What is a Load Bank?
A load bank is a device that is intended to accurately mimic a load that a power source will see in an actual application. Such a load bank can be built with resistive, inductive, or capacitive elements. It is a resistive device that dissipates some amount of energy in the form of heat, which can be removed by natural convection, forced air, or a water cooling system.
Why Use a Load Bank?
An individual would use a load bank to simulate an electrical load to test an electric power source without connecting it to its normal operating load. Load banks are the best way to replicate, prove and verify the real-life demands on critical power systems. They ensure the quality and reliability of emergency power sources, such as generators, uninterruptible power sources (UPS), renewable energy and more. In essence, load banks verify the accuracy of the power sources’ output capacities by creating a variety of different loads that replicate real-world use.
Load Bank Applications
There are two main categories of applications, namely Energy and Power. If the load of energy absorbed is over a short period of time (less than a minute or 60 seconds as a rough rule of thumb), it is called as an Energy Application wherein Joules (Watts x seconds) are consumed by the load. This kind of a load bank focuses on the active mass of the resistor as the primary design criteria. Subsequently, the required ratings for the resistor (current handling/ Ohms/voltage, etc.) helps guide design to the appropriate style of the resistor required. Many different styles may be applicable.
Similarly, if the load of energy happens continuously (more than a minute or over 60 seconds), it falls under the category Power Application and measures Watts dissipated in the load. This kind of a “load bank” uses the Watt (volts x current) dissipation capability of the resistor as the main design criteria. Again, the ratings needed on the resistor helps decide which style of resistor is best suited for the continuous power application. Multiple different styles may be applicable.
Resistors for Energy Applications
Resistors are two-terminal devices that absorb electric energy and convert it into thermal energy. A range of resistive products are available that can handle high energy events like transients, overloads, and in-rush current. From composition to unique thick-film designs, manufacturers offer multiple product series that can take thousands of Joules. They come in axial, radial and surface mount designs to suit all needs and energy levels (Figure 1)
FIG 1 Power resistors come in axial, radial, and surface mount designs to suit all needs and energy levels. Source: Ohmite
Listed below are some applications where energy and active mass drive the design.
- Neutral grounding [low resistance grounding (LRG)]
- Transit Braking
- Industrial Braking.
- Capacitor Pre-charge – resistors used with capacitors to limit the inrush current.
- Capacitor Dis-charge- resistors used with capacitors to discharge and make the capacitor safe.
For the above applications, engineers have designed and manufactured resistor styles to handle maximum energy needed. In case of neutral grounding LRG, for instance, resistors constructed of a heavy resistance alloy mounted on ceramic insulators and supported by a metal mounting bracket are recommended, as well as custom solutions to limit the fault current in a system in the event of a line fault.
These rugged power resistors (Figure 2) come in both Edgewound and Round-Wire formats, and can handle 400 to 1600 Watts of power. They are designed to withstand harsh environments, and thermally enhanced to handle high power dissipation. While the Edgewound resistors are tailored for very low resistance grounding with high power capacity, the Round-Wire resistors offer higher resistance with lower power capacity. Typically, the Edgewound style resistors deliver values ranging from 0.100 Ohm to 8.50 Ohms. Similarly, the Round-Wire style resistors offer resistance values from 11.0 to 25.0 Ohms. The datasheet table gives the current handling capability and Wattage for each style.
Figure 2 The rugged Edgewound series resistors. Source: Ohmite
Metal parts, except for the resistance element, are heavily plated to prevent corrosion and oxidation at high operating temperatures and to prevent corrosion. The mounting bar has slots on each end to help with installation. Edgewound units provide clamp-type terminals, permitting a reliable connection which can be moved along the resistive element to obtain intermediate values.
Also, for the Transit braking application, engineers recommend power resistors that are built to perform in rugged environments. They come with features like corrosion resistant stainless steel insulator supports, solid nickel terminals, and special electroless nickel-plated solid copper terminal supports. Terminals are welded or silver brazed to the oval, spiral Edgewound resistance element.
Figure 3 Rugged Edgewound resistors are built to offer more wattage in smaller packages. Source: Ohmite
Oval Edgewound design allows for more wattage in a smaller package (Figure 3). For the Transit braking application, the user can choose from a range of bank wattages, such as 8-kW, 12-kW, 15-kW, or 24-kW with a variety of current ratings per bank. In addition, resistor banks can be produced in grid arrays of 2 x 4 to 4 x 4. All resistors in one grid are the same length. Frames are constructed of heavy gauge zinc plated steel. Either “bolt up” or “bolt down” type frames are available. Frames may be grounded if voltage potential is below 1000 Volts.
