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WO1996005651A1 - Systeme numerique d'optimisation de puissance pour moteurs a induction a courant alternatif - Google Patents

Systeme numerique d'optimisation de puissance pour moteurs a induction a courant alternatif Download PDF

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Publication number
WO1996005651A1
WO1996005651A1 PCT/US1995/009507 US9509507W WO9605651A1 WO 1996005651 A1 WO1996005651 A1 WO 1996005651A1 US 9509507 W US9509507 W US 9509507W WO 9605651 A1 WO9605651 A1 WO 9605651A1
Authority
WO
WIPO (PCT)
Prior art keywords
turn
amplitude
inrush current
voltage
directional switching
Prior art date
Application number
PCT/US1995/009507
Other languages
English (en)
Inventor
David L. Williamson
Ryan Corley
Original Assignee
Williamson David L
Ryan Corley
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Application filed by Williamson David L, Ryan Corley filed Critical Williamson David L
Priority to AU33596/95A priority Critical patent/AU3359695A/en
Publication of WO1996005651A1 publication Critical patent/WO1996005651A1/fr

Links

Classifications

    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02P—CONTROL OR REGULATION OF ELECTRIC MOTORS, ELECTRIC GENERATORS OR DYNAMO-ELECTRIC CONVERTERS; CONTROLLING TRANSFORMERS, REACTORS OR CHOKE COILS
    • H02P27/00—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage
    • H02P27/02—Arrangements or methods for the control of AC motors characterised by the kind of supply voltage using supply voltage with constant frequency and variable amplitude

