GB1604067A

GB1604067A - Method of and apparatus for damping rotor oscillations of rotary field ac motors

Info

Publication number: GB1604067A
Application number: GB22930/78A
Authority: GB
Original assignee: Licentia Patent Verwaltungs GmbH; Licentia Oy
Current assignee: Licentia Patent Verwaltungs GmbH; Licentia Oy
Priority date: 1977-06-09
Filing date: 1978-05-26
Publication date: 1981-12-02
Also published as: DE2726410A1; NL7805962A

Description

(54) A METHOD OF AND APPARATUS FOR DAMPING ROTOR OSCILLATIONS OF ROTARY FIELD A.C. MOTORS (71) We, LICENTIA PATENT VER WALTUNGS G.m.b.H., of 1 Theodor Stern-Kai, 6 Frankfurt/Main 70, Federal Republic of Germany, a German body corporate, do hereby declare the invention, for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement: The invention relates to a method of damping the rotor oscillations of rotating field A,C motors, preferably three-phase current hysteresis motors which are fed via intermediate circuit converters comprising a controllable rectifier, a direct current intermediate circuit and an inverter.

When enriching uranium by the centrifuge separation method, hysteresis motors are used in order to drive the gas ultracentrifuges. With the centrifuge separation method, an isotope mixture is dissociated by means of a high gravitational field which is produced in its inner chamber by means of the high rotational speed of the centrifuge.

Since the separating capacity of a centrifuge depends to a large extent on its peripheral speed, it is desired to get as high a speed as possible within the mechanical limitation.

Because of the large dependence of the separating power on the peripheral speed of the centrifuge, not only is a very high operational speed demanded, but also a very high speed of accuracy so that the degree of enrichment is equal in all of the separating stages in a large plant and thus losses from mixing cannot occur. On the other hand, owing to the high speed, the constructional size of a centrifuge is limited and consequently so is its separating capacity, so that a large number of centnfuges is necessary for industrial exploitation of uranium enrichment. The number of the drive motors is just as large because each centrifuge is driven by its own motor.

A hysteresis motor fulfils the requirements best as the drive of a centrifuge, because it is characterised by a combination of features which are not present in electric motors of classical construction. The hysteresis motor is a self-starting motor and goes over automatically into synchronism after it has reached normal speed. When using the hysteresis motor in fast-running drive systems, the very simple and rotationally symmetrical shape if its solid and winding-free rotor is of particular advantage.

The medium frequency supply to the hysteresis motors takes place via static converters whereby a large number of motors is started up and is also operated with one converter.

Rotors of hysteresis motors tend to oscillate as do synchronous A.C. motors in general and this has its effect in a low frequency fluctuation in the input of effective power from the supply mains. Since the effective power consumed can fluctuate over a large range, supply mains cannot be used where such fluctuation is present.

In synchronous operation, the torque developed by the motor is proportional to the stator current I, the magnetic flux < D and the sine of the rotor angle #. The stator current and the magnetic flux are in turn functions of the stator field voltage. A damping torque can be achieved by controlling one of the three magnitudes.

It is known to apply the necessary damping torque via a phase angle displacement of the inverter output voltage (German Auslegeschrift No. 25 14 557). The noticeable disadvantage involved in this method is that instability arises during run up of the hysteresis motors into synchronism, this instability making it impossible for all of the motors to be accelerated together by the converter to the synchronous operating point. The motors must be connected up rather in small groups shortly before the entry into synchronism via appropriate switching safety devices, one after the other, to the converter.

The present invention seeks to avoid rotor oscillations of rotary field motors, such as hysteresis motors used for the drive of ultra high speed gas centrifuges.

According to a first aspect of the invention, there is provided a method of damping rotor oscillations in rotating field A.C.

motors fed by an intermediate circuit converter comprising a controllable rectifier, a direct current intermediate circuit and an inverter, said method comprising producing a torque necessary for damping the oscillations by varying the stator field voltage of the motors by control of the rectifier output voltage.

According to a second aspect of the invention, there is provided an apparatus for damping rotor oscillations in rotating field A.C. motors fed by an intermediate circuit converter comprising a controllable rectifier, a direct current intermediate circuit and an inverter, said apparatus comprising means for sensing the output of the inverter, means for producing a signal dependent thereon, and means for supplying this signal to the control circuit for the rectifier to vary the output voltage thereof to produce a torque necessary for damping the oscillations.

