the induction motor. In addition, parasitic noise and harmonic currents from remote sources conducted along the branch distribution circuit can interfere with proper switching operation of the controller itself, resulting in loss of power control.

These factors not only reduce the power factor of the branch load, but also interfere with motor operation and inject harmonic currents back into the distribution branch and distribution grid. In addition, harmonic distortion generated by the controller increases the rms value of the load current in the distribution branch, which is the basis for determining utility service charges, thus increasing energy costs to the customer.

Disclosure of Invention

An improved power controller system is provided for increasing the operating efficiency and performance of a conventional ac induction motor by receiving operating power from an electronic controller using a fast switching circuit to control the application of ac power to the motor stator windings. The improved controller system operates efficiently to drive nonlinear mechanical loads under light torque load and full rated torque load conditions, reduce harmonic currents from remote sources, reduce controller induced harmonic currents and load induced harmonic currents.

The primary low-pass filter is connected in series between the branch phase line and the power controller. KVAR (kilovolt ampere reactive) capacitors are connected across the output terminals of the power controller in the bypass (shunt) to a neutral relationship. The KVAR capacitor value is coordinated with the stator winding inductance value to form a secondary low pass filter through the controller output terminals. The primary and secondary low pass filters isolate the power controller from the induction motor for spurious noise and harmonics generated by local and remote sources and also improve the actual power conversion efficiency from the power generating source to the induction motor.

According to an aspect of the invention, there is provided a power controller system comprising one or more supply input terminals for receiving an alternating voltage at a basic distribution frequency from one or more selected supply phases of an alternating current power source and one or more supply output terminals for conducting an alternating current to one or more stator phase windings of an alternating current induction motor, the power controller system comprising:

an electronic power controller including one or more power input terminals for receiving an alternating voltage at the base distribution frequency from one or more of the controller system power input terminals, one or more power output terminals electrically connected to conduct alternating current to one or more of the controller system power output terminals, and a switching device coupled between the power input terminals and power output terminals for controlling the conduction of current to one or more of the system power output terminals;

a primary low pass filter circuit connected in series between the ac power source and the electronic power controller, the primary low pass filter circuit including one or more input terminals coupled to one or more of the power input terminals, and one or more output terminals coupled to one or more power input terminals of the electronic power controller; and

one or more bypass capacitors in bypass connection with the one or more selected supply phases, the one or more bypass capacitors connected to one or more of the controller power output terminals, wherein a capacitance value of each of the one or more bypass capacitors is selected such that the capacitance value of each of the one or more bypass capacitors is coordinated with an inductance value of the stator phase winding, thereby providing a secondary low pass filter between the power output terminal of the electronic power controller and the stator phase winding.

According to another aspect of the present invention there is provided a method for controlling the application of ac voltage at one or more phases of the ac voltage from a power supply to one or more stator phase windings of an ac induction motor to match the power requirements of a mechanical load driven by the motor, the method comprising for each phase the steps of:

coupling a gate controllable switch in series between a selected phase of the alternating voltage and a selected motor stator phase winding, wherein the gate controllable switch comprises first and second control gates, each for applying each polarity of the alternating voltage to the switch and the motor;

alternately triggering the gate controllable switch to a conducting state during each alternation of the alternating voltage;

-inhibiting the gate controllable switch from turning on during each alternation of the alternating voltage for a time interval proportional to: the start of the interval is when the alternating current voltage in the stator phase winding alternately passes a first zero crossing, and the end of the interval is when the corresponding alternating current in the stator phase winding alternately passes a second zero crossing;

filtering the alternating voltage conducted by the gate controllable switch;

connecting a bypass capacitor to one of the power output terminals, the bypass capacitor being in bypass connection with the one or more selected supply phases, wherein a capacitance value of the bypass capacitor is selected such that the capacitance value is coordinated with an inductance value of one of the stator phase windings, thereby forming in combination a secondary low pass filter circuit when such a connection is established.

Drawings

FIG. 1 is a simplified electrical schematic diagram showing the connection of an AC power controller for dynamically adjusting the operating power applied to an induction motor to match the nonlinear load demand;

FIG. 2 is a simplified schematic diagram of a nonlinear load application in the form of a ganged sucker rod pumping system with a sucker rod jack rack, which is powered by the power controller system of FIG. 1;

FIG. 3 illustrates a typical induction motor torque load and sucker rod travel displacement produced by the linkage pumping rack and sucker rod pump system of FIG. 2;

FIG. 4 illustrates exemplary voltage and current waveforms generated in a representative stator phase winding during controlled operation of the induction motor of FIG. 2;

FIG. 5 is a front perspective view showing the physical arrangement of the controller system components inside the enclosure.