Definitions

  • Alternating current (AC) induction motors are designed for optimum efficiency operation at full rated load and minimum specified voltage. Losses are due to the resistance of the windings through which the current flows or copper losses as well as hysterisis losses or iron losses, due to the inherent inefficiency of the conversion of electric energy into magnetic field energy. Therefore the induction motor is inefficient when it is used at less than full load or more then its minimum specified voltage or any combination of the two.
  • the stator windings have the full voltage applied at all times irrespective of load and hence both the resultant copper and iron losses are large even when the load is small. This results in excessive heating of the motor, shortened life and increased maintenance and operating costs. Increased heating causes a further increase of resistance, thus further decreasing efficiency.
  • One of the ways of reducing losses when operating an AC induction motor at less than full power is to reduce the applied voltage below its specified minimum.
  • a known method used to accomplish this is to interrupt the AC power source for part of each cycle. The motor under less than full load will continue to run at its normal speed due to its rotational momentum even though each half cycle of power applied is less than a full half cycle.
  • a common device used to interrupt the AC power source is a bi-directional silicon controlled rectifier switch such as a Triac installed in series with the stator windings. The Triac can be turned on by an electrical signal for varying amounts of the AC power half cycle and can also be switched in either direction. The effective applied voltage may thus be reduced, which results in a reduction of stator current.
  • stator current is a difficult matter in practice, particularly if we seek to optimize the efficiency of a motor whose load is varying over time.
  • the usual approach is to sense the load on the motor and to cause a voltage or signal to be produced which varies with the load.
  • the load sensitive signal is then used to vary the duration during each AC cycle that the Triac is on. In this manner when the motor load is heavy, the signal will keep the Triac on for most of the AC cycle and more power will be delivered to the motor stator resulting in additional torque, while as the load decreases, the signal will be such as to reduce the duration of time the Triac is on and thus resulting in less power to the motor. Since there is no current to the stator during part of the low load AC cycle, heat and hysterisis losses are reduced, improving the overall efficiency of the motor.
  • the designer of an efficient motor control system is faced with the issue of what parameters to measure to ascertain the conditions existing in or about the motor, and then how to utilize the information thus obtained to control the current through the motor or more particularly in a Triac controlled system, to control the Triac firing cycle.
  • Such methods for improving the efficiency of AC induction motors have been described in previous patents, e.g. Parker U.S. Patent No. 4,864,212, Hedges U.S. Patent No. 4,636,702, and Fisher U.S. Patent No. 5,200,684.
  • the previous methods describe means for controlling the current flow through the motor to achieve improvements in either speed control or load control, by measuring parameters such as the back EMF or the phase lag between the voltage and current zero crossings. These methods are designed for particular conditions of load and motor parameters and are not readily adaptable to continuously varying loads.
  • the present invention is an improvement as it teaches a method for automatically tuning the motor to obtain the optimum efficiency from AC induction motors under varying load and other operating conditions.
  • the present invention consists of an induction motor which has a Triac and an inrush current sensing device placed in series with the AC power source, however, the Triac is regulated or controlled by a microprocessor which is programmed to turn the Triac on and off for optimum efficiency under varying conditions.
  • a digital control system together with embedded software allows for automatic optimization of the efficiency of operation under all possible variations of load and power input. Optimum efficiency is achieved when the stator current is equal to the rotor current under idealized conditions. All motor control systems are efforts to reach or achieve such a condition, or given certain external parameters to approach such an operating state for most of the conditions of a motor. Even motors that operate with steady-state loads have at least three different operating conditions.
  • the present invention uses an improved method of maintaining operation of the motor at optimum efficiency, by measuring different parameters than previously measured. By measuring these different parameters and using the information to set the Triac turn-on delay, an improved efficiency of operation results.
  • a further advantage is that by measuring these different parameters, the system can be continually tuned or adjusted to maintain optimum efficiency of the motor in real time, through variations of motor load, input voltage and other operating condition.
  • the invention utilizes the addition of a saturable transformer in series with the motor stator, the Triac and the AC voltage source.
  • the output of the saturable transformer produces a narrow voltage spike corresponding to the inrush current following the turn-on of the Triac.
  • a microprocessor measures the voltage spike amplitude for each half cycle and compares the amplitude of said voltage spike with prior amplitudes. As the Triac turn-on delay increases for each half cycle, the voltage spike will be of a constant amplitude, as the delay increases further the amplitude of the voltage spike increases, then decreases and increases again.
  • the system searches for a decrease in spike amplitude as a function of increasing Triac turn-on delay.
  • the non-linearity of these parameters as a function of Triac turn-on delay time gives rise to the ability of automatic tuning of the system to operate in an area where the spike amplitude decrease slope is of a certain magnitude which corresponds to optimum efficiency of motor operation.
  • the time interval between the turn-on of the Triac and the voltage spike representing the inrush current for the optimal efficiency conditions is measured and stored in memory. Once said time interval is established, maintaining said time interval constant or nearly so will assure optimally efficient operation as the load varies.
  • the time interval is inversely proportional to the load, thus as the time intervals decreases e.g. when load increases, this variation will be sensed and the Triac turn-on delay will be reduced. By reducing the Triac turn-on delay the time interval will increase regaining efficient operation.
  • the control is maintained by a microprocessor with built in memory. The memory stores the instruction for performing the measurements, comparing them to the prior data and adjusting the Triac firing delay. An external clock is utilized for timing measurements.
  • Inherent in the present invention is the ability to provide the AC induction motor with a soft start.
  • the microprocessor provides for a gradual ramping up of the source voltage to limit the starting surge current present in previous designs.
  • this invention provides for a shutdown under brown-out or low voltage conditions, thus preventing possible damage to the motor.
  • the use of a contactor and an overload cut-out with fusible links, such as commonly present in today's applications are no longer necessary.
  • Figure 1 is a block diagram of the control system.
  • Figure 2 is a graph showing the negative slope of the peak amplitude demodulated inrush current pulse as a function of Triac firing angle where the firing angle of the Triac should be maintained to achieve optimum efficiency of operation.
  • Figure 3 is a detailed circuit diagram of the control system.
  • An AC induction motor may be connected in series with a full wave switch to limit the voltage applied across the stator, and thus to also limit the current through the stator.
  • a saturated core transformer is also placed in series with the induction motor stator, and the output of said transformer will be used to monitor the peak inrush current through the stator.