For Industrial Braking application, engineers suggest building resistive load banks using custom steel elements to minimize inductance and heating problems associated with other resistors. They can be designed to withstand harsh environments, and include metal plating to prevent oxidization of metal parts. Although, these load banks are custom made to fit a specific application, they are cost effective because they can be built utilizing standard high quality resistors.
Similarly, for the Capacitor Pre-Charge application, experts advise ceramic-based resistors. In this application, axial leaded non-inductive bulk ceramic resistors provide excellent performance where high peak power or high-energy pulses must be handled in a small size. Thanks to bulk construction, it produces an inherently non-inductive resistor, which allows energy and power to be uniformly distributed through the entire ceramic resistor body. They come in distinctly different ceramic materials to allow designers to meet the most demanding requirements. These resistors are recommended alternatives to carbon composition resistors.
Similarly, for Capacitor Dis-Charge applications, ceramic-based disk resistors are recommended.
Resistors for Power Applications
Resistors are primarily building blocks of Load Banks. They all absorb energy, dissipate power, limit current, and drop voltages for the applications. Some common applications for continuous duty power where wattage dissipation drive the design include:
- Motor starting – large motors need to step voltage up more slowly to get going.
- Load testing
- Motor braking
- Generator testing and maintenance– run a generator and put the power created by the generator through a resistor
- Battery testing
For starting DC motors, load bank resistors are designed to maximize Watt dissipation. They are tough, versatile, and incorporate elements like nickel-copper, nickel-chrome or stainless steel wire wound in a double helix around the center core (Figure 4). All have ceramic cores with a spiral groove to grip the coiled wire. The 90° flat bus style terminals are corrosion resistant stainless steel.
Figure 4 Tough and versatile ceramic based power resistors for motor starting and speed control. Source: Ohmite
Likewise, for motor braking (10% - 100% Duty), the duty cycle determines the power rating of the braking resistor. Each braking load bank is assembled using heavy gauge galvanized steel with a wired terminal block and provisions for an optional thermostat. These assemblies are constructed with harsh environments in mind. Furthermore, all the above resistors described for motor starting, load testing, and motor braking are recommended for generator testing and maintenance.
Load Bank Configurations
There are five main configurations for load banks. These include:
- Roof or Duct-Mounted
- Custom Design
If you have a permanent power source or generator on-site, consider a stationary load bank for your system. These models have power ratings up to 3,000 kW and system voltages up to 600 VAC. Stationary models are permanent solutions for regular testing and also for supplementing light loads, and typically require more space to accommodate. Safe for use outdoors and free-standing, stationary models offer the best solution for frequent testing needs.
If you need a more mobile option, portable banks go wherever you need them to. Portable models will fit through most doorways while still boasting 100 kW up to 1,000 kW power ratings. Portable models work best for testing lower power levels and can be moved easily between sites. Portable units may weigh as little as 70 pounds and have optional cages to protect the units between uses.
Trailer-mounted load banks work best for companies that require remote testing of generators on-site without purchasing separate load banks for every location that needs tests. They tend to offer larger testing capacities than portable models with power ratings up to 3000 kW. It allows a truck to transport it easily.
Roof-mounted or duct-mounted models are permanently installed on the generator itself. For specific needs not met by other load bank designs, a customized model will serve as a solution.
Capacity in kilowatts (kW) and how you use the load banks will determine the best options for your business.
Types of Load Banks
The three most common types of load banks are resistive, inductive, and capacitive. They offer varying power factors. Amongst all, resistive load banks are the most popular with a unity power factor. Also, per testing requirements, the inductive and capacitive load banks normally supplement resistive solutions.
Resistive load banks mimic the operational load. They convert electrical energy to heat using power resistors and dissipate the heat using air or water. They come in a wide array of options, which include portable, stationary, and a variety of power ranges. These options allow you to test according to your specific design criteria and operational needs. Resistive load banks may best be used for generators with a capacity of less than 200 kVA, portable generators, small generators, and UPS systems.
Safety: Load banks are vulnerable to potential hazards inherent in high-energy and high-power applications. Consequently, resistive load banks employ a variety of protection circuits for safe operation of the system.
Load Bank Testing
A load bank test is a method of validating the true operational performance of a power source, such as the UPS or a generator. It is carried out as preventative maintenance necessary to avoid future damages or breakdowns in power sources. In addition, such load bank tests are used to ascertain the current conditions of UPS batteries as well as battery sets to show whether any of the cells are holding the charge, approaching the end of their working life or whether they are about to fail. This helps replace a failing unit before it causes a critical application problem.
Metallic load bank resistors are typically used for high-power load testing of emergency power systems including generators, UPS, turbines, battery systems and dynamic braking power dissipation for generators and large motors.
