WO2002089307A1

WO2002089307A1 - Well pump motor driver control apparatus and method

Info

Publication number: WO2002089307A1
Application number: PCT/US2002/013348
Authority: WO
Inventors: Thomas E. Polk
Original assignee: Special Technology Products, Inc.
Priority date: 2001-04-27
Filing date: 2002-04-29
Publication date: 2002-11-07
Also published as: AR035873A1

Abstract

The present invention relates to a circuit for providing conditioned alternating current to a motor for running a well pump reciprocating motor which experiences uneven torque. A system is provided for conditioning the primary electrical supply 310, bussing the conditioned primary service 318 to a variable frequency drive apparatus VFD 336 which alters the line frequency of the alternating current to a controlled frequency to change the speed of the motor in response to a signal received from a well controller 330, and, if the system begins to inductively regenerate current when the speed of the rotor exceeds the line frequency from the drive, to condition and shunt the excess direct current R to the primary supply side 318 of the variable frequency driver where it may be reused.

Description

WELL PUMP MOTOR DRIVER CONTROL APPARATUS AND METHOD

BACKGROUND OF THE INVENTION

Field of Invention

This invention relates to AC motor frequency driver for use in well pumping operations and, in particular, to controlling an AC motor to minimize costs and maximize well production. Background and Prior Art

Oil production throughout the world has historically been augmented by artificial lift means to increase the movement of oil from the production zone to the surface. One common method to provide artificial lift utilizes a walking beam/sucker rod, reciprocating device. Numerous control methodologies have been developed to control the electric motor which is the prime mover of the walking beam /sucker rod pump system in response to various measurable quantities such as load, position, motor current, changes in load characteristics, or the passage of time. If oil is pumped from the annulus of the well faster than the rate of inflow into the annulus, the pump mechanism experiences what is characterized as fluid pound thereby causing severe pump rod loading in compression. The absence of oil in the annulus sufficient to cover the rod actuated pump quickly leads to mechanical failure of the pumping system because the pump experiences fluid pound as it hits the top of the fluid column in the annulus. Historically, well motor controllers were devised to sense this condition and shut the power to the motor for some period to allow the annulus to refill with fluid. The pump controllers used in the past have detected fluid pound and instructed the motor to shut off for a period of time to allow refilling of the annulus when the motor would be restarted to continue pumping. This "pump-off" condition has been dynamically detected from the sensors, previously described, thereby saving the rod system from compressive loading, but which reduces the flow of oil from the well bore.

It was eventually recognized that slowing the reciprocation of the rod pump to match the infilling of the annulus with oil would be preferable to shutting the well down for a period. Additionally, rapid downstrokes place the rod string in compression and lead to premature mechanical failure. There are a number of other benefits from slowing down, but not stopping, the well when the unit starts. These benefits are described in United States Patent No. 4,973,226 to McKee, that details a method of controlling the pumping speed — through the use of a variable frequency controllers - on a rod pumping unit to provide a substantially constant amount of filling of the down hole pump and control the fluid level, thereby avoiding a "pumped off" condition and the need to turn the motor off. See also United States Patent No. 4,661,751 to Werner also describing a well pump control system for a variable frequency motor.

Variable frequency drivers have experienced substantial operational problems in these applications because of the unbalanced load characteristics of a well with walking beam units. Although installers have long sought to balance the units on installation, the dynamic response of the well to pump and other uncontrollable factors prevents the system from remaining mechanically balanced. Additionally, the variability of the electrical supply and demand changes the electrical dynamics and may introduce unexpected impedances and harmonics into the system. AC drivers that accept the primary line current and energize the motor in an appropriate manner have long provided a DC bus to draw off generated electromotive forces from unbalanced motors to prevent overload and overheating of the driver. In order to prevent overloading or overheating of the driver itself, this excess energy was shunted to extensive resistor grids that dissipated the energy as heat resulting in a substantial waste of energy. The present invention eliminates the need for these resistor grids.

