What are power resistors?

Power Resistor

Power resistors are designed to withstand and dissipate large amounts of power. They are made from materials with a high thermal conductivity, allowing efficient cooling. Lomacor resistors are designed to be coupled with heat sinks to be able to dissipate the high amount of power. Some might even need forced air or liquid cooling while under maximum load.

Some are wire wound, some are made from wire grids for ease of cooling, but the common thing for all power resistors is that they are built to dissipate the most power while keeping their size as small as possible. An example use for power resistors are load banks used to dissipate power generated during engine braking in vehicles using electrical motors.

Typical applications

Power resistors are used when there is a need to safely convert large amounts of energy into heat using electrical energy as a medium. They are used as controllable power dissipation devices, protective devices and devices that simulate real world loads.

Engine braking

High power resistors are used in locomotives and trams to safely convert kinetic energy of the vehicle to heat. Since the energies involved in stopping heavy vehicles moving at high speeds, classic disc brakes would wear out too fast and their maintenance would be too expensive. Because of that, regenerative braking tends to be used. In regenerative braking, kinetic energy is transformed to electrical energy and fed back into the supply network.

However, when regenerative braking is not available, power resistors are used. Resistance brakes offer controlled braking power without introducing wear to the parts. It is often necessary to dissipate many kilowatts for extended periods of time.

Load banks

Load bank resistors are devices used to safely simulate a real-world load. They are used to load-test generators, turbines and battery UPS systems. Resistive load banks provide a known adjustable resistance value in a compact package as opposed to real loads which can be dispersed over a large area, random in value and are usually not exclusively resistive, but may have an inductive or capacitive component as well.

AC load banks can withstand and dissipate as much as 6 megawatts of power, which is quite a lot, but such large load banks can be the size of a room, and they are equipped with active cooling to prevent thermal damage.

Neutral grounding resistors

Neutral grounding resistors are power resistors used in power grounding of Y-connected generators. They are used to limit the fault current as well as transient over-voltages and allow protective relays to be used in such applications. Neutral grounding resistors are rated at up to 8 kilo amperes and are mostly used in medium-voltage AC distribution systems.

When these resistors are used, even if a grounding fault occurs, it is much easier to locate because of a limited number of possible fault locations.