Abstract:
An induction motor control system senses the zero-cross angle of a current waveform applied to the motor&#39;s phase windings and computes the difference between the sensed current zero-cross angle and a predetermined demand voltage angle to estimate a power factor angle. The estimated power factor angle is compared to a predetermined desired power factor angle, and the voltage applied to the phase windings is adjusted in response to the difference between the estimated power factor angle and the desired power factor angle.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The invention relates generally to control of a rotating electric machine, and more specifically, to a load responsive speed regulation device and method for an inverter fed variable speed induction motor drive system based on power factor angle estimation and slip control.  
           [0003]    2. Description of Related Art  
           [0004]    Many applications for electric motors require variable speed motor operation, and to this end, various speed control solutions have been proposed. Induction motors are popular for several reasons, including high robustness, reliability, low price and high efficiency. A typical induction motor includes a stationary member, or stator, that has a plurality of windings disposed therein. A rotating member, or rotor, is situated within the stator to rotate relative thereto. In a three-phase induction motor, for example, a rotating magnetic field is established by applying three-phase sinusoidal alternating voltages to the stator windings. The rotating magnetic field interacts with the rotor windings to effect rotation of the rotor.  
           [0005]    Power conversion systems are commonly used to provide the multiphase AC power to the induction motor for variable speed applications. An example of such a power conversion system is a DC-to-AC inverter bridge, which typically includes inverter switching devices connected in a bridge formation between the DC bus lines and output terminals of the power conversion system. The inverter switching devices are controlled to connect the power on the DC bus lines to the system output terminals in a desired pattern such that AC output signals having the desired frequency and amplitude are synthesized from the DC power on the DC bus lines. Various modulation strategies may be employed for controlling the inverter switching devices to deliver power, including sine wave Pulse-Width Modulation (“PWM”).  
           [0006]    The desired speeds are achieved by setting the excitation frequency and a corresponding voltage. Low cost speed control solutions are often implemented using constant or schedule based volts-per-hertz algorithms, and speed regulation is based on feedback from a tachometer on the motor shaft. Elimination on the tachometer, however, is desirable not only from a cost perspective, but also for reliability reasons. Processing the speed information from tachometers requires extra signal conditioning circuits and takes up additional input-output pins on the processor chip.  
           [0007]    Moreover, an induction motor&#39;s speed slips relative to synchronous speed as the load increases. In applications where the load on the motor shaft is varying (for example, a washing machine), it is also important that proper stator voltage be applied to enable efficient operation and avoid over-heating of the motor due to saturation and iron losses. Meeting these requirements requires a load responsive control system that can provide near constant speed operation of the motor and the right amount of the voltage to the stator, thus preventing saturation in the motor core.  
           [0008]    Solutions have been proposed for efficient operation of an induction motor based on controlling the power factor of the motor (generally, the power factor is calculated based on the phase difference between the voltage and currents). Such solutions, however, may not be satisfactory in low cost solutions. Additionally, cost effective solutions for a variable speed induction motor drive systems fed by PWM inverter bridges that use simple volts-per-hertz control are not generally available.  
           [0009]    The present invention addresses shortcomings associated with the prior art and proposes a low cost solution to achieve the same results.  
         SUMMARY OF THE INVENTION  
         [0010]    The present invention provides a load responsive variable speed induction motor drive system and method based on power factor angle sensing and slip control to maintain speed regulation without the need for a tachometer.  
           [0011]    In one aspect of the invention, a method for controlling an induction motor is provided. The induction motor has a rotor and a stator with a plurality of phase windings therein to which AC power is applied to cause rotation of the rotor relative to the stator. The method includes sensing the zero-cross angle of a phase current waveform in the phase windings and computing the difference between the sensed current zero-cross angle and a known stator voltage angle to estimate a power factor angle. The estimated power factor angle is compared to a command power factor angle, and the voltage applied to the motor is adjusted in response to the error between the estimated power factor angle and the command power factor angle. The input frequency may also be adjusted in response to the difference between the estimated power factor angle and the desired power factor angle during certain load conditions to keep speed constant.  
           [0012]    In accordance with other aspects of the present invention, an induction motor system includes a stator and a rotor situated relative to the stator to rotate relative to the stator. A plurality of phase windings are situated within the stator, and a power source is connected to the windings to output AC power thereto. In exemplary embodiments, a DC-AC inverter provides the AC power to the motor. A controller is connected to the inverter, and is programmed to compare a sensed current zero-cross angle to a predetermined demand voltage angle to estimate a power factor angle. The voltage, and in some embodiments, the frequency of the voltage applied to the motor are adjusted in response to the difference between the estimated power factor angle and a predetermined desired power factor angle. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]    Other objects and advantages of the invention will become apparent upon reading the following detailed description and upon reference to the drawings in which:  
         [0014]    [0014]FIG. 1 is a block diagram conceptually illustrating aspects of an induction motor in accordance with the present invention;  
         [0015]    [0015]FIG. 2 is a schematic diagram illustrating additional aspects of the inverter shown in FIG. 1;  
         [0016]    [0016]FIG. 3 is a block diagram illustrating a speed control system in accordance with an embodiment of the present invention; and  
         [0017]    [0017]FIG. 4 is a schematic diagram illustrating a phase current zero-cross angle detection circuit in accordance with an embodiment of the invention.  
