Choosing motor controls and drives

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.