Improving Efficiency with Decentralized Machine Drive Topologies

Oct. 24, 2011
Understanding how the use of decentralized frequency inverters along with motor gear sets can be used to solve simple machine applications and enhance the performance of existing equipment.

Machines used for specialized applications such as intralogistics (the in-house flow of materials used for production) or strip processing typically require drive-based automation solutions. These are intelligent drive systems that also contain control functionality. Conversely, applications in the fields of packaging, material handling, or robotics are better served using a controller-based architecture.

The total efficiency of either type of drive system is based on how much electric energy is required for a defined process. Therefore, when evaluating energy use, it’s important to consider the entire drive system, including the inverter, motor and gearbox.

Machine design and operation often focus on increasing the efficiency of the electric motor, even though greater energy savings can typically be obtained by adapting the drive to the operating process. As an example of this, consider the synchronized motor drives commonly used for continuous material stored on a reel, unwound for processing and wound back onto a reel at the end of the process. With synchronized drives, the ratio between the speed or angle of several drives is fixed via an electronic gearbox. This also applies to machines fabricating continuous material, such as paper, films, textile yarns and webs, sheet metal and wires.

Intermittent drives can be used to improve the efficiency of machines such as those used for cutting, punching, bonding, coating, welding, bending and forming processes. In these machines, drives with electronic cam functions transform linear position information relating to a master axis into curved motion profiles via a path-controlled profile generator. Optimized camming functions are the key to achieving high speeds and therefore result in higher cycle rates.

Frequency Inverters for Electronic Speed Control

A drive design generally comprises integrated motors, inverters and gearboxes. A high level of energy efficiency in all components characterizes an efficient solution. For multi-axis applications, centralized topologies are suitable. However, in automated machine systems, a decentralized approach using an intralogistics model would feature “intelligent” frequency inverters.

Frequency inverters are used for electronic speed control on machine drives. Available in a range of basic voltage models, with single- or three-phase power, operating a 230V or 480V or 600V motor, machine drive selection is contingent on motor type, voltage, current rating, input source and 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.

For those planning to equip a plant with decentralized technologies, there are a number of robust drive solutions that can optimize the benefits of decentralization. Frequency inverters combined with motors and gears provide the basis for these solutions.

Speed variability is the primary advantage of a frequency inverter. Rather than running a motor directly across the available power supply, the inverter converts the supply voltage to variable voltages and frequencies at varying stages during operation, depending on the machine and process requirements. An HVAC fan, for example, may not need to run at full speed 24/7. An inverter can reduce the voltage, frequency and energy usage of the fan.

Machines using advanced motor controls can usually process or convey equivalent volumes of products with substantially improved efficiency, while expending less energy and reducing mechanical wear and maintenance. Decentralized servo technology means that synchronous servo motors are fitted with an integrated servo inverter for precise operation at low speeds allowing improved efficiency and reduced installation costs. Compared to mechanical and pneumatic solutions, servo technology is not only more responsive and accurate, but also a better value and more wear resistant.

The latest inverter drives can achieve servo-like performance at the cost of a basic frequency drive. As a tool for controlling synchronous motors, inverters can reduce the capital and operating costs associated with servo solutions. Working in parallel with highly efficient gearboxes and motors, the Lenze 8400 frequency inverters, for example, enable intelligent adjustment of the magnetizing current. The proprietary voltage, frequency control energy-saving function automatically adjusts the motor's magnetizing current to operating conditions in the partial load range, thus decreasing energy consumption up to 30 percent.

Variable Frequency Drives in Rugged Applications

For machine applications requiring high load, slow motor speeds, standard ac 3-phase motors may not be the best choice. Standard ac 3-phase induction motors are not designed to run below 50 percent of base speed and should only run at speeds based on designed voltage. Reduced voltage applications will slow the motor cooling fan, preventing motor cooling and eventually lead to the overheating and eventual burn-out condition often seen in these traditional motors. Selection of high efficiency vector motors right sized to the inverters will be the best choice.

Advances in variable frequency drives (VFD), coupled with sensorless vector technology, make these drives a strong value proposition for more challenging machine applications. While a VFD delivers close to 100 percent starting torque, vector technology can offer 200 percent starting torque to overcome the initial load. As a result, an engineer can significantly decrease motor size, thus reducing application cost.

At the core of sensorless vector technology are sophisticated algorithms that achieve optimal torque production, speed and process control. Vector-controlled drives provide control of the magnetizing and torque producing flux components to the motor, allowing significant improvements in dynamic positioning and speed. While a standard drive typically has a 10:1 motor speed range, flux vector mode operates at a 60:1 factor to speed, with superior motor and torque control even at very low speeds. A higher starting torque places less demand for current drawn by the motor at lower speeds, thereby reducing the risk of burning up the motor. Another advantage of sensorless vector control is that it doesn’t require closed-loop feedback. Open-loop speed regulation eliminates the additional cost of a feedback device associated with closed-loop systems.

Advanced vector drives on the market today can be used with 3-phase AC induction motors and are available in NEMA 1 (IP31) and NEMA 4X (IP65). Programmable digital and analog I/Os allow the drive to be configured for many application-specific tasks, such as multiple preset speeds, electronic braking, and motor jogging. Benefits include high starting torque, auto-tuning, advanced low-speed control, and dynamic speed regulation. With a power range up to 20 HP at 230v and 60 HP for 480v and 600v in NEMA-1, the sensorless vector drives excel in environments where inverter technology was once considered too costly, including HVAC equipment, packaging, food processing, and material handling machinery.

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