Choosing motor controls and drives

By Loren K. Jones, Contributing Writer“FASTER, FASTER, you fool, you fool!” Those immortal words from comedian Bill Cosby seem to have been the mantra for industry since the beginning of the industrial revolution, and nothing I’ve seen indicates that it is going to change any time soon. From Henry Ford’s first assembly lines to today’s advanced robotic arm, automation has been a drive for speed. After all, as the old saying goes, “Time is Money.”Industry is fighting for time, driving to get as much production per hour as it can. It’s simple economics. Plants cost money, so each plant needs to produce as much as it can. The people who run the machines cost money, so each person needs to produce as fast as he or she can. Running a plant at or above what the engineers say is the designed capacity can boost profits and save the cost of building a new facility. All of that boils down to the “Faster, Faster” desires of management.In the not so distant past, the only way to control the speed of manufacturing lines was mechanically. New gearboxes. Different sized pulleys and belts. Different sized drive rolls. The drawback of those methods was precision. The precise size of pulley required might not be available, so the closest available was used. Matching speeds on a high-speed manufacturing line can be as crucial as the speed itself. A speed mismatch can result in the process being slowed down or production lost due to damage from the machines. Unfortunately, these methods were just about the only way to do it. Across-the-line AC motor starters didn’t offer any speed control, and although the early DC motors and drives weren’t right for many applications, they were the only game in town.Early DC motor controls offered industry the ability to regulate the speed of manufacturing lines without retooling the plant. Manual rheostat controls gave operators the ability to tweak the process, but had the drawback that some rube could lean on them and mess everything up. The advent of computer controls, especially the development of the programmable logic controller (PLC), eliminated the rheostats.The PLC gave companies the ability to control their new drives with far more accuracy. The development of the gated SCR drive gave industry the needed ability to match, change, tune and fiddle with the speeds of their lines. Gone was the need for complex calculations on pulley size and gear ratio. If a roll was wearing and the speed was off a little, just boost the drive a bit. If timing was an issue, speed this one up and slow that one down and the bottleneck is just a memory. The problem was that the drives were big. Whole cabinets and even rooms full of cabinets were devoted to the drives. Of course, there was also the maintenance problem.Having been in the field for the last twenty years, I’ve worked on those big drives. The SCRs were subject to failures that cost hours a week to fix. The venerable Reliance MaxPack and CardPack drives were my introduction to this aspect of process control. I could probably still strip and replace all 12 SCRs in one of those drives in less than an hour.The brushes in the DC motors were also an issue. They had to be checked regularly, replaced often, and the list of things that could potentially damage the commutater, either directly or through chemical interaction with the copper and electricity, is too numerous to list.In the last ten to fifteen years, that has all changed. The emergence of variable frequency AC (VFAC) drives has resulted in changes throughout the industry. DC, the standard by which all others are measured, is rapidly becoming a thing of the past. VFAC drives (See Figure 1) offer the same benefits without many of the drawbacks that plagued DC. In DC, exceeding the motor’s rated speed was difficult, involving field weakening and a resultant loss of torque. FIGURE 1: VFAC IN CHAINS

This Reliance model SP5005 hp VFAC controls a chain conveyor. Source: James Hardie Industries

VFAC drives can generally be run up to three times a motor’s rated speed at full torque with very little loss of efficiency, although that is usually frowned upon. Motors can also be run at far lower than rated speeds as long as a separate cooling fan is provided for the motor. For a company trying to balance spare parts inventory against need, installing a handful of drives that match those used in other parts of the factory and being able to eliminate the different RPM spares of the same frame and horsepower motor can save money. Small motors, such as 1, 2, or 3 hp motors (See Figure 2), allow a higher horsepower motor to be used in all applications because the drive will limit the current to the motor and avoid damaging the equipment. Everyday 1750 RPM motors can be used in place of 1180 RPM and 3600 RPM motors, reducing the amount of money that is sitting on a shelf.Regular motor maintenance is less frequent and far less involved for AC motors as well. Motors can be totally enclosed fan cooled induction motors off the shelf. They are also generally cheaper, as are the drives themselves. Different drives, however, have different characteristic strengths and weaknesses. That’s when the decision of which drive to use comes into play.“One of the biggest mistakes people make in industry is not matching the drive to the application,” says Denise Chenoweth, an engineer at DC Technologies, a systems integrator in Tampa, FL. “I’ve had customers who look at the Allen-Bradley drives and say, ‘Ugh, Allen-Bradley? I’ve had nothing but trouble with Allen-Bradley.’ I’ve also had customers who have the same drives and say they have no problems with them. It’s all in the application.” For people like Denise, the drives she uses make all the difference in the world. “Most drives are basically the same. You just have to match the drive to the application. It all depends on what you are controlling.” FIGURE 2: SMALL IS GOOD

