Abstract:
A pulse width modulation circuit ( 10 ) has a first set of windings with a first inverter circuit coupled thereto and a second set of windings having a second inverter circuit coupled thereto. A pulse width modulation controller pulse width modulates the first inverter circuit and the second inverter circuit so that the first inverter circuit has a predetermined phase shift therebetween to reduce input ripple.

Description:
TECHNICAL FIELD 
     The present invention relates generally to rotating electrical machines, and more particularly, to a method and circuit for reducing ripple current in a multiple inverter system. 
     BACKGROUND 
     Multiple electrical machines or a single electrical machine having two windings or more that are each connected to a separate inverter circuit are typically operated in a fixed manner. Oftentimes, the switching of the inverter circuits occurs simultaneously resulting in a high ripple current in the DC bus. The result of a high ripple current is that a large capacitor must be used to help reduce the ripple current. In automotive applications, the DC bus is often connected to a battery which is sensitive to ripple current. That is, if a high ripple current is present on the DC bus the life cycle of the battery may be reduced. Also, the durability of the capacitor coupled to the DC bus is also reduced by a high ripple current, forcing the use of a higher ripple current rated capacitor which increases its cost. A high ripple current also increases the temperature of the components on the DC bus and increases the amount of electromagnetic interference generated by the circuit. 
     Particularly in automotive applications in which the numbers of units produced is extremely high, it is desirable to reduce the costs of components. The capacitors used to reduce ripple current in drive circuits are expensive components. The expense increases as the size of the capacitor increases. Therefore, it would be desirable to reduce ripple current and therefore reduce the size of the capacitor to decrease the cost of the circuit. 
     Known systems for reducing ripple current include operating the first inverter and a second inverter to cancel harmonics. Oftentimes the systems are operated with transformers that require a minimum inverter frequency at a maximum voltage. Therefore, one example of a phase displaced multiple inverter bridge circuit with waveform notching is described in U.S. Pat. No. 5,168,473. However, such a system is operated with a fixed phase difference between the inverters. 
     Because operating conditions, particularly in an automotive application, are continually varying, it would also be desirable to provide a system that varies the phase difference between the switching of the inverter circuits. 
     SUMMARY OF THE INVENTION 
     It is therefore one object of the invention to provide a circuit for controlling an electrical machine that can vary the phase between the switching of the inverters in response to the varying operating conditions of the circuit. 
     In one aspect of the invention, a circuit for controlling an electrical machine has a first set of windings with a first inverter circuit coupled thereto and a second set of windings having a second inverter circuit coupled thereto. A pulse width modulation controller pulse width modulates the first inverter circuit and the second inverter circuit so that the first inverter circuit has a predetermined phase shift therebetween to reduce output ripple. 
     One feature of the invention is that the pulse width modulating controller varies the phase difference between the first inverter circuit and the second inverter circuit in response to an operating condition of the electrical machine. 
     In a further aspect of the invention, a method for operating an electrical machine comprises the steps of pulse width modulating a first inverter circuit to have a first electrical phase angle; 
     pulse width modulating a second inverter circuit to have a second electrical phase angle; 
     sensing an operating condition of the electrical machine; 
     controlling the steps of pulse width modulating a first inverter circuit and pulse width modulating a second inverter circuit to reduce a ripple current in response to the predetermined operating condition. 
     One advantage of the invention is that the teachings of the present invention may be applied to control circuits for electrical machines that have more than two sets of windings and inverters. That is, three or more sets of windings and inverters may be simultaneously controlled by a pulse width modulating controller to reduce ripple current in the DC bus capacitors and associated components. 
     Other objects and features of the present invention will become apparent when viewed in light of the detailed description of the preferred embodiment when taken in conjunction with the attached drawings and appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic view of an embodiment of a control circuit for an electrical machine according to the present invention. 
     FIGS. 2A,  2 B, and  2 C are respective plots of two inverter switching signals without phase shift and the resultant capacitor current and battery current without a phase shift in the inverter circuit. 
     FIGS. 3A,  3 B, and  3 C are respective current plots of two inverter switching signals with 90 degrees phase shift and the resultant capacitor current and battery current with a 90 degrees phase shift in the inverter circuit. 
     FIGS. 4A,  4 B,  4 C, and  4 D illustrate a capacitor current comparison between a respective single inverter circuit, a dual inverter circuit with no phase shift, a dual inverter circuit with a 45 degrees phase shift, and a dual inverter circuit with a 90 degrees phase shift. 
     FIGS. 5A,  5 B,  5 C, and  5 D illustrate a battery current comparison between a respective single inverter circuit, dual inverter circuit with no phase shift, a dual inverter circuit with a 45 degrees phase shift, and a dual inverter circuit with a 90 degrees phase shift. 
     FIGS. 6A and 6B are plots of battery current with a capacitor typical of a system capacitance. 
     FIGS. 7A and 7B are plots of capacitor current using a larger capacitor similar to that in a typical system. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring now to FIG. 1, the following description is provided with respect to a two winding circuit having two associated inverter circuits coupled thereto. Those skilled in the art will recognize that more than two winding circuits and inverter circuits may be used following the teachings of the present invention. Also, the present invention applies equally to two or more electrical machines each with a set of windings coupled to the same direct current bus. 
