Variable-speed drives

Productivity is a shared goal in industrial applications involving designers, component makers, and end users. Everyone plays a role. In this report, the editors of Motion System Design polled variable-speed drive experts for their advice on optimizing productivity. Here are their responses on both sides of the issue, which we believe you'll find most helpful.

What particular design/construction features in variable-speed drives contribute to higher productivity, and why?

Markus/B&R USA: Design features contributing to higher productivity include motor type, housing, and materials selected. Built-in components that facilitate high-speed operation also help. An integral braking function is one example, providing safe, fast stops from maximum speed to standstill under maximum load.

Drives that are easy to upgrade also improve productivity. A networked PLC, for example, can remotely update drive firmware and application data through a single machine interface. If a drive must be exchanged in the field, its operating system automatically updates from the PLC.

Paul/Hitachi: Downtime is generally the converse of productivity. In applications utilizing inverters, downtime is primarily caused by trips or faults, and failures. Minimizing these is key to maximizing the inverter's contribution to higher productivity. Thanks to powerful microprocessor technology and advanced algorithms, sophisticated trip-avoidance functions are now also being incorporated in inverters. Such functions adjust output frequency and voltage to keep the inverter running through system disturbances that might otherwise have caused the inverter to trip out. Every trip avoided means uninterrupted production.

Drew/Siemens: Variable-speed drives must be able to communicate with other devices and in many cases control entire processes. By precisely controlling process speed, for example, a drive can continuously modify throughput to meet demand changes. To coordinate more than one aspect of a process, variable-speed drives must be able to communicate with many devices. To illustrate, on a bottle labeling system, the drive must be able to precisely coordinate the upper level controller, bottle feed, and labeler to produce a quality product.

Energy savings is key to productivity. On variable torque applications, such as a fan for airflow control, normally the motor connected to the fan operates across the line. This means the motor always operates at a fixed speed: 1,800 rpm for a four-pole motor, for example. The result is consistent energy demand from the power system. However, if the system requires varied airflow, then using a variable-speed drive can save a lot of energy. The power requirement varies as the cube of the speed, so if airflow requirements are only half the air volume, then the variable-speed drive can operate at half the output frequency — resulting in motor speed at a low 900 rpm. The resultant power requirement is only 1/8th of that required to operate at full speed.

Philippe/Leeson: Ease of communication is a must. Variable-speed drive packages must be able to talk to everything from PLCs to the simplest switches. Speed optimization features also improve productivity, as do remote operator interfaces that link and coordinate multiple drives.

What can designers do to ensure higher productivity from the variable-speed drives they place in machines?

Markus/B&R USA: When possible, designers can stack or mount drives close together to save cabinet space. Drives with built-in functions, such as a restart inhibit, further save space as well as the cost of additional components. (A restart inhibit guarantees that if the enable signal is removed, the motor will not start unexpectedly.)

Paul/Hitachi: Even the most sophisticated drive functions cannot help prevent trips and failures if the motor and/or drive is not properly sized for the application. It is important that the selected inverter have enough current capacity to deliver the torque needed by an application across the speed range. Recall that torque is not just required to keep the load moving against friction; it's also required to accelerate and decelerate the load. If rapid speed changes are required, peak torque requirements increase as well. Therefore, it may not be sufficient to say that the drive matches the motor horsepower rating. A motor can safely draw many times its full load nameplate current for short times with no adverse effects, while the same is not true for inverters. That said, while it may be necessary to oversize a drive in certain applications, each case has to be reviewed in detail.

Drew/Siemens: The key to achieving higher productivity with variable-speed drives is to look at the entire system. Proper design based on boundaries such as overall speed requirements, accuracy, inertia, and process coordination must be considered; it is especially important to match motor and variable-speed drive design with load requirements. Other considerations — such as the environment in which the variable-speed drive and motor are to be placed — are also important.

Philippe/Leeson: With ongoing cost pressures and limited design cycle time for new products, OEMs have to be fast and flexible with new designs. Especially today, designers need to work more closely with end user specifications and applications on the “front end” to better position design criteria in new products. If an end user is calling for “scaled down features,” designers can maximize “value add” by providing just that.

What can end users do to ensure higher productivity from the variable-speed drives on their machines?

Markus/B&R USA: One way to raise productivity is to take advantage of plug-in slots on the bottom of some drives, adding encoder or resolver feedback for precise motor positioning. Other slots can be used for modular high speed I/O in case programmable limit switches are needed. Monitoring temperature is another trick, making it possible to operate the drive safely up to its limit. The resulting overload capability provides sufficient reserves for short load or acceleration spikes. End users can also request maintenance-on-demand features employing internal function blocks that monitor parameters to detect electromechanical drive train wear. If there is a problem, the drive can notify service technicians via PLC.

