Improvement of reliability, output and efficiency are the core responsibilities of plant management, operation and maintenance. The use of the latest technologies in online electrical testing is proving that additional monitoring capabilities are crucial for cost-saving operations.

Online monitoring tools designed for field use must be safe and easy to use if they are to be used frequently and reliably. Connections to medium or high-voltage applications can be achieved safely by hookups to existing current transformers (CTs) and potential transformers (PTs). A general rule of thumb in predictive maintenance says that the quality of a predictive maintenance (PdM) program is about equal to the quality of the tools used times the frequency with which they are applied. In other words, only top-of-the-line tools and frequent monitoring are likely to yield effective plant reliability program results.

Online monitoring is too often performed with unsafe procedures, usually to get a job done as quickly as possible. Responsible plant operation, however, allows for only the following two methods for safe performance of online testing:

a) Lockout procedures using protective gear.

b) Dedicated hardware in critical motor control cabinets.

The crucial need for plant reliability requires frequent monitoring, which can't be achieved with the first option if cost-effective long-term measures and ease of operation are desired. Only additional hardware installed in critical motor control cabinets (MCCs) will yield reliable plant operation in an easy, safe, and cost-effective manner.

VOLTAGE QUALITY AND LOAD LEVEL

 As stated previously, online monitoring has evolved considerably from when state-of-the-art electrical monitoring was confined to current and voltage levels. Power analysers introduced the capability of monitoring power quality several years ago, and they are now capable of identifying and logging voltage imbalances, distortions and transients.

Poor voltage conditions are a leading cause of overheating in motors that are not running over-loaded. However, comparisons of the severity of sub-optimal voltage quality with its effect on the motor are not possible with power analysers. Evaluations of the influence that real-world poor voltage conditions exert on motors at different load levels allow a maintenance professional to ensure that the motor is running at the proper NEMA derating. Only the addition of very accurate load estimations to power quality analysis will offer useable results from a motor PdM standpoint.

The concern in the field is this: is a given motor operating under too much load under the particular voltage conditions it encounters? This can only be answered reliably if accurate load estimations and power quality measurements are put together using the applicable professional standards and guidelines.

VOLTAGE LEVELS AND ESTIMATION ERRORS

A frequent circumstance in the field is that voltage busses are operated at an over-voltage exceeding 5%. The reasons for this practice are twofold: on the one hand, a higher voltage level on the voltage bus can ensure that the voltage that reaches the motor terminals is sufficient after subtracting the voltage drop in the lead resistance. On the other hand, over-voltage will induce lower currents, which are frequently preferred in the field. The reasons behind this preference are more for comfort than necessity. Stator currents are frequently used for rough load estimation. Additionally, stator currents are known as the source of I2R losses in the motor. Increased voltage levels are frequently used to artificially drop the current level. 

This promotes a feeling of reduced losses to the motor and a cooler and healthier operation. This condition, however, only marginally improves the operational efficiencies of motors while causing severe deterioration of the operating power factor. Only substantial efficiency increases will reduce the motor's operating temperature and lengthen its life. A higher voltage level only raises the motor's efficiency marginally, and does not strongly change the operating temperature or expected life. However, artificially lower stator currents lead to erroneous conclusions if current levels are used as a measure of load.

In addition to nameplate inaccuracies, this method also depends upon voltage level. As mentioned above, over-voltages are very common in the industry. Load estimation based upon the current level can incur severe errors, prompting a false sense of security if a motor is running with an overvoltage and rated stator currents. In reality, motors under these conditions are operating into their service factor, introducing overheating conditions and rapid deterioration.

A CONVEYOR PROBLEM SOLVED

At a pulp and paper mill in Canada, a 60hp, 1,170 rpm 460V motor is run by a variable frequency drive (VFD). This motor runs a conveyor belt at varying speeds. The conveyor belt feeds logs into a saw. When no log is in the saw, the conveyor belt runs at a higher speed; when a new log approaches the saw blade, the conveyor belt has to slow down to a speed tuned for cutting. Shortly after it detects that a log has left the saw, the conveyor returns to maximal speed again.

Figure 1 graphically depicts the described process with torque and frequency over time. Frequency is shown in red, and voltage in blue. Data was captured over a period of 7.5 seconds. The VFD runs the motor at 60Hz, then slows down to 12Hz for cutting. In this case, the cutting of a log takes less than 1.4 seconds, and the VFD ramps the frequency back up to 60Hz.

The particular control of this VFD is of the V/f type. The voltage level is kept proportional to the operating frequency. This is a very common control type of control for low-cost implementations.

Figure 2 shows torque level (red) versus time and the motor's operational speed (blue) versus time. It reveals that the maximum speed of the motor is 1,200 rpm, while the speed of the motor during the cutting of the log is only 215 rpm. The torque level is constant when the conveyor operates at a constant speed. When the VFD backs the voltages down, the motor speed also slows down in response to reduced torque. As soon as the low speed is reached, the torque level normalises to the steady-state level. During the following acceleration process, it is necessary to increase the torque level, which falls back to the steady state once acceleration is completed.

First, during deceleration, the torque drops severely. It even drops below the zero-torque line, which means the motor is employed as an electrical brake during this time. What this also means is that the log has actually slowed down rapidly (reducing the speed of the motor to about 1,000 rpm in just one second). This information is telling because it ensures that the conveyor is not only designed for pulling but can also withstand pushing.

Secondly, it is noteworthy that over a period of about one second, the operational torque rises above the rated torque, as shown in figure 3. This means the motor is stressed during acceleration. The torque required to accelerate the log to 1,200 rpm back from 215 rpm is too high, especially if it has to be achieved in less than one second. The solution to this dynamic over-torque problem is simple. The VFD is programmed with a constant, which is called Hz/sec. This constant sets the maximum acceleration and deceleration rates. In this case, the maximum acceleration rate is too high. Reducing the VFD's Hz/ sec setting brings the acceleration torque down to healthy levels. A disadvantage of this is that acceleration and deceleration processes take a bit longer (which can translate into a reduction of production over time). In this case, the settings were changed such that both the acceleration and the deceleration took an additional 0.2 sec, which has little to no effect on production.

Another notable issue can be identified upon inspection of the last three seconds of data. The operational torque over this period oscillates, a typical symptom of an improperly tuned feedback loop. The application is 'chasing' or cannot maintain a steady state speed free of oscillations. This oscillation fatigues the conveyor system and often leads to premature wear and failure. This type of oscillation can be avoided by installing a PID controller and proper tuning. However, this can only be done once such a problem is clearly identified.

The three issues determined by analysis of data on this graph have relatively straightforward solutions, yet they can't be identified with standard motor test and monitoring instrumentation. Only instrumentation capable of displaying the dynamics of a VFD application can serve as a tool to ensure systems can be maintained and operated reliably.

This example shows that it is possible to monitor, correct and maintain the most dynamic applications in modern plants: variable frequency drives. There are tools available on the market now that allow these newer applications to be debugged and maintained so that they keep running tomorrow. Case studies like this are the reason why more companies are adopting aggressive and high-quality online monitoring predictive maintenance programs.