24-volt drive technology in continuous conveyor systems

In addition to the standard 400 V drive technology, 24 VDC technology for conveyor systems has become increasingly popular in recent years.

Today’s continuous conveyor systems can be used in many different sectors and branches of industry. As well as classic 400-volt drive technology, 24-volt drive technology has increasingly been used over the past few years. Since 24 V and 400 V drive technology share some common distinguishing features, the two drive technologies will be compared. Possible distinguishing features include the operating mode, drive-train design and conveyer task.

Application areas for 24-volt drive technology

24-volt drive technology is primarily used in accumulating conveyor systems these days. Each module of a longer conveyor is fitted with a roller drive that is separately powered and with a decentralised control unit that can be directly activated. Each drive has an interface to an external master control system. This means various modules can communicate with each other to facilitate material flows. In each case, the drive is only switched on and ready for action when a unit needs to be moved across that particular module of the conveyor. Once the unit has gone, the conveyor module is immediately switched off again. Long conveyors can thus be divided into segments that automatically switch themselves on and off again.

Such transportation tasks are common in many different sectors. In the packaging industry, the main tasks are dynamic positioning and precise delineation of the distances between transported units. This ensures high availability of the conveyed goods at the unloading points. The challenge in such cases is usually that the loading frequency of the conveyed goods is asynchronous with the unloading frequency.

Similar key tasks are handled by distribution centres or mail order firms with manual picking systems. Roller conveyor systems with 24 V drive technology are also suitable for use in such instances.

As a general rule, 24 V drive technology can be used in logistics centres handling throughputs of between 100–1000 unit loads per hour, eg, in the entry zones of automated high-rack warehousing systems.

Advantages of 24-volt drive technology

Fundamentally, 24 V drive technology can be described as one of the safest options for powering continuous conveyors, since the lower voltage involved makes the systems easier to maintain and service.

The ability to divide longer conveyor lengths into individual modules with decentralised control units that allow information to be shared with other modules can also increase the flexibility of the system. Any changes as a result of remodelling or extending the system to include extra modules can thus be implemented relatively simply, like ‘plug-and-play’ solutions.

Integrated roller drives represent a low-maintenance, low-noise option and the advantages of such technology also lie in its compact, space-saving design. The fully enclosed unit protects all transmission elements such as bearings and couplings from external environmental influences such as dust, water, grit, chemicals, fat, oil and the high-pressure steam typically used to clean conveyor systems.

Simple integration of the conveyor rollers with their built-in drive facilitates quick maintenance of each module, and the redundancy of the system allows a fault within the system to be more easily rectified. Since it is possible to simply replace one defective module, the fault no longer has to be fixed within the entire system, as the unit in question can be taken away and checked for identification and repair of any problems. This means the conveyor process is only interrupted for a brief period.

The combination of a decentralised roller drive with modern, decentralised control technology offers great potential, for conveyor modules can be strategically switched on and off without having to be centrally administered within the overall control system.

Whenever electrical power is converted to mechanical energy, a certain degree of heat is also generated. The use of new materials in roller drives lowers the surface temperature of the 24 V system by generating less friction than comparable, centrally controlled, gear-motor combinations. This means less heat has to be expelled and at the same time the degree of power loss is also significantly reduced. Ultimately, this lengthens the service life of the drives and extends the lifespan of the entire system.

Disadvantages of 24-volt drive technology

The task of many distribution centres is to make products available 24/7 so that goods can be taken from the warehouse and distributed to customers at any time. If high throughput is required at the same time, the entire continuous conveyor system is usually operated in non-stop mode. For such usage, 400 V drive technology with central gear-motor combinations is still the recommended option, since these motors have a high power density, long lifespan and, when operated at nominal power, very good efficiency.

Because long conveyor lengths are divided up into smaller conveyor modules when 24 V technology is used, it also means a larger number of motors is used. Every additional motor-gear combination increases the potential for problems to arise. The overall availability of such a system, compared to that of an otherwise constant, non-segmented conveyor, is therefore lower. As well as the use of more drives, more sensors and control units are also used. The required basic output, due to the increased incidence of standby power, thus also increases.

