Electricity: measuring and testing – Electrical speed measuring – Including motor current or voltage sensor
Reexamination Certificate
1998-11-02
2001-03-27
Patidar, Jay (Department: 2862)
Electricity: measuring and testing
Electrical speed measuring
Including motor current or voltage sensor
C324S163000, C324S173000, C378S094000
Reexamination Certificate
active
06208132
ABSTRACT:
BACKGROUND
This invention relates generally to measuring a motor speed for an induction motor, and specifically for measuring motor speed using a sensing coil to determine a rotor frequency.
The rotor frequency is used with a stator frequency to calculate the motor speed. In a two pole motor, for example, such as used for X-ray tubes, the speed (in rotations per minute) is equal to sixty times the difference between the stator and rotor frequencies (in hertz). The stator frequency is easily obtainable from the external leakage flux or the stator current spectrum. The determination of rotor frequency has been more difficult.
Flux sensing coils used to measure rotor frequency must often be placed far from the motor, sometimes with metal casings intervening. Additionally, if the motor is mounted on a rotating platform, the Earth's field may give rise to confusion in the signals.
Flux sensing coils have been used to measure motor speed via rotor leakage flux, as described, for example, in commonly-assigned U.S. Pat. Nos. 5,049,815 and 4,761,703; in other U.S. Pat. Nos. 534,596, 3,943,356, 4,839,585, and 5,530,343; in J. Penman et al., “Condition monitoring of electrical drives,” IEEE Proceedings, Vol. 133, Part B, No. 3, May 1986; and
M. S. Erlicki et al., “Leakage Field Changes of an Induction Motor and Indication of Nonsymmetric Supply,” IEEE Transactions on Industry and General Applications, vol. IGA-7, no. 6, November/December 1971. But these approaches typically require intrusive mounting of the sensing coil inside the motor housing, or are prone to errors and external signal interference due to their methods of measuring speed.
SUMMARY OF THE INVENTION
It is therefore desirable to non-intrusively mount a flux sensing coil on the outside of a motor housing, and to obtain a reliable speed determination therefrom despite weak signals and interference.
Briefly, according to one embodiment of the invention, a method for measuring an operational rotor frequency of an induction motor including a rotor and a stator comprises: analyzing a motor flux spectrum in a first vicinity of a specified frequency of the rotor to identify at least one 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 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.
REFERENCES:
patent: 534596 (1895-02-01), Atwood et al.
patent: 3943356 (1976-03-01), Schmutzer et al.
patent: 4761703 (1988-08-01), Kliman et al.
patent: 4839585 (1989-06-01), Bicknell
patent: 5049815 (1991-09-01), Kliman
patent: 5530343 (1996-06-01), Bowers, III et al.
patent: 5828210 (1998-10-01), Kliman et al.
J. Penman et al., “Condition monitoring or electrical drives,” IEE Proceedings, vol. 133, Part B, No. 3, May 1986.
M. S. Erlicki et al., “Leakage Field Changes of an Induction Motor and Indication of Nonsymmetric Supply,” IEEE Transactions on Industry and General Applications, vol. IGA-7, No. 6, Nov./Dec. 1971.
Kliman Gerald Burt
Mallick John Andrew
General Electric Company
Patidar Jay
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