GB2207017A - Phase locked loop - Google Patents

Abstract

A phase-locked loop PLL including a phase comparator (13, Fig 2), an analogue switch (16) and a voltage controlled oscillator (14) has an open loop operation when the phase difference between the input and output is less than a predetermined value. Also described is a method of detecting rotor bar faults in e.g. a squirrel cage induction machine comprising generating a pure frequency signal at the same frequency as the fundamental oscillating output signal of the machine such as by using a PLL generator 2 (Fig 2) and matching the amplitudes thereof; subtracting the pure signal from the output signal so as to suppress the fundamental oscillation in the output signal, and analysing the remaining output signal. <IMAGE>

Description

DETECTING ROTOR BAR FAULTS The present invention relates to a system for detecting faults in the rotor bars of e.g. a squirrel cage induction machine1 and also to a device for generating a signal at the fundamental frequency of a signal having many harmonics.

It is not uncommon for the rotor bars of squirrel cage induction machines to develop faults, e.g. fractures, due to the high stresses imposed each time the machine is started. Normally, a fault in one rotor bar does not cause catastrophic failure of the machine, unlike stator faults, but instead the machine will continue to operate. However, the remaining rotor bars then experience significantly higher currents, stresses, and temperatures, increasing their likelihood of failure. Thus, the development of a fault in one rotor bar will increase the probability of a fault developing in another rotor bar, and therefore the machine tends to deteriorate progressively. It is desirable, therefore, to detect rotor bar faults, so that repairs may be carried out at a suitable time.

The current waveform from a squirrel cage induction machine has a fundamental frequency at the driving frequency (e.g. 50 Hz) but has a large number of harmonics.

It has been found that failure of one or more rotor bars cause unique peaks in the spectrum of the current, and therefore detection of those peaks would provide a way of detecting rotor bar faults in a non-intrusive way. The problem, however, is that those peaks are found to be very close to the fundamental frequency, and of an amplitude much less than the amplitude of the fundamental frequency.

Therefore, it is extremely difficult to detect those peaks by standard techniques. Effectively, the peaks are swamped" by the fundamental frequency.

It would be possible to carry out direct Fourier analysis on the signal but the relative magnitude of the peaks of interest (e.g. 0.1 of the fundamental) would mean that the analysis equipment would have to have a very large dynamic range, and such equipment is currently very expensive.

Therefore, the present invention seeks to provide a system which enables those peaks to be analysed without requiring such expensive equipment. The present invention proposes that a "pure" frequency signal, closely matched to the fundamental in frequency, phase and amplitude, is generated from the current signal from the squirrel cage induction machine, and that signal is subtracted from the current signal. In this way, the fundamental can be substantially eliminated, and therefore the peaks of interest can be investigated. This, in its widest aspect, represents a first aspect of the present invention.

In order to obtain maximum ease of use, in a nonintrusive way, it is necessary to generate the signal for subtraction not from the supply voltage to the machine, but from the current signal from the machine, i.e. the signal containing the fundamental and many harmonics. Since the frequencies generated by a faulty rotor are very close to the fundamental frequency, they cause a slow modulation of the zero-crossing of the current signal. This modulation means that the signal for subtraction cannot be generated by a standard phase-locked loop, which would otherwise be satisfactory. The use of standard phase-locked loops would involve those circuits having extremely long integration times, which would necessitate a long period of operation before the signal could be analysed.

Therefore, the present invention proposes a modified phase-locked loop arrangement, which avoids the need for long integration times. This phase lock loop arrangement, although designed for use in detecting the faults in rotor bars of a squirrel cage induction machine, is thought to have other applications and is therefore a second, independent, aspect of the present invention.

In this phase-locked loop arrangement, the input signal is compared with the signal generated from an oscillator to determine when the two signals are out of phase, and the output of the comparator fed via a switching circuit to the oscillator, and also to a circuit which detects the duration of the output. When that output time is less than a predetermined amount, the switch is maintained open, so that the oscillator output remains the same. However, when the duration of the signal from the phase comparator increases beyond that predetermined amount, the switch is closed and the signal from the comparator then modifies the frequency signal generated by the oscillator.Effectively, therefore, the phase-locked loop is open provided the phase difference between the input and the signal from the oscillator is less than a predetermined amount, and the loop is closed only when the phase shift is greater than that amount.