         [0018]    While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0019]    Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions must be made to achieve the developers&#39; specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.  
         [0020]    A constant slip speed in a motor ensures constant speed of the rotor at a constant input voltage frequency. Maintaining a constant slip also leads to a constant power factor operation. This can be concluded from the equivalent circuit description of an induction motor. Thus, by estimating the power factor of the motor and varying the input voltage to the motor to maintain a constant power factor, a constant rotor slip can be maintained. Consequently, the motor will also have a constant rotor speed.  
         [0021]    Rather than calculate the actual power factor by determining both the actual current and voltage phase angles, the power factor angle is estimated by sensing the zero crossing of the phase current of the motor and the command voltage to the inverter bridge powering the motor. FIG. 1 is a block diagram of a rotating electric machine, such as a three-phase induction motor system  10 , in accordance with aspects of the present invention. The motor  10  includes a stator  20  and a rotor  22 . The stator  20  includes a plurality of windings  24  that receive AC power from an inverter  26 . The inverter  26  receives DC power from a power source  28 . A controller  30  includes a schedule of voltage and frequency constants, and provides control inputs to the inverter to vary the voltage and frequency to achieve the desired speed. The controller  30  may be implemented, for example, with any type of digital controller such as a digital signal processor (DSP) chip, microcontroller or microprocessor. An example of a suitable controller is a model ADMC328 from Analog Devices.  
         [0022]    [0022]FIG. 2 illustrates portions of an exemplary inverter  26  in accordance with aspects of the present invention. The exemplary three-phase inverter  26  includes three inverter legs  31 , 32 , 33  corresponding to the windings  24  of the three motor phases. Each leg  31 , 32 , 33  includes upper and lower switching devices  36  connected in a bridge formation between the positive and negative lines  41 , 42  of the DC bus. The switching devices  36  may comprise any suitable switching device, such as bi-polar devices, power MOSFETs, IGBTs, etc.  
         [0023]    The switching devices  36  of the three inverter legs  31 , 32 , 33  are driven by corresponding gate drivers  38  so as to connect the power on the DC bus lines  41 , 42  to the motor windings  24  in a desired pattern, such that AC output signals having the desired frequency and magnitude are synthesized from the DC power on the DC bus lines  41 , 42 . In certain embodiments of the invention, PWM schemes are used for controlling the inverter switching devices  36 . In the illustrated embodiment, the gate drivers  38  have inputs connected to receive the output of a PWM generator  127  implemented by the controller  30 .  
         [0024]    In known induction motor drive systems, a tachometer would typically provide an indication of the actual motor speed to the controller  30 , which would compare the actual speed to a desired speed value and adjust the inputs to the motor windings  24  accordingly. However, in accordance with the present invention, the motor power factor is estimated by calculating the difference between a voltage phase angle and the phase current zero-cross angle to adjust the voltage and frequency constants, and thus, the magnitude of the voltage and the frequency applied to the motor  20 .  
         [0025]    [0025]FIG. 3 is a block diagram illustrating a speed control system  100  in accordance with aspects of the present invention. In exemplary embodiments of the invention, this is implemented digitally in the controller  30 . The estimated, or computed power factor angle  110  and a desired or command power factor angle  112  are input to a summing junction  114 , which outputs a power factor angle error value to a controller  116 , such as a proportional-integral-derivative (PID) controller. In certain embodiments, the estimated power factor angle  110  is low pass filtered to increase noise immunity. The controller  116  converts the power factor error value to a voltage compensation value  118 , which may be limited to an upper and a lower limit to avoid saturation of output. This range is established so as to handle the expected load changes on the motor  20 . The voltage compensation  118  is also low pass filtered in exemplary embodiments of the invention.  
         [0026]    The voltage compensation  118  is negated with the original command voltage computed from the motor&#39;s constant volts per hertz schedule  120  at a summing junction  122 . The voltage schedule  120  is derived based on motor design parameters in a known manner. The summing junction  122  outputs the corrected voltage that is applied to the PWM generator  127 . The PWM  127  generator generates switching signals that are applied to the switches  36  of the inverter  26 . In this manner, the power factor angle is regulated to a command angle by varying the terminal voltage to the motor  20 .  
         [0027]    By varying the magnitude of the voltage applied to the motor windings  24  to maintain constant power factor, the slip and thus the speed of the motor  20  is regulated to a near constant value. Based on the voltage compensation  118 , input frequency may also be varied to provide additional speed regulation at certain load conditions. The input frequency to the motor  20  is increased for all positive errors fed into the controller  116  and decreased for all the negative errors. Hence, at light loads the input frequency to the motor will decrease enough to provide constant speed operation of the motor. The voltage compensation  118  is converted to a frequency compensation value  124 , for example, by applying a gain factor thereto. The frequency compensation  124  is applied to the input frequency in a summing junction  126  to provide an adjusted frequency to the motor  20 .  
