Adaptive tools, engineering can reduce drive system energy consumption

Tricks and Tips: Design tools and electric drive components can reduce energy use and pollution, conserve resources, and lower energy costs without sacrificing productivity. Overall cost effectiveness of the drive system should be assessed as part of a lifecycle costs (LCC) analysis.

07/30/2012


Industry consumes almost half of the world’s electric energy use, with electric drives representing a majority of that power consumption. Energy efficiency has gained support of every industrialized nation and top management levels of leading industrial enterprises. Machine automation in continuous operations, such as automotive, material handling, packaging, and food and beverage industries, has heightened demand for energy-efficient machinery. Design tools and electric drive components can reduce energy use, resulting in less pollution, conservation of resources, and lower energy costs without sacrificing productivity.

Almost every part of the production process, automated transport, and factory logistics use electric drives. Most drives found in industrial applications have a power output between 100 W and several megawatts. Process engineering plants are dominated by drives with high output power. In contrast, factory automation and logistics centers use a larger number of drives with lower output power. Thousands of drives can be found in a typical automotive manufacturing plant or an automated distribution center. Even an average industrial plant usually has several hundred drives operating in processes and machines.

Lenze L-Force MF ac motors offer an economical solution to close the large performance gap between conventional ac motors and servo motors. Courtesy: Lenze Americas

Calculating total drive energy

The calculation of energy costs in drive systems often equates to no more than a recount of procurement costs. The overall cost effectiveness of the drive system can only be assessed as part of a lifecycle costs (LCC) analysis. Although a commonly used tool in business management, LCC analyses are rare for drives. With rising energy costs, that is changing. In the future, machine operators will increasingly include running costs in their purchasing decisions and expect adequate information from the supplier.

Multiple parameters determine drive energy efficiency. In evaluating total energy efficiency for a defined process, the entire drive system, comprising inverter, motor, and gearbox, merits consideration. All drive components work with a comparatively poor efficiency in the partial load operational range and thus generate high losses in proportion to the mechanical process. The more precisely the machine requirements and the load-dependent power requirement are defined, the better the drive components can be selected. Optimum efficiency of drive systems often lies in a narrow band around the rated power. Despite this, too often drives are oversized as a safeguard measure. An oversized drive costs more and operates below its rated power, with commensurately lower efficiency. Accurately sizing machine drives to the maximum mechanical energy required by the application is a critical first step.

Motor and drive selection

Powerful engineering and configuration tools can help machine engineers set the right course in the design and development phase. New software helps design engineers select the right drives and motors for optimal machine performance. Software enables the exact determination of the process variables, evaluates components, and helps optimization coordination. Software can size components based on user-entered machine torque, time, and motion profiles, and generate data specifying where and when and by which means efficient savings can be achieved. Results show the energy consumption of the main drivetrain components calculated by differentiated loss models.

Some software requires recalculations to compare scenarios. Ideally software should work within a concise and comprehensible graph format clearly showing usage by each component, with a comparative analysis and payback for multiple design scenarios. The software should streamline the design and sizing process and convert drive energy savings into kilowatts used, fuel cost, and wasted CO2. Software also should provide reliable data by quickly calculating solution variants on the basis of mechanical performance figures. These values can be used to determine the energy costs and CO2 emissions. By comparing solutions, the user can identify the optimum combination of components and the best motion sequence for the drive task. Optimized mechanics and reduced inertias and frictions fundamentally reduce the power requirement to be met by the drive.

Table 1: Optimization potential and effects for the complete system

Power supply / power recovery

 

Inverter with

 

Motor

 

In dc-bus connection ++

 

Energy optimized motor control ++

 

Use of synchronous motor ++

 

With power recover to the main ++

 

Moderate switching frequency, such as 8 kHz 0

 

Use of IE2 or 120 Hz motor ++

 

 

 

2-switch modulation, such as inverter drives 0

 

Use of speed setting range +

 

 

 

 

 

87 Hz operation for standard motors +

 

Gearbox with...

 

Energetically optimized mechanical components with

 

High efficiency ++

 

Low friction ++

 

Low number of stages +

 

Low inertias +

 

Avoiding very high drive speeds 0

 

Optimized motor profile +

 

Key: ++ = high, + = medium, 0 = low

The table above shows an overview of energy savings potential and provides tips for optimizing the energy efficiency of drive systems. Courtesy: Lenze Americas

Efficient motors

The most widely used class IE1 motors have been phased out and prohibited in some new installations. Software can help with motor selection as machine markets transition to the required higher efficiency motors. Certain motors help design engineers avoid increases in frame sizes and complex design adaptations for the migration to Class IE2 ac motors. Open and closed loop controlled operation can be achieved with frequency inverters, with some ac motors allowing a higher nominal speed than conventional 4-pole motors.

