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7. The Attendant Problems.

Question 7.1.

In paper and pulp mils, metallurgical plants, and other industries, the most important equipment often has low rotating frequencies - from 2-3 up to 5-100 RPM. In diagnostics of equipment with such low rotating speeds, as we know the vibration diagnostics does not give as expected high efficiency as, for example, the method of rotating speed pulsation analysis. And the number of measurement points have to be much more than for example in the conveyer monitoring system by speed pulsation where only one tacho transducer on the driver electric machine is used. Is not it worthwhile to refuse to use on-line monitoring systems using vibration and to substitute monitoring systems analyzing the rotating speed pulsation for them?
(The question was asked during a meeting of specialists in diagnostics from the main Russian paper and pulp mills.)

The question is answered by Alexej V. Barkov:

To give a detailed answer on this question, it is worth dividing it into several questions.

The first question: Is the vibration diagnostics of low speed machines efficient?

Really, at first it would seem that the vibration diagnostics for low speed machines is inefficient. First of all, a machine, rotating with such low frequency (up to 5-10 Hz) oscillates as a single unit on its foundation on which, as a rule, are mounted many sources of oscillating forces. Consequently, it is very difficult to investigate so many vibration components from different sources. Secondly, at such low frequencies, the shock pulse method does not work because, even in units without any defect such as rolling element bearings, because of frequent breaks of the oil film, many shock pulses occur that superimpose on each other and distort the form that is representative for the shock pulses of the high frequency vibration.

But this is only at the first glance. In reality, there are several more methods of vibration analysis including the vibration excited by shock pulses that remain effective for diagnostics of low speed machines. The peculiarities of some of them are described in the publications on this home page.

The second question - is the monitoring of low speed rotating machines by the frequency speed pulsation efficient?

Yes, for the diagnostics of many types of rotating machines and their units, rotating frequency pulsation analysis is used successfully, for example, in the case of rotors of the induction motors diagnostics. Machine diagnostics by angle fluctuation of a rotor was developed most intensively in 1970s and 1980s, but most often the measurements of the angle deviation of the rotating frequency was conducted, not directly using the sensors of the angle oscillations, but indirectly by modulation of such processes as vibration, electric current and others. For example, in Russia in 1982, a method of squirrel cage diagnostics of induction motor using the modulation of the induction motor vibration by the double frequency of rotor slip was patented. Several years later, a similar patent appeared using modulation, not of vibration but of the current in the windings of the induction motor that also is one of the methods of the rotor rotating frequency pulsation measurement. The latter method, though it has no advantages compared the first one, is used as the main method of electric machines diagnostics by such a well known company as CSI, Inc.

No less important is the use of the speed pulsation analysis method to control whole systems with a large number of shafts driven by one driver, for example in low speed conveyers of different types. The analysis of low frequency pulsation of speed (or current) of the driver electric motor enables the detection of the appearance of anomalistic loads much before the action of driver protection system using the value of load. But especially important in this analysis is the conveyer quality control after its installation or maintenance when other methods of rotating machine diagnostics are yet not so efficient. The most efficient method of such systems as driving system rotating speed pulsation analysis is the spectral analysis and its modifications.

The third question - can the method of rotating speed pulsation analysis in one point of a system with large number of rotating machines connected with each other for the purpose of monitoring and diagnostics substitute for the vibration analysis in many points of each rotating machine?

Unfortunately such substitution is impossible as many types of machine defects do not lead to the appearance of alternating components of loads and hence, to the pulsation of rotating speed even up to an alert condition. At the same time, the usage of the transducers of the machine rotating speed pulsation in the monitoring system ensures receiving additional information about the machine condition with a limited number of vibration control points, especially not long before an accidental failure.

The forth question - why is it expedient to measure the speed pulsation only on low speed machines?

Measurement of the machine speed pulsation with sufficient accuracy is possible if the value of the pulsation (the frequency deviation) is not less than 10-4 - 10-3 of its mean value. As the rotor's inertia moment of a high speed machine is very large, to reach such values of deviation it is necessary that the pulsating moment is very large or the pulsating frequency is low (103 104 times less than the rotating frequency). As the real pulsating loads, necessary to be detected during monitoring, do not exceed tenths of a per cent of the mean load, solving such problem is possible, first of all, for low speed machines.



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