How AC drives can reduce maintenance costs
June 14, 2007 by Doug Weber
Manufacturers are always looking for ways to cut costs and increase the efficiency of their plants. One way to do so is through the use of AC drives.
Many users of AC drives are aware of the significant reduction in energy consumption that is offered when an AC drive is used to control the speed of a centrifugal fan or pump. Using a drive to operate these devices at, for example, 80 per cent of their rated speed, can cut energy costs in half. What is often overlooked is the far-reaching impact on the overall health of automated systems that AC drives can offer. In fact, converting a process from fixed speed to variable speed in itself can reduce wear and tear and maintenance requirements for mechanical systems by reducing start/stop cycles and eliminating vanes, dampers, valves and other mechanical system components. In addition to these benefits, understanding the features offered by a variable speed drive can significantly reduce system maintenance and overall operating costs.
Drives have somewhat limited monitoring capabilities compared to devices exclusively dedicated to preventive maintenance. By nature, however, they do monitor motor current and speed, and can perform protective functions based on that information. In an example of an intelligent motor control approach, PLCs and other controllers connected to a drive via a communications network can also monitor these values and provide warnings and reminders to maintenance personnel that something in the process has changed. By monitoring the motor current and speed, it may become obvious that a motor is loaded more heavily than normal and that the mechanical system should be checked before a failure occurs. Networked data helps users decrease downtime and protect machinery.
In more extreme situations, the drive itself will act to protect the motor. Almost all drives today have a built-in electronic motor thermal overload feature. When a motor is in a state of severe exertion, beyond its safe operating limits, the motor overload feature can reduce the output current or shut off the motor and protect it from thermal damage or catastrophic failure. Motor overload software uses an algorithm incorporating motor current, speed and time as inputs to model the temperature of the motor. This may also be done with thermister feedback directly from devices buried in the motor windings, using actual temperature readings to determine motor stress. Multi-motor applications–those using one AC drive and more than one motor – will require the motor overload to be disabled since the drive would be unable to distinguish each individual motor’s current to provide protection for individual motors. These applications require more advanced machine monitoring devices that can accept data from multiple sources to alert personnel of impending faults and failures.
S curve and controllable acceleration and deceleration are other, often-overlooked drive features that can help to improve process performance and reduce maintenance. When a load transitions from steady state speed to accelerating or decelerating, the transition is usually instantaneous. The same is true when the transition is reversed. While not as dramatic, it is the industrial equivalent of “popping the clutch” on a stick-shift car. This jerking action puts considerable stress on mechanical components. In belt-driven systems, belts can fly off or break. In geared systems, the process can wear or break gear teeth.
AC drives can control this phenomenon through the S curve feature. Using the same analogy, it is the process equivalent of “feathering or slipping the clutch” to ease into acceleration or deceleration. S curve controls the jerk or rate of change of acceleration. It has long been recognized as an aid in the handling of very light conveyor loads, such as in a bottling line, but it can also play a significant role in extending the life of mechanical components. Lower mechanical stress means lower maintenance costs.
The flying start feature is used to reconnect the drive to a motor that is already spinning and, as quickly as possible, resume normal operation with minimal impact on load or speed. When a drive executes a normal start, it initially applies zero hertz and ramps up to the commanded frequency. If the drive is started in this mode with the motor already spinning, large currents will be generated and an overcurrent trip may result if the current limiter does not react quickly enough. The likelihood of an overcurrent trip is further increased if there is a residual flux on the spinning motor when the drive starts. Even if the current limiter is fast enough to prevent an overcurrent trip, the end result is still to effectively decelerate the motor to a very slow speed and then re-accelerate it to the desired frequency. This can place extreme mechanical stress on the application, potentially causing costly downtime and repair costs while decreasing productivity.
In flying start mode, the drive’s response to a start command will be to identify the motor’s speed and begin its output synchronized in frequency, amplitude and phase to that of the spinning motor. The motor will then be reconnected at its existing speed and smoothly accelerated to the commanded frequency. This process eliminates overcurrent tripping and significantly reduces the time for the motor to reach its desired frequency. Since the motor is “picked up” smoothly at its rotating speed and ramped to the proper speed, little or no mechanical stress is present.
In some applications, such as large fans, wind or drafts may rotate the fan in the reverse direction when the drive is stopped. Flying start will determine, not only the speed of the fan, but also its direction of rotation. For faster uptime, it executes a safe, controlled deceleration to zero speed and then accelerates to the commanded system speed.
Some machinery may have mechanical resonance points that must be avoided to minimize the risk of equipment damage. Many of us have experienced a severe “shimmy” in the steering wheel because the car’s front end is out of alignment. Experience shows us that this shimmy can be severe at one speed, but speeding up or slowing down by just a few miles per hour will make the vibration stop. The car’s misaligned front end has a mechanical resonance only at specific speeds. All rotating mechanical systems have these resonant points and many can be damaged if allowed to operate continuously at these speeds, resulting in system downtime and increased maintenance costs.
Drives offer a feature called skip frequencies or critical avoidance frequencies to ensure that the motor will not continuously operate at one or more of these vibration points. The frequency that causes the resonance is programmed into the skip frequency parameters, and a bandwidth is programmed around the frequencies to create a skip band that avoids the vibration-causing areas. Most drives offer multiple skip frequency parameters to accommodate different resonance points.
Normal acceleration and deceleration are not affected by the skip frequencies. The drive output will ramp through the band uninterrupted. When, however, a command is issued to operate continuously inside the established band, the drive will alter the output to remain outside the band until a new command is issued.
When mechanical resonant frequencies are identified and drives are programmed to avoid continuous operation at those frequencies, wear and stress from vibration are greatly reduced.
An AC drive can control the amount of current it supplies to a motor. Current limiting functions are often used to prevent mechanical damage. By limiting current or shutting down the operation, AC drives can reduce mechanical damage. In addition to current limiting to reduce torque, many drives have a feature called an electronic shear pin – a modern take on an old concept. Outboard boat motors, for example, are usually equipped with mechanical shear pins. Rather than break the propeller if it were to strike a rock, the shear pin breaks, mechanically disconnecting the propeller from the motor and saving the mechanical system (and the boat owner’s pocketbook). Similarly, a drive’s electronic shear pin feature can define a current limit level that would cause damage. If the torque in the motor ever exceeds the set limit, the drive will automatically shut off the motor.
By limiting torque to a set level, AC drives provide good protection for systems that can become jammed. Chain breakage and other damage can be avoided by not allowing a motor to power through the jam.
With the increasing complexity of manufacturing systems, users want quick configuration to shrink development time. Once a project is installed and running, they also want access to diagnostics and maintenance data to maximize uptime. With the ability to integrate into one information platform, users can consolidate control disciplines into a single, integrated environment that offers considerable time- and cost-saving advantages.
Spreading the word
Taking advantage of system architecture and drive features doesn’t require a great deal of special training. Users need only be aware of the features and the benefits that the drives provide. By taking full advantage of the wide array of techniques already available, plant engineers can minimize the stress and abuse placed on valuable plant machinery, increase equipment uptime and reduce maintenance costs – key elements for boosting the bottom line.
Doug Weber is the marketing manager for the Allen-Bradley drives business at Rockwell Automation in Mequon, Wis.