The inverter of the converter may be controlled at a very constant clock pulse frequency which can be produced preferably with the aid of a quartz oscillator and changes in the effective power of the motor may be cancelled out using the controlled rectifier of the converter. A voltage may be supplied to a control amplifier for controlling the rectifier which voltage is obtained by differentiating the effective power.

Because of the very constant clock pulse frequency, one of the causes of effective power oscillations is removed. Nevertheless, if the frequency fluctuates slightly, for example, at an operating frequency of 1000 Hz, by a few Hertz, then the effective power input to the motor is suddenly increased, because the motor transfers from the synchronous into the asynchronous operating range.

With an idling hysteresis motor, upon a frequency change of a few Hz, the effective power can increase from the idling power loss up to the hysteresis power loss which has a value which is a multiple of the value of the idling power loss.

If oscillations are caused by the load (changes in the load) or by the mains (changes in the supply voltage) then the resulting fluctuations in the effective power may be compensated for by control of the controlled rectifier of the converter. Since it is not the actual value of the effective power but only the change in the effective power which cause oscillations, the change in effective power may be derived from the actual value of the effective power using a differentiating element. Detection of the effective power may take place by measuring the voltage and the current of the motors.

The invention will now be described in greater detail, by way of example, with reference to the drawing, the sngle figure of which shows schematically one embodiment of the invention.

Rotary field AC motors 7 are supplied by means of an intermediate circuit converter comprising a controllable rectifier 1, a direct current intermediate circuit having a smoothing inductance 3 and a smoothing capacitance 4 and an inverter 2. The clock pulse frequency of the inverter 2 is predetermined highly accurately and constantly by an oscillator 18 via an amplifier stage 19. The torque relquired in order to damp the rotor oscillations is produced by adjusting the stator field voltage of the motors by varying the output voltage of the rectifier 1.The signals for controlling the semiconductor elements of the rectifier 1 are the result of the output of a control amplifier 12 to which a constant desired stator field voltage signal Vn and the actual stator field voltage signal VA are supplied, the actual voltage being measured via sensors 11. A magnitude related to the effective power applied to the rotary field motors is connected as an additional input magnitude of the control amplifier 12. This requires a measurement of the actual current taken by the rotary field motors, which measurement takes place via current sensors 9.

The power input to the rotary field motors is additionally applied to the control amplifier 12 in the form of a signal which corresponds to the first differentiation of the effective power formed in a differentiating element 16 after the effective power has been formed in a multiplier 15. The output of the control amplifier 12 is compared in a subordinate control circuit to the actual current of the intermediate circuit of the converter and the result of this comparison is supplied to a controller 13. Finally the output of the controller 13 supplies the ignition signals for the semiconductor elements of the rectifier 1 via the control device 14 of the rectifier 1 in which the synchronous control voltage of the supply mains is also taken into account.

It will be understood that while the above described embodiments relate to hysteresis motors, the principle is also applicable to other rotating field motors.

WHAT WE CLAIM IS: 1. A method of damping rotor oscillations in rotating field A.C. motors fed by an

