GB2157102A - Method and apparatus for amplitude control of varying parameters - Google Patents

Abstract

The amplitude of a varying parameter is controlled by sensing the rise or fall in the amplitude of the parameter, determining therefrom the rate of rise or fall (p, q) and, when the rate is indicative that an amplitude (P, Q) is developing which is greater or lesser as the case may be than a predetermined level, then eliminating, or modulating by attenuation or amplification as the case may be, the amplitude to prevent an amplitude above or below the predetermined level as the case may be from developing. The parameter may be electrical power, current or voltage, or torque or acceleration, especially in a three-phase induction motor. <IMAGE>

Description

SPECIFICATION Method and apparatus for amplitude control of varying parameters The present invention relates to a method and apparatus for controlling the amplitude of varying parameters.

It is an object of the present invention to provide an efficacious method and apparatus for controlling the amplitude of a varying parameter.

The present invention is based on the realisation that many systems giving rise to varying parameters impose a characteristic on the variation of the parameter such that the value or level assumed by the variation can be predicted over a short period of time i.e. that the rate of rise or fall of the amplitude of the parameter at any given instant is indicative of a future instantaneous value or level of that parameter.

There is provided by the present invention, a method for controlling the amplitude of a varying parameter, which comprises sensing the rise or fall in the amplitude of the parameter, determining therefrom the rate of rise or fall and when the rate is indicative that an amplitude is developing which is greater or lesser as the case may be than a predetermined level, and then eliminating, or modulating by attenuation or amplification as the case may be, the amplitude to prevent an amplitude above or below said predetermined level as the case may be from developing.

The present invention also provides apparatus for controlling the amplitude of a varying parameter, which comprises means for sensing the rise or fall of a varying parameter, means for determining therefrom the rate of rise or fall as the case may be and when the rate is indicative that an amplitude is developing which is greater or less as the case may be than a predetermined level, and means for eliminating or modulating, by attenuation or amplification as the case may be, the amplitude to prevent an amplitude above or below said predetermined level as the case may be from developing.

It will be understood that the present invention may entail sensing merely the rise in amplitude or merely the fall thereof or sensing both the rise and fall at appropriate instants; and that, in the practice of the invention, the sensing means, the determining means and the modulating means may be con- stituted accordingly notwithstanding the fact that such means capable of dealing with both rising and falling amplitudes could be used in all instances.

It will be understood that the variation of the parameter may be represented graphically or by a waveform and that the rise or fall during a given interval may therefore be determined from the slope of the graph or waveform at an earlier instant i.e. it may be expressed as a ratio or as a scaled angular measure.

The analogue value of the amplitude may be digitised and a microprocessor employed as the determining means.

The sensor chosen to sense the amplitude may vary in dependence on the parameter to be sensed. In the case where the parameter is electrical power, current or voltage, the sensor could, for instance, be an elector-magnetic device the field intensity of which varies with the parameter. In the case where the parameter is torque, the device could be an electro-mechanical transducer. In the case of acceleration, an accelerometer may be employed preferably one providing an electrical output signal.

The present invention is considered to be especially useful when the parameter is electrical and, in this case, the invention has particular application for the protection of the circuits against excessive currents, and for controlling torque and/or power dissipation and/or transients in rotating electrical machines and, especially, in large rotating electrical machines.

The present invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 (a) is a waveform diagram of current waveform showing a transient having a peaked value in excess of an acceptable limit of current; Figure 1 (b) shows the same waveform in accordance with the present invention; and Figure 1 (c) shows the same waveform after modulation by a known method to avoid the transient peak; Figures 2 (a) and 2 (b) are again waveform diagrams which compare respectively computed and practical values of a starting torque characteristic for a three-phase induction motor; and Figure 3 again shows the computed characteristic shown in Figure 2 (a) together with modified characteristics due to amplitude modulation.

A common form of a large electrical rotating machine is a three-phase induction motor and, for the purpose of exemplification of the present invention, the present invention will now be described with reference to such an electrical motor.

During the starting, braking and re-switching of such motors, very high values of currents are demanded from the supply and exceptional values of peak transient torque may develop. From the viewpoint of the electrical supply authorities, short-duration high currents are undesirable since the resulting voltage drop in the supply lines affect other consumers, for example, light dimming. Indeed, the authorities may prohibit a consumer from connecting a motor if it exceeds a given rating. High peak values of transient current and torque may also have undesirable effects upon the motor in the form of damage to the end-windings and mechanical transmission system.

Present methods of alleviating the problems encountered during the starting of electric motors resort to either reducing the applied voltage or including current-limiting elements (resistors or inductors) throughout the entire starting procedure.

While these methods do limit the starting current, they also suffer the well-known severe drawback that the mean starting torque is greatly reduced.

Consequently, the run-up time is increased, and therefore currents which may still be in excess of rated value are demanded for a prolonged period of time, and this may raise opposition from the supplying authorities. The present invention enables all the advantages which are allied to existing methods of start-up to be obtained while enabling the disadvantage of reducing the mean starting torque to be overcome.

