Non-intrusive speed sensing for induction motors
First Claim
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1. A method for measuring an operational rotor frequency of an induction motor comprising a rotor and a stator, comprising:
- analyzing a motor flux spectrum in a first vicinity of a specified frequency of the rotor to identify at least one identified rotor frequency candidate for the operational rotor frequency by identifying rotor current frequencies and harmonics in the first vicinity of the specified rotor frequency;
additionally analyzing the motor flux spectrum in a second vicinity of a specified frequency of the stator to derive at least one derived rotor frequency candidate for the operational rotor frequency by identifying and deriving fundamentals and sidebands of the stator in the second vicinity of the specified stator frequency; and
comparing the at least one identified rotor frequency candidate with the at least one derived rotor frequency candidate to determine the operational rotor frequency.
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Abstract
A sensing coil affixed to the outside of an induction motor housing uses flux signals from the motor to measure motor speed. In particular, components of the rotor current are measured and compared to analysis from stator fundamentals and sidebands to determine the true rotor frequency which can then be used to determine the motor speed. A shield and/or a compensating coil can optionally be used to filter out extraneous signal noise.
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Citations
17 Claims
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1. A method for measuring an operational rotor frequency of an induction motor comprising a rotor and a stator, comprising:
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analyzing a motor flux spectrum in a first vicinity of a specified frequency of the rotor to identify at least one identified rotor frequency candidate for the operational rotor frequency by identifying rotor current frequencies and harmonics in the first vicinity of the specified rotor frequency;
additionally analyzing the motor flux spectrum in a second vicinity of a specified frequency of the stator to derive at least one derived rotor frequency candidate for the operational rotor frequency by identifying and deriving fundamentals and sidebands of the stator in the second vicinity of the specified stator frequency; and
comparing the at least one identified rotor frequency candidate with the at least one derived rotor frequency candidate to determine the operational rotor frequency. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
placing a compensating coil substantially on an opposite side of the shield from the pickup coil to obtain an interfering flux signal; and
prior to analyzing the motor flux spectrum, using the interfering flux signal to remove the effect of the interfering flux from the motor flux signal.
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8. The method of claim 3, further comprising compensating for the effects of an externally interfering flux by:
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placing a compensating coil substantially in line with the pickup coil at an additional distance from the pickup coil to acquire an interfering flux signal, the additional distance selected such that the compensating coil links substantially none of the motor flux but does link the interfering flux; and
prior to analyzing the motor flux spectrum, using the interfering flux signal to remove the effect of the interfering flux from the motor flux signal.
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9. The method of claim 3, used for measuring the speed of an X-ray anode,
wherein attaching the pickup coil to the outside of the housing comprises affixing the pickup coil to an outside of a housing of an x-ray tube comprising the X-ray anode.
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10. An apparatus for determining an operational rotor frequency of an induction motor comprising a rotor and a stator, comprising:
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a pickup coil for acquiring a motor flux signal from the motor; and
a computer for;
analyzing a motor flux spectrum in a first vicinity of a specified frequency of the rotor to identify at least one identified rotor frequency candidate for the operational rotor frequency by identifying rotor current frequencies and harmonics in the first vicinity of the specified rotor frequency;
additionally analyzing the motor flux spectrum in a second vicinity of a specified frequency of the stator to derive at least one derived rotor frequency candidate for the operational rotor frequency by identifying and deriving fundamentals and sidebands of the stator in the second vicinity of the specified stator frequency; and
comparing the at least one identified rotor frequency candidate with the at least one derived rotor frequency candidate to determine the operational rotor frequency. - View Dependent Claims (11, 12, 13, 14, 15, 16)
a compensating coil placed substantially on an opposite side of the shield from the pickup coil to obtain an interfering flux signal;
whereinthe computer is adapted to use the interfering flux signal to remove the effect of the interfering flux from the motor flux signal.
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16. The apparatus of claim 11, further comprising:
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a compensating coil placed substantially in line with the pickup coil at an additional distance from the pickup coil to acquire an interfering flux signal, the additional distance selected such that the compensating coil links substantially none of the flux from the motor but does link the interfering flux; and
wherein the computer is adapted to use the interfering flux signal to remove the effect of the interfering flux from the motor flux signal.
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17. An apparatus comprising:
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an X-ray anode;
a motor including a rotor and a stator;
a housing enclosing the motor;
a pickup coil affixed to an outside of the housing for acquiring a motor flux signal from the motor; and
a computer for;
analyzing a motor flux spectrum in a first vicinity of a specified frequency of the rotor to identify at least one identified rotor frequency candidate for an operational rotor frequency by identifying rotor current frequencies and harmonics in the first vicinity of the specified rotor frequency;
additionally analyzing the motor flux spectrum from the motor in a second vicinity of a specified frequency of the stator to derive at least one derived rotor frequency candidate for the operational rotor frequency by identifying and deriving fundamentals and sidebands of the stator in the second vicinity of the specified stator frequency;
comparing the at least one identified rotor frequency candidate with the at least one derived rotor frequency candidate to determine the operational rotor frequency; and
using the operational rotor frequency to determine a motor speed.
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Specification