Small AC drives can perform many tasks. This 5 hp Baldor VFAC drive controls a batching screw. Source: James Hardie Industries

What’s Out There?A simple inverter drive that rectifies the incoming AC voltage and then inverts the resultant DC to a controlled frequency AC output is sufficient in most applications where speed regulation is desired but is not critical. They are easy to use and are generally rugged enough to handle power spikes and varying loads. They are not, however, very precise on their speed control when the load changes. The slip factor of induction motors, the most common type, changes slightly when the load changes but the drive doesn’t compensate. It is putting out a set frequency, based on the entered data and the speed control signal, which should result in a certain RPM from the motor. If the load is changing, that speed is changing as well.A more precise form of VFAC drive and motor is the Flux Vector set. These drives are very good at maintaining a precise speed. The motor is equipped with an encoder that feeds back to the drive and gives it an exact measurement of the motor speed. They are also very strong, and provide greater starting and stopping torque than most inverter drives, even those equipped with dynamic braking.Under conditions of changing load, such as conveyors that are vacated and then rapidly reloaded, the flux vector motor will increase or decrease its output to maintain the speed. In applications where rapid speed changes occur, such as part inspection stations, the flux vector can perform nearly immediate stops and starts, within the limitations of the machinery and the acceleration and deceleration parameters programmed into the drive. Flux vector drives can also hold a motor stationary in response to a zero speed signal, eliminating the need for an external brake.The most precise speed control comes from servo drives (See Figure 3). Servo drives are primarily used for positioning systems. The high-resolution feedback from the motor allows for exact, repeatable positioning of parts, robotic arms, and sensors. They also allow for exact speed matching between systems where a slight mismatch could result in damaged parts. The drawback of servo drives is that most of them require an outside servo controller. Some newer drives contain integrated controllers, but most require something else to tell them what to do. Because of this, they are also the most expensive and difficult drives to use. FIGURE 3: DRIVE HE SAID

Allen-Bradley Kinetix 1394 Digital Servo Controllers with SERCOS modules control servo-driven belt conveyors. Source: James Hardie Industries

What Do I Use?Once you decide what kind of drive is needed, you have to choose a supplier. There are many manufacturers out there, with familiar names like Baldor, Allen-Bradley, Reliance, and Saftronics.In new design applications, using “the ‘best in class’ and building from the ground up,” as Dave Schmitz of the Minster Machine Company, an OEM of stamping and forming presses in Minster, Ohio, puts it, works out fine. Unfortunately, in existing facilities the brand name will often be chosen to match what is already installed. Then the best drive that is available from that company has to be used.Standardizing the types of motor controls and drives in a facility is one way companies cut maintenance costs. Fewer spares mean less money sitting on a shelf collecting dust. Standardization also helps the electricians and technicians who service the equipment. At two o’clock in the morning, the last thing a service tech wants to do is figure out how to wire and program a new brand of drive while some production supervisor stands over him looking at his watch.For maintenance manager Shawn Stevens of James Hardie Pipe in Plant City, FL, potential downtime often outweighs initial cost when deciding on a drive, but there’s always the bottom line. “If you’re not changing them out proactively, changing to a new kind of drive because one breaks down can take a lot longer,” he says. “If I can get something my guys are already familiar with, I’ll do it. I don’t have anything against new technology, but if I can get the same results from the equipment I have, why change?”Other problems can arise when controlling the equipment after it is installed. Field service technician Keith Oliver with Siemens Electric expresses it this way: “We use ProfiBus to control our equipment, and all the equipment we get says it’s ProfiBus compatible, but there’s always something that isn’t quite right and you spend hours figuring it out.” At rates of one hundred dollars or more per hour for a drives service tech, the hours spent figuring out how to get a drive to talk can be annoyingly expensive.For drives that don’t use communication protocols like ProifiBus or ModBus, the control wiring can also be a factor in deciding which drive to use. Using a drive that requires a 24VDC signal in a facility that uses exclusively 120VAC controls can earn you a big frown from the maintenance manager while his guys scramble to rig an interface, such as a relays, that the other drive didn’t need. Flexibility in analog control signals is also critical. I’ve been in facilities that used 4-20mA, 0-20 mA, 0-10 VDC, and +/- 10 VDC in different portions of the plant.In any industrial control application, using the right drives and motor controls can make or break the project. End users want the best price they can get, but they want reliability as well. Drives aren’t always necessary or desirable. Using across the line magnetic starters works fine in applications that don’t require speed change or regulation. However, even some traditionally single-speed applications, such as pumps and fans, are being regulated now to provide controlled outputs for processes. As the processing power of today’s PLCs increase, the possibilities for controlling every aspect of a process expand, and drives are going to be used in more and more non-traditional applications. Which drive is used is going to make a huge difference in how well those applications perform.