Detailed Description

Referring to fig. 1, a conventional ac distribution network 10 provides power from a high voltage ac power source 12 to a step-down distribution transformer 14. The distribution transformer feeds the stepped down power to a distribution panel 16, the distribution panel 16 including conventional three-phase distribution breakers 18, 20 and 22. Alternating current at fundamental frequency 60 hertz, 480VAC phase to phase (277VAC phase to neutral) is conducted via 4 wires, the 4 wires shared by neutral branch circuit 26, neutral branch circuit 26 including alternating phase lines 28, 30, 32 and shared neutral line 34.

The three-phase alternating current is applied to the input terminals N1, N2, and N3 of the electronic power controller 36 through branch circuit conductors. The power controller 36 applies a controlled amount of ac power to the input terminals S1, S2 and S3 of the three-phase induction motor 38 through its output terminals M1, M2 and M3. The electric motor 38 is mechanically coupled in torque power transmission to a mechanical load 40. The power controller 36 detects the instantaneous power demand of the mechanical load and adjusts its power output to dynamically match the load demand during each half-cycle of the applied power waveform. The ac power applied to the ac induction motor 38 is automatically increased or decreased as needed to match the non-linear load demand.

The power Controller 36 is preferably constructed as described in U.S. patent 6,400,119 entitled "Energy switching Motor Controller," which is hereby incorporated by reference in its entirety. As described in that patent, first and second gate controlled switches (silicon controlled rectifiers) 42, 44; 46. 48 and 50, 52 are interconnected in parallel, with opposite polarity in each phase of the applied ac voltage. In response to detecting the timing of zero crossing events of the ac voltage and ac current waveforms in the respective stator winding phases of the induction motor, a trigger pulse generator couples a trigger control signal to the respective gates of the silicon controlled rectifier switches.

Each phase of the first and second silicon controlled rectifier switches is alternately triggered into a conducting state during each alternation of the applied ac voltage and alternately disabled for a time interval proportional to a measured time difference between an ac voltage zero crossing and a corresponding ac current zero crossing, obtained by comparing the sequential first and second interrupt time differences corresponding to the zero crossing events to a continuous run time basis.

Referring to fig. 4, the measured time difference between an ac voltage zero crossing and a corresponding ac current zero crossing in each half cycle of the Φ a waveform is indicative of the instantaneous load demand. The power controller 36 detects this difference and outputs it to dynamically match the load demand power level during the next half cycle of the applied ac waveform. Fast switching circuits 42, 44 in each power phase of controller 36; 46. 48 and 50, 52 alternately conduct and interrupt ac power applied to the ac induction motor 38 in proportion to the measured difference.

By employing this configuration, the power applied to the motor is automatically increased or decreased as needed from one half cycle to the next of each phase, thereby matching the instantaneous power demand of the load 40. The current in each phase is interrupted during an interval proportional to the measured phase difference, which is the phase difference at which the voltage and current waveforms zero-cross in the previous half-cycle. Thus, the current is interrupted in only one phase at a time, while the power regulation is continuously performed in the three phases Φ a, Φ B, and Φ C.

Harmonic currents from the remote source are reduced by the primary low pass filter 54, which primary low pass filter 54 includes three identical LC filter sections in series with the branch distribution conductors 28, 30 and 32 at input terminals N1, N2 and N3, respectively, of the controller 36. The controller 36 and the induction motor 38, as well as all other components that may be connected to the filtered side of the branch power distribution circuit 26, are isolated from external noise and spurious signals generated by remote devices in other phases or other branches of the power distribution network 10.

Each low pass filter section comprises an inductor (L1, L2, L3) in series with the phase conductor (28, 30, 32) and a capacitor (C1, C2, C3) connected to the bypass of the neutral phase. Each LC section of the primary low pass filter 54 has very low attenuation from direct current up through the fundamental distribution frequency (60 hz) to the cutoff frequency (e.g., 300 hz), and attenuates substantially all other signals above the cutoff frequency (including harmonic components up through and beyond 11 th order).

