Patent Publication Number: US-6037686-A

Title: Current compensated choke filter for multi-phase motor drives

Description:
TECHNICAL FIELD 
     This invention relates to a choke filter which separates the main current flow through a multi-phase motor from the motor leakage currents by leaving only flux created by the leakage current to induce inductive reactance that blocks the leakage current from motor current measuring devices. 
     BACKGROUND ART 
     Elevators of moderate speed, typically used in medium height buildings, are conventionally geared machines. The ripple in torque which may be created by leakage currents in the motor is fairly well damped out by the effect of the gears, which not only reduce the physical effect on the cab through the gear ratio, but also have a significant damping effect. However, the same is not true in higher speed gearless machines. It has been found that the leakage currents resulting from inevitable ground capacitance of each of the phases of a multi-phase motor driven by a fast switching frequency inverter introduce errors into the measurement of current of each phase, which is used to control the motor drive. This systematic current error can cause torque ripple at low, mechanically-relevant frequencies, thus leading to considerable elevator vibration. 
     DISCLOSURE OF INVENTION 
     Objects of the invention include elimination of current measurement errors in the control of multi-phase motors; reduction of vibration in gearless elevators; reduction of torque ripple at low, mechanically-relevant frequencies in systems driven by multi-phase motors in response to fast-switching frequency inverters; and improved compensation for multi-phase motor leakage currents. 
     This invention is predicated on the concept that the sum of the torque-producing phase currents flowing into a multi-phase motor must equal zero, whereas the leakage currents flowing into the motor return to the source via a ground path. 
     According to the invention, the torque-producing current is separated from the leakage current in each phase by using a current-compensated choke in the measurement filter; the choke has an equally proportioned winding for each phase on a highly permeable ring core, each phase having its own core as part of a leakage current filter, the currents of all the phases and therefore the corresponding flux induced in the cores, all sum to zero. 
     According to the present invention, a filter using a qualitative difference between the torque-producing phase currents of the motor and the capacitative leakage currents allows measurement of the torque-producing currents without pollution by the leakage currents, which are bypassed. 
     With the invention, the magnetic flux induced in the core by torque-producing phase current flowing through one phase of the motor is counteracted by flux induced in the core of the current flowing through the other phases of the motor. The resulting flux generated by torque-producing phase currents is always zero, so the inductance of the coil is very low for the torque-producing currents. In contrast, the leakage currents, being not counteracted by any adverse currents in the coil, cause large flux variations in the core. Thus, the coil presents a large inductive reactance to the leakage currents, thereby substantially blocking the leakage currents. The resulting filter allows measuring the torque-producing current with essentially no pollution by leakage current. 
     The invention is more effective than a filter using only frequency as a criterion for separation of the currents. The invention responds to relatively small, easy to handle currents, and therefore is smaller and cheaper than primary-side filters. The invention is less expensive and more effective than the reduction of leakage capacitances in the motor which could be affected by improvements in motor design and manufacture. 
     When used in gearless elevator motor systems, current feedback control is improved by eliminating errors caused by leakage current in the measurements, thereby significantly reducing vibrations and improving the quality of the ride. 
     Other objects, features and advantages of the present invention will become more apparent in the light of the following detailed description of exemplary embodiments thereof, as illustrated in the accompanying drawing. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a view of a current compensated choke according to the invention. 
     FIG. 2 is a schematic block diagram of a measurement circuit connected to a three-phase motor drive system utilizing filters employing the choke of FIG. 1. 
     FIG. 3 is a partial alternative to the diagram of FIG. 2, illustrating windings on the cores of the chokes. 
    