Reciprocating pumping systems typically operate between 4 to 20 strokes per minute. Depending on the well depth (and thus the rod load and liquid column supported) as well as the counterweight installed on the system, motor torque may vary greatly during a complete cycle. When the frequency of the AC motor is greater than the frequency of the AC line or the variable speed drive providing power to the motor, it becomes a generator. The energy generated by the motor when it enters this "generator" mode must be dissipated by the resistor grids or by a regeneration system which shunts this energy from the DC bus back to the primary side of the drive unit.

With the present invention, the continuous variable speed drive permits motor adjustment to maintain fluid level at the preset optimum point in the annulus and allows the motor to operate at its maximum power output. The driver increases or decreases stroke per minute of the reciprocating system as well characteristics and motor-loads vary. Since the motor may be selectively slowed during* the down stroke to allow time for the rods to move to the bottom of the stroke cycle, the rods are not put into unnecessary compression during normal operation. Additionally, as a further result of the selective slowing of the rods during the entire stroke cycle to permit infilling of oil into the annulus, the driver reduces rod stress and traveling valve wear from fluid pound or other anomalies. Further, since the motor functions for longer periods of time without stoppages or time-outs to allow well infilling, the controller minimizes belt, sheave and gear box wear. Finally, because the motor operates for longer periods of time without unnecessary shutdowns to allow infilling, the control results in less paraffin build up, which results from natural cooling in the well bore during the period of time that oil is not being pumped from the annulus.

Significantly, the use of the driver of the present system permits the use of NEMA (National Electrical Manufacturers Association) B designed motors, which result in significantly lower capital costs since these motors are approximately 20% less costly than NEMA D type motors. The operating characteristics of the motors are significantly improved for the present purposes as a direct result of the system of the present invention.

For example, using the driver system of the present invention, NEMA D design motor slippage can be maintained at between 20-30% and NEMA B design motor slippage can be increased to around 15-20%. Slippage is the difference between the speed of the rotating magnetic field in the motor (normally called the synchronous speed) and the rotor speed expressed as a percentage. Often this quantity is expressed derived as: S=(NS-NR)/NS x 100, wherein S is the percent slippage, NS is the synchronous speed in rpm, and NR is the rotor speed under load in rpm.

Torque of an induction motor generally depends on the strength of the interacting rotor and stator fields and the phase relations between them. Throughout normal range of operations, the torque is normally directly proportional to rotor current. Since rotor current increases in almost direct proportion to the motor slip, as slip increases from close to zero to between 20% and 30% torque increases to its maximum rated for the motor. After this maximum is reached (normally referred to as the breakdown torque), any movement away from this point decreases power and causes heat to be induced in the motor. The unbalanced load factors that affect the motor controlled by the drive system of the present invention can be accommodated without substantial movement away from maximum torque, thereby minimizing motor heating and premature failure of the motor. Power factors using the driver of the present invention typically are maintained at the 95-97 percent range through the operating cycle of the system. These electrical characteristics result in a substantial reduction in the ratio of kilowatt-hours per barrel of oil. Other types of proposed systems monitor the DC link of the driver and adjust the motor speed to reduce braking power (and heat buildup) in the drive. The present invention avoids lowering the efficiency of the system to avoid overloading or overheating of the driver. SUMMARY OF THE INVENTION

The present invention provides an AC isolation transformer providing low impedance, a variable frequency drive, and regeneration unit for accepting dynamic control signals from oil well control circuitry and using such control signals to slow or speed up a AC induction motor while further accepting and recycling AC voltages from the DC bus of the driver to prevent abnormal heating or overload characteristics in the motor when the torque on the motor becomes unbalanced. The apparatus for driving a variable frequency motor for use with a pumping well comprises an isolation transformer connected to a primary source of electromotive force; a signal controller adapted to accept control signals from a variety of sensors on a pumping well; a variable frequency drive for controlling an alternating current motor connected to said transformer in response to a control signal from the signal controller; and, a voltage regenerator connected to a direct current bus of the variable frequency drive for shunting or gating electromotive force resulting from movement of a rotor past a stator faster than line frequency.

