1. Field of the Invention
The present invention relates generally to motor current feedback measurements, and relates more specifically to a computational reconstruction of motor current obtained through measurement of a DC bus current.
2. Description of Related Art
Inverters for three phase motor drives are well known in the industry. Typically, a DC bus supplies switched power to different phases of an AC motor. A design approach used to supply switching commands and sequences to the inverter involves the use of space vector modulation. For example, a switch vector plane is illustrated in
With this type of motor control, it is desirable to accurately measure motor phase current to provide a high performance control. However, it is often difficult to accurately measure motor phase current over wide current and temperature ranges. For example, Hall effect sensors can be used, but are inherently bulky and costly. In a pulse width modulated (PWM) inverter drive system, motor phase current can be determined from measurement of the DC bus current when non-zero basic vectors are used. Each basic vector is assigned a specific time in a PWM cycle to generate the command voltage vector. However, if a basic vector is used only for a very short period of time, motor phase current cannot be directly determined from the DC bus current. This lack of observability of motor phase current is due to practical considerations in the implementation of the PWM inverter drive system. For example, time delays caused by A/D converter sample and hold times, slewing of voltage during turn on, and other delay factors prevent the effects of basic vectors used for a very short time from being observed.
In the space vector plane shown in
The present invention provides an algorithm for the reconstruction of motor currents from measurement of DC bus current. The non-observable operation of motor phase current is restricted to a much smaller domain with the use of the reconstructing algorithm. 2-phase space vector modulation permits the minimum time for an observable effect of a basic vector to be decreased. In practical application, the time constraint related to non-observability is cut in half. By reducing this minimum time, the available time for measurement of phase current according to this technique is doubled. When the voltage vector angle is large than 30°, the zero vector 111 is used instead of 000. By using the different zero vector, a switched phase pulse time is maximized.
When 2-phase space vector modulation is used, and motor current exceeds a certain threshold level, dead time need not be inserted and the time constraint can be further reduced.
When command voltage falls inside the non-observable domains, the command voltage vector is formed from two vectors generated in two PWM periods. One generated vector is a voltage vector having a phase equal to 30° and a magnitude equal to two times the width of the non-observable section. Use of this vector insures the observability of two of the three motor phase currents. The second vector is added to form the resulting command voltage vector that falls inside the non-observable domain. The time average of the two combined vectors are equal to the time average of the command voltage vector. Using the combination of the two vectors permits the controller execution cycle to be reduced by half.
The present invention is described in detail below, with reference to the accompanying drawings having appropriate reference numeral designators, in which:
The present invention provides an algorithm to reconstruct 3-phase motor current information from measurement of a DC bus current supply. A voltage space vector plane 10, as illustrated in
Referring to
In a two level PWM inverter drive system, eight possible basic voltage vectors can be produced, and any desired command voltage vector can be formed by the eight basic voltage vectors. The desired command voltage vector is limited by the maximum output voltage of the inverter, as 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 basic vectors are used. Each basic vector is assigned a specific time in a PWM cycle to generate a command voltage vector. If the command voltage vector is used only for a very short period of time, the motor current cannot be observed from the DC bus current. The shortness of this time constraint results from time delays associated with A/D conversion, including sample and hold times, in addition to voltage slewing resulting from device turn on. It is this time constraint that forms the non-observable regions in the voltage space vector plane illustrated in
Referring again to
Referring now to
With the use of a 2-phase voltage space vector modulation, a further reduction in the time constraint can be achieved. When motor current is higher than a given threshold, the need to insert dead time is eliminated. Typically, the time constraint can be written as a minimum time Tmin as follows:
Accordingly, when Td is equal to zero, Tmin is reduced accordingly.
Referring now to
Referring now to
Although the present invention has been described in relation to particular embodiments thereof, many other variations and modifications and other uses will become apparent to those skilled in the art. It is preferred, therefore, that the present invention be limited not by the specific disclosure herein, but only by the appended claims.
The application is based on and claims benefit of U.S. Provisional Application No. 60/368,860, filed on Mar. 28, 2002, entitled Motor Current Reconstruction Via DC Bus Current Measurement, to which a claim of priority is hereby made.
Number | Date | Country | |
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60368860 | Mar 2002 | US |