     Referring now to FIG. 1, a circuit for controlling an electrical machine generally represented by reference numeral  12  is illustrated. The electrical machine has two sets of windings  14 ,  16 , each of which have three phases illustrated. The first set of windings  14  has three phases A 1 , B 1 , and C 1 . The second set of windings  16  has three windings A 2 , B 2 , and C 2 . The first set of windings  14  and the second set of windings  16  are illustrated coupled in a wye formation. However, the present invention is equally applicable to various types of configurations including both delta, or one delta one wye, or other combinations with more than three sets of windings. 
     Circuit  10  further includes a first inverter  18  and a second inverter  20 . Each inverter has a plurality of switches that are used to convert a DC input into an AC output in a conventional manner. Inverter circuits  18 ,  20  are coupled to a DC bus  22  that is coupled to a battery  24  and a capacitor  26 . First inverter  18  has a first set of switches a 11  and a 10  coupled to first phase of the first set of windings  14 . The first inverter  18  also has a second set of switches b 11  and b 10  and a third set of switches c 11  and c 10  coupled to a respective second and third phase of the first set of windings  14 . As will be evident to those skilled in the art, preferably the circuit  10  has inverter circuits which are equivalent and machine windings that are also equivalent. 
     Likewise, the second inverter circuit  20  has a first set of switches a 21  and a 20 , a second set of switches b 21 , b 20 , and a third set of switches c 21 , c 20  coupled to a respective first phase A 2 , a second phase B 2 , and a third phase C 2 . 
     Circuit  10  also includes a pulse width modulating controller  28  that is used to control the switching of the various switches of the first inverter  18  and the second inverter  20 . As illustrated, pulse width modulating controller  28  has outputs  30  that are generally labeled with the switches that are controlled thereby. Each of the outputs  30  are coupled to the switches, but for simplicity, the actual direct connections have been removed. Pulse width modulating controller  28  has voltage command inputs  32 . Voltage command inputs  32  receive the desired voltage for the various windings. Pulse width modulating controller  28  controls the operation of switches in response to the voltage command inputs to provide the desired voltage at the windings. 
     Pulse width modulating controller  28  may also include an “other” input  34 . “Other” input  34  is illustrated as a separate input, and the function of this may be incorporated into voltage command input  32 . Other inputs  34  are used to represent that other inputs may be used to control the pulse width modulation of the various circuits. Other inputs  34  may include other operating current parameters of the vehicle to which it is attached or to the operating conditions of the circuit. Other inputs  34  may correspond to, for example, the amount of voltage or the magnitude of voltage at the voltage command input  32 , the average magnitude of current flowing on the DC bus circuit  22  from the battery  24 , or the magnitude of the phase currents in the first set of windings  14  or the second set of windings  16 . 
     The pulse width modulating controller  28  generates pulse width modulated signals for all inverters and is capable of setting phase differences among the pulse width modulating signals to the inverters. The pulse width modulating signals can be generated by well-known sine-triangle method, space vector techniques, or any other means. A phase shift between first inverter  18  and second inverter  20  means that the relative shift of the control signals sent from the pulse width modulating controller to the corresponding switch in the first inverter is shifted by an amount relative to the corresponding switch of the second inverter  20 . By providing the relative phase delay, the ripple current on the DC bus  22  is reduced which allows the potential for reducing the size of capacitor  26 . 
     In operation, voltage command inputs  32  are provided to the pulse width modulating controller which in conjunction with optional other inputs  34  controls the pulse width modulation for each of the switches connected to the first set of windings  14  and the second set of windings  16 . A relative phase difference between the switching of the switches connected to the first set of windings  14  and the second set of windings  16  is controlled by the pulse width modulation controller  28 . By controlling the phase difference, the ripple current is reduced on the DC bus  22 . 
     Referring now to FIGS. 2A-2C and FIGS. 3A-3C, an inverter is operated with no phase shift as is commonly done in the prior art. As illustrated, the corresponding capacitor current in FIG.  2 B and the corresponding battery current in FIG. 2C is relatively large when compared to FIG. 3 in which the switches of the first inverter  18  and the switches of the second inverter  20  are operated with a 90 degrees phase shift. 
     Referring now to FIGS. 4 and 5, respective simulation results based on a dual inverter system shown in FIG. 1 has been plotted with respect to respective capacitor current ripple and battery current ripple. In FIGS. 4A and 5A, a single inverter system is used for comparison with a no phase shift system in FIGS. 4B and 5B. A system according to the present invention is illustrated in FIGS. 4C and 4D and FIGS. 5C and 5D. As can be seen, the voltage ripples of FIGS. 4C and 4D and FIGS. 5C and 5D have been significantly reduced over the ripples of FIGS. 4A and 4B and FIGS. 5A and 5B. 
     Referring now to FIG. 6, the battery current for a system with no phase shift is compared to a system with a 90 degrees phase shift such as that shown in FIG.  1 . In FIG. 6, the capacitor was sized to be about 0.01 Farads to simulate a capacitor of the size typically used in such systems. As can be seen, the variation about the averages of each of the signals is lower in the system shown in FIG. 6B corresponding to the present invention. 
     Referring now to FIG. 7, a capacitor current is shown in FIGS. 7A and 7B in a similar manner to those of FIG. 6 relative to battery current. 
     While particular embodiments of the invention have been shown and described, numerous variations alternate embodiments will occur to those skilled in the art. Accordingly, it is intended that the invention be limited only in terms of the appended claims.