The present invention enables selective use of applied voltage reductions or current-limiting elements throughout the starting process. It is only during those periods when a current or torque is about to become excessive that suppression is needed. Fig. 1 (a) shows an uncontrolled current wave which may develop during a transient condition. Using present methods of limiting the current peaks to acceptable values, all of the peaks would be scaled down to produce the wave shown in Fig.

1 (c); it is clear that the acceptable peaks (R and S) have also been reduced to values which will have detrimental effects upon good torque production.

Reference to Fig. 1 (a) shows that the unacceptable peaks at P and Q are preceded with corresponding high rates of amplitude change at p and q. Thus, by monitoring the rate of change, an early warning is given that an unacceptable peak is about to develop and that preventive action should be taken.

Fig. 1 (b) shows the modulation of the Fig. 1 (a) waveform when current-limiting elements are employed selectively based upon "rate of change" information.

Figs. 2 (a) and 2 (b) compare computed and practical values of starting torque characteristics for a three-phase induction motor, and it is shown that an undesirable relatively high peak torque is developed, followed by a negative (braking) torque which is detrimental to the run-up of the motor.

The computed characteristic is again shown in Fig.

3 together with modified characteristics due to the presence of current-limiting resistances in the stator circuits during starting. Curve (b) results when the resistances are included throughout the run-up, as in present-day practice and it is clear that the oscillatory characteristic now has a relatively low mean value responsible for a prolonged run-up time. Curve (c) in Fig. 3 shows a substantial improvement which results when the current-iimiting resistors are employed only when the torque growth rate warns that an unacceptable value of peak torque is about to develop. Selective use of the resistors has achieved a reduction in the torque peaks, and the negative (braking) torque has been eliminated; the resistors were in only 10% of the total run-up period, and therefore can be beneficially short-time rated.

The above described apparatus serves to modulate the amplitude of the varying parameter to maintain the amplitude of said parameter at a predetermined level or within predetermined limits.

The said apparatus may further include a shutdown facility, for example a switch which, when operated by the apparatus detecting an unacceptable rise in the amplitude of the varying parameter, opens the circuit carrying the varying parameter to prevent the overload from damaging the circuit.

In a further arrangement (not shown) the means for modulating the amplitude of the varying parameter may be omitted and a sensed unacceptable change in the amplitude of the varying parameter may simply serve to actuate the shutdown facility.

Claims (15)

1. A method for controlling the amplitude of a varying parameter, which comprises sensing the rise and/or fall in the amplitude of the parameter, determining therefrom the rate of rise or fall and, when the rate is indicative that an amplitude is developing which is greater or lesser as the case may be than a predetermined level, then eliminating, or modulating by attenuation or amplification as the case may be, the amplitude to prevent an amplitude above or below said predetermined level as the case may be from developing.

2. A method according to Claim 1, wherein the variation of the parameter is represented graphically or by a waveform and that the rise or fall during a given interval is therefore determined from the slope of the graph or waveform at an earlier instant.

3. A method according to Claim 2, wherein said determination from the slope of the grpah or waveform is expressed as a ratio or as a scaled angular measure.

4. A method according to any of the preceding claims, wherein the analogue value of the amplitude is digitised.

5. A method according to any of the preceding claims, wherein said elimination of said amplitude is effected by stopping generation of said parameter.

6. Apparatus for controlling the amplitude of a varying parameter, which comprises means for sensing the rise or fall of a varying parameter, means for determining therefrom the rate of rise or fall as the case may be and when the rate is indicative that an amplitude is developing which is greater or lesser as the case may be than a predetermined level, and means for eliminating or modulating, by attenuation or amplification as the case may be, the amplitude to prevent an amplitude above or below said predetermined level as the case may be from developing.

7. Apparatus as claimed in Claim 6, wherein the sensing means, the determining means and the modulating means are constituted such as to be capable of dealing with rising or falling amplitudes or with both rising and falling amplitudes.

8. Apparatus according to Claim 6 or 7, wherein the case where, the analogue value of the amplitude is digitised, the determining means comprises a microprocessor.

9. Apparatus as claimed in Claims 6, 7 or 8, for use in controlling an electrical power, current or voltage being said parameter wherein the sensor is an electro-magnetic device the field intensity of which varies with the parameter.

10. Apparatus as claimed in Claims 6, 7 or 8, for use in controlling torque being said p zrameter, the sensor is an electro-mechanical transducer.

11. Apparatus as claimed in Claims 6, 7 or 8, for use in controlling acceleration, wherein said sensor is an accelerometer.

12. Apparatus according to Claim 11, wherein the accelerometer is ore providing ar electrical output signal.

13. Apparatus according to any of the preceding claims 6 to 12, wherein said means for eliminating said'amplitude comprises means for shutting down the generation of said parameter.

14. A method for controlling the amplitude of a varying parameter, substantially as described with reference to the accompanying drawings.

15. Apparatus for controlling the amplitude of a varying parameter, substantially as described with reference to the accompanying drawings.