Energy-Saving Drive Solutions
Almost half of the electric energy produced in the world is used by the industry sector. And, electric drives are responsible for approximately two-thirds of this power consumption.
When evaluating energy use, the entire drive system—comprising an inverter, motor, and gearbox—should always be considered. The total efficiency determines how much electric energy is required for a defined process. While work often focuses on increasing the efficiency of the electric motor, greater energy savings can typically be obtained by optimally adapting the drive to the operating process.
Saving energy is one of the biggest challenges we face today and in the future. Consequently, Lenze is acting responsibly and has implemented effective ways to use drives to save energy.
3 efficiency approaches
There are a number of parameters which determine the energy efficiency of drives. And there are just as many potential starting points with a correspondingly large number of options for increasing energy efficiency. The appropriate measures can be determined by analyzing the mechanical process and its energy requirement.
Three approaches can be used for improving energy efficiency in drives:
1. Electric energy used intelligently
2. Converting energy with high efficiency
3. Using recovered braking energy.
The greatest potential for improving energy efficiency can be found in the intelligent provision of electrical energy (75%). This is followed by the use of drive components with a high level of efficiency (15%). Further potential lies with using braking energy (10%).
Electric energy intelligence
To make effective use of the energy available, the mechanical power output by the electric drive must meet the actual needs of the application. Both the maximum amount of power needed and fluctuations in operations should be taken into account.
An intelligent, needs-oriented provision of energy therefore requires the:
- Drive to be designed in accordance with the maximum amount of mechanical power needed, and
- Mechanical power output to be adapted to the prevailing need, which fluctuates greatly in many applications.
To effectively implement this, it is important to ensure an accurate design, the use of speed-controlled drives, and diagnostics at the inverter.
The optimum efficiency of drive systems often lies in a narrow band around the rated power. However, many drives are oversized to "be on the safe side." As a result, the drive is operated far below its rated power and efficiency is significantly reduced. As oversizing also means higher procurement costs, the first measure worth taking when improving energy efficiency is to accurately orientate the drives to the maximum mechanical energy required by the application.
The amount of energy needed varies in virtually every mechanical process. This is especially the case in cooling and heating systems where the output power of pumps and fans depends on the prevailing ambient temperature. Large fluctuations in the output power needed also may arise in material handling technology if the throughput isn’t constant.
To improve efficiency, the power output by the motor must be adapted to these different needs. An inverter is used to change the motor speed and therefore the output power, the product of speed and torque. Using an inverter greatly improves energy efficiency in virtually all applications. And, savings of up to 60% are common in applications with pumps and fans.
Motion profiles, diagnostics
Dynamic motion sequences can be designed so that energy efficiency is as high as possible. For example, many positioning tasks don’t always need the maximum acceleration and braking times. Adjusting to the dynamics actually greatly reduces losses in the motor.
In processes that tend to be static, adjusting the motor’s operating point to the actual load can minimize losses. Using a frequency inverter to adjust the motor voltage produces better efficiency, in particular for partial load operations with standard three-phase ac current motors.
In controlled drives, inverters record the status of the drive. This can be used for preventive maintenance, and the designer can reduce over-dimensioning in the design.
Converting energy can be more economical when higher efficiency standard three-phase ac motors are used. The most commonly used standard three-phase ac motors are available with different efficiency classes. Since 2011, only motors of efficiency class IE2 or higher may be used in Europe. Currently, most widely used motors of class IE1 are not allowed in new installations.
Motors of efficiency class IE3 are significantly larger and more expensive than those of class IE2, with the same power output, and should therefore only be used in applications where they are permanently operated at rated speed and high load. Usually 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.
Synchronous servo motors
As a general rule, controlled drives with asynchronous motors can also be used with synchronous servo motors. As the magnetization of the motor on a permanently excited synchronous motor is not generated via in-fed reactive current, but by permanent magnets, the motor currents are lower. This results in better efficiencies than can be achieved with a corresponding asynchronous motor. And, the amount of energy needed for typical positioning applications drops by 30%.
Lower motor currents, however, also mean that a smaller power loss occurs in the inverter, and if necessary, a smaller inverter can be selected, which in turn increases the total efficiency of the drive. It is therefore well worth checking all applications with controlled drives to see whether a synchronous servo motor with improved energy efficiency would not offer a better solution.
Gearboxes adjust the high motor speed to the mechanical process. A ratio of around 20 is most commonly used.
This can be achieved at very high levels of efficiency with two-stage helical gearboxes. Worm and bevel gear teeth are used for right-angle gearboxes. While worm gearboxes generally cause high losses, bevel gearboxes can be used with higher degrees of efficiency.
Additional increases in efficiency can be achieved if an inverter or lower power motor can be used thanks to improved efficiency of the gearbox.
Today, inverters have an auxiliary power of approximately 15 W, which is primarily required to supply the control electronics. In addition there are power-dependent losses in the output stages of the inverter, which are determined by the level of motor current. The selected switching frequency and the length of the motor cable also have a significant influence on total losses. Inverters currently reach a very high efficiency of between 94% and 97%.
Most inverters cannot supply braking energy back to the mains as this process incurs additional costs and is not necessary in many cases. If it is worth recovering power to the mains, an extra regenerative module must be connected to the dc bus of one or more inverters. Therefore, it makes economic sense to use a regenerative module if the average regenerative power exceeds 5 kW.
In many applications with noteworthy braking power, other drives are running in motor mode at the same time, such as synchronized drives and unwinders in continuous production lines. In such cases, the inverter’s dc buses should be linked (dc-bus connection) to allow energy to be exchanged directly. A dc-bus connection can also be used to make joint use of a central regenerative unit with several drives and thereby save money.
Another way in which braking energy can be used is to store it in a capacitor and then make it available again during the next acceleration or lifting procedure. Compared with a regenerative unit, this option is less expensive, but the storage capacity of the capacitor is limited. Energy storage is currently cost-effective for very fast cycled drives.
– Mariusz Jamroz is senior OEM commercial engineer, Lenze. Edited by Mark T. Hoske, content manager CFE Media, Control Engineering, Plant Engineering, and Consulting-Specifying Engineer, email@example.com
Motor-Driven Systems Efficiency Update