The apparatus can also provide an isolation transformer having a low- impedance of between 3 and 4 percent. Additionally, the apparatus may provide a switch for disconnecting the motor on response to a signal from a remote temperature detector if excessive heat is detected in the motor, and the voltage regenerator provides a rectified line to primary electrical service through to the drive.

The isolation transformer is electrically connected to a primary electrical service supplying AC service (which ma be either single phase or three phase) to a variable frequency driver. A control signal for selectively increasing or decreasing frequency of alternating current voltage supplied to an alternating current motor in response to said signal is sent to the variable frequency driver which delivers the current and the signaled frequency by converting the alternating current to direct current, converting the direct current to a frequency appropriate to the signal and thereafter reconverting the direct current to alternating current to be supplied to the motor. A line reactor is electrically connected between the variable frequency driver and the motor to condition the alternating current from the driver to the motor to avoid spikes which may damage the motor. A regeneration unit is connected between a direct current bus on the variable frequency drive and the primary-side electrical connection to the variable frequency drive to recycle generated electromotive force resulting from the generation cycle of the motor to the electrical supply of the driver. The electrical motor is further provided with remote temperature detectors that provide a signal to the driver to disconnect the motor from electrical service upon experiencing overheating.

Finally, the present invention provides a method for controlling a variable frequency alternating current motor on a well pump comprising the steps of: converting alternating current to direct current to chop the current to a controlled frequency and thereafter reconverting such energy to alternating current at a desired frequency; and, when generated by the motor, provides gating of the direct current from a variable frequency driver associated with the motor to a regenerator circuit to direct the generated current back to an alternating current supply line.

This method may further comprise the step of conditioning the input of electricity to the variable frequency driver, and filtering alternating current from the variable frequency driver to prevent damaging line spikes from reaching the alternating current motor. Another feature of the present invention may comprise the step of disconnecting electrical energy to the motor when a remote temperature detector signals abnormal heating in the motor. BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 is a schematic representation of walking beam /sucker rod pump system connected to a power grid and control circuit according to the present invention.

Fig. 2 is a schematic one-line representation of a drive control system according to the present invention.

Fig. 3 is a schematic representation of a typical drive control line system according to the present invention.

Fig. 4 is a schematic block diagram of the regeneration unit according to the present invention.

Fig. 5 is a schematic diagram of an alternative active front end regenerative controller according to the present invention. DESCRIPTION OF A PREFERRED EMBODIMENT

Referring now to the drawings, Fig. 1 is a schematic representation of a walking beam /sucker rod pump system. The well 40 is provided with a motorized walking beam pump unit 10 that provides a walking beam 12, counterweights 14, and gear box 16, all of which are actuated by electric motor 18. Motor 18 turns the gears 16 which move the crank arm (not shown) which is reciprocatingly attached to walking beam 12 having the weight offset by counterweights 14, all in a manner and with equipment well known in the industry of artificial lift oil production. In such arrangements, the walking beam moves the polished rod 42 vertically through interconnection with yoke 44. The movement of the polished rod fills a downhole pump thereby moving a column of oil to the surface for collection and transportation in a manner again long known in the trade. Other types of electrically driven reciprocating pump systems, such as rocking arm, walking beam or lifting chain^' systems can also be controlled by the present invention.

Fig. 1 also shows the source of motive power constituting a three-phase electrical source 30, which is connect through intermediate switches 32 to the rod

pump control box 20 by cabling 22 which is further grounded by stakes 36. Lightning strike protection is a significant consideration in all walking beam pump systems and lightning protection system 38, which is grounded in butt wrap pole ground 34 and grounding stakes 36, provides important protection to the control circuitry of the rod pump control box 20.

