Low-current resistors
In this area "Ohm" is

• Load/utility resistance,
• matching resistance,
• loss resistance,
• internal resistance (of a voltage source),
• voltage divider (switching resistor),
• Measuring resistor,
• reactance and characteristic impedance,
• lead resistance

expected.

Heavy current/power resistors - Overview
Their use is determined by their function and wide ranges of dielectric strength and power. These include

• Charging and discharging of capacitors, inductors, batteries (use on both the three-phase and DC sides, with balancing function if necessary),
• Connecting to earth (earthing resistor),
• Testing systems (the range of use as a test resistor is particularly wide due to the unlimited number of applications),
• Damping of vibration processes (damping resistor),
• Heating (flexibly mountable resistor, e.g. for heating electrical systems),
• Use as a starting, braking or load resistor,
• Use as a filter or protective resistor (combination with inductive component to replace small mains chokes),
• Damping of load peaks during the de-excitation of the generator magnetic field.

Low-power PTC resistor ≥ 35 W for small frequency inverters (Koch, www.bremsenergie.de)

Many tasks are now performed much more cost-effectively, energy-efficiently and reliably by power/electronics (examples: Replacing starting resistors with soft starters, test/load resistors http://www.megger.de ). Often, however, the functional requirements can still be met economically and robustly with customized resistor devices.

Requirements
The conditions vary greatly depending on the application. Compact and vibration-resistant, power resistors must be easy to dimension and install. Due to their prescribed operating mode (usually S2 short-term operation, S3 intermittent operation), they are often subject to pulsed current loads (ED ≥ 1 %). Very high temperatures on the material surface require effective, rapid heat dissipation. With voltages above 1000 V DC, strong energy pulses must be absorbed quickly. The required degree of protection (e.g. IP67) must be guaranteed. Certain functions require mobility (e.g. discharge resistors). Power resistors are available from low wattages up to limit values ≥ 0.5 MW(http://www.frizlen.de, http://www.bremswiderstand.de).

Designs
A wide variety of designs and resistance technologies are available to meet the requirements, e.g. as

• Steel grid, cast iron resistor (high performance, meander punched, cast),
• encapsulated flat design,
• Resistance wire wound on ceramic carrier, open or perforated sheet metal housing (standard technology for low, medium power),
• PTC ceramic for low power (self-protecting, suitable for installation).

Discharging resistor FDLZ for mobile use (Fritzlen, www.fritzlen.com).

Application examples
If motors reduce their speed, the inverter-fed machine operates briefly as a generator in the II. or IV. quadrant of the speed-torque characteristic. In this state, the drive supplies braking energy. If it is not fed back into the mains and converted in the machine itself (regenerative braking), it is dissipated as heat in a pulse-controlled braking resistor (braking chopper), which is connected in parallel to the capacitor of the intermediate circuit. This component is characterized by high pulse strength with low duty cycle ED (≥ 1 %) and the ability to withstand short-term overloads. Compact, powerful, easy-to-install and easy-to-cool ballast resistors can be installed in the inverter or operated externally in a housing with the highest degree of protection and high dielectric strength (>< 1200 V DC). They are often used as reserve braking devices in regenerative systems.

Wind turbines in particular require a wide variety of designs in the most diverse functional groups. In the case of high-current systems, the choice usually falls on their rail-shaped, stable design. Although line resistances are usually undesirable, they must be calculated correctly and included in the system function.

Dr.-Ing. habil. Joachim Krause

Resistance = electrical resistance This quantitative term is also used for other specialist areas, e.g. as thermal or magnetic resistance.

Operating mode Behaviour (of a drive), load (of a power resistor) Basic rated operating modes S1...S10 are defined in DIN EN60034-1/VDE 0530 Part 1. They are characterized by typical time characteristics of current, torque and speed (e.g. continuous operation S1, short-time operation S2, intermittent operation S3) and facilitate the optimized dimensioning of devices, motors, etc., as the heating influenced by the operating mode is taken into account. The actual load curve must be in good agreement with the rated operating mode.

PTC Positive Temperature Coefficient. In ohmic resistors with PTC (positive temperature coefficient), the resistance value increases by several powers of ten as the temperature rises. Component for temperature protection.

Brake chopper The drive of a certain speed has kinetic energy. As a result of braking, energy flows into the intermediate circuit as regenerative current. Above a voltage threshold, the chopper is switched on. The excess energy is then converted into a thermal state via the braking resistor(Figure 2).