Each section of the primary low pass filter circuit 54 preferably includes inductors (L1, L2, L3) and capacitors (C1, C2, C3) that are tuned to the current high impedance and attenuate signals at 300 hertz and higher, and that result in a low impedance with little attenuation or loss when passing from dc through the ac power distribution frequency in the 50 hertz-60 hertz range. Each section of low pass filter 54 provides a 40: 1 attenuation ratio or cutoff frequency for high frequencies, thus isolating controller 36 and its connected components from external noise and spurious high frequency signals.

For an AC power distribution of 60 hz and a cutoff frequency of 300 hz, the preferred value for each capacitor C1, C2, and C3 is 3uF, each capacitor is rated to operate at 600V AC, and the preferred value for each inductor L1, L2, and L3 is 0.86 mH. Preferably, each inductor L1, L2, and L3 is a core inductor rated for 56 amps and 40hp, 480VAC, 60 hz service. This allows the 60 hertz ac power to pass through virtually without attenuation, thereby delivering clean, filtered three phase ac current and voltage at 60 hertz to the power controller 36.

In accordance with an important feature of the present invention, clean, filtered ac current is provided as operating power from the primary low pass filter 54 to the internal power supply of the power controller 36. This prevents interference from remote noise sources and ensures stable operation of its microprocessor, comparators, trigger circuits and other components that require a stable voltage level. In addition, because of the bi-directional operation of primary low pass filter 54, harmonics and other noise signals generated by the operation of the switching elements of power controller 36 or induction motor 38 are attenuated and suppressed, thereby preventing injection back into power distribution network 10.

The power factor of the induction motor 38 is improved and the connection of KVAR (kilovolt ampere reactive) capacitors C4, C5 and C6 to the neutral relationship across the bypass controller output terminals M1, M2 and M3 reduces the effect of harmonic currents generated by the induction motor under nonlinear mechanical load conditions. The KVAR capacitor values are selected and coordinated with the stator phase winding W1, W2, and W3 inductance values to provide a secondary low pass LC filter section in series between the power controller output terminals M1, M2, and M3 and the induction motor input terminals S1, S2, and S3.

Each section of the secondary low pass filter 59 has very low attenuation from direct current up through the fundamental distribution frequency (60 hz) to the cutoff frequency (e.g., 300 hz or the 5 th harmonic) and attenuates substantially all other signals above the cutoff frequency (including up through and above the 11 th harmonic component).

KVAR capacitors C1, C2, and C3 serve two purposes: the power factor of the induction motor 38 is improved and the current flowing into the induction motor 38 is filtered while suppressing the backflow of harmonic currents generated by the motor. The secondary low pass filter 59 prevents controller generated harmonics from being injected into the induction motor 38 and prevents induction motor generated harmonic currents from being injected into the controller 36 and the power distribution network 10.

The actual power conversion efficiency is improved by the impedance transformation effect of the primary low-pass filter 54 and the secondary low-pass filter 59. The primary low pass filter 54 transforms the primarily inductive power supply impedance to an effective power supply impedance Z_SWhich acts as a balanced LC impedance within the passband of the primary low pass filter 54. The secondary low pass filter 59 has the same effect on the high inductance input impedance of the induction motor 38. The secondary low pass filter 59 transforms the induction motor impedance to an effective load impedance Z_LWhich acts as a balanced LC impedance inside the passband of the secondary low pass filter.

According to the maximum power transfer theorem, when the load impedance Z is adjusted_LLimited to be equal to the power supply impedance Z_SMaximum power transfer is achieved. Correction of power factor and power transfer for optimum power during 60 Hz operation of a three-phase, 40HP induction motor having a 480VAC three-phase power supplyFor efficiency, the preferred value for each KVAR capacitor C4, C5, and C6 is 5uF, rated for 600VAC service. The values of the KVAR power factor correction capacitors C4, C5, and C6 are preferably selected to improve the power factor of the motor, provide low pass filtering action, and optimal power delivery.

KVAR capacitors C4, C5 and C6 connected in combination with stator winding inductors W1, W2 and W3 define a secondary low pass filter circuit 59. These secondary filter sections convert the high inductance motor load to a balanced effective load impedance Z_LThis impedance may be in conjunction with the effective source impedance Z provided by the primary low pass filter 54 at the power controller input_SA comparison is made. Careful selection of the KVAR power factor correction capacitors C4, C5, and C6 for a given induction motor will transform the load impedance presented by the motor, and thus according to the transformed load impedance Z_LMatching the transformed power supply impedance Z_SProportionally improves power transfer.