**WARNING** end of DESC field may overlap start of CLMS **.

Claims

**WARNING** start of CLMS field may overlap end of DESC **. entry into synchronism via appropriate switching safety devices, one after the other, to the converter. The present invention seeks to avoid rotor oscillations of rotary field motors, such as hysteresis motors used for the drive of ultra high speed gas centrifuges. According to a first aspect of the invention, there is provided a method of damping rotor oscillations in rotating field A.C. motors fed by an intermediate circuit converter comprising a controllable rectifier, a direct current intermediate circuit and an inverter, said method comprising producing a torque necessary for damping the oscillations by varying the stator field voltage of the motors by control of the rectifier output voltage. According to a second aspect of the invention, there is provided an apparatus for damping rotor oscillations in rotating field A.C. motors fed by an intermediate circuit converter comprising a controllable rectifier, a direct current intermediate circuit and an inverter, said apparatus comprising means for sensing the output of the inverter, means for producing a signal dependent thereon, and means for supplying this signal to the control circuit for the rectifier to vary the output voltage thereof to produce a torque necessary for damping the oscillations. The inverter of the converter may be controlled at a very constant clock pulse frequency which can be produced preferably with the aid of a quartz oscillator and changes in the effective power of the motor may be cancelled out using the controlled rectifier of the converter. A voltage may be supplied to a control amplifier for controlling the rectifier which voltage is obtained by differentiating the effective power. Because of the very constant clock pulse frequency, one of the causes of effective power oscillations is removed. Nevertheless, if the frequency fluctuates slightly, for example, at an operating frequency of 1000 Hz, by a few Hertz, then the effective power input to the motor is suddenly increased, because the motor transfers from the synchronous into the asynchronous operating range. With an idling hysteresis motor, upon a frequency change of a few Hz, the effective power can increase from the idling power loss up to the hysteresis power loss which has a value which is a multiple of the value of the idling power loss. If oscillations are caused by the load (changes in the load) or by the mains (changes in the supply voltage) then the resulting fluctuations in the effective power may be compensated for by control of the controlled rectifier of the converter. Since it is not the actual value of the effective power but only the change in the effective power which cause oscillations, the change in effective power may be derived from the actual value of the effective power using a differentiating element. Detection of the effective power may take place by measuring the voltage and the current of the motors. The invention will now be described in greater detail, by way of example, with reference to the drawing, the sngle figure of which shows schematically one embodiment of the invention. Rotary field AC motors 7 are supplied by means of an intermediate circuit converter comprising a controllable rectifier 1, a direct current intermediate circuit having a smoothing inductance 3 and a smoothing capacitance 4 and an inverter 2. The clock pulse frequency of the inverter 2 is predetermined highly accurately and constantly by an oscillator 18 via an amplifier stage 19. The torque relquired in order to damp the rotor oscillations is produced by adjusting the stator field voltage of the motors by varying the output voltage of the rectifier 1.The signals for controlling the semiconductor elements of the rectifier 1 are the result of the output of a control amplifier 12 to which a constant desired stator field voltage signal Vn and the actual stator field voltage signal VA are supplied, the actual voltage being measured via sensors 11. A magnitude related to the effective power applied to the rotary field motors is connected as an additional input magnitude of the control amplifier 12. This requires a measurement of the actual current taken by the rotary field motors, which measurement takes place via current sensors 9. The power input to the rotary field motors is additionally applied to the control amplifier 12 in the form of a signal which corresponds to the first differentiation of the effective power formed in a differentiating element 16 after the effective power has been formed in a multiplier 15. The output of the control amplifier 12 is compared in a subordinate control circuit to the actual current of the intermediate circuit of the converter and the result of this comparison is supplied to a controller 13. Finally the output of the controller 13 supplies the ignition signals for the semiconductor elements of the rectifier 1 via the control device 14 of the rectifier 1 in which the synchronous control voltage of the supply mains is also taken into account. It will be understood that while the above described embodiments relate to hysteresis motors, the principle is also applicable to other rotating field motors. WHAT WE CLAIM IS:

1. A method of damping rotor oscillations in rotating field A.C. motors fed by an

intermediate circuit converter comprising a controllable rectifier, a direct current intermediate circuit and an inverter, said method comprising producing a torque necessary for damping the oscillations by varying the stator field voltage of the motors by control of the rectifier output voltage.

2. A method according to Claim 1, wherein the inverter is controlled at a constant and highly accurate clock pulse frequency.

3. A method according to claim 1 or 2, wherein a control signal for varying the rectifier output voltage is obtained from a first differentiation of the effective power consumption of the motors.

4. An apparatus for damping rotor oscillations in rotating field A.C. motors fed by an intermediate circuit converter comprising a controllable rectifier, a direct current intermediate circuit and an inverter, said apparatus comprising means for sensing the output of the inverter, means for producing a signal dependent thereon, and means for supplying this signal to the control circuit for the rectifier to vary the output voltage thereof to produce a torque necessary for damping the oscillations.

5. An apparatus according to claim 4, wherein the change in the effective power of the rotating field motors is supplied to a control amplifier for controlling the stator field voltage of the motors.

6. An apparatus according to Claim 5, wherein the controlled stator field voltage is controlled from a comparison of a desired voltage and the actual voltage.

7. An apparatus according to Claim 5 or 6, wherein the actual current of the intermediate circuit of the converter is measured and is compared to the output of the control amplifier for controlling the stator field voltage of the motors.

8. An apparatus according to Claim 7, characterised in that the control deviation is supplied to a current controller.

9. A method of damping rotor oscillations in rotary field A.C. motors substantially as described herein with reference to the drawing.

10. An apparatus for damping rotor oscillations in rotating field A.C. motors substantially as described herein with reference to the drawing.