Paul/Hitachi: End users should be sure the inverter is installed in an environment that offers suitable protection based on the design. Humidity, dirt, and dust can bridge leads or foil traces and cause shorts; they can also build up on components or cooling fan blades and interfere with heat expulsion. For maximum productivity, end users should also make sure input power is as clean as possible. If power to the inverter fluctuates significantly, is prone to large transients, or goes out of the specified range, this can damage the inverter.

Drew/Siemens: End users often decide who supplies their machines. Based on information submitted regarding characteristics required for an application, they can also influence the machine's design. By working closely with OEMs, end users can influence the system to be implemented, as well as the quality and reliability of variable-speed drive components chosen.

Philippe/Leeson: End users can make their needs known, asking for specific programming features and communication options. Productivity gains from things like this will impact them the most.

What are the typical shortcomings in the design/construction of variable-speed drives, and how do they affect productivity?

Tom/Automation Direct: Improvements are long overdue on the input side of the drive: active input stages on drives could impart the ability to perform power factor correction and solid-state soft starts. Benefits would include power cost savings, further reductions in package size, and increased reliability and safety.

Other power input enhancements could include line reactors, EMI filters, and semiconductor fuses. While these devices might increase the price of the drive itself, they are required for many applications. Specifying, mounting, and wiring these additional components into a drive system is certainly not as productive as buying a drive with those parts designed-in. While such built-in extras might lead to increased productivity for the users, the price pressure on manufacturers discourages them from including these advanced features.

Ease-of-use issues are still prevalent with many VFDs. Defaults for all parameters should allow the drive to operate “out-of-the-box” with a minimum of programming. Some manufacturers force the user to enter dozens, if not hundreds, of parameters to spin the motor. Entering nameplate data for a few key settings is all that should be required to bring a drive system online quickly and productively.

English-language error messages are much more useful than cryptic error codes. Instead of looking up a code (or looking for the manual) users can begin problem diagnosis immediately. Imagine a drive that e-mails an alert about increased frictional load, allowing the replacement of a faulty bearing during scheduled downtime, as opposed to an unexpected bearing failure during a critical runtime period. Drives with self-diagnostic and predictive-maintenance capabilities offer additional productivity gains.

Gilbert/KB Electronics: Drives can also impair productivity when a program is not sophisticated enough to meet application requirements. For example, in a web application, the drive's PID parameters must be tuned to the optimum level in order to achieve maximum output without affecting the end product reliability. If the drive designer does not incorporate into the software a simple way to tune these parameters, an optimum result will not be obtained. This can cause a direct reduction in productivity.

Lou/Danaher Motion: Some drives are designed and built without certain electrical and mechanical features needed to survive harsh industrial environments, affecting productivity in a negative way. These include:

Enclosure design — An enclosure with NEMA-4 rating can protect against dust, sprayed liquids, and other contaminants present in a typical industrial environment, where a less expensive NEMA-1 design allows these substances to contact the inner workings of the drive, causing the potential for premature failure or overheating.
Non-isolated user inputs — The lack of input signal isolation makes drives more prone to “noise” which can cause nuisance tripping, or catastrophic failure if the input signal wiring is inadvertently grounded.
Lack of line conditioning components — Drive designs that omit EMI/RFI suppression and line conditioning components are more likely to fail due to power line problems, and can also cause problems with other electronic equipment by polluting the connected power line with electrical noise. Drives lacking these features have an initially low purchase price, but adding external filters or chokes often causes installation price to exceed that of a drive incorporating these features.

John/Oriental Motor: One known shortcoming of dc-input brushless-motor drive systems is the length limitation of the motor-to-drive cable. When compared to ac input systems, the 24-Vdc input systems' cable length may be limited to as much as 20% of the length of a similar ac input system. The main reason for the shorter extension length is due to voltage losses in the power lines, resulting in a drop in torque. The loss in torque, however, occurs more so in the higher speeds (at about 2,000 to 3,000 rpm), which may in turn lower productivity.

Douglas/Bosch Rexroth: Typical shortcomings of variable-speed drives are that they cause noise in the motor power cables and only offer limited speed accuracy. As update times within the drive get faster speed accuracy is limited.

What are the most common mistakes designers make with regard to variable-speed drives, and how do these mistakes affect productivity?

