Electric drive decisions

How do electric drives contribute to productivity in today’s automation environment?

Rich • SEW: The most basic principle for achieving productivity gains by using variable frequency drives (VFDs) is control: not just motion, but when to move, based upon what condition, how fast, to where, when to stop, and how? (If it asks why, get a new VFD). Simply installing a VFD into the system is useless without control. You probably don’t just need variable speed; you need motion — well coordinated and executed motion. That’s productivity.

Len • Bodine: In the past, when an application required variable speeds, designers were forced to turn to brush-type dc or costly servomotor drive systems. The availability of small adjustable speed drives (ASDs) and of variable speed ac gearmotors and motors now allows equipment designers to leverage their unique characteristics to improve productivity. Unlike brushtype motors that require routine brush replacement, ac inverter duty, three-phase gearmotors and motors are virtually maintenance free. And contrary to variable-speed dc drive systems, ac drives and motors can operate at higher-than-rated motor speeds, meaning that manufacturing can be accelerated if the need arises. Ac drives offer a variety of functions as well, including start, stop, reverse, acceleration ramp, deceleration ramp, and timed operations.

Steven • Lenze-AC Tech: Drives boost productivity with the ability to gain precise control of processes. Whether it’s speed, torque, or position control (or any combination), drives offer the ability to  fine tune and control electrically driven machines to make them more ef - cient for high volume, high-quality production. Controllability and tuning make machines more adaptable and compatible with changing production materials and processes. Drives also allow easy monitoring of machine production through digital and analog signals as well as serial communication. Drives often reduce energy costs when used to decrease speed or torque, particularly with centrifugal-type loads such as fans or pumps.

Rick • Baldor: Drives are usually used for one of two reasons: To control the speed of a process because the process simply requires variable speed to function properly, or to provide energy savings. In the  rst case, drives allow speed motor variation (which in most cases cannot be varied without a drive) so that mechanical speed changing devices are not needed. In the second case, the system (often a fan or pump) can always run at full speed and work in the application; however, considerable energy can be saved if motor speed is reduced. These savings are governed by the Af nity Laws, but in general, the amount of energy consumed is proportional to the cube of speed. So, if a motor can be operated at half speed and still perform the required job, then required energy is reduced by seven eighths.

Chris • Leeson: In today’s rapidly changing automated production environment, the drive is key: Its (correct) use improves application pro tability through the drive’s ability to control speed, resulting in reduced energy consumption. Drives can also optimize performance and quality through programmability and the ability to communicate with other devices. Also, because inverters are “soft start” devices, they reduce stresses on equipment.

Specs and sizing

What’s your best advice on specifying, sizing, and applying adjustable speed drives where productivity is the main goal?

Rich • SEW: Following are some considerations to keep in mind:

• Multitasking. Using a VFD may allow you to do several things at once instead of sequentially.

• Safety. Many VFD systems implement certi ed safety control; this means you may be able to allow motion to continue in areas where you previously required it to stop because of personnel safety. And even if stopping is required, say to clear a jam, certi ed integrated safety may prevent untimely and expensive procedures for preventing accidental motion.

• Decentralized control. Placing VFDs near motors instead of in central control cabinets allows for much faster servicing. It also accelerates initial installations, so the system is productive sooner. Decentralized intelligence, the practice of putting motion control into drives instead of in central PLCs, results in simpler PLC programs that interface with smart drives rather than complicated programs that are large and dif cult to handle.

• Electronic line shafting. In contrast with machines in which all components are linked together like a big watch, individual control of multiple axes electronically provides ultimate  exibility. How is this helpful? In applications such as packaging, where user needs change on an unpredictable schedule, retooling and changeovers waste the only thing users can’t make more — time. Electronically controlled systems have signi cantly fewer mechanical components, so even in everyday operations such as cleaning and jam clearing, these compact, modern designs allow better access for shorter interruptions. Changeovers are done with touch screens, not tools, and recipes rather than wrenches.

• Blame game. When component faults, failures, and mishaps occur, across-line starter systems can’t tell a designer what the conditions were just prior to the fault. Do they have Ethernet capability to allow remote access by personnel for quick problem solving? Can they monitor motor currents and other data for predictive maintenance? Probably not. In contrast, these functions are standard on many VFDs. Unfortunately, many designers don’t leverage all of these features.