Motors & Drives

Nov. 1, 2003
Motors and drives are where the action is. They convert electrical power into the torque and speed necessary to keep industry turning

Motors and drives are where the action is. They convert electrical power into the torque and speed necessary to keep industry turning. The editors of Motion System Design conducted a survey among industry experts, asking for their feedback and analysis. Here are their responses, which we hope you’ll find useful.


When using motors and drives, what applications present the most challenges in terms of machine productivity and why?

JOHN/BALDOR: In any application, the greatest challenge comes when the system integrator, designer, or user must select the best motor technology. After doing this, probably the most challenging applications are when the user tries to cut corners. Too many compromises in performance or repeatability are the result. This may lead to disappointing machine performance and productivity. In short, results of compromises from shortcuts must be understood. Communication and clearly defined expectations are a must.

MAX/LEESON: The motor and drive applications that present the most challenges in terms of machine productivity are those in washdown environments in the food processing industry. Productivity in this industry depends on the up-time of the machines. Because an electric motor is frequently the prime mover for a machine, a rugged and reliable motor is required to ensure maximum up-time. In a food-processing washdown environment, all equipment — including the motor — is frequently exposed to harsh chemicals such as bleach, caustics, acids, and surfactants. If not properly excluded, these chemicals can penetrate the motor and damage internal components. Eventually, the motor will fail.

DOUGLAS/BOSCH REXROTH: Machines that require high accuracy, high force and high speed simultaneously, such as grinding machines or large mold-making machines, can be very challenging. You can always use a ball screw to achieve high force, but you will be limited by the speed. Therefore, linear motors are a perfect solution for these types of applications.

CHRIS/DANAHER: The most challenging applications are generally those requiring extremely high levels of accuracy and throughput at high operational speeds. Challenges stem not so much from the motor-drive design, but rather from the design engineer’s need for an exact solution without added lead-time or cost.


What are the worst cases of improper design and implementation you’ve seen?

CHRIS/DANAHER: Coupling high-performance motion components to a load with ordinary gearboxes, compliant couplings, or even belts, is quite common and easily preventable. These arrangements defeat the purpose of utilizing high-performance components and can create significant control issues.

• Another problem, which can be difficult to diagnose, is using moderate or low-quality cables between the drive and motor. Time and again, these lower-quality cables are responsible for intermittent electrical noise issues that have a significant negative impact on system operation. It simply doesn’t make sense to have high performance motion components connected with cables that are not up to the task.

• A third common error is not matching motors and drives, which results in low performance, overheating, or both. The drive should be specified along with a motor.

• A fourth common error is utilizing inappropriate feedback devices. Using the wrong feedback device keeps the system from operating at its intended level of performance.

DOUGLAS/BOSCH REXROTH: Improper sizing of a linear motor is, by far, the most improper implementation that we have seen. With a rotary motor, you can typically up-size your motor, add a gearbox or modify gearing to overcome inertia mismatches. This is not possible with linear motors; they are an integral part of the machine.

MAX/LEESON: The worst cases of improper design and implementation occur when the wrong motor is selected for the application. For instance, a generalpurpose motor is much less expensive than a washdownduty motor — this is especially true of stainless steel motors. However, general-purpose motors will last days or weeks in a washdown environment, whereas a washdown-duty motor will last months or years.

Best Practices

Describe best practices in designing with motors and drives.

DOUGLAS/BOSCH REXROTH: Proper sizing of both motor and drive is key. For the motor, you want to make sure that the inertia of the motor is equal to the load inertia. Too high load inertia can lead to lost control, and if the motor inertia is too high, you can waste energy just to accelerate the motor.

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GEORGE/BOSCH REXROTH: When sizing a drive, make sure that it can supply the current necessary for the motor to perform as required. The gains should be properly tuned for optimal performance. If not, lost production and unstable performance — such as excessive overshoot — can result.

MAX/LEESON: Don’t undersize the motor, or its life will be reduced. It’s equally important to avoid over-sizing the motor. The extra “service factor” may look like it will extend motor life. However, when a motor is operated significantly below its full-load rating, it is no longer operating at the most efficient point on the load curve. The cost of the additional power usage offsets any savings from longer life.

CHRIS/DANAHER: Design engineers should take as much time as needed to define the machine structure and motion performance needs based on mass, force, and motion profiles. It is important to consider that existing designs may not be able to handle the higher performance levels offered by newer motor technology.

Designers should work with suppliers that offer a breadth of product options to meet specific needs, whether from a catalog standard, modified standard, or completely customized solution.

Designers should also utilize tools that manufacturers make available, such as software that can help solve motion system requirements and identify components that will meet those requirements. Then, manufacturer application engineers can help with intangibles that aren’t necessarily covered in the software.

JOHN/BALDOR: Usually the biggest challenge for a designer is obtaining complete information and a clear description of the job to be performed. Get a list of all parameters for the application and define the machine and its electromechanical components.

MAX/LEESON: Continue to offer products with the right feature set at the right price. This will help designers that are under tight budget constraints to be able to select the right motor for the application.

JOHN/BALDOR: The first step manufacturers can take to improve productivity is to continue development of products.

For example, a new brushless servomotor can develop up to 50% higher torque than previous models while reducing package size. This additional torque can increase a machine’s throughput. Other product improvements are in the software arena, where the newest is 95 times faster (9500% faster) than its predecessor.

