Condition Monitoring (CM) has long been used to improve plant performance and reduce costs, by helping to diagnose faults and optimise maintenance schedules. There is a well understood kit-bag of CM tools and techniques, of which the leading method is vibration analysis, with thermography, ultrasound, oil analysis, motion amplification and others also widely used. None of these tools provides a universal solution to every situation, despite what some of the vendors would try to claim. Rather, like any kit of tools, the skilled practitioner understands the capabilities of each tool, and selects the most appropriate tool, or combination of tools, for the particular task in hand.
Nowadays, with the increasing emphasis on sustainability and reducing carbon footprint, there is a new generation of condition monitoring solutions that can also provide energy monitoring and energy optimisation capabilities, based on electrical measurements to give information on electrical, mechanical and operational problems all in one go.
Electrical CM concepts
A variety of electrical condition monitoring techniques have been widely used for years, including Partial Discharge (PD) to detect early deterioration of insulation, Motor Current Signature Analysis (MCSA) which has mainly been used to identify specific motor faults, Motor Start-Up Current Analysis and others. A new technique called Model-Based Voltage and Current analysis (MBVI) blends the benefits of some of these electrical techniques with vibration analysis, to give useful information on a range of mechanical problems, like bearing faults, belt drive problems, looseness and rubbing etc, electrical problems with the equipment like motor rotor or stator problems, issues with the electricity supply such as phase imbalance or high harmonic distortion, and perhaps most interestingly, energy efficiency assessment.
Because these systems work by measuring voltage and current being drawn by the motor, all connections are carried out in the switchgear, which is almost always in a clean, dry, non-flammable area, so these systems avoid the need for expensive ATEX ratings and are ideal for monitoring the condition of inaccessible equipment. The systems automatically compensate for load and speed, and even make automated allowances for distorted waveforms that can be present with inverter driven equipment.
An example from Faraday Predictive
An example of this new technique and the kind of outputs they can provide comes from Faraday Predictive and is available in both portable and permanently installed forms. The impressive capability of this technology can be seen in some of the outputs that these systems provide, which include both current and predicted condition of the equipment at an overall level and the current and predicted status of individual faults up to three months into the future, allowing specific maintenance plans to be drawn up with the right materials for the right work at the right time.
These systems automatically provide information on each fault detected, describing the nature of the fault, how the system has detected it, the impact of this fault, and the recommended corrective action, in addition to the predicted future condition to allow timing of maintenance work to be optimised.
As well as providing detailed electrical measurements such as the voltage and current, the supply frequency, power factor / phase angle, active and reactive power, phase imbalance on both voltage and current, and Total Harmonic Distortion on both voltage and current, the system also assesses the energy impact of any faults present. The proportion of the total energy being consumed by each fault is displayed, allowing cost-justified decisions on corrective action.
Many of these electrical parameters also have implications for mechanical reliability – for example, a phase unbalance on a 3-phase current (meaning that not all phases are the same amplitude) creates a twice per cycle oscillation in torque created by the motor – which puts additional stress on things like shafts and couplings. And harmonic distortion, which represents current flowing in and out of the motor but not doing any effective work in turning the shaft, creates additional heating in the motor which is not good for reliability. A widely used rule of thumb is that a 10oC rise in motor temperature halves the life of the motor windings.
How MBVI systems work – subtle changes in the relationship between voltage and current
Most engineers are familiar with the fact that an electric motor will draw more current when it is under a higher load, and a lower current when it is under lower load, even though the supply voltage is constant. What they may not be aware of is that this current variation occurs not just with major load changes like a pump discharge valve being opened, it also applies to smaller, shorter duration effects that may occur as fast as several times per rotation of the motor shaft, for example caused by phenomena such as rolling element bearing problems. As the figure 1 shows, the current waveform can end up significantly distorted relative to the voltage waveform. Note that in this case the voltage waveform (pink) is itself significantly distorted away from a pure sinusoidal shape, even though this motor is driven direct from the mains supply. This is not unusual – the voltage waveform in many industrial situations can be distorted by a variety of non-linear loads connected to the network. The current waveform (red) is noticeably more distorted than the voltage waveform – which is caused by the behaviour of the motor and the driven equipment.
The small wiggly brown line across the middle of the diagram represents the distortions on the current waveform which have not been caused by distortions on the voltage waveform, and therefore must have been caused by phenomena in the machine. This residual current signal can be thought of as directly equivalent to a raw vibration signal – and can be analysed in the same way, analysing the magnitude and frequency of these distortions to identify the underlying causes and their severity.
Use in practice
Portable MBVI systems such as the Faraday Predictive P100 shown in figure 2 can be used to collect data from a wide range of equipment – like a portable vibration data collector. They can provide an easy means of becoming familiar with the technology, prior to committing to a permanent installation.
For users who want 24x7 monitoring of their equipment, systems such as the Faraday Predictive S200 (figure 3) are designed for permanent installation inside the motor starter cabinet.
These systems are already providing benefits to users in a range of industrial applications, including Oil and Gas, Energy, Food and Drink, Utilities and others.
If you would like to join them in using this technology, or just want to find out more, contact Faraday Predictive on 0333 772 0748 or
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Geoff Walker, Operations Director of Faraday Predictive, is Chairman of the Electrical Condition Monitoring working group of the British Institute for Non-Destructive Testing (BINDT). He also sits on the BSI and ISO working groups responsible for standards for Condition Monitoring and Diagnostics of Machine Systems. A Mechanical Engineer by background, he has decades of experience in automated fault diagnostics and the optimisation of maintenance strategy.
