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...
What is the power rating of a resistor? The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. As is stated by Joule’s first law, the generated electrical power is related to the voltage and current: When the electrical power equals the dissipated heat (by radiation, convection and conduction), the temperature of the resistor will stabilize. The temperature is not equal across the resistor. The resistor body is slightly hotter than the terminals, with the highest temperature at the center of the body. The higher the rate of heat dissipation to the environment, the lower the temperature rise will be. Larger resistors with a bigger surface area can generally dissipate heat at a higher rate. If the (average) power dissipation is larger than the power rating, the resistor may be damaged. This can have several consequences. The resistance value can shift permanently, the lifetime can significantly be reduced or the component is completely damaged resulting in an open circuit. In extreme cases the excessive power can even cause a fire. Special flameproof resistors are available, that cause a circuit brake before the temperature reaches a dangerous state. Power rating definition The power rating of a resistor defines the maximum energy a resistor can (safely) dissipate. Resistor derating The nominal power rating is defined for a certain ambient temperature in free air. Note that the amount of energy that a resistor in practise can dissipate without causing damage, is strongly dependent on the operating conditions and therefore not equal to the nominal power rating. For example, a higher ambient temperature can significantly reduce the power rating. This effect is referred to as derating. It should be taking into account by the designer. Often the power rating is chosen largely above the electric power. Typically resistors are...