The low pass filter circuit 54, power controller 36 and KVAR capacitors C4, C5 and C6 are enclosed in a common enclosure 55, as shown in fig. 5. An air-cooled heat sink (not shown) is thermally connected to the iron core wire inductors L1, L2, and L3 at the back of the shielding box.

The induction motor 38 is a conventional three-phase induction motor having an operating rating of 40 hp. Ac power connected to the neutral at 60 hz, 480VAC line is applied to three phase stator windings W1, W2 and W3, which are connected in a Y winding configuration and are disposed in stator slots symmetrically spaced 120 degrees from each other. Rotational torque is transmitted by a squirrel cage rotor R which is magnetically coupled into a rotating magnetic flux field produced by three-phase alternating current flow in the stator windings W1, W2 and W3. The rotor R transmits torque to an output drive shaft 58 coupled to the load 40. The load 40 may be a non-linear mechanical load such as a beam pumping unit 60 shown in fig. 2.

Referring to fig. 2, the power controller system 56 of the present invention receives ac operating power from the three-phase branch power cord 26. The power controller system 56 provides a controlled amount of ac operating power to the beam pumping unit 60. The pumping unit is sometimes referred to as a ganged pumping jack, which reciprocates the sucker rod 62 and the downhole pump. The pumping unit draws formation fluid on each upstroke of the sucker rod, while oil (formation fluid) F flows into the pumping unit on the downstroke, is supplied to the wellhead fitting on the upstroke, and then repeats the upstroke and downstroke.

The pumping unit 60 includes a swing beam type linkage pumpback 64 having a conventional swing beam 66 and horse head 68. The swing beam 66 is mounted on a pivot 72 of the a-frame 70. The counterweight 74 and crank 76 are driven by the ac induction motor 38 through a gearbox 78. The rotor R of the induction motor is mechanically coupled to the gearbox 78 by the power transmission shaft 58. The wire hanger 80 is attached to the horse head 68 by a short cable 82. The lower end of the hanger 80 is secured to the sucker rod 62. The polished section of the sucker rod 62 extends through the surface wellhead fitting 84 and is connected to a sucker rod string which extends from the wellhead through a tubing string 86 into the subterranean formation.

A conventional timer control unit 88 is connected to one phase of a 480VAC, 60 hz three-phase power supply for providing operating power to the internal pumping cycle timer 88. The internal timer is set to match a known reservoir fill rate, automatically starts pumping cycle operation of the pump unit 60 for a first predetermined pump-on interval, and then interrupts ac power to the controller 36 during a predetermined pump-off interval. The timer control unit 88 includes a step-down transformer that provides 110VAC, 60 hz operating power to the internal timer and relay circuits.

The timer control unit 88 also includes circuitry for automatically interrupting the ac power to the controller 36 and resetting the timer to the pump-off period in response to the pump-off control signal 90. The pump-off control signal is generated in response to a condition that temporarily depletes or evacuates formation fluid from the wellbore. A conventional fluid impact sensor on the wire hanger 80 on the upper end of the sucker rod 62 detects the drop hammer impact of the oil plunger. Pumping action is stopped until the reservoir replenishes the well bore to production levels.

Referring to FIG. 3, waveforms 92, 94 represent representative values of induction motor load and stroke (stroke) displacement, respectively. During normal pumping operation, the pump unit pumps at a fixed rate, such as 6.6 stroke cycles per minute (9 seconds for a peak-to-peak stroke cycle). The motor torque load 92 applied by the sucker rod load is a complex non-linear function of time, including positive and negative ramp functions as well as some spiral-up or oscillation function.

These torque waveform components are generated during four separate load phases. The pump is loaded with formation fluid at a positive slope load and then transitions through a zero load slope at the stroke apex when the counterweight 74 transitions through top dead center, producing some ringing or oscillation at the stroke apex at a relatively high torque level. The pumping unit load then transitions along the negative load ramp to the bottom of the stroke. The torque load waveform thereafter transitions through zero tilt at the bottom of the stroke, producing some ringing or oscillation at relatively low torque levels as the counterweight 74 transitions through bottom dead center at the bottom of the stroke.

These non-linear mechanical load fluctuations result in strong harmonic currents that may interfere with the operation of the controller 36 and may be injected back into the distribution network through the distribution branches. This increases the rms value of the load current in the distribution branch, which increases the energy costs to the customer as it determines the utility service costs. The power controller system 56 reduces or mitigates these harmonic currents that may be caused by the rapid switching action of the thyristor switches in the power controller 36 or by the nonlinear mechanical load applied to the induction motor 38.