  • Figure 1 a block diagram, shows the full wave switch in the form of a Triac 10, and the load current sensor 11, in the form of a saturated core transformer.
  • Figure 3 shows the detailed schematic of the preferred embodiment.
  • Induction motors are typically equipped with starter windings to enhance the initial start or rotation of the motor.
  • the present invention is designed to monitor the motor as it goes through its starting mode, and to start its controlling functions following the start mode. Induction motors are designed to switch out of the starting mode automatically.
  • the control system senses the presence of large voltage pulses, representing the inrush current, at the output of the saturable core transformer, which pulses would be limited by the zener diode.
  • the start mode ends, as it is normally designed to do automatically, the voltage pulses would decrease below the level of the zener limiting voltage and the control system would sense that the motor has entered its operating mode. If the motor starting mode fails to automatically end after a pre-programmed length of time, usually a few seconds, but variable for different motors, the control system will assume a locked rotor condition exists and it will shut the motor down requiring a manual restart.
  • the Triac will be turned on with an arbitrary delay of approximately 400 ⁇ sec . That is the time interval between the line voltage zero crossing and the Triac turn-on. Some time later, within that voltage cycle, depending on the characteristics of the circuit a current will flow, which will produce a voltage spike or pulse in the output of the saturable transformer.
  • An analog to digital converter in the microprocessor is used to measure the amplitude of the decoded pulse, and a number assigned to this amplitude, which number is stored in memory. The system then proceeds in what is referred to as the capture mode.
  • the microprocessor On the subsequent half cycle the microprocessor will cause the Triac turn-on to be delayed by an additional 25 ⁇ sec, and the voltage pulse representing the inrush current will be measured. The number representing the amplitude of the successive pulses is compared to the previous ones. When the amplitude of the current pulse starts increasing illustrating that the stator and rotor currents are close to equal and motor efficiency is nearing its optimum, the microprocessor will end the capture mode and initiate the tune mode. On subsequent voltage half cycles the Triac turn-on delay will be increased by 1 ⁇ sec increments and the amplitude of the resultant pulse representing the inrush current will again be measured. The amplitude of the inrush current pulses will peak as additional delays are introduces, will decrease if still additional delays are introduced and eventually resume increasing as yet additional delays are introduces. The optimum operation is on the down slope after reaching the initial peak amplitude.
  • the microprocessor is programmed to sense the reversal in gradient, from increasing amplitude to decreasing amplitude, and stop the addition of delay increments, to the time interval between voltage zero crossing and Triac turn-on.
  • the system will now measure the time interval been the Triac turn-on time and the voltage pulse representing the in-rush current. This time period is also recorded in the microprocessor memory and the circuit now shifts to an operating mode to maintain this latter time interval.
  • This derived interval will be unique for a given motor and its operating conditions.
  • the system will attempt to maintain the interval by turning the Triac on earlier, as measured from the voltage zero crossing. As the load decreases and less current is required, the system will still try to maintain the same interval by turning the Triac on later, as measured from the voltage zero crossing.
  • the microprocessor causes an error flag to be sent and the system goes through a re- start mode. Capture, tune and maintenance cycles are repeated.
  • the system will thus automatically seek and locate the optimally efficient point of operation and then maintain such operation as changes in load or input voltage occur.
  • the advantages of the system are the reduced power consumption and the resultant reduced temperature of the motor housing and windings while operating, resulting in further incremental improvements in efficiency, or conversely in the ability to obtain greater power output from smaller motors.
  • the entire control device consisting of Triac, saturable transformer and all control circuitry can be packaged in one box or container, such that it may be plugged into available voltage source and the motor plugged into the control device.
  • the control circuitry is further described as follows .
  • the output of the saturated core transformer is demodulated and rectified by the demodulator 12 in Figure 1 and results in voltage pulses that are proportional to the inrush current through the stator.
  • the microprocessor 13 is controlled by a clock 38, and is programmed to measure the amplitude of the voltage pulse representing the inrush current as a function of the increasing time interval between the voltage zero crossing of each half cycle of the input line voltage and the Triac turn- on time.
  • the characteristic of the circuit is such that when the ratio of the demodulated pulse amplitude is plotted against the Triac firing angle a curve containing an intermediate peak and a null point occurs, as shown in Figure 2.
  • the down slope approaching the null point represents the optimum firing angle for the Triac, as a function of AC induction motor load, at which point the applied stator current and induced rotor current are matched. It is important to note that the circuit in searching for the optimum delay in turning on the Triac must change from increments in the order of 25 microsecond to those of only 1 ⁇ sec as the point of non-linearity is approached.
  • This invention makes use of the anomaly in the ratio of modulated current amplitude and Triac firing angle to automatically locate and select the null point, and maintain the firing angle on the negative slope of the curve approaching the null point. In this manner optimum efficiency is maintained.
  • microprocessor 13 which has built in memory, where the operating instruction may be encoded.
  • the narrow voltage pulses representing the inrush current which are sensed at the output of the saturatable core transformer, are demodulated by a full wave diode bridge 31, the output load being a resistive divider 32, which is further limited and filtered by a zener diode 33 and a capacitor 34.
  • the resultant output is fed to the on board analog to digital converter of the microprocessor 13.
  • the input voltage waveform protected by a current limiting resistor 35, and modified by a diode 36 and a zener diode 37 limiter, is also introduced to the microprocessor.
  • the diode 36 shunts the negative excursion of the input wave form.
  • the zener diode 37 clamps the positive excursion at five volts.
  • the result is a square wave, the edges of which are used by the microprocessor 13 to determine the voltage zero crossing, which serves as the reference from which all timing measurements are made. Under brownout or low input voltage conditions, existing devices simply shut down the motor if the voltage drops below a pre-programmed level.
  • the current invention is not dependent on a programmed voltage.
  • an overload condition will be sensed.
  • the overload could be due to low input voltage e.g. a brownout or to a mechanical overload. In either instance, if the condition persists for a programmed length of time the motor will be shut down. If the reason for shut down was low input voltage, the microprocessor will restart the motor as soon as the under voltage is corrected. If the reason is a mechanical problem, then the system must be manually re-started.
  • the microprocessor 13 has a clock frequency maintained by an external crystal 38, there are also two light emitting diodes 39 and 40 which display the status of the microprocessor. One displays the presence of an inductive load, and the other indicates when the circuit is tuned to optimum efficiency.
  • the Triac is isolated from the microprocessor by the use of an optically coupled firing circuit 41.
  • the power for the microprocessor is provided by a DC power supply 14 which is capacitively coupled to the source voltage.

