Variable speed drives are often viewed as new technology that allow users to save energy in electrical motor applications by varying frequency and voltage. Electric motors that operate at speeds based on demand, rather than continuously at full speed, can provide a quick payback on investment for many applications.
Yet, with the ever-growing demand for energy efficiency from industry, these simple variable frequency drives (VFDs) are evolving into intelligent controllers with more technical capacity, functionality and higher processing ability, enabling advanced networking with a variety of systems and replacing the need for secondary components.
An emerging trend in VFD technology is the inclusion of not only predictive drive maintenance functions but also motor monitoring. Traditionally, vibration sensors, motor winding and load monitoring functionalities have required external components to be integrated as separate items in a system. These extra components require installation and integration into other control networks, not to mention additional space, which may result in increased capital costs. Often, panel space and economics push these items out of specifications, leaving processes susceptive to unexpected failures, downtime and emergency breakdown costs.
However, increasing availability of cost-effective technological components has enabled VFD manufacturers to incorporate new features in a standard drive unit. One of the latest innovations has been the ability for some manufacturers to include motor condition-based monitoring as part of their standard offering.
This technology allows for vibration sensors, load envelope monitoring and winding integrity sensing to be integrated directly into the VFD without requiring external components. This means users now can benefit from fewer components and reduced space requirements and integration into other control systems. This may open the door for more facilities to add these predictive maintenance functions into their day-to-day operations.
Condition-based monitoring allows users to extract data that assists in answering critical questions about the condition of their equipment. Such technology helps facilities transition from reactive maintenance to proactive maintenance, thereby making room to schedule repairs. By highlighting potential problems and notifying systems of abnormal behavior, condition-based monitoring in VFDs allows users to take proactive steps. For instance, knowing that there is increased vibration on a motor can allow users to act on a failing bearing before it becomes catastrophic and damages both the motor and whatever item it is connected to.
There are three main features of condition-based monitoring. The first one is load envelope monitoring. It allows information about a process to be programmed into a VFD and defines the anticipated motor load when operating in normal conditions.
The second function, vibration monitoring, creates a way for users to wire in a vibration sensor and then set warnings and alarms that correlate to acceptable levels for the motor setup.
Lastly, winding integrity constantly monitors the feedback from the pulse width modulation (PWM) waveform and can identify if there are small shorts or end turn failures beginning to appear.
Before exploring each of these features individually, a user will need to discern the standard normal operating conditions.
For many units, this can be done by establishing a baseline and having that information populate in the drive. It will typically involve running the drive for a fixed amount of time when first installed to establish what is acceptable and what is not.
From there, the VFD is flexible enough to allow the user to define the operating range before a warning is generated, or when the VFD needs to shut down to protect the process. Instead of taking a rigid framework and trying to have a process fit into it, a good condition-based monitoring program can often adapt to what is normal for each specific process.
Lastly, in cases where it is not possible to run a fixed baseline, some VFDs will allow an end user to run the drive over a few days to gather the information in an “online” scenario.
Here is more on the three features of condition-based monitoring:
1. Load Envelope Monitoring
Load envelope monitoring establishes the expected load at a given speed. When the baseline is run, the drive records the load across the speed range. From there, the user can establish a window that will determine when the motor is pulling too many, or too few, amps.
An example of this could be a centrifugal pump. If the current draw of the motor gets too high, it could indicate an issue is developing with a bearing and should be addressed. If the motor is not drawing the expected number of amps, then there could be a problem with flow, whereby the pump is not receiving enough fluid. This too could trigger the drive to stop and prevent the pump from running dry.
2. Vibration Monitoring
With vibration monitoring, the user can now connect sensors directly into the unit, rather than another monitoring device.
This simplifies installation and keeps all of the motor control and monitoring in the VFD.
Vibration monitoring is another tool used in predicting problems. When the vibration exceeds limits, it can indicate that a motor bearing, coupler, gearbox or sheave is beginning to fail and should be replaced. Further, if the motor is directly coupled to equipment such as a fan, a growing imbalance could also be picked up and maintenance performed before the motor takes on damage.
3. Winding Integrity Monitoring
The last component to a conditioned-based monitoring system is the winding integrity monitoring. As part of the baseline establishment, the drive can determine proper feedback from the motor. With many winding failures starting off as small shorts, the feedback the drive receives will change. As a result, the drive can in turn interpret these changes and recognize that a small short has developed. By identifying this early, the user can plan a shutdown and do a motor swap, sending the old unit out for repair. This can be more cost effective than having to work these problems out in a rush, or in an emergency breakdown situation. Also, preventing further damage can mean fewer repair costs.
Overall, condition-based monitoring systems assist users in better protecting and predicting the life cycle of both the drive and the motor. As VFD technology continues to advance, these controllers will provide users with more ways to be efficient, reduce costs and be better prepared when the unexpected does occur.
With load envelope, vibration and motor winding integrity monitoring, VFDs may once again change the landscape for motor control. Alongside increased efficiency, VFDs can also reduce the items required for a complete system, predict both drive and motor life cycles, and find new ways to protect investments.