Extensive field testing has been conducted on power controller systems 56 installed on private oil wells. The test results are summarized in tables 1 and 2.

The operational data summarized in tables 1 and 2 was extracted from logs on different dates spaced two months apart, which relate to two separate tests conducted on the same induction motor 38 and pumping unit 60. When the motor initially needs repair and is in poor working condition, a first test is performed with bearing problems. Two months later, after the motor had been repaired with a new bearing and verified to be in good working condition, a second test was performed on the same motor. The induction motor 38 installed on the field test pumping unit was a 40Hp induction motor rated for three phase 480 volt 60 hz ac service. The motor was connected to a walking beam type pumping unit 60, which was already in service for 15 years at the time of field testing.

Oil, water, gas production and power consumption of the wells were continuously stable for 15 years prior to testing. The pumping unit 60 used for the field test is located at the end of the irregular four-wire three-phase branch distribution line 26 and is susceptible to severe spike and power surge caused by frequent lightning storms. All branch power lines are open lines with no insulation from the transformer 14 to the service pole.

The parameter recorder employed during these field trials was a model 1231A recorder by rusrak range. The data logger is calibrated to reflect readings in terms of kilowatt-hour meters on the utility pole. To consult the manufacturer for calibration specifications. There are no interruptions during the period of logging.

The recorded data shown in tables 1 and 2 reflect the induction motor 38 performance with and without the controller system 56. It should be noted that the voltage remains consistent with and without the controller system 56 installed, while with the controller system 56 installed, the current is reduced, the reactive power is reduced, the real power consumption is reduced, and the motor power factor is also improved.

The harmonic current distortion in the third and fifth order is 5.0THD to 7.0THD without the controller system installed, and on average is about 3.0THD to 4.0THD with the controller system installed. This indicates that the controller system 56 is functional. It should be noted that with the controller system 56 installed, the magnitude and stability of the current values for the three-phase induction motor 38 are closely balanced.

TABLE 1

TABLE 2

Claims (9)

1. A power controller system (56) comprising one or more supply input terminals (F1, G1, H1) for receiving an alternating voltage at a basic distribution frequency from one or more selected supply phases (Φ a, Φ B, Φ C) of an alternating current power supply (12) and one or more supply output terminals (S1, S2, S3), the supply input terminals (F1, G1, H1) for conducting the alternating current to one or more stator phase windings (W1, W2, W3) of an alternating current induction motor (38), the power controller system (56) comprising:

an electronic power controller (36) comprising one or more power input terminals (N1, N2, N3), one or more power output terminals (M1, M2, M3) and switching means (42, 44; 46, 48; 50, 52), said power input terminals (N1, N2, N3) being adapted to receive an alternating voltage at said basic distribution frequency from one or more of said controller system supply input terminals, said power output terminals (M1, M2, M3) being electrically connected to conduct an alternating current to one or more of said controller system supply output terminals, and said switching means (42, 44; 46, 48; 50, 52) being coupled between said power input terminals (N1, N2, N3) and power output terminals (M1, M2, M3) for controlling the conduction of a current to one or more of said system supply output terminals (S1), s2, S3);

a primary low pass filter circuit (54) connected in series between the ac power source (12) and the electronic power controller (36), the primary low pass filter circuit (54) including one or more input terminals coupled to one or more of the power input terminals (F1, G1, H1), and one or more output terminals (F2, G2, H2) coupled to one or more power input terminals (N1, N2, N3) of the electronic power controller (36); and

one or more bypass capacitors (C4, C5, C6) connected in bypass with the one or more selected supply phases (Φ a, Φ B, Φ C), the one or more bypass capacitors (C4, C5, C6) being connected to one or more of the controller power output terminals (M1, M2, M3), wherein a capacitance value of each of the one or more bypass capacitors (C4, C5, C6) is selected such that the capacitance value of each of the one or more bypass capacitors (C4, C5, C6) is coordinated with an inductance value of the stator phase windings (W1, W2, W3) providing a low pass filter between the power output terminals (M1, M2, M3) of the electronic power controller (36) and the stator phase windings (W1, W2, W3).