Some special features of metallic load elements include:
. Use of special alloys for longer element life and consistent operating performance
. Load elements are fully supported across their entire length by vibration-resistant stainless steel support rods with high-temperature ceramic insulators
. Ceramic insulators have excellent thermal shock resistance making them ideally suited for use in extreme climates
. Load elements assembled in discrete trays designed for modularity
. Spiral element design allows for much greater power density and heat dissipation
Likewise, key resistor specifications are shown below:
. Load elements designed for compatibility with all typical voltages
. Each load element rated for 8.33 kW 240/480V
. Maximum metallic alloy overload temperature: 1300 °C
. Maximum operating temperature as applied in load system: 600 °C.
Load Banks and Renewable Power
Because load banks manage and shed power overloads from energy-generating processes, they are also finding use in the renewable energy sector, especially in wind power production. Advancements in technology have dropped the cost of wind power production to below that of coal, natural gas, and nuclear. As a result, wind power has become much more popular for industrial use in recent years. Here, load banks are employed to handle an overload of power. Wind power is vulnerable to spikes in electricity that can lead to the power lines overloading. Overloaded power lines need to shed their load somewhere, and that is where the load banks come into play.
Designing a Load Bank
A load bank can be designed with off-the-shelf components or can be custom designed by power resistor manufacturers like Ohmite. If engineers and labs prefer to build their own solution, for reasons like cost, we have a variety of power resistors available from Ohmite. All the datasheets, including packaging and price, for power resistors offered by Ohmite, are available for designers.
Custom designs are available for needs that are not covered by the standard offerings. Often these needs originate from energy/power levels that exceed the standard offerings, space restrictions that are not conducive to standard offerings, or ease of use considerations when you don’t want to build your own. A highly professional engineering team can design and quote custom solutions for virtually any requirement. Ohmite’s skilled team of engineers can meet nearly any power rating, energy rating, resistance value rating, tolerance, package customization, and controls requirement required. Customized solutions are also offered to cater to the specialized needs of customers.
Cooling Load Banks
Load banks employ a variety of techniques to cool resistive load elements. By efficiently removing the heat created by the resistive load, cooling systems enable smaller packages for load bank resistors. The most common methods used for cooling load elements include natural convection air, integral fan-forced air, radiator airflow, or water flow. Integral fan-forced air systems for resistive load elements consist of an aluminum fan blade (masked for high efficiency) directly driven or belt driven by a motor. The fan motor is energized by a control contactor and protected by a circuit breaker. Motors for outdoor load banks are totally enclosed. Motors for indoor load banks are open-drip-proof.
Similarly, the radiator airflow systems utilize the radiator fan airflow of an existing generator.
Test engineers have observed that life expectancies of higher resistances (>20 Ohms), may vary from 1 to 5 years depending on the water purity. Higher resistance values cause increased conduction in the water which deposits contaminants, like minerals from the water, on the resistor. These deposits reduce heat transfer, deteriorate the silicone seal and inhibit water flow, all of which can cause premature failure.
Good quality drinking water may give 1 to 2 years of life whereas high purity water may increase life by a factor of 5. Assemblies are tested at 85 psi. However, for long-term reliability, it is recommended that the operating pressure be at 42 psi or lower.
Furthermore, direct and indirect water-cooled non-inductive resistors are capable of dissipating more power in a smaller package than most water-cooled resistors in the market.
Building a Custom Load Bank
When it comes to building custom load banks, engineers and designers can work, modify, and adjust existing and established designs/platforms for larger and more complex custom requirements. Qualified engineers are highly capable of creating and designing efficient and simple solutions using standard offerings.
Custom load bank needs are as numerous as the number of companies that exist and Ohmite is here to help each company get the solution that works best for them. With over 95 years of experience in producing resistive products, Ohmite has the knowledge and know-how to design the most efficient and cost-effective solution.
By replicating the real-world load, resistive load banks ensure the quality and reliability of emergency power sources, such as generators, UPSes, renewable energy and other similar critical power supplies. From axial, radial and surface mount designs, they can be manufactured in a wide range of resistor styles to address two main categories of applications, namely Energy and Power. Whether the application is consuming Joules or Watts, resistive load banks can be configured in various ways to meet your needs.
Ohmite is the leading power resistor manufacturer and your load bank solution. We can custom design a resistive load bank and integrate forced air cooling, controls, switches, and more into your design. We have over 95 years of experience and will deliver you the best, cost-efficient solution.
 “Offshore Wind Market Report: 2022 Edition,” The U.S. Department of Energy, Office of Energy Efficiency & Renewable Energy, August 16, 2022 https://www.energy.gov/sites/default/files/2022-09/offshore-wind-market-report-2022-v2.pdf