It may be noted that in many remote producing fields, single phase electrical service may be the sole source available to the oil producer. In this situation, single phase primary electrical service can also be accommodated by the present invention without departing from the spirit or scope of the invention claimed.

Referring again to Fig. 1, the present invention relates to a control circuit for controlling a variable speed pump motor 18. Most pump motors are provided with three phase electrical service from field power distribution grids typically through lines 30 which are connected to isolation transformer for matching and conditioning the nominal distribution voltage to the design voltage for such motors, most commonly 480 VAC (volts alternating current). The electrical service may provide lightning protection systems 38, with appropriate grounds 34 and 36 for isolating surges associated with lightning strikes, along with disconnect junction 32 for disconnecting service to the well in a manner well known to the industry.

Electrical service 30 is provided to the rod pump control box or enclosure 20 which provides the means of connecting the service to the well motor and associated control circuits and ancillary equipment such as heaters, fans, and shutters for isolating and conditioning the enclosure from the elements. The enclosure 20 can be a standard NEMA box to conform to appropriate national electrical codes for harsh hazardous environments as is well known to those having ordinary skill in the art of providing electrical service in the oil and gas industry.

Power to the electrical motor is provided by approved cabling 24 to motor 18 which can typically range from 15 to 150 horsepower (HP) rod pump motor available from a number of suppliers such as GE, ABB, Toshiba, US Motors and TEACO, all of which are well known suppliers of such motors to the industry. The motor or prime mover 18 drives the gear box 16 that is connected with a crank arm and counterweight 14 for reciprocating the walking beam 12 vertically about the Sampson post 13. The walking beam supports the polish rod 42 on the harness 44 for reciprocating movement into the well head 40. The polish rod 42 is connected to rods (not shown) that reach to the production zone of the well and support the downhole pump well known to those in the oil production industry.

The operating conditions of the well may be monitored utilizing well known technologies to provide input to the controller 20 to change the speed of the motor 18, for example, to slow the motor down if the oil pumps from the annulus faster

than oil flows into the annulus from the formation. The load on the polish rod 42 can be used to detect loading of the pump. Position transducers (not shown) can be used to determine the relative position of the polish rod in the total stroke to correlate load with position to determine whether the oil is reaching fluid pound. Other sensors can determine the hydrostatic pressure downhole to determine the depth of oil in the annulus. Once the pressure falls below a trigger point, the sensor would signal the controller to slow down the pump frequency to permit the infilling of oil.

Controllers that can be used for this purpose include the CAC Model 8800 Rod Pump controller Controller available from CAC, Inc. of Kingwood, Texas, and the Delta-X controller. Other similar controllers from other manufacturers of well control circuits can also be used for the detection and processing of the signals from the well sensors without departure from the spirit of the present invention. The present drive system is flexible enough to use any number of acceptable control products to monitor and control the producing well and its associated prime mover motor system.

Fig. 2 is a schematic one-line view of an embodiment of the well motor control system connected to a primary electrical source and providing energy to a complete well pumping system. Primary electrical service 310 is generally 480 volt 3-phase alternating current (480 VAC). A transient voltage surge suppressor (TVSS) 33 may be placed between primary service 310 and the main control breaker 312 and the balance of the system to prevent damage from voltage spikes resulting from nearby lightning strikes or the like. Regular service connections may be tapped onto the primary service bus for heaters, fans (not shown) and through a standard transformer 320 to the well controls 330, powered by buss line 330', such as those which may purchased from CAC or the like. The control circuitry, heaters and lights to illuminate or condition the enclosure and wellhead area are well known to one having ordinary skill in the art of oilfield electrical service and are therefore not specifically shown in Fig. 2. Signal controller 330 may be connected to a load cell LC 332 and position transducer POS 334 for continuously monitoring the production status of the well. A signal from the controller 330 can be used to initiate a change in motor speed through the variable frequency drive VFD 336. This signal is typically

a 4-20 milliamps generated by the signal controller 330 proportional to the speed adjustment requested. The VFD 336 external control connection accepts this signal to alter the speed of the motor in proportion to the signal.