Some motors can bridge the gap between conventional servo motors and high-efficiency IE2 Class ac motors, with nominal frequency of 120 Hz and a speed-setting range of 1-24. Integrated high-ratio gearboxes can achieve higher output speeds, up to 3,500 rpm. With efficiency ranging from 94% to 98%, the right-angle and axial gearboxes ensure almost loss-free energy conversion. Low inertia translates into less energy consumption during speed changes. During rated operation, some three-phase motors surpass the minimum efficiency of Class IE2 motors but are unaffected by IEC 60034-30. Certain motors can be specified up to two sizes smaller than IE2 motors of equivalent power. Multifunction capability can reduce motor inventory as one motor can fit applications that may have required multiple conventional motors (of varying frame sizes and power ranges).

Motors from the IE3 efficiency class are significantly larger and more expensive than those of the IE2 class with the same power output and should, therefore, only be used in applications where they are permanently operated at rated speeds and high loads. Often the better solution for achieving higher energy efficiency is the use of an inverter that adapts the output power of the drive to the application.

Table 2: Lifecycle Costs

  1. Procurement costs – costs for drive components
  2. Operating costs – running energy costs
  3. Servicing costs
  4. Disposal costs

Opportunities exist to minimize lifecycle costs in stages 1 and 2. Courtesy: Lenze Americas

Inverter advantages

To improve efficiency, the power output by the motor must be adapted to different needs. Energy efficiency depends on right-sizing and selection, and on the adaptation of efficient products to the individual application case. Using an inverter greatly improves energy efficiency in virtually all applications. Adjusting the motor’s operating point to the actual load in processes that tend to be constant can greatly diminish losses. Dynamic motion sequences can be designed such that energy efficiency is as high as possible. For example, a lot of positioning applications don't always need the maximum acceleration and braking times. Adjusting to the dynamics actually needed greatly reduces losses in the motor.

Using a frequency inverter to automatically adjust motor voltage produces better efficiency in partial load operations with standard three-phase ac motors. Normally, in partial load operation, three-phase ac motors are still supplied with a greater magnetizing current than actually required by the operating conditions. Additional energy savings can be yielded in combination with high-efficiency gearboxes and inverter drives with built-in energy saving software for voltage frequency control.

This energy-saving feature makes it possible to reduce energy consumption by up to 30%. Designed for centralized and decentralized frequency inverters, the software uses load and torque measurements to adapt to partial loads by automatically reducing the magnetizing current of the motor to the actual requirement. This feature can be temporarily disabled for manual control or full load operation.

In the case of load changes, this operating mode delivers better dynamic performance than other products on the market. In applications with long, extreme partial load phases, a voltage reduction enables reduction of the average required power. That makes this feature particularly practical in applications with great partial load operation, low requirements with regard to the dynamic performance, and infrequent load changes, as commonly found in material handling roller conveyors, conveying belts, pumps, and fans.

It is well worth checking all applications with controlled drives to see whether a synchronous motor with improved energy efficiency can offer a better solution. Controlled drives with asynchronous motors can always be implemented using synchronous motors. The motor currents in such drive systems are lower as a permanently excited synchronous motor is magnetized—not by the supplied reactive current, but by permanent magnets. This results in better efficiencies than can be achieved with a corresponding asynchronous motor. But lower motor currents also mean less power loss in the inverter. Depending on the application it may be possible for a smaller inverter to be selected, thereby reducing total drive efficiency.

Maximum energy efficiency, low acquisition costs, and short payback periods are just some of the challenges machine engineers face in today’s global marketplace. The basis of an intelligent and economical use of energy is the knowledge of the steady state or variable requirements of certain processes. Given the high proportion of total energy represented, improving the efficiency of electric drives is the best targeted approach to reduce overall energy consumption. All the described possibilities for increasing energy efficiency can be calculated and compared in software, yielding a design template for an energy-efficient complete machine. The benefit resulting from this quickly becomes evident: less power leads to smaller and, therefore, more economical components and reduced energy consumption.

For technology details, see: Increase system energy efficiency with motor and drive tools.

- Mariusz Jamroz is senior OEM commercial engineer, Lenze Americas; edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer, mhoske(at)cfemedia.com.

http://www.lenzeamericas.com 

 

ONLINE extra

Increase system energy efficiency with motor and drive tools

Energy-saving drive solutions



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