Currently, 24 V drive technology is capable of transporting lightweight goods in the 30–50 kg range. In order to be able to move heavy pallets higher-powered drives are required, at least according to the current state of technology. Both heavy and lightweight goods can be easily transported with the aid of 400 V drive technology since, depending on the transportation task, the conveyor can be adapted by adding tried and tested versatile modular products to certain drive elements to improve the energy efficiency and throughput rates.

Comparison of 24-volt and 400-volt drive technology

A comparison of 24 V and 400 V drive technology can be made using various criteria. To be more specific, energy consumption, the costs of installation, operation and maintenance, the overall flexibility and system performance are the most important criteria.

Energy

To compare the energy usage of the different drive concepts, the transportation of a unit load weighing 50 kg was analysed. In order to quantify the required energy consumption for other transportation tasks, a simulation model was developed at the Institute for Material Handling and Logistics (IFL), with which the energy consumption of both 400 V and 24 V drive technology can be quantified for the different scenarios. This model is not limited to a particular type of conveyed goods or roller conveyor as it is individually adaptable to various tasks.

Figure 1: Comparative layouts of 24 V drive technology (left) and 400 V drive technology (right). For a larger image click here.

The model of the roller conveyor is basically designed to suit the relevant drivetrain. The stand-by consumption of such elements as current converters, frequency inverters and control units is included in the calculations.

Performance measures on real systems were undertaken to verify the model and determine any missing parameters such as resistance coefficients. As well as the latest advances of contemporary current systems, with the help of the model, the energy-savings potential of new technologies and the influence of various operating strategies can be determined.

Table 1: Overview of comparative parameters of 400 V and 24 V drive technology.

For a direct comparison of both technologies, the layout of Figure 1 consists of a straight-line conveyor of 32 m in length, 24 V modules of 0.8 m and 400 V modules of 16 m. Other parameters are shown in Table 1.

Using this overview of the parameters and comparative layouts, three different tasks were investigated:

• Task 1 — continuous operation: The entire conveyor was switched on without any conveyed goods. Then a low-load carrier weighing 50 kg was transported across the entire length of the conveyor. When the unit reached the end of the line, the conveyor was switched off. This task was carried out on both modules of the 400 V and all 40 modules of the 24 V system.
• Task 2 — intermittent service: This task was used to test the intermittent mode described in section 2.2. For both forms of drive technology, the conveyor modules required to transport goods were turned on and then immediately switched off again once they were no longer needed. Here, too, a low-load carrier weighing 50 kg was transported and its energy consumption measured.
• Task 3 — accumulation: To investigate the accumulation process, individual modules or the conveyor with the weight of one low-load carrier were switched off after 16 m and then accelerated again with that weight for the purpose of further transportation.

The resultant energy consumption levels are shown in Figure 2.

Figure 2: Comparison of energy consumption for each layout when transporting one low-load carrier for the three tasks. For a larger image click here.

For Task 1, when continuous transportation occurred, the theory that less energy consumption was required using 400 V technology was confirmed. This is primarily due to the fact that in the case of 24 V drive technology, 40 modules and thus 40 motors were switched on at the same time and operated for the entire period the weight was carried, which was about 41 seconds. In the case of the 400 V system, only two drive units were involved.

In the case of Task 2, more energy-efficient transportation occurs when 24 V technology is used in intermittent mode. Unlike continuous operation, this mode of operation with 24 V technology enables energy consumption to be reduced by 88% through intelligent control of energy consumption alone. Even when 400 V technology is used in intermittent mode, by halving the length of the relevant conveyor required to transport the goods, the required energy consumption can be reduced by 9%, as shown in the sample layout.

Task 3 investigated what happens when the weights involved are decelerated and re-accelerated. It is clear here that in the case of 400 V technology significantly more energy is required when switching on the longer conveyor lengths. For this task, there is a 53% difference between 24 V and 400 V technology.

