Electricity: motive power systems – Positional servo systems – Unwanted harmonic or voltage component elimination...
Reexamination Certificate
1999-07-07
2001-05-08
Ip, Paul (Department: 2837)
Electricity: motive power systems
Positional servo systems
Unwanted harmonic or voltage component elimination...
C318S696000, C318S685000, C318S432000
Reexamination Certificate
active
06229277
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to a and to an apparatus for damping vibration of an m-phase alternating-current motor.
The moment of an AC machine is generated by the voltage supplied to the armature circuit, normally a stator, and magnetisation of the excitation circuit, normally a rotor. The supply voltage is fed into the stator windings, which are fitted in slots in the stator. Magnetisation is developed by means of permanent magnets or excitation windings fitted in the rotor. The smoothness of motor operation depends on how closely the magnetic field developed in the air gap of the motor approaches the sinusoidal form. The harmonic components of a non-sinusoidal curve form produce moments that result in mechanical vibration and noise, the frequency of such moments depending on the frequency of said components.
Besides being dependent on the frequency of the supply voltage, the form of the air gap field also depends on the form of the field produced by excitation, among other things. Moreover, in machines with a small number of slots per phase and field pole, the so-called slot-harmonic field produced by the current flowing in each slot is stronger. For example, in a three-phase machine with one slot per phase, the six-fold supply frequency is a problem.
SUMMARY OF INVENTION
An object of the invention is to achieve a solution for substantially reducing the vibration and noise produced by the harmonic moments of a motor. To implement this, the procedure of the invention includes feeding the motor with a second voltage having a frequency equal to the basic frequency multiplied by a quantity
2
. Correspondingly, the apparatus of the invention is characterized by apparatus to produce the above function.
In the solution of the invention, a field is generated in the air gap at a frequency that compensates the largest deviation from the sinusoidal curve form produced by the supply frequency. Therefore, the overall curve form of the air gap field approaches the sinusoidal form, resulting in a smooth moment. By suitably selecting the amplitude of a second voltage fed into the motor, an appropriate degree of compensation is achieved. In particular, the moment of a slot frequency corresponding to a six-fold supply frequency is compensated with a frequency equalling five times the supply frequency when the number of slots per phase is one. Accordingly, a slot harmonic corresponding to a twelve-fold supply frequency is compensated with an 11-fold frequency.
Especially in axial motors in which a large moment is to be generated at a maximal distance from the axis of rotation, the moment produced by slot harmonics can be substantially reduced.
According to a preferred embodiment of the invention there is a phase shift between the first and the second voltage. Further according another preferred embodiment the phase shift is controlled on the basis of the load of the motor and the voltage reference. For each load it can be found a phase shift where the vibration moment generated by the harmonics has its minimum value. With this embodiment the vibration is minimized when the load is changing.
Yet in another preferred embodiment the amplitude of the second voltage is controlled on the basis of the load. The required amlitude is small when the load is low and the required amplitude increases when the load increases.
According to a preferred embodiment, the moment curve of the motor is measured for a normal supply voltage and, based on this measurement, the moment of the second voltage compensating the harmonics is determined.
In another embodiment, the moment curve is measured continuously and the amplitude of the second voltage is determined based on this.
The supply of the second voltage is preferably implemented by adding to the modulation frequency of the frequency converter generating the motor supply voltage a component corresponding to the frequency of the second voltage.
According to yet another embodiment, the second voltage is fed at a phase shift relative to the supply voltage, allowing the moment compensation to be applied with a maximal accuracy.
REFERENCES:
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patent: 5872440 (1999-02-01), Brabant et al.
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patent: 6040676 (2000-03-01), Nordquist et al.
patent: 6064172 (2000-05-01), Kuznetsov
Electronic Machines and Drives, Gordon R. Slemon, University of Toronto, 1992, ISBN 0-201-57885-9, chapter 10.7.
Aulanko Esko
Hakala Harri
Kurronen Panu
Ip Paul
Kone Corporation
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