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]

Ohm’s law

 
Ohm's law

What is Ohm’s law? Ohm’s law states that the electrical current through a conductor is proportional to the potential difference across it. Furthermore, the electrical resistance of the conductor is constant. This leads to the mathematical equation: where I is the current in amperes, V the voltage in volts and R the resistance in ohms. To illustrate: a resistor of one ohm subjected to a current of 1A has a voltage difference of 1V across its terminals. The equation is named after Georg Ohm. In 1827 he published his findings that form the basis of the formula that is used today. He performed a large series of experiments that showed the relation between applied voltage and current through a conductor. The law is therefore empirical. Although Ohm’s law is one of the fundamentals of electrical engineering, at the time of publication it was received with criticism. The ohm is adopted as the official SI unit for electrical resistance. Gustav Kirchhoff (known from Kirchhoff’s circuit laws) made a generalization that is more used in physics: where σ is the conductivity parameter (material specific), J is the current density in a location of that material, and E the electric field in that location. Ohm’s law and resistors Resistors are passive elements that introduce resistance to the flow of electric current in a circuit. A resistor that functions according to Ohm’s law is called an Ohmic resistor. When current passes through an Ohmic resistor, the voltage drop across the terminals is proportionally to the magnitude of resistance. Ohm’s formula stays also valid for circuits with varying voltage or current, so it can be used for AC circuits as well. For capacitors and inductors the law can of course not be used, since their I-V curve is inherently not linear (not Ohmic). Ohm’s formula [… read more]