    
     BEST MODE FOR CARRYING OUT THE INVENTION 
     Referring to FIG. 1, a choke 4 of the present invention comprises a highly permeable magnetic ring core 5 having three identical windings 10-12 thereon. The choke 4, as applied to one phase of a three-phase motor, would have currents for each of the phases lu, lv, lw applied to corresponding coils 10-12 as shown. The sum of the currents lu, lv, lw, is zero, as is known. Therefore, the flux generated by the current lu in coil 10, illustrated by the arrow 14, is compensated for by the summation of the flux created by the current lv in the coil 11, represented by the arrow 15, with the flux created by the current lw in the coil 12, represented by the arrow 16. The arrows 14-16 indicate polarity of flux in response to currents flowing in the direction of the arrows within the coils 10-12. At any point in time, the current in one coil is generally of opposite polarity to the currents in the other two coils, and the vector summation of currents in the three coils 10-12 is zero. Thus, the net flux in the core 5 as a consequence of the torque-producing currents is zero. However, the leakage current to ground within the motor, resulting from lu through the coil 10 will return to the drive circuitry through ground (rather than through the other coils) so it will produce an uncompensated flux illustrated by the arrow 18. Since the net flux represented by arrows 14-16 is zero, the core 5 will be highly permeable (totally unsaturated) to the leakage currents, thereby highly magnetically responsive thereto. As a consequence, the time varying leakage currents will induce significant inductive reactance within the coil 10 from the resulting inductance of the highly permeable core 5. 
     The choke 4 therefore represents a selective choke, being essentially invisible (except for small ohmic losses) to the torque-producing currents, but having a large retardant effect on the leakage currents. 
     Referring to the measurement apparatus of FIG. 2, for each of the three phases U, V, W, a current lu, lv, lw is provided by a corresponding, conventional current-to-current transducer 19, such as a conventional LEM transducer, to decouple the measurement apparatus from the motor drive circuit. The current provided to the measurement apparatus is a low current proportional to the motor phase current provided to the three phase motor 20 by a three phase motor drive 21. Each phase has a filter 26, adjacent to a current measuring device, such as a measuring resistor 25, which includes a current compensated choke 4 of the invention and a series resistor/capacitor shunt 22 to ground which provides a current path for the leakage currents that are blocked by the choke 4. As seen in FIG. 3, on each of the chokes 4, there is a winding Uu, Vv, Ww for a corresponding phase, and a winding Uv, Uw; Vu, Vw; and Wu, Wv for each of the other phases. Thus the chokes 4 are provided windings for the phase currents as described hereinbefore with respect to FIG. 1. Therefore, each of the chokes 4 is invisible (essentially) to all of the currents corresponding to phase currents, since all of the flux resulting from the phase currents, that is, the flux corresponding to the torque-producing currents, is cancelled out. On the other hand, in each of the chokes 4, the flux resulting from motor leakage currents (shown by arrow 18 in FIG. 1) is significant, thereby generating significant inductive reactance so that the leakage currents are shunted to ground through the shunts 22 around the measuring resistors 25 so the leakage currents cannot affect the voltages eu, ev, ew across the measuring resistors 25. 
     Due to the selective properties of the current-compensated choke, the cutoff frequency for the torque-producing currents in the measurement branch is much higher (beyond any frequencies utilized to drive motors) than is the cut-off frequency for the leakage currents. As a result, the torque-producing phase currents to a three-phase motor 20 may be measured by corresponding measuring resistors 24 without any significant measurement components attributable to leakage currents. 
     In the embodiment of FIG. 2, the filters 26 are connected to the three-phase motor drive 20 by LEMs 19. The LEMs 19 not only allow use of low-current capacity measuring apparatus, but also as current sources, stabilizing the current in the choke coils of each phase, so peak values of leakage current blocked by the choke in any phase does not couple into the other phases. 
     In the embodiments herein, the core 5 is shown as a circular toroid; however, it could be a square or other shaped toroid, and its cross-sectional configuration is essentially irrelevant to the invention. Thus, the parameters of the core 5 can be chosen to suit any implementation of the present invention, so long as it has high permeability. The current between each choke and ground can be measured by another suitable measuring device, such as a Hall device, rather than measuring the voltage across the measuring resistors 25. The invention may be used to measure current in less than all of the phases of a multi-phase motor. 
     Thus, although the invention has been shown and described with respect to exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without departing from the spirit and scope of the invention.