The effect of this is that the modulation of the zerocrossing of the current signal from an induction machine are ignored, at least while they remain a relatively minor perturbation. For large perturbations, the phase-locked loop may' be set up so that it has a relatively short integration time, so that it can rapidly change its output frequency.

In this way, it becomes possible to generate from the current signal from the squirrel cage induction machine, a signal which is relatively "clean" at, and in phase with, the fundamental frequency, which by suitable amplitude matching may then be subtracted from the current signal.

For simplicity, the switch in the phase-locked loop between the the phase comparator and the oscillator may be a simple analogue switch controlled by a circuit which forms an analogue analysis of the output of the phase comparator, but alternatively digital techniques can be used, in which the output of the phase comparator is converted to a digital signal, which is then used to determined whether or not the oscillator should be operated. However achieved, the aim is to provide a range of phase differences over which the phase-locked loop is effectively open.

An embodiment of the invention will now be described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows a block diagram of a system for detecting faults in a squirrel cage induction machine; Fig. 2 shows a circuit diagram of the arrangement for generating the signal for subtraction from the current signal; Fig 3 shows a circuit diagram of a circuit for matching the amplitude of the signal from the circuit of Fig. 2 with the fundamental of the current signal; and Fig. 4 shows a circuit diagram of a frequency shifting circuit.

As has been mentioned above, in order to provide a non-intrusive system, the present invention proposes that the system for detecting rotor bar faults be based on a signal processed from the current output of the machine.

Therefore, a suitable current transducer 1, e.g. a Hall effect probe, is connected to the current output, and that transducer generates a signal which is fed to a circuit 2 for generating a "clean" signal at the fundamental frequency. The output of that generator 2 is fed to a circuit 3 which matches its amplitude with the fundamental of the current signal, to generate a signal with the fundamental suppressed. That signal is then shifted in frequency, in order to make it easier to analyse with standard analysing arrangements, by a frequency shifting circuit 4. With the frequency shifted, an analyser 5 analyses the spectrum of the signal, and a computer 6, or other suitable means, may then be used to detect when the results of the analysis show that peaks have been generated corresponding to rotor bar failure.This may be achieved by comparing the amplitude of pulses at the expected frequency corresponding to rotor failure with fixed thresholds, and with each other, to enable a faulty motor to be detected.

The generator 2 of Fig. 1 will now be described in detail with reference to Fig. 2. The current signal from the current probe 1, is applied to an input 10 of the generator 2 and then passes through a DC blocking filter 11. The signal is then "squared" by a suitable circuit 12 and fed to a phase comparator 13. A voltage controlled oscillator 14 generates a frequency signal, which after passing via a frequency divider 15 forms the other input to the phase comparator 13. The output of the phase comparator 13 forms a measure of the difference in frequency between the output of the voltage controlled oscillator 14 and the fundanental of the current input, and in a normal phase locked loop would be used to control the voltage oscillator 14. However, in this embodiment of the present invention, an analogue switch 16 receives the output of the phase comparator 13.That analogue switch 16 is controlled by a circuit 17 which also receives the output from the phase comparator 13, and which determines if the duration of that output is greater than, or less than, a predetermined time 20 us. If the pulse from the phase comparator 13 is shorter than that time, the analogue switch 16 remains open, and no signal is passed to the oscillator 14. It is only when the output from the comparator 13 has a duration greater than the predetermined amount, is the switch 16 closed, and a signal fed via low pass filter 18 to the oscillator 14.

The circuit 17 thus acts as a "narrow-pulse removing circuit", and operates by means of a monostable 19 which generates a signal of predetermined length, against which the output of the comparator 13 is compared by logic element 20.

Thus, the phase locked loop is open for phase perturbations less than the predetermined amount, but operates normally for perturbations above that amount.

As was mentioned with Fig. 1, the output of the generator shown in Fig. 2 is then matched with the amplitude of the fundamental frequency of the current signal, and one subtracted from the other. Fig. 3 shows a circuit diagram of the amplitude matching circuit, in which it is the amplitude of the current signal which is adjusted. The current signal is received at an input 30, and fed via an amplifier 31 to a circuit which adjusts its gain in dependence on suitable input. To generate that signal, the output of the circuit 32 is fed via a rectifier circuit 33 and a low pass filter 34 to a summing circuit 35. That circuit sums the signal with a "set point" signal appropriately set as shown at 36, and the output fed via an integrator 37 to the circuit 32.The output of circuit 32 is fed to a subtraction circuit 38 which subtracts the current signal from the signal generated by the circuit of Fig. 2, received at input 39, to give a signal in which the fundamental has been substantially removed. A band pass filter circuit 40 then eliminates higher harmonics, so that the output from output 41 can be analysed relatively easily, to detect if peaks due to rotor bar faults are present.