The primary service line 310 connects through main run circuit breaker 312 to line contactor Ml to a multi-tap isolation transformer 315 providing a delta on the primary side and a wye on the secondary side of the transformer. Line contactor Ml provides a means for isolating the entire system for servicing of any of the components and is a standard magnetically activated switch thrown by the operator from the control panel. Similarly, run contactor M2 provides additional isolation for the purpose of service on the motor system.

The isolation transformer 315 provides the operator with the opportunity to match the primary line 310 voltage to the secondary line voltage on a one-to-one ratio for the center lead and provides two additional taps or poles on either side of the center throw allowing a 2.5% difference from the adjacent tap's voltage. This permits the operator to fine tune and match the primary line feed to the secondary demand. This three phase drive isolation transformer is designed to provide low impedance of not greater than 4%. Ideally, this transformer should have an impedance no greater than 3%. The transformer should also be rated with a K-factor of no greater than 4. The secondary side of the isolation transformer is connected to the variable frequency drive VFD 336 that controls and supplies the variable frequency motor 324 with AC through line 321 that is conditioned by reactor 323. Intermediate the variable frequency drive and the motor, reactor 323 conditions the alternating current thereby preventing line spikes from reaching the motor 324. The line between the drive and the motor is also protected from transient voltage spikes by another TVSS 33' to prevent abnormal voltage spikes from reaching and destroying the variable frequency motor. As may be readily appreciated from the one-line drawing of Fig. 2, significant efforts have been made to protect the drive and the motor from transient voltage spikes that may damage or destroy the system.

Overheating protection on the motor is monitored by remote temperature detector (RTD) 325 consisting of four temperature sensors circumferentially and evenly placed on each bearing housing within the motor to detect excess heating of the bearings. When excessive heating is detected a control signal is sent to the VFD 336 which provides a control block for receiving and shutting off energy to the motor to prevent bearing failure in the motor. The remote temperature detector 325 (RTD) is linked to the variable frequency drive by control line 326 as an additional control feature to shut the motor down before abnormally high temperatures destroy the motor.

Also connected to the variable frequency drive is a regenerative controller R that offloads or shunts electromotive force generated by the motor as it progresses through the low torque portion of the pump cycle. Any motor in which the rotor moves past the stator at a higher frequency than the nominal line voltage frequency on the stator will generate a direct current. This current must either be absorbed by the motor and driver or shunted or gated off the motor and driver to prevent overloading thereby tripping the protection circuits. The VFD provides a means on its DC bus connection for drawing this current from the motor. Historically, variable frequency drives dissipated this excess energy by dumping the current into a resistor grid which heated and thereby dissipated this generated energy by Joule heating.

In the present invention, the VFD is provided with a regeneration circuit that draws from the DC bus of the variable frequency drive and feeds back to the primary side as AC power. This energy may be used to power other portions of the pump installation equipment or may be run back through the isolation transformer to the primary feed thereby reducing the total power used by the system. Previously, a variable frequency drive provided a resistor grid connected to the DC bus of the drive to provide a means for regenerated voltage to dissipate this generated voltage. As previously noted, this need for regeneration results when the motor frequency is greater than the frequency of the AC line or the variable frequency drive providing power to the motor. This regenerative control circuit R can be used on any AC variable frequency drive that utilizes a fixed bus system, such as the bus system used by pulsed width modulator drives. This circuit passes regenerated energy from the DC bus of the variable frequency drive VFD to the primary AC line, thereby providing line regulation of the DC bus and preventing the drive from tripping on voltage generated by the motor. Bus diodes on the regeneration circuit isolate the regeneration DC bus from the drive DC bus thereby assuring that only regeneration energy is transmitted and avoiding tripping the drive's ground fault detection circuitry. DC bus line inductance on the regeneration unit also provides current surge protection.