Another role for manufacturers is to provide technical support to educate designers. Most motor and drive manufacturers already have a technical application group to assist with handling of questions regarding proper product selection. Beyond that, customers should be trained on how to use and apply products for the right applications. For example, lack of training on what servo can accomplish often limits what a designer may put into a new design. Additionally, unfamiliarity with programming motion controllers limits incorporation of time-saving devices.

GEORGE/BOSCH REXROTH: New products increase speeds, feeds, and accuracies, creating higher throughput with less waste. Some servo drives can be ordered with industry-specific firmware to provide for optimized, applicationspecific functionality built into the drive. In addition, the latest servo drives use the same setup and diagnostic software to provide quick and easy startups and extensive diagnostics.


What can designers do to improve productivity?

JOHN/BALDOR: First and foremost, designers, users, and integrators can improve productivity by learning of a product’s full capabilities; the best way is to attend an educational class. A variety of classes for manufacturers, system integrators, designers, and distributors can help participants learn about the latest technology and what it can do for their application. Labs and hands-on learning help individuals learn how to use the equipment, what its capabilities are, and how to write programs for their applications — thus improving their machines’ productivity. Designers know their application best and their machine best. Giving them the tools allows them to implement productivity improvements.

Second, in selecting motors and drives, designers should check out stocked products and specify these stocked items for the machine, as opposed to specifying customdesigned units. Machines are up, running, and productive sooner with a stocked item.

GEORGE/BOSCH REXROTH: Some motors have specifications permanently stored in feedback memory. This allows the drives to recognize the motors, read in these parameters, and adjust control-loop gains and settings to optimize system performance. This feature reduces startup time and required expertise. In addition, if the application requires, these control settings in the drive can be manually overridden to allow for custom settings.

CHRIS/DANAHER: Designers can partner early in the design process with suppliers who have a breadth of expertise, engineering resources, and available product to help them select the solution that will meet their application requirements.

MAX/LEESON: Designers should follow manufacturers’ recommendations.

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End-User Advice

What should end users do to maximize life and productivity?

CHRIS/DANAHER: The simplest thing that end-users can do to maximize machine life and productivity is to follow the maintenance recommendations of the machine. They can also demand from machine builders systems that put life-cost ahead of upfront- cost. As an example, systems that employ direct drive brushless motors will have a higher up-front cost than systems using gearboxes, drives, or other mechanical components, such as ballscrews. But direct-drive systems require significantly less maintenance over the life of the system. This maximizes machine uptime and minimizes maintenance costs.

JOHN/BALDOR: Increased life results from following manufacturer’s maintenance guidelines. Be sure to observe the motor’s radial and side loading parameters. Operating within the design limits will improve reliability and extend life. Also, do not try to increase cycles and machine load without calculating if that will overload the machine. Notify the manufacturer about ambient conditions in which the motor and drive will operate. The manufacturer can then suggest a motor better suited for the environment if he or she is aware of those conditions.

MAX/LEESON: Follow the manufacturers recommendations for proper installation. Don’t throw the instructions out with the box! Washguard motors have multi-position drain plugs on the endbells that need to be opened in the proper orientation to allow condensation to drain from the motor. Many times this does not happen, and the motor’s life is unnecessarily shortened. Another consideration is that the product’s warranty may be void if installation instructions are ignored.

DOUGLAS/BOSCH REXROTH: Many of our motors and drives are equipped with blowers to keep them cool. Filters help keep cooling fans free of debris in the incoming air. It is crucial to keep these filters clean to prolong the life of the blower, and keep it running to avoid motor shutdowns.

Some of our high-performance motors and drives have liquid cooling to maintain temperature. With these types of products, it is important to follow the specified maintenance schedules. Maintaining the pH level of the coolant is extremely important.

Another recommendation is to perform vibration analysis as preventive maintenance on the motor to determine if bearings are about to go bad. The latest drive systems offer built-in intelligent preventive maintenance specifically for this function.

Component Interactions

How do choices made involving motors and drives affect other areas of the machine or system?

GEORGE/BOSCH REXROTH: Some drives support open system interfaces, which enables them to communicate with a variety of control platforms. The latest drive systems can be equipped with built-in, EN 954- 1 category-3 compliant safety technology. This popular European safety standard is gaining worldwide popularity.

The mechanical system has to be able to handle the dynamics of its motor. The mechanical system is always the weakest link and determines how high it gain can be set. The gains on a well-designed machine mechanically can be set so high that they can actually move the machine if it is not properly secured.

MAX/LEESON: The motor is the prime mover in the drive train of many machines and that motor may be controlled by a drive. Making the wrong choices when selecting or installing this equipment affects the performance of many components downstream from the motor. Ultimately, performance of the entire machine will suffer.

Today, with the cost of downtime so high, most machines require high performance, so fewer designers consider making trade-offs that will affect the performance. In fact, designers today require better performance and lower costs ... there are no trade-offs there.

JOHN/BALDOR: When attempting incremental motion, for example, a larger motor only adds to the system inertia that must be started and stopped. “Just enough” is the right selection. Components of a machine need to be lightened if faster cycles are required, it may cease to be a “more motor” solution.

CHRIS/DANAHER: Motor and drive selection has a significant impact on overall machine size, as well as reliability and machine throughput. There really is no more significant consideration for the machine or system.

Trade-offs include considering up-front costs versus long-term costs, as well as the productivity and performance of various options. Conventional mechanical devices that create linear motion can keep up-front costs down and are a good option in many cases, but they can also limit bigger-picture performance in systems that use higher-performance technologies in their other axes of motion. The key is selecting the optimum product for each axis based on an understanding of total machine requirements.

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