Six ways VFDs can improve motion control applications

Inverter and variable motor control technologies are being used to solve application challenges and improve efficiency and cost-effectiveness in unexpected ways.

By Craig Dahlquist, Lenze Americas Corporation October 3, 2017

Variable frequency drive (VFD) technologies—also called inverters, variable speed drives, or ac drives—have been used to control many machine tasks and automated robotics in everything from manufacturing and processing plants to warehouses and other logistics facilities. There are a number of reasons why OEMs or large end-user companies developing their own machinery internally might employ VFDs for motion control applications—it really comes down to what they are trying to achieve.

Regardless whether they’re used in material handling, machining, or pump and fan applications, VFDs are an affordable option that can help optimize performance, save energy, and permanently lower machine and robotic lifecycle costs. VFDs are available in a range of basic voltage models, with 3-phase power operating a 230 V, 480 V, or 600 V motor. Machine drive selection is contingent on motor type, voltage, current rating, input source, and input/output (I/O) requirements. Sizing depends on a number of application-specific factors, including the full-load rating and maximum voltage under full load conditions for the motor.

By varying the frequency and voltage supplied to an electric motor, VFDs, in their most basic applications, allow operators to match motor speed to load requirements, operate motors at the most efficient speed for a specific application, and reduce energy consumption. Considering that electric motors consume more than 65% of all the power used by industry, the everyday impact of VFDs can’t be underappreciated.

It is the more complex or unusual uses for VFDs, however, that reveal a whole host of potential efficiencies available to creative OEMs and end users. Newer ways of using VFD technology can help solve specific motion control application challenges or make them more economical and profitable. Here are six real-world use cases for tackling advanced motion control applications with VFD solutions: 

1. Conveyors with changing loads

From airports to factories, conveyors with changing loads are a chronic challenge and a significant drain on energy resources. Conveyors that run empty don’t need full power, but do need to be responsive as they get loaded over time and the demands on the motor change.

Conveyors with changing loads can be run with VFDs to greatly reduce power consumption (see Figure 1). Inverters sense lighter loads and adjust the power factor of the motor to run efficiently even at low load cycles. This kind of "eco-mode" minimizes the amount of power used when not required and allows the motor to power up and run at peak performance when a heavier load is added.

Depending on the specific use case, OEMs will need to choose between centralize implementation where power is local to the central control cabinet or decentralized implementation where power is local to the motor. VFDs built right on top of the motor have the advantages of space-savings and more efficient power control.

For large factories or automated systems that are spread out, decentralized VFDs eliminate the time and cost (both material and labor) required to run cables back to a control cabinet. Additionally, it is easier—and most cost effective—to drop power from a power bus as close to the motor as possible. 

2. Simplifying inter-logistics

While VFDs are essential for some use cases, some systems can be optimized with an even simpler solution. New inverters that have multiple fixed speed selections rather than an automatically variable speed can reduce greatly the number of different geared motor combinations in inter-logistic applications by the ability to vary the motor speeds.

In the case of a big warehouse where all the conveyors are connected in a large network, requirements include different conveyor speeds at different locations. Historically, this has meant numerous gearboxes installed at various sections of the system with unique power ratios to make each section of conveyor run at the right speed. The result, however, is that a lot of different gearbox ratios are being used to support the same power requirements.

Rather than employing 20 different gearbox sizes in such a situation, just four or five inverter/motor/gearbox combinations might suffice. The frequency can be adjusted to control the speed, allowing operators to optimize each combination rather than relying on across-the-line single speed motor contactors.

Major airports and warehouses can use fewer parts-an average of five rather than 20-and the simpler, programmable inverters are even less expensive than tradition VFDs. 

3. Operate induction motors at higher frequencies

Normal induction motors are designed to run off the line at 60 Hz, but that isn’t necessarily the most optimal design for an application. With VFDs, OEMs can design a motor that goes down to 20 Hz, for winding applications for example, or all the way up to 100 to 600 Hz for a much higher power density.

In other words, because power is a factor of speed times torque, OEMs can design motors that are smaller but with the same power as a traditional induction motor. These higher frequency motors are, on average, two motor sizes smaller than their 50/60 Hz counterparts but with the same amount of power. Additionally, with less inertia in the motor, VFD-enabled induction motors have the ability to offer more dynamic system capabilities. 

4. Run induction motors in servo mode

Servo control requires high precision of speed and position. It requires accuracy. As such, permanent magnet motors are the equipment of choice when it comes to performing servo functions. But, as they rely on rare metals, they also are very expensive.

With the proper feedback, inverter-controlled induction motors can be run in servo mode, offering a much less expensive alternative to the traditional permanent magnet servo motor. While VFDs are most often used in open-loop speed control and are not necessarily considered exceptionally precise, the technology sufficiently can control the motor rotor position for many servo applications.

The power density isn’t quite as good and the motor will be slightly larger, so it is essential to carefully consider system needs, motor size, and capabilities. For example, a VFD induction servo motor can’t accelerate as quickly as a permanent magnet motor, but does your application really require that capability?

While the solution might be slightly less dynamic, the substantial cost savings could provide a significant advantage in the marketplace. 

5. Run permanent magnet motors without feedback

Permanent magnet motors are some of the most efficient motors in the common marketplace but have traditionally required feedback to keep track of the pole position and properly commutate the motor.

VFD technology now can run permanent magnet motors without feedback and still attain positioning accuracy within 5 degrees. With VFDs, the pole positions are calculated when the motor is in a stopped position and the motor can then be commutated for proper control.

Positioning applications without any feedback eliminates the need for a cable and a more expensive servo inverter, replacing both with a less expensive and more efficient VFD. Powered permanent magnet motors with VFD technology also means applications can be run in a speed mode when reducing power consumption is more critical. 

6. Reduce panel space, cable length

One of the simplest and most overlooked ways to use VFDs is in space-saving efforts. Integrated motor-drive combinations offer the ability to reduce control panel space and motor cable length on the facility floor. In applications where motors are spread out or in a system where a moving component shares a common power rail with other moving components, having the inverter drive integrated into the motor just makes sense (see Figure 2).

Rather than running all cables back to a central cabinet, system engineers can realize a decentralized system that relies on individually-driven motors with only control cables run from a main control out to the different parts of the machinery. 

Craig Dahlquist has been an application engineer at Lenze Americas Corp., Uxbridge, Mass. for the past 14 years. He has a BS and an MS in Electrical Engineering from Western New England University in Springfield, Mass., and is a technical expert specializing in automation applications and motion control.

This article appears in the Applied Automation supplement for Control Engineering and Plant Engineering.

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