KR100638866B1 - Motor current reconstruction via dc bus current measurement - Google Patents

Abstract

Techniques for reproducing phase currents based on measurement of DC bus current are disclosed. By using two-phase space vector modulation, the size of the unobservable regions of DC bus current samples for reproducing phase currents is reduced. These unobservable regions can be further reduced by omitting the insertion of dead time for switch actuation when the output current is greater than a predetermined threshold. In order to obtain an observable DC bus current indicative of the phase current, the voltage command vectors of the unobservable regions can be formed by two additional vectors having different phases and magnitudes. These additional vectors have the same combined time average value as the voltage command vector.

DC bus current, motor current, voltage command vector, unobservable region, dead time

Description

Reproduction of motor current by DC bus current measurement {MOTOR CURRENT RECONSTRUCTION VIA DC BUS CURRENT MEASUREMENT}

Related Applications

This application is based on US Provisional Application No. 60 / 368,860, filed Mar. 28, 2002, entitled " Motor Current Reconstruction Via DC Bus Current Measurement. &Quot; It is to claim its priority.

BACKGROUND OF THE INVENTION The present invention relates generally to motor current feedback measurements, and more particularly to computational reconstruction through calculation of motor current obtained through measurement of DC bus current.

Inverters for driving three-phase motors are well known in the art. Typically, the DC bus supplies switched power to different phases of the AC motor. Design efforts used to supply switching commands and sequences to the inverter require the use of spatial vector modulation. For example, FIG. 1 shows a switch vector plane, where certain switch states appear at the vertices of the hexagon.

Using this type of motor control, it is desirable to accurately measure the motor phase current in order to provide high performance control. However, it is often difficult to accurately measure motor phase current over a wide current and temperature range. For example, Hall effect sensors can be used, but they are inherently bulky and expensive. In a pulse width modulated (PWM) inverter drive system, the motor phase current can be determined by measuring the DC bus current when non-zero basis vectors are used. To generate a command voltage vector, each base vector is assigned a specific time of PWM period. However, if the base vector is only used for a very short time period, the motor phase current cannot be determined directly from the DC bus current. This lack of observability of motor phase current is due to practical considerations in the implementation of a PWM inverter drive system. For example, the time delay caused by the A / D converter sample hold time, voltage slewing during turn-on and other delay factors make it impossible to observe the effects of the fundamental vectors used for a very short time. .

In the space vector plane shown in FIG. 1A, the unobservable regions are shown as located along the edges of the sections of the space vector plane. If motor phase currents cannot be observed during their control periods, it is difficult to achieve robust high performance motor drive.

The present invention provides an algorithm for reproducing motor currents from the measurement of DC bus current. Unobservable behavior of the motor phase current is limited to a much smaller domain using a reproducing algorithm. Two-phase spatial vector modulation can reduce the minimum time for the observable effect of the base vector. In practical applications, the time constraints related to unobservability are cut in half. By reducing this minimum time, the time available for measuring phase current in accordance with the present technology is doubled. If the voltage vector angle is greater than 30 ^° , zero vector 111 is used instead of 000. By using different zero vectors, the phase pulse time to be switched is maximized.

If two-phase space vector modulation is used and the motor current exceeds a certain threshold level, dead time does not need to be inserted and time constraints can be further reduced.

If the command voltage is in the unobservable region, the command voltage vector is formed from two vectors generated in two PWM periods. One vector generated is a voltage vector with a magnitude of 30 ^O and twice the width of the unobservable section. Using this vector ensures observability of two of the three motor phase currents. The second vector is added to form the resulting command voltage vector that is within the unobservable region. The time average of the two combined vectors is equal to the time average of the command voltage vector. By using a combination of two vectors, the execution cycle of the controller can be cut in half.

The present invention will now be described in detail with reference to the accompanying drawings, with appropriate reference signs.

1A shows a voltage space vector plane with typical unobservable regions.

1B shows a modified voltage space vector plane according to the present invention.

2 shows reduced unobservable regions through two phase commutation.

3 illustrates a current sampling technique in accordance with the present invention.

