Braking resistor

 
Braking resistor

What is a braking resistor? The property of resistors to dissipate heat can be used to slow down a mechanical system. This process is called dynamic braking and such a resistor is called a dynamic braking resistor. To decelerate an electric motor, kinetic energy is transformed back into electrical energy. This energy is dissipated by using a power resistor. Dynamic braking can be rheostatic and regenerative. In rheostatic braking the energy is dissipated as heat in a resistor. In regenerative braking, the electric power is fed back in the system. The last option generally has a higher cost. Brake resistors are used for (small) motion systems, but also for large constructions such as trains or trams. A big advantage over friction braking systems is the lower wear and tear and faster deceleration. Advantages of dynamic braking resistors over friction braking: Lower wear of components. Control motor voltage within safe levels. Faster braking of AC and DC motors. Less service required and higher reliability. Resistor technology Brake resistors have relatively low ohmic values and a high power rating. Therefore, the wirewound resistor is a popular solution. Often they have a ceramic core and are fully welded. They are usually encased in a frame to create a safe distance to other parts. To increase dissipation capability, the frames are often executed with cooling fins, fans or even water cooling. Brake resistors for variable frequency drives Most DC motors will behave as generators as soon as they are removed from the power supply. This is due to their permanent magnets. The generated energy can be dissipated by connecting a power resistor as load. AC induction motors don’t have permanent magnets. In these motors, the rotating magnetic field in the stator induces a magnetic field. Braking resistors are used for applications where the motor [… read more]

Power resistor

 
Power resistor

What are power resistors? Power resistors are designed to withstand and dissipate large amounts of power. In general they have a power rating of at least 5 Watt. They are made from materials with a high thermal conductivity, allowing efficient cooling. They are often 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, such as locomotives or trams. Definition A power resistor is a resistor designed and manufactured to dissipate large amounts of power in a compact physical package. Types and construction Wirewound resistors Wire wound resistors are made by winding a metal wire around a solid form, often made of ceramic, fiberglass or plastic. Metal caps are attached to the end of the winding and metallic leads are attached to the ends. The end product is often coated with a non-conductive paint or enamel to offer some protection from the environment. Wire wound resistors can be built to withstand high temperatures, sometimes up to 450 °C. These resistors are often built to tight tolerances thanks to the material used, an alloy of nickel and chrome called Nichrome. The body of the device is then coated with a non-conductive paint, enamel or plastic. Winding types There are several winding methods. Some of them are: helical winding, edge-winding and bifilar winding. The helical type is the ordinary winding [… read more]