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.