Tom/Automation Direct: Designers must fully understand the mechanical systems that the VFD is driving. The effects of friction, temperature, and gearing are all crucial to the proper application of variable frequency drives. In many installations, VFDs need to be protected from electrical noise and unreliable input power. The proper usage of branch protection, line reactors, and semiconductor fuses is often learned only after a drive failure.

Gilbert/KB Electronics: Drive designers should consider adding simplified PLC functions. This is particularly useful for applications where a PLC (or programmable relay) might enhance efficiency and process productivity but would not be cost effective.

Lou/Danaher Motion: Choosing the wrong type of variable-speed drive for an application can greatly affect productivity. An example of this would be using a volts-per-Hertz drive where the accuracy and extended speed range of a flux vector drive would allow more consistent results. Also, incorrectly sizing a drive can cause nuisance tripping and unwanted shutdowns. Peak overload and the drive's overload capacity should be taken into account.

John/Oriental Motor: One potential mistake that designers can make is not considering speed regulation of the variable-speed products they are choosing. Tighter speed regulation (typically ±0.05%) is often the benefit of an encoder-based variable-speed drive, but costs more up front. Lower speed regulation (typically -3% to -5%), which is offered by various ac induction motor systems, may suit the application fine, while saving costs for the designer. Matching the required speed regulation of the product to the application always leads to increased productivity.

Douglas/Bosch Rexroth: Inexpensive drives use small IGBTs with fast switching times and simple filters. The fast switching times translate into an increased efficiency for the IGBTs themselves, but on the downside, cause electrical noise that can harm motors. Improperly filtered noise combined with long cables can cause motor problems and lower productivity.

What are some of the mistakes end users make with variable-speed drives that lead to lower productivity?

Tom/Automation Direct: Many end users fail to post a schematic in the panel or near the drive. When problems occur, the personnel who respond may not have documentation for the system. Most drive inputs are now programmable, and their purpose is not easily deduced without proper documentation. That's why the control mode and other control-specific information should be prominently displayed, along with I/O connections. Time lost while troubleshooting due to poor or non-existent documentation is non-productive and unnecessary.

End users should maintain an offline backup of all drive parameters and settings. This can be helpful when a damaged drive must be replaced, or when checking to see if any parameters have been changed. The ability to quickly revert to a known, good set of drive parameters can be critical to reducing downtime and increasing productivity.

Gilbert/KB Electronics: Designers are constantly challenged with minimizing drive size to lower cost and reduce space requirements. This can make the drive less effective for the installer. In many cases, terminations on terminal blocks for signal inputs are so close in proximity that wire shorting is inevitable. Other times, mounting tabs are located too close to the drive's enclosure, making installation difficult. Wiring compartments have also diminished in size, complicating the wiring procedure.

In short, designers shouldn't neglect the installation process; they should design drives that are easier to install, thereby enhancing cost effectiveness of the installation process.

Lou/Danaher Motion: Some of the most common problems can be avoided by closely following the manufacturer's guidelines for installing and wiring the drive, such as following proper grounding techniques, using shielded cable where recommended, and routing wiring correctly. Failure to install line-conditioning apparatus (such as line reactors or chokes) where less than perfect plant power is available and running overly long motor leads without adding output impedance correction are factors of which end users should be mindful.

Mounting the drive in an area that is too hot, cold, or moist, or failure to provide sufficient space around the drive for proper cooling can also be problematic. Again, following the manufacturer's recommendations — and selecting a drive that provides the right amount of protection demanded by its environment — are key to reducing lost production caused by failure.

John/Oriental Motor: Not all motors offer the same value to the end user. End users should not simply look at the initial cost of the motor system; rather, they need to consider the effort and costs associated with servicing motors in the final equipment. If you take brushed-type dc motors as an example, they are almost always lower in initial costs compared to a similar brushless dc motor system. At a later time, however, the end user must be able to access the brushed-type dc motor, which sometimes is buried inside the machine and hard to get to, in order to change out brushes. In such cases, this is not simply an inconvenience, but also leads to lower productivity.

Douglas/Bosch Rexroth: Using cables that are too long (or that have poor shielding) is a common mistake made by end users with variable-speed drives. The resulting noise destroys motor insulation and causes eventual winding shorts. Having to replace an unnecessarily damaged motor is a sure way to lower productivity.

An even more prevalent mistake is contamination of the drive. A drive is often protected by a simple yet effective air filter. If the filter gets clogged, the drive heats up unnecessarily, causing it to age more quickly.