         [0028]    The frequency adjustment at certain load conditions is effective in further regulating the speed of the motor  20 . Thus, for a configured load point, near zero speed regulation is achieved for load variations on the motor shaft. Moreover, in further embodiments of the invention, the frequency and voltage inputs may be compensated for temperature variations. Temperature of the motor may be estimated, for example, by measuring the stator resistance. Stator resistance is estimated by applying DC voltages to the motor phases for short intervals of time. Based on the estimated stator resistance, voltage and frequency compensation is calculated and applied to the motor.  
         [0029]    To calculate the estimated power factor  110 , the current zero-cross instants are determined using a zero-cross detection circuit  200 . Referring to FIG. 2, a resistor  204  is included between the lower switching device  36  of one of the inverter leg  31  and the negative DC bus  42 . Using a single inverter leg, such as inverter leg  31 , to determine the zero-cross angle greatly simplifies the circuitry required to implement the current zero-cross detection, though any or all of the inverter legs  31 , 32 , 33  may be used for the zero crossing detection. The current zero crossing is determined by the zero-cross detection circuit  200 , then provided to the controller  30  to calculate the power factor angle. In this implementation scheme a level change in the signal is used to signify the zero cross of the phase current, though other schemes for detecting the zero crossing are contemplated, such as through use of the PWM signals.  
         [0030]    [0030]FIG. 4 is a schematic diagram of a zero-cross detection circuit  200  in accordance with an embodiment of the present invention. As noted above, the resistor  204  is included between the lower switching device  36  of one of the inverter legs  31 , 32 , 33  and the negative DC bus  42 . For sake of simplicity, the first inverter leg  31  is referenced in this description, though any of the inverter legs  31 , 32 , 33  could be used for this purpose.  
         [0031]    The signal obtained from the resistor  204  is fed to conditioning circuitry  202  that produces a square wave type output signal based on the zero crossing of the phase current. The output is connected to the controller  30 . In one embodiment, the output is connected to a level sensitive interrupt pin on the DSP implementing the controller  30 , providing an output edge to the DSP each time the current crosses zero. The phase angle is stored in the DSP memory when interrupted. To increase the noise immunity in one embodiment, the interrupt is enabled only for 60° before and after the zero crossing of the phase command voltage. This window of operation works effectively for most of the practical working conditions of the drive system.  
         [0032]    The conditioning circuitry  202  functions by converting the current flowing in the inverter phase leg  31  to a voltage signal using the current shunt resistor  204 . The first step in processing the signal is to buffer the signal from the resistor  204  and remove the high frequency PWM switching noise. In the illustrated circuit  202 , this is accomplished using an operational amplifier  212 , two resistors  214 ,  216 , and a capacitor  218  in an inverting opamp filter configuration. A PNP transistor  220  is inserted in the feedback path of the opamp  212  to differentiate between the positive portion of the phase current and the negative portion of the phase current. This is accomplished using the inherent base emitter diode in the PNP transistor  220 . When the voltage across the current shunt resistor  204  becomes positive, the voltage at the output of the opamp  212  is pulled down which turns off the transistor  220 . Alternatively, when the voltage across the current shunt resistor  204  becomes negative, the transistor  220  is turned on.  
         [0033]    An NPN transistor  222  translates the current flowing in the transistor  220  into a logic level voltage signal, which can be read by the controller  30 . When the PNP transistor  220  is ON, current flows in the base of transistor  222 , which turns it ON and produces a low-level signal at the input of the controller  30 . When transistor  220  is OFF, transistor  222  turns off which produces a high level signal at the input of the controller  30 . The final result of the circuit  202  is to turn the bipolar sinusoidal current waveform following in the shunt resistor  204  into a logic level signal with transitions at the current to zero crossing points.  
         [0034]    Estimation of power factor angle is the second step in implementing this algorithm. By definition, the power factor angle is the phase difference between the phase current and terminal voltage of the motor. Since the PWM algorithm accurately reproduces the commanded voltage, the generated phase angle in the controller is used in this computation. By using the voltage phase angle and the sensed zero crossing instant of the current, the power factor angle can be computed. Once the zero-cross angle of the current is sensed, the difference between this angle and the zero-cross angle of the voltage is computed to get the power factor angle. This power factor angle is low pass filtered to increase noise immunity and to avoid sudden changes in the command voltage of the motor. The filtering may be done by any acceptable means—the power factor angle is low pass filtered by software in one exemplary embodiment. This results in a cost effective control scheme for power factor angle estimation.  
         [0035]    The present invention described herein provides a load responsive variable speed induction motor drive system and method based on power factor angle sensing and slip control, thus maintaining near constant speed regulation without the use of a tachometer. Varying the input winding voltage helps in the efficient operation of the motor and also in maintaining control over the slip of the motor. Additional compensation to the frequency of the input motor voltage enables near constant slip for a varying load on the motor shaft.  
         [0036]    The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.