How to properly size the variables of an electrical environment

The evolution of on-board electronics increases the need to understand the electrical system of mobile equipment to ensure performance and safety in off-road operations

Utilized in mining and heavy construction, large mobile machines have to drive equally large motors, which in turn require electrical currents that are consistent with the size of that equipment. In addition, these machines are equipped with an increasingly greater number of electronic components, such as on-board computers and countless sensors, comprising a set of features that raise the criticality of the electrical power supply required to enable full and proper operation of the machine. And, at the core of this complex structure, there lies a critical, sensitive component responsible for managing and distributing electrical loads: the electrical panel.

Before presenting the technologies available in the market, as well as the standardization and care in maintenance that such systems require, the specialists interviewed by M&T magazine conceptualize the different types of electrical panels used in equipment. “Alternating current (AC) panels represent an evolution when compared to earlier direct current (DC) models and, historically, form the base of the drive systems used in mobile equipment,” says Carlos Alberto Favato, product manager for Belaz products at equipment dealer Brasif.

According to him, the older technology for the control of machines utilized electromechanical devices such as relay switches, contactors and resistors, which evolved into solid-state components. As a rule, such systems were preferred for their greater ease of installation compared to AC systems. “That technology, however, was overtaken by progress in semiconductors, which led to the development of electronic power switches that have been promoting the popularization of the AC models, especially as applies to large equipment,” adds Favato.

Favato classifies as ‘large’, excavators and loaders with capacities (payloads) in excess of 20 tons and off-road trucks with capacities of over 150 tons, in which the application of AC electrical panels is already a reality worldwide. “For smaller machines, it is still usual to use DC panels,” the Brasif manager states. “But Belaz is demystifying all this with the production of a 100 t off-road truck equipped with an alternating current panel.”

DETAILING

Whatever the type of electrical panel, to Daniel Farion, director of contracts at Instalo - a company specializing in the installation and maintenance of electrical assemblies for construction equipment, the use of this type of system is directly associated with the high power requirements of the equipment. “Therefore, every care possible should be taken when designing the product so that the main factor that guides the decision to purchase a machine is its technical characteristics and not merely its cost,” he says

The caveat made by the executive is irrefutably justified: incorrect installations can lead to serious accidents; so serious in fact that they have the potential to cause the death of equipment operators by electrocution. Therefore, specialists strive to conceive systems of electrical panels with the utmost possible in detail, demonstrating that the foremost requirement in dealing with these components is to know them deeply. “In this regard, we must begin by distinguishing electrical panels for mobile equipment from those designed for industrial systems,” Favato underscores.

According to him, both in industrial machinery and in off-road equipment, designers need to fit the panels into the limited space available in the equipment’s fairing or housing. Although it seems obvious, this information is particularly relevant in the case of mobile equipment where it is not always possible to find enough space to place the panels to power the various controls of the motor.

“The allocation of space is simpler in industrial systems, where each electrical panel is just one more item in a spacious electrical-panels room where as many panels as required can be accommodated to form a single complete system,” says Favato. “In mobile equipment, however, there’s a greater challenge since the electrical panels have to fit into the limited space allocated to them, which makes it commonplace to apply a single panel to perform the roles of a rectifier, chopper and inverter all at the same time.” The specialist adds that these three components form a complete system of speed control in AC-driven machines (see frame / sidebar below).

SYSTEMS

The same project design rules for electrical panels must be taken into consideration when choosing a new system to meet the needs of a motor with resized characteristics. “The panels for a new sizing of electrical power should be established according to the project and, then, the load connected to the panel must not be altered without prior assessment in order to understand whether the sizing in question can handle the new load,” says Farion. He adds that this type of change without adjustment can cause serious damage because an undue increase in load in an installation may, for example, lead to an increase in temperature of the system, compromising cables, buses, equipment and even other parts of the electrical installation since the electrical panels are connected to transformers, generators and even other panels.

In turn, Vinícius Utsch, engineering manager of Shock Engenharia, reminds us that electric motors are considered a source of energy by the mechanical circuit and, contrarily,  as a load by the electrical circuit. Therefore, the specialist explains that the sizing of a particular motor to meet the needs of a mechanical load relates directly to the correct sizing of all protective circuits and current conduction circuits that will enable the full and complete operation of the equipment.

Thus, it is contradictory to speak of electrical resizing without talking about the modification of electrical panels. “However, it is common that, in some cases, the resizing of the mechanical equipment is related to the technical characteristics of the motor,” he says. “And that may mean having to replace a motor, maintaining its power characteristics but changing its speed at the end of shaft.” Utsch provides, as an example, the change of speed in a conveyor belt or in an air compressor in an effort to improve the performance of the equipment. The detail, the specialist points out, is that this modification can only be done in one of two ways.

PRACTICES

The first is by changing the frequency of the power and the other is by making a change in the number of poles in the motor. “To change the frequency, a user will often  install a frequency inverter in the electrical panel, while to change the number of poles it is necessary to replace the motor with one of similar characteristics, such as its power and housing, especially” explains Utsch. “It so happens, however, that users do not always take into account factors related to the electric current, which is, simply put, the ‘thermometer’ of the installation. Thus, an improper change may result in disruptions or failures in the operation of the equipment and even accidents of irreversible proportions.”

To avoid this problem, Utsch - the specialist of Shock Engenharia - recommends a consensual practice between mechanics and electronics to align the procedures to be performed. Thus, according to him, the replacement of a motor to change the performance of equipment will also determine the need to evaluate the entire electrical system into which the motor will be inserted. “After all, a simple circuit breaker subjected to a condition beyond the nominal values can cause severe damage to the technical characteristics of the equipment,” he says.

When the time comes to resize a motor and, consequently, the electrical system connected to it, it is also important to observe the conditions of temperature, altitude, and relative humidity. According to Utsch, these factors may indicate that a piece of equipment is inadequate for the environment for which it was designed, resulting in loss of performance and consequences ranging from mechanical failures to the overall compromising of the electrical installation.

“Therefore, we say that sizing all the variables of an electrical environment properly means assuring all users that the system is robust and reliable, so that all the protections, conductors and loads are always aligned on behalf of one and the same goal: productivity with safely,” adds the manager.

Systematics of electrical panels

The disposition of electrical panels as well as the role each one plays and the overall configuration of the speed system control of an AC motor depends on factors such as the number of motors to be driven, the role of the motors, the power involved, space available and, evidently, the affinity between the various subsystems. Understand the role that this technology plays by examining the diagram below which illustrates an AC motor speed control system with power input via a diesel engine or via an AC power network.