When throughput is increased to 10 boxes of 50 kg each (see Figure 3), Task 1 and Task 3 present a similar picture. In the case of Task 2 it is clear that decentralised 24 V drive technology requires more energy than 400 V technology. It can thus be shown that when throughput is increased, both technologies should be explored further in relation to the task and operating mode.

Overall these investigations show that the use of 24 V drive technology over short conveyor distances in intermittent service and accumulating mode requires significantly less energy than comparable use of 400 V drive technology. This means the decision whether to use 24 V drive technology or 400 V drive technology should be based on customer-specified throughputs and transportation tasks and the appropriate technology selected accordingly.

Figure 3: Comparison of energy consumption for each layout when transporting 10 low-load carriers. For a larger image click here.

Costs

For a general roller conveyor system, various costs can be identified and quantified. The life cycle of the system can be used as a reference for classifying the costs. Manufacture of the system requires some initial outlay, operation or use of it leads to operating and maintenance costs and removal of it incurs recycling or waste disposal costs.

In this case, the initial outlay, operating and maintenance costs will need to be studied in more detail. Calculation of the recycling or disposal costs would require further detailed information about the material used to build the system.

As far as the operating and maintenance costs are concerned, a direct link is identifiable between energy consumption and energy costs.

Flexibility

As mentioned earlier, 24 V drive technology with decentralised drives and decentralised control units enables the use of shorter, modular conveyor segments. As well as the aspect of reduced wear and tear with intermittent use and the possibility of longer maintenance intervals, the conveyor can be extended or remodelled in a similar way to a plug-and-play solution.

Roller conveyors thus no longer represent physical obstacles that are fixed in one position for several years but rather versatile systems that can be altered to meet the changing seasonal demands of a distribution centre. Thanks to the added safety of low-voltage DC solutions, such changes can even be made by in-house staff.

System performance

As described, there are two main operating modes for roller conveyor systems — continuous and intermittent. Depending on the required throughput, the two operating modes can be directly compared with each other and evaluated according to ecological and economic principles before one mode is selected.

For intermittent service, which can be used for throughputs of 100–1000 units per hour, it makes sense to use 24 V technology with decentralised drives and decentralised control units in order to reduce not only the costs but also the energy consumption of the system. On the other hand, for continuous operation and piece numbers of several thousand units per hour, it pays to use 400 V technology with central drives and controls.

Summary

An in-depth comparison has been made between 24 V drive technology and 400 V drive technology for roller conveyor systems. In principle, 24 V and 400 V technology can be described using similar classification criteria. Based on this overview, which reflects the current state of roller conveyor technology, the advantages and disadvantages of 24 V technology were able to be determined. Apart from safe usage, this technology stands out for the increased degree of flexibility derived from dividing one long conveyor into smaller, decentralised modules, which leads to energy-efficient operation and immediate cost savings. Over longer conveyor distances, the use of 400 V drive technology is still indicated, since 24 V drive technology involves a host of motors which cause increased maintenance and servicing costs and significantly diminished availability of the system as a whole.

As a general rule, 24 V drive technology can be used in many different application areas. Thanks to the fundamentally modular system of continuous conveyors, 24 V technology is thus ideal for certain transportation tasks.

Authors

Dipl.-Ing. Meike Braun works as an academic assistant at the Institute for Material Handling and Logistics (IFL), within the Karlsruhe Institute of Technology (KIT). She focuses in her research work on analysing and enhancing the energy efficiency of various types of intralogistics systems.

Dipl.-Ing. Peter Linsel is head of the Warehouse and Material Handling Technology Department of the IFL. He focuses in his research field on energy-efficient, lightweight conveying systems.

Prof. Dr.-Ing. Kai Furmans is the Director of the Institute for Material Handling and Logistics (IFL).

Image credit: ©stock.adobe.com/Vaclav Janousek