The regenerative control circuit R also provides control logic to monitor the DC bus reference and the drive DC bus feedback to determine the need to enter the regeneration phase. When the reference exceeds the feedback by approximately 5%, the circuit commences regeneration. The regenerative control unit R also controls transistor switching sequence and synchronizes the switching with the AC line. Regenerated energy ma be conducted back to the AC line during the peaks of the AC rectified line. The frequency of switching is 360 Hz for a 60 Hz input line. The conduction angle is modulated by the controller between 40^Ω to 55^Q of the 60^Ω window.

The regenerative control circuit R, which can be adapted from a Model M3345 Regen Control Module from Bonirron, Inc. of Nashville, Tennessee, shunts generated voltage to the line 318 between the low impedance secondary transformer and the VFD. When the primary line potential is less than the regeneration line potential, voltage is shunted to the primary side of the transformer. When the potential on the VFD is less than the primary line potential, the voltage on the AC line supplied by the regenerator is used by the VFD.

The variable frequency drive receives a control signal from the control circuitry 330 on the well that measures load and position through load cell LC and the position transducer POS in a manner well known to those in the pump control industry. Based upon the control logic of the signal controller 330, a control signal is issued to the VFD to increase or decrease speed by a certain percentage of the existing speed. Ideally, the load on the polish rod (and the associated torque on the motor) will increase on the upstroke and decrease on the downstroke until the oil is pumped out of the annulus. A variable frequency driver permits the operator to set the parameters of the control circuitry to maintain the speed of the motor during most of the upstroke at a fixed rate to prevent undue tensile loading of the rod string down the annulus and then to slow (at the very top of the upstroke) the speed of the motor to allow the rod string to be slowly lowered into the annulus to prevent undue compression of the rod string on the down-stroke. This added feature was not available in prior art "pump-off" controllers providing signals to fixed frequency motors and has only been intermittently available to existing variable frequency drive and motor systems because of the inherent problems created by such specific control programs on motor and driver heating and overloading. Another suitable variable frequency driver which may be adapted for the use described herein is the Model ACS600 from ABB Industrial Systems, Inc. of New Berlin, Wisconsin. Fig. 3 is a schematic of the rod pump drive control system and power distribution system. Three phase 480 VAC power is supplied by the primary service connection. TVSS provides lightning isolation on the primary side of the electrical service. The primary service line 310 may be tapped to provide electrical power to various points. In this case, a lOOhp rod pump motor is connected through contactor Ml that may be energized remotely. Once the line contactor Ml is activated, line voltage is directed through isolation transformer 315 to variable frequency drive VFD. The VFD accepts signals from the signal controller 330 in a manner well known to those in the pumping well control industry. The frequency controlled by the VFD is advanced or retarded by signals received by the signal controller in a manner well known to those skilled in the art of pump off controls. AC current is provided at the appropriate frequency through cabling 321 to reactor 323 and thence to motor 324 as previously described. Further line voltage spike protection is provided by connecting a second transient voltage surge suppressor (TVSS) 33' between the reactor and the motor. Further protection is provided to the motor by temperature detector 325 which signals the VFD to shut down if abnormal heating is experienced in the motor bearings thereby preventing motor destruction or damage.

Heater HTR and fan F may also be used to condition the enclosure for

environmental conditions and may be tapped onto the primary electrical source 310. Step-down transformer, such as that shown at 320, may be used to provide power to control circuitry 330, or to lighting and communication equipment in a manner well known to those in the electrical service to the oil industry.

Fig. 4 is a schematic block diagram of the regeneration unit R manufactured as Model M3345 by Bonitron, Inc. of Nashville, Tennessee. The DC bus 328 is shown. The DC current is rectified and selectively shunted back to the three phase service on bus lines 318. As may be readily appreciated by one having ordinary skill in the art, the power control module converts the DC current into AC current for the selected polarity line of the AC service.

