Electricity: motive power systems – Induction motor systems – Primary circuit control
Reexamination Certificate
1999-03-22
2001-06-05
Masih, Karen (Department: 2837)
Electricity: motive power systems
Induction motor systems
Primary circuit control
C318S801000, C318S807000, C318S524000, C318S496000, C318S503000, C318S727000
Reexamination Certificate
active
06242884
ABSTRACT:
FIELD OF THE INVENTION
This invention pertains generally to the field of electric motors and to drives for such motors, and particularly to induction machine drives.
BACKGROUND OF THE INVENTION
The use of a common magnetic structure which is shared by two sets of stator windings was first introduced in the late 1920s as a way to increase the power capability of large synchronous generators. See, P. L. Alger, et al., “Double Windings for Turbine Alternators, ” AIEE Transactions, Vol. 49, January, 1930, pp. 226-244. Since that time, dual stator machines have been used in many other applications. These include synchronous machines with AC and DC outputs. P. W. Franklin, “A Theoretical Study of the Three-Phase Salient Pole-Type Generator with Simultaneous AC and Bridge Rectified DC Output, ” IEEE Transactions on Power App. and Systems, Vol. PAS-92, No. 2, March/April 1973, pp. 543-557. Dual stator machines have also been used as current source inverters to large pumps, compressors and rolling mills driven by induction machines. T. Kataoka, et al., “Dynamic Control of a Current-Source Inverter/Double-Wound Synchronous Machine System for AC Power Supply, ” IEEE Transactions on Industry Applications, Vol. IA-17, No. , 3May/June 1981, pp. 314-320. Another purpose for the use of dual stators has been to improve reliability at the system level. See, e.g., J. R. Fu, et al., “Disturbance Free Operation of a Multiphase Current Regulated Motor Drive with an Open Phase, ” IEEE Transactions on Industry Applications, Vol. 30, No. 5, September/October 1994, pp. 1267-1274; J. C. Sahnon, et al., “A Split-Wound Induction Motor Design to Improve the Reliability of PWM Inverter Drives, ” IEEE Transactions on Industry Applications, Vol. IA-26, No. 1, January/February 1990, pp. 143-150.
Dual stator machines are normally constructed by “splitting ” the stator winding into two displaced but identical windings. See, e.g., E. F. Fuchs, et al., “Analysis of an Alternator with Two Displaced Stator Windings, ” IEEE Transactions on Power App. and Systems, Vol. PAS-93, No. 6, November/December 1974, pp. 1776-1786. However, splitting the stator winding in this manner results in mutual coupling between the stators, causing circulating harmonic currents. K. Gopakumar, et al., “Split-Phase Induction Motor Operation from PWM Voltage Source Inverter, ” IEEE Transactions on Industry Applications, Vol. 29, No. 5, September/October 1993, pp. 927-932. Such split stator winding machines have thus had a major drawback because the circulating currents add extra stator losses and demand larger semiconductor device ratings. In addition, there is coupling between the electromagnetic torques produced by each stator winding. See, T. A. Lipo, “A d-q Model for Six Phase Induction Machines, ” International Conference on Electric Machines, Athens, Greece, 1980, pp. 860-867.
SUMMARY OF THE INVENTION
A dual stator winding induction machine in accordance with the invention has two polyphase windings with input terminals available to be supplied separately with drive power. The two stator windings are wound with a different number of poles to essentially eliminate the magnetic coupling between the two stator windings and to decouple the torques produced by each set of stator windings. In addition, circulating harmonic currents encountered in conventional dual stator winding machines due to the so called mutual leakage coupling are completely eliminated. Since the output torque corresponds to the algebraic sum of two independent torques, the stator frequency is no longer determined uniquely by the rotor speed and the slip frequency, but also by the added variable of a second torque component, adding an additional degree of freedom to the system for greater control flexibility.
The dual stator winding machine supplied with power from two separate variable frequency inverter drives in accordance with the invention provides two independently controllable torque components, thereby allowing the low frequency operation of the machine —including at standstill —to be improved. Such a characteristic is particularly important for constant volts per hertz control operation at zero speed, where the influence of the stator resistance becomes dominant. In the present invention, zero speed operation does not require zero excitation frequency for the two power drives, thus significantly reducing the effect of the resistance voltage drop. The dual stator machine of the present invention can be built with minimal modifications to standard winding configurations, requiring no structural modifications of the stator frame. The provision of the two stator windings also increases the reliability of the machine over standard single stator winding machines, while improving the magnetic material utilization for normal operation.
A particular advantage of the present invention is the ability to drive the machine at zero and low speed operation without the need for a rotor position encoder to provide rotor position and speed feedback. Zero speed operation can be obtained by applying drive power to the two windings at a frequency and power level to provide opposing, balanced torques to the rotor. Implementation of sensorless vector control is thus facilitated since the drive power supplied to the two stator windings is always above zero frequency.
The stator of the machine of the present invention is constructed by dividing the normal single polyphase (typically three-phase) winding into two separate (e.g., three-phase) windings wound for a dissimilar number of poles. Although any combination of dissimilar pole numbers may be used, to best utilize the magnetic material, and avoid localized saturation and additional stator losses, in accordance with the invention it is preferred that a 2 pole and 6 pole combination be utilized. For best magnetic material utilization, this pole number configuration provides a nearly trapezoidal magnetomotive force (MMF) distribution while limiting the maximum number of poles to provide good power factor and efficiency.
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P.L. Alger, et al., “Double Windings for Turbine Alternators,” AIEE Transactions, vol. 49, Jan. 1930, pp. 226-244.
P.W. Franklin, “A Theoretical Study of the Three Phase Salient Pole Type Generator with Simultaneous AC and Bridge Rectified DC Output,” IEEE Transactions on Power App. and Systems, vol. PAS-92, No. 2, Mar./Apr. 1973, pp. 543-557.
J.R. Fu, et al., “Disturbance-Free Operation of a Multiphase Current-Regulated Motor Drive with an Opened Phase,” IEEE Transactions on Industry Applictions, vol. 30, No. 5, Sep./Oct. 1994, pp. 1267-1274.
J.C. Salmon, et al., “A Split-Wound Induction Motor Design to Improve the Reliability of PWM Inverter Drives,” IEEE Transactions on Industry Applications, vol. IA-26, No. 1, Jan./Feb. 1990, pp. 143-150.
E.F. Fuchs, et al., “Analysis of an Alternator with Two Displaced Stator Windings,” IEEE Transactions on Power App. and Systems, vol. PAS-93, No. 6, Nov./Dec. 1974, pp. 1776-1786.
T.A. Lipo, “A d-q Model for Six Phase Induction Machines,” International Conference on Electric Machines, Athens, Greece, 1980, pp. 860-867.
T.J. Jahns, “Improved Reliability in Solid-State AC Drives by Means of Multiple Independent Phase-Drive Units,” IEEE Transactions on Industry Applications, vol. IA-16, No. 3, May/Jun. 1980, pp. 321-331.
Lipo Thomas A.
Muñoz-Garcia Alfredo Rodolfo
Foley & Lardner
Masih Karen
Wisconsin Alumni Research Foundation
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