4 shows the insertion of a known voltage vector to obtain a reference command voltage vector.

The present invention provides an algorithm for reproducing three-phase motor current information from measurements of a DC bus current supply. As shown in FIG. 1B, the voltage space vector plane 10 includes unobservable regions near the edges of the sector. According to the invention, the voltage space vector plane 11 has reduced unobservable regions.

Referring to FIG. 2A, a conventional three phase inverter provides three phase voltage space vector PWM modulation. In FIG. 2B, the same command voltage can be formed using the two phase voltage space vector PWM, as shown in the rectification diagram 21. With two-phase vector modulation, the minimum time constraint on unobservable is halved.

In a two level PWM inverter drive system, eight possible base voltage vectors can be generated, and these eight base voltage vectors can form any desired command voltage vector. This preferred command voltage vector is limited by the maximum output voltage of the inverter, which is determined by the DC bus voltage level. In a PWM inverter drive system, motor phase current information can be determined from the DC bus current when non-zero basis vectors are used. Each base vector is assigned a specific time of PWM period to generate the command voltage vector. If the command voltage vector is only used for a very short time period, the motor current cannot be observed from the DC bus current. This shortening of time constraints results from the time delays associated with A / D conversion, including sample and hold times, in addition to voltage distortion resulting from device turn-on. This time constraint forms unobservable regions in the voltage space vector plane shown in FIG.

Referring again to FIG. 2A, the command voltage vector appears in the unobservable region of sector 1. In this case, all three phases of the motor are PWM driven in the PWM period T _pwm . The total time T2 occurs at two different points in the PWM period T _pwm .

Referring now to FIG. 2B, the unobservable area is reduced by using two-phase voltage space vector modulation. The same command voltage as that formed in the system of FIG. 2A can be formed by the two-phase PWM of FIG. 2B. However, in two phase voltage space vector modulation, the unobservable time constraint period is cut in half. The total T2 time is locally limited at one point. Thus, the effective time for measuring current is doubled. If the voltage angle is greater than 30 ^o , zero vector 111 is used instead of zero vector 000 to maximize T1 time.

By using two-phase voltage space vector modulation, time constraints can be further reduced. If the motor current is larger than a predetermined threshold, there is no need to insert a dead time. Typically, the time constraint can be written as the minimum time T _min as follows:

Therefore, if Td is zero, T _min is reduced accordingly.

Referring now to FIG. 3, the DC bus current is sampled three times every PWM period. In FIG. 3, samples idc1 and idc3 are taken from the same motor phase, but they are taken at different time instances. Current samples idc1 and idc3 are calculated based on the equations presented in FIG. 3, providing timing synchronization with sample idc2.

Diagram 40 of FIG. 4 shows the insertion of a known voltage vector. The insertion of this voltage vector is used to form the command voltage vector V _ref . If the command voltage is within unobservable sector bands, the command voltage vector is formed by two vectors generated in two PWM periods. The voltage vector V1 has a magnitude of 30 ^o and twice the width A of the unobservable sector. By forming the vector V1 in accordance with these constraints, the observation of the two motor phase currents is ensured. Vector V2 is added to vector V1 to form the resulting command voltage vector V _ref . The time average of V1 plus V2 is equal to the time average of V _ref as shown in FIG. 4. By forming the command voltage vector with these vectors V1 and V2, the execution period of the controller is reduced by half when the command voltage enters the unobservable sector.

While the present invention has been described with reference to specific embodiments, many other variations, modifications, and other applications will be apparent to those skilled in the art. Accordingly, the invention is not limited to the forms disclosed in the foregoing patent application, but is defined only by the appended claims.