Although the fundamental of the current signal will be sinusoidal, the signal generated by the oscillator 14 of the circuit of Fig. 2 may be a square wave. Subtraction of one from the other will eliminate the fundamental, but will introduce harmonics at higher frequencies. The band pass filter 40 then removes these, to enable the resultant output to be limited to a frequency range close to the fundamental, which range contains the peaks of interest.

As was described with reference to Fig. 1, it is desirable to shift the frequency of the signal, to make it easier to analyse, and the circuit for this is shown in Fig. 4. The circuit receives an input from the generator 2, which is applied to a loop circuit consisting of a phase comparator 51, a low pass filter 52, a voltage controlled oscillator 53, and a circuit 54 which divides the frequency signal by a suitable number, e.g. eight. The resulting output of this loop is fed via a circuit 55 which divides the signal by a suitable number, e.g. 10, and a band pass filter 56 to a multiplier circuit 57 which receives the signal from the amplitude matching circuit 3 via an input 58. This multiplier circuit 57 shifts the frequency of the current signal received at input 58, which is then passed via a low pass filter 59 to a suitable analysis arrangement.The circuit shown in Fig. 4 is a generally conventional one for producing a multiplying signal, and effectively shifts the centre frequency of the spectrum of the current signal to a region providing sufficient resolution to discriminate the harmonics of interest. A centre frequency of 10 Hz has been found to be suitable for analysis by a CML 2105 analyser.

This analyser performs a Fourier analysis on the current signal (with fundamental suppressed), and the results analysed by a suitable computer program. In practice, the fundamental may not be completely suppressed, but an intensity reduction of e.g. 30 dB or more will reduce the fundamental sufficiently to enable conventional Fourier analysis to be carried out with relatively inexpensive arrangements.

The circuit loop arrangement discussed above is applicable not only to the analysis of a signal from a squirrel cage induction machine, but it may be used, for example, in phase-controlled rectifiers, e.g. to detect the zero-crossings of the incoming voltage to control thyristor firing.

Claims (9)

1. A phased-locked loop device comprising a signal controlled oscillator having an output for generating an output signal, and a phase comparator having one input connected to the output of the oscillator and a second input for receiving an input signal, the output of the phase comparator being connected to the input of the signal controlled oscillator, there being means for disconnecting the phase comparator and the oscillator when the magnitude of the phase difference between the input signal and the output signal is less than a predetermined value.

2. A device according to claim 1, wherein the phase comparator is adapted to generate pulses whose width corresponds to the difference in phase between the input and output signals, and the means for disconnecting the phase comparator and oscillator includes means for detecting the pulse width of those pulses.

3. A phase-locked loop device substantially as herein described with reference to and as illustrated in the accompanying drawings.

4. An apparatus for detecting faults in an electrical machine, comprising: a device according to any one of claims 1 to 3, the input of which is connected to the output of the machine; means for matching amplitude of the output signal of that device with the output of the machine, and for subtracting the output signal of that machine from the sample signal; thereby to generate a signal with the frequency of the output of the oscillator suppressed.

5. An apparatus according to claim 4, wherein the output of the matching and subtracting means is connected to a spectrum analyser.

6. A method of isolating peaks in a sample signal having a predominant fundamental oscillation, comprising: generating a pure frequency signal from the sample signal, which pure signal has the same phase and frequency as the fundamental oscillation; matching the amplitudes of the pure frequency signal and the fundamental oscillation; and subtracting the amplitude-matched pure frequency signal from the sample signal, thereby to suppress the fundamental oscillation in the sample signal and generate a measurement signal.

7. A method according to claim 6, wherein the pure frequency signal is generated by: generating an output signal from a signal controlled oscillator; supplying the output signal and the sample signal to a phase comparator; detecting the phase difference between the output signal and the sample signal; and modifying the output of the oscillator when the phase difference is greater than a predetermined value, the modified output forming the pure frequency signal.

8. A method of detecting faults in an electrical machine, comprising: isolating pulses in the output of the machine according to the method of claim 6, or claim 7, with the output of the machine corresponding to the sample signal; and analysing the measurement signal to detect faults in the machine.

9. A method of detecting faults in an electrical machine substantially as any one herein described with reference to the accompanying drawings.