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Control Of Ac Motors In General (AREA)

Abstract

L'invention concerne un procédé et un dispositif servant à améliorer l'efficacité du fonctionnement de moteurs à induction à courant alternatif. Ce procédé et ce dispositif sont basés sur le réglage du système de commande du moteur, de façon à maintenir constant l'intervalle temporel entre la mise en fonction d'un moyen de commutation bidirectionnel et le moment de détection du courant de démarrage résultant à travers le stator, simultanément à la variation de la charge.
PCT/US1995/009507 1994-08-08 1995-07-28 Systeme numerique d'optimisation de puissance pour moteurs a induction a courant alternatif WO1996005651A1 (fr)

Priority Applications (1)

Application Number Priority Date Filing Date Title
AU33596/95A AU3359695A (en) 1994-08-08 1995-07-28 Digital power optimization system for ac induction motors

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
US28712994A 1994-08-08 1994-08-08
US08/287,129 1994-08-08
US08/492,832 1995-07-24

Publications (1)

Publication Number Publication Date
WO1996005651A1 true WO1996005651A1 (fr) 1996-02-22

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Application Number Title Priority Date Filing Date
PCT/US1995/009507 WO1996005651A1 (fr) 1994-08-08 1995-07-28 Systeme numerique d'optimisation de puissance pour moteurs a induction a courant alternatif

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AU (1) AU3359695A (fr)
WO (1) WO1996005651A1 (fr)

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001071901A1 (fr) * 2000-03-10 2001-09-27 Melexis N.V. Dispositif de commande de triac, a regulation de la vitesse pour un demarrage en douceur

Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4297628A (en) * 1980-07-03 1981-10-27 Louis W. Parker Energy economizer for induction motors
JPS57180393A (en) * 1981-04-30 1982-11-06 Toshiba Corp Controller for induction motor
US4710692A (en) * 1986-10-16 1987-12-01 Square D Company Self calibration of the thyristor firing angel of a motor controller using a current window to determine a final value of a reference current lag phase angle

Patent Citations (3)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4297628A (en) * 1980-07-03 1981-10-27 Louis W. Parker Energy economizer for induction motors
JPS57180393A (en) * 1981-04-30 1982-11-06 Toshiba Corp Controller for induction motor
US4710692A (en) * 1986-10-16 1987-12-01 Square D Company Self calibration of the thyristor firing angel of a motor controller using a current window to determine a final value of a reference current lag phase angle

Non-Patent Citations (2)

* Cited by examiner, † Cited by third party
Title
PATENT ABSTRACTS OF JAPAN vol. 007, no. 027 (E - 156) 3 February 1983 (1983-02-03) *
PROPESCU VIOREL ET AL.: "Energy saving controller for induction motors", ELECTRONICS ENGINEERING, vol. 57, no. 703, LONDON, GREAT-BRITAIN, pages 43 - 44 *

Cited By (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
WO2001071901A1 (fr) * 2000-03-10 2001-09-27 Melexis N.V. Dispositif de commande de triac, a regulation de la vitesse pour un demarrage en douceur

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Publication number Publication date
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