Claims (20)

In a method of reproducing a motor phase current from a DC bus current on a DC bus of a PWM inverter motor drive system, Forming a command voltage vector in accordance with a space vector modulation configuration for controlling the motor; Measuring the DC bus current on the DC bus that powers the inverter to obtain an indication of motor current; Determining when the command voltage vector is in an inverter switching state such that the measurement of the DC bus current does not accurately represent the motor phase current; Forming a voltage vector sum from two additional vectors, wherein the vector sum is substantially equal to the command voltage vector, and at least one of the additional vectors indicates that the measurement of the DC bus current indicates the motor phase current. Indicating a switching state to be present; Applying the at least one additional vector to the space vector modulation in a switching period of the PWM inverter drive system; And And applying another additional vector to the space vector modulation in a subsequent switching cycle of the PWM inverter drive system, such that the time average of the additional vector is equal to the command voltage vector. How to. In the method of determining the phase current of the motor by measuring the DC bus current supplied to the inverter of the state vector motor drive system, Determining when the command voltage vector is located in an unobservable sector of a voltage space vector plane defining an inverter switching state such that an accurate measurement of the phase current cannot be obtained by measuring the DC bus current; Applying a first voltage vector having an angle and magnitude to the state vector motor drive system such that the first voltage vector is outside of the unobservable sector; Applying a second voltage vector having a phase and magnitude to the state vector motor drive system to derive the command voltage vector from a time averaged sum of the first and second voltage vectors; And Measuring the DC bus current during a time period during which at least one of the first and second voltage vectors is applied to the state vector motor drive system to provide an indication of the motor phase current. How to determine the current. The method of claim 2, Wherein the first voltage vector has a phase of 30 ^° +/− N × 60 ^° and wherein N = 0,1, ..., 5. In the method of obtaining the measurement of the phase output current from the power inverter by measuring the supplied DC bus current, Determining when the phase current from the power inverter becomes unobservable by the measurement of the DC bus current by forming a command voltage vector in the inverter; Applying a first voltage vector to the power inverter such that the phase current is observable by measurement of the DC bus current; And Applying a second voltage vector to the inverter for coupling with the first voltage vector in a time average sum such that the result is approximately equal to the time average of the command voltage vector. How to get a measurement of the output current. In a method for reproducing a motor phase current in an inverter drive system for a motor, Measuring a current applied to a DC bus powering an inverter to obtain an indication of the electrical phase current of the motor; Determining when three-phase PWM inverter modulation causes motor switching that makes the motor phase current undeterminable through measurement of the DC bus current; And Dynamically changing from the three-phase PWM modulation to a two-phase PWM modulation during the switching period of the inverter drive system to reduce the time interval during which the motor phase current cannot be measured by measuring the DC bus current. A method for reproducing a motor phase current, characterized in that. The method of claim 5, The step of changing from the three-phase PWM modulation to two-phase PWM modulation comprises: from three phase phase modulation during any switching period in which the motor phase current is determined to include a switching event that cannot be determined through measurement of the DC bus current. A method of reproducing a motor phase current comprising changing to phase modulation. The method of claim 5, Changing from three phase phase modulation to two phase phase modulation when the motor is operating at a relatively high speed. The method of claim 5, Removing the insertion of dead time when the motor current is greater than a predetermined threshold, thereby reducing the time interval at which the motor phase current cannot be determined through the measurement of the DC bus current. How to reproduce. The method of claim 5, And using an alternating zero vector when the voltage angle is greater than 30 ^o to maximize the time for current measurement. The method of claim 9, And a zero vector 111 is used as the alternating zero vector. A method of reproducing a motor phase current based on a DC bus current measurement in an inverter motor drive system having a PWM period, Sampling the DC bus current at least twice during the PWM period; Determining an average of the two samples; And Calculating a motor phase current using the average. The method of claim 11, And said inverter motor drive system performs three-phase PWM modulation. The method of claim 11, And said inverter motor drive system performs two-phase PWM modulation. The method of claim 11, Sampling the DC bus current at least three times during each PWM period. The method of claim 14, Average of two of said samples to determine a one-phase current measurement and a third sample to determine another phase current measurement. A space vector PWM motor drive system operating according to the method of claim 1. A space vector PWM motor drive system operating according to the method of claim 2. A space vector PWM motor drive system operating according to the method of claim 4. A space vector PWM motor drive system operating according to the method of claim 5. A space vector PWM motor drive system operating according to the method of claim 11.

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