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Resistive, Inductive, and Capacitive Load Banks

 
Load banks place electrical load on power sources to test their capacity to deliver electricity or to adjust the characteristics of current. Load banks create power demand using resistive, inductive, and capacitive elements. This brief summarizes their differences.

Overview

In an alternating current system, current occurs in sine waves according to the frequency of the power source. In a circuit unaffected by inductance or reactance, voltage and current would rise and fall together during each cycle. This condition, known as unity, is shown in Figure 1.
 
In practice, circuits present inductive or reactive characteristics that cause voltage and current to peak at separate times in an ac cycle. In inductive circuits, voltage leads current, as shown in Figure 2. In capacitive circuits, voltage lags current, as shown in Figure 3. Greater amounts of time between current and voltage peaks indicate a greater amount of inductive or capacitive load, and either condition increases the work needed to deliver the required amount of real power to loads.
 
 
The extent to which the voltage and current peak at separate times is quantified by the power factor. For purely resistive loads, power factor equals 1. Increasing variance from this value indicates decreasing amounts of real power available for work.

Types of Load Bank Elements

Resistive Load Elements

The most common load banks use resistive load elements. Resistance is generated when current passes through conductors in a load bank element, producing heat and placing a corresponding electrical load on the power source. Resistive load elements can produce precise amounts of load at a power factor equaling 1.

Resistive load elements generate large amounts of heat that must be quickly dissipated to prevent overheating. Consequently, load banks use forced air to cool resistive elements, which is provided by a dedicated power circuit and one or more blowers.

Loading the prime mover, typically a diesel engine, can identify problems in its fuel, exhaust, cooling, and other systems. Because resistive elements operate at a unity power factor, they do not test the reactive power produced by a power source. Because most facility power distribution systems operate at a lagging power factor near 0.8, a resistive unit can apply load up to 100% of a generator’s nameplate power kW rating. However, a resistive load elements will not test the generator against any inductive or reactive load on the circuit.

Inductive Load Elements

Known also as reactive load elements, inductive elements use wire coils to create inductive fields. The power used to create and maintain these fields loads the power source under test. Compared to resistive loads, inductive load current peaks after voltage. Consequently, inductive coils produce lagging power factors.

Because they produce lagging power factors, inductive load elements are used whenever the power factor of a test load must be reduced. For instance, the power factor in a hospital’s power distribution system may be near 0.8. However, during generator tests, load banks may be used instead of live building load to avoid disrupting power to the facility. Because resistive load banks provide a power factor of 1, they are unable to test a power source at its rated kVA. Adding an inductive load bank can adjust the power factor to the value needed for full-capacity testing.

Capacitive Load Elements

Capacitive load elements use capacitors that store electrical charge. They resist changes in voltage, which causes current to peak before voltage during each electrical cycle. As a result, capacitive load elements provide a leading power factor, and can be used to raise power factors of circuits.

Combined Load Element Designs

Combined load banks typically provide both resistive and inductive load elements in a single enclosure. For generators, this enables testing at 100% of kVA rating. The resistive and inductive load elements may be independently controlled to produce purely resistive or inductive loads or to adjust the power factor as required.

Notably, load banks with more than one type of element can accommodate the widest range of applications. Combined load banks are used to test turbines, switchgear, rotary UPS, generators and UPS systems. These load banks can be especially suited for use by rental companies, which may be required to accommodate different load types as equipment is relocated from site-to-site.

Summary

Load banks are available with resistive, inductive, and capacitive load elements. Resistive units test power sources without changing the power factor. Inductive and capacitive load elements can be used to simulate for non-unity loads and to adjust the power factor of circuits. Load banks with combined load bank elements offer the greatest range of functionality, which can be especially suited to applications where load banks are moved from site-to-site. For additional detail regarding types of load elements and their applications, refer to the ASCO Power Technologies white paper entitled Resistive, Inductive, and Capacitive Load Bank Elements – Function, Design, Application.