11 recommendations for selecting motors and drives

July 11, 2013
Motors and drives are two of the foundation components of any automated system. Here are a few tips and best practices for achieving improved motor control:
1. Think system. When doing an equipment upgrade, think of the drive train as an integrated system and choose components with similar efficiency ratings. It makes no sense to couple a high-efficiency pump to an inefficient worm gear. The motor used to drive the load could be half the horsepower with a more efficient gear reducer because of the reduced energy losses.

2. Matching firmware. When changing a drive, make sure the firmware of the drive or motion controller matches the one you are replacing.

3. Intelligence improves uptime. When you want to improve uptime, think about replacing standard IEC electromechanical starters with an intelligent overload with a hybrid contactor. Now you have the best of both worlds - a programmable, intelligent overload that has motor load feedback (alarm back to operator as well as shutdown parameters) with a hybrid contactor. The hybrid contactor extends the time between end-of-life motor switching by a factor of ten. The intelligent overload will remain in service indefinitely.

4. Proper way to select motors. Selecting a motor needs a bit more than just calculating the maximum rotational speed and torque. The best way to select a motor is by drawing a speed-torque diagram of the load and the maximal allowed speed-torque diagram of the motor (as defined for continuous operation). If the diagram of the load characteristics is fully enclosed by the diagram of the motor characteristics, the motor is adequate.

5. Manual mode. The manual mode, which allows maintenance to check if one specific motor has a problem or not, is usually forgotten. If the motor will not run correctly, put the drive into manual and run the motor for a short time. This eliminates the drive from the problem and points to the controller or, if local control does not work, the drive could be suspect.

6. Do your homework. Always check for the proper rating and proper type of drives or motors for different type of application. Ask the supplier about the fail-safe function of the system and always check for proper grounding of the system. Make sure you inform the supplier about the features you want and the process to be controlled.

7. Magnetically coupled drives. For variable speed control of rotating equipment, such as drives for an agitator motor or a chiller compressor, take special care to place the drive in a hardware panel installed in a dust-free location, preferably air-conditioned. A better solution for harsh environments that have maintenance people with minimum skills might be a magnetically coupled drive with variable speed option. Magnetically coupled drives have the advantage of decoupling of vibration and potential overloading of the motor or driven equipment due to the air gap between the motor shaft and the other shaft, which can reduce costly damage to mechanical seals on equipment.

8. Economical spares. Try to use the same type of drives to maintain a common inventory. Also, instead of stocking multiple spare cards for the drive, keep one spare module. This will be an economical solution.

9. Deciding factors. With the sophistication of drives technologies, it often becomes difficult to decide what to choose as the right technology for positioning applications. In the past, servo systems were the only choice available for achieving reliable positioning performance. Today, however, many applications, excluding CNC machines, can be implemented using AC flux vector drives. The determining factor in the final choice of technology is the positioning accuracy requirements and the ability to deliver full torque at zero speed.

10. Drive failures. The reliability of the core components, such as the PLC or a variable frequency drive, in a control system is important. Drive failure may be caused by many factors: PCB calibrating, IC chips failure, mishandling during installation, operating environment, etc. Even though the manuals of some drives claim that using certain technologies, such as MOV, common mode choke, common mode capacity and others, can make the drive more EMI compatible, it can also make them easier to damage.

11. Good housekeeping. Keep motors and drives clean and free of foreign contaminants to keep them running for a long period of time.

Performance packed into compact drives

Advancements in today's AC drives, including compact drives, require that engineers more thoroughly review how drives with new, performance-enhancing features can contribute to automation projects.

Criteria for selecting a compact drive still requires an understanding of the most appropriate control technology - volts per hertz, sensorless vector, etc. - but while those control technologies have remained relatively unchanged over the last decade, there have been dramatic technological leaps in other areas.

Similar to trends in consumer electronic products, compact drives now pack more intelligence and user-focused design in smaller sizes. Engineers tasked with implementing an automation project need to understand the latest compact drives features and how the right mix of features can ease data sharing, improve safety, minimize programming and installation time, and, ultimately, lower total overall cost.

To find a compact drive that can do more and maximize value, engineers should examine features in the following four key areas:
Communications. Seek compact drives with multiple communication options, dual port connectivity supporting ring topologies with device level ring (DLR) functionality, and support for other open industrial networks.Safety. Seek embedded safety as a standard feature to help protect personnel and equipment, meeting ISO 13849-1 standards with ratings up to and including SIL 2/PLd Cat 3.Programming. Seek simple local and programmable automation controller (PAC)-integrated programming to reduce development time. Features such as descriptive scrolling text on LCD human interface modules (HIM) and intuitive software with standard connectivity help speed up drive configuration.Physical size. Seek compact, modular designs that allow simultaneous configuration and installation while offering flexible mounting options with small footprints, high ambient operating temperatures and low clearance requirements, saving valuable panel space.

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