2. The power controller system (56) of claim 1, wherein

SelectingA capacitance value of each bypass capacitor (C4, C5, C6) is coordinated with an inductance value of the stator phase winding (W1, W2, W3) of an induction motor (38) to be connected to the power controller system, so as to transform an effective load impedance presented by the induction motor to an electrical impedance (Z impedance) when such a connection is established_L) The electrical impedance (Z)_L) And an effective source impedance (Z) presented at an output of the primary low-pass filter circuit (54) when the primary low-pass filter circuit (54) is coupled to an AC power source (12)_S) Are equal.

3. The power controller system (56) of claim 1, wherein

Each bypass capacitor (C4, C5, C6) is rated for kilovolt-ampere reactive service.

4. The power controller system (56) of claim 1 wherein

The primary low-pass filter circuit (54) comprises one or more low-pass LC filters (L1, C1; L2, C2; L3, C3), the low-pass LC filters (L1, C1; L2, C2; L3, C3) being connected between one or more of the supply input terminals (F1, G1, H1) and one or more of the power input terminals (N1, N2, N3).

5. A power controller system (56) according to claim 1, the switching means comprising first and second gate controllable switches (42, 44; 46, 48; 50, 52), each having a respective control gate and being connected in parallel, the polarity relationship of the switches being opposite to each other between a first node and a second node for each phase of the alternating voltage, wherein the first node is electrically coupled to one of the system supply input terminals (F1, G1, H1) and the second node is electrically coupled to one of the system supply output terminals (S1, S2, S3).

6. A method for controlling application of ac voltage from a power source (12) in one or more phases (Φ a, Φ B, Φ C) of ac voltage to one or more stator phase windings (W1, W2, W3) of an ac induction motor (38) to match power requirements of a mechanical load (40) driven by the motor, the method comprising the steps of, for each phase:

coupling a gate controllable switch (42, 44; 46, 48; 50, 52) in series between a selected phase (Φ A, Φ B, Φ C) of the alternating voltage and a selected motor stator phase winding (W1, W2, W3), wherein the gate controllable switch comprises first and second control gates, each for applying each polarity of the alternating voltage to the switch and the motor;

alternately triggering the gate-controllable switches (42, 44; 46, 48; 50, 52) to a conducting state during each alternation of the alternating voltage;

-inhibiting the gate controllable switch from turning on during each alternation of the alternating voltage for a time interval proportional to: the start of the interval is when the alternating current voltage in the stator phase winding alternately passes a first zero crossing, and the end of the interval is when the corresponding alternating current in the stator phase winding alternately passes a second zero crossing;

filtering the alternating voltage conducted by the gate controllable switch;

connecting a bypass capacitor (C4, C5, C6) to one of the power output terminals (M1, M2, M3), the bypass capacitor (C4, C5, C6) being in bypass connection with the one or more selected supply phases (Φ a, Φ B, Φ C), wherein a capacitance value of the bypass capacitor is selected such that the capacitance value is coordinated with an inductance value of one of the stator phase windings (W1, W2, W3) so as to form in combination a secondary low pass filter circuit when such a connection is established.

7. The method according to claim 6, comprising the steps of:

selecting a capacitance value of the bypass capacitor (C4, C5, C6) such that the capacitance value is equal to the fixed capacitance valueThe inductance values of one of the sub-phase windings (W1, W2, W3) are coordinated to provide a secondary low-pass filter circuit (59), the secondary low-pass filter circuit (59) transforming the electrical impedance presented by the induction motor (38) into an effective electrical impedance (Z)_L) The effective electrical impedance (Z)_L) And an effective source impedance (Z) presented at an output of the primary low-pass filter circuit (54) when the primary low-pass filter circuit (54) receives an alternating voltage from an alternating current power source (12)_S) Are equal.

8. A method according to claim 6, comprising the step of controlling the operation of said gate controllable switches (42, 44; 46, 48; 50, 52) by an electronic controller (36); and further comprising the step of applying the filtered voltage output from the primary low pass filter (54) as operating power to the electronic controller.

9. The method according to claim 6, comprising the steps of:

selecting a capacitance value of the bypass capacitor (C4, C5, C6) such that the capacitance value is coordinated with an inductance value of the stator phase winding (W1, W2, W3) to transform an electrical impedance presented by the induction motor (38) into an effective impedance Z_LThe effective impedance Z_LSubstantially identical to the effective source impedance Z presented at the output of the primary low-pass filter circuit (54) when the primary low-pass filter circuit (54) receives an alternating voltage from an alternating current source (12)_SAre equal.