Fig. 5 is a schematic diagram of the alternative active front end regenerative controller found in the ABB ACS 600 Motor Control devices adaptable for use in this application. The active front end (or line side) controller provides a transistor bridge circuit permitting four quadrant switching. This configuration allows nearly sinusoidal AC current at a unity power factor and further permits reversible current flow through the controller's intermediate circuit DC bus-bars 328 to the line or network side of the system 318. Both the line side and the motor side of the ACS 600 drive system provide six insulated gate bipolar transistors with free wheeling diodes. High frequency switching and fast incremental changes in current minimize distortion of the voltage waveform at the input of the controller. The front end controller therefore minimizes harmonic distortion of both voltage and current throughout the operating cycle of the drive.

As previously noted, both Fig. 2 and Fig. 3 may be readily modified to show a single phase service. Single phase electrical service may also be accommodated using a similar circuit providing a single phase isolation transformer and other obvious changes to the circuit to accommodate the single phase nature of the electrical service provided.

The foregoing disclosure and description are intended to be illustrative and explanatory of the invention and various changes in the size, shape, and materials, as well as the details of the illustrated operation and construction may be made without departing from the spirit and scope of the invention.

Claims

CLAIMSWhat is claimed is:

1. An apparatus for driving a variable frequency motor for use with a pumping well comprising: an isolation transformer connected to a primary source of electromotive force; a signal controller adapted to accept control signals from sensors on a pumping weE; a variable frequency drive for controlling an alternating current motor connected to said transformer in response to a control signal from the signal controller; and, a voltage regenerator connected to a direct current bus of the variable frequency drive for shunting electromotive force resulting from movement of a rotor past a stator faster than line frequency.

2. The apparatus of claim 1 wherein the isolation transformer provides low- impedance.

3. The apparatus of claim 1 wherein the variable frequency drive provides a switch for disconnecting the motor on response to a signal from a remote temperature detector if excessive heat is detected in the motor.

4. The apparatus of claim 1 wherein the voltage regenerator provides a rectified line to primary electrical service through to the drive.

5. An alternating current driver circuit for a well pump motor comprising: an isolation transformer electrically connected to a primary electrical service; a variable frequency driver electrically connected to the isolation transformer and to a control signal for selectively increasing or decreasing frequency of alternating current voltage supplied to an alternating current motor in response to said signal by converting the alternating current to direct current, converting the direct current to a frequency appropriate to the signal and thereafter reconverting the direct current to alternating current to be supplied to the motor; a line reactor electrically connected between the variable frequency driver and the motor to condition the alternating current from the driver to the motor; and, a regenerative control unit connected between a direct current bus on the variable frequency drive and the electrical connection to the variable frequency drive to recycle generated electromotive force resulting from the generation cycle of the motor to the electrical supply of the driver.

6. The circuit of claim 5 wherein the isolation transformer is low impedance.

7. The circuit of claim 5 wherein the electrical motor is provided with remote temperature detectors that provide a signal to the driver to disconnect the motor from electrical service upon experiencing overheating.

8. A method for controlling a variable frequency alternating current motor on a well pump comprising the steps of: converting alternating current to direct current to chop the current to a controlled frequency and thereafter reconverting such energy to alternating current at a desired frequency; and, when generated by the motor, gating direct current from a variable frequency driver associated with the motor to a regenerative control circuit to direct the generated current back to an alternating current supply line.

9. The method of claim 8 further comprising the step of conditioning the input of electricity to the variable frequency driver.

10. The method of claim 8 further comprising filtering alternating current from the ^w variable frequency driver to prevent damaging line spikes from reaching the alternating current motor.

11. The method of claim 8 further comprising the step of disconnecting electrical energy to the motor when a remote temperature detector signals abnormal heating in the motor.