Resistance:
Electrical resistance is a measure of the degree to which an object opposes an electric
current through it, measured in ohms. Its reciprocal quantity is electrical conductance,
measured in siemens. Assuming a uniform current density, an object's electrical
resistance is a function of both its physical geometry and the resistivity of the
material it is made from: R = l·p / A, where "l" is the length, "A" is the cross
sectional area, and "p" is the resistivity of the material. Electrical resistance shares
some conceptual parallels with the mechanical notion of friction. The SI unit of
electrical resistance is the ohm, symbol p (greek letter rho). The resistance of an
object determines the amount of current through the object for a given potential
difference across the object: R = V / I, where "R" is the resistance of the object,
measured in ohms, equivalent to J·s/C2, "V" is the potential difference across the
object, measured in volts, and "I" is the current through the object, measured in amperes.
For a wide variety of materials and conditions, the electrical resistance does not
depend on the amount of current through or the amount of voltage across the object,
meaning that the resistance R is constant.
Resistor:
A resistor is an electrical device that opposes an electric current by producing a
voltage drop between its two ends that can be approximated by Ohm's law: V = I x R.
The electrical resistance is equal to the voltage drop across the resistor divided by
the current through the resistor at constant temperature.
Power resistor:
A power resistor is a resistor used in high power industrial applications. This type of
resistor varies from the size of a shoe box to the size of a car. In terms of power it
goes from the hundreds to the thousands of watts.
Applications:
Power resistors are used in applications such as dynamic braking for locomotives and
other types of machiney; neutral grounding for industrial power distribution; motor
control for cranes and heavy equipment; charge and discharge for capacitor banks,
reactors and superconductive magnetic coils; and harmonic filtering for electric
substations.
Neutral grounding resistor:
A Neutral grounding is a type of power resistor that is generally installed between the
neutral terminal of transformers or generators and the ground mesh in an electrical
industrial installation. It is used to limit ground fault current and therefore limit
ground fault damage, to limit overvoltages and in general to protect people, equipment
and production against short circuits.
Dynamic Braking:
Technique for braking in which mechanical energy is converted to heat or electrical
energy in order to slow or stop motion.
Motor control:
Regulating the speed and torque in an electric motor. This can be achieved by inserting
and removing power resistors with the help of contactors and a timing mechanism.
Harmonic Filters:
Devices designed to stop harmonic distortion in electric circuits for protection
purposes. The most used harmonic filters are combinations of resistors, inductors and
capacitors.
Discharge resistors:
Power Resistors used to drain electric current previously stored in capacitors at a
predetermined rate.
Types of power resistor elements:
Many different types of power resistors have been developed according to the required
ohmic value and current-carrying capacity such as Wirewound Power Resistors for high
ohmic value and low current, Edgewound Power Resistors for medium ohmic value and
current and Stamped Grid and Ribbon Power Resistors for low ohmic value and high
current.
Power resistor components:
The main components are resistive, insulating, supportive and connective and
protective elements.
Resistive elements:
A resistive element is generally a metallic alloy with a precise ohmic value and
current-carrying capacity. Wirewound Power Resistors use metallic wire. Edgewound
Power Resistors use metallic ribbon and Stamped Grid Resistors use punched metal
sheets.
Insulating elements:
Non-conductive components such as porcelain and mica insulators are used as an
electrical barrier between metallic elements.
Supportive elements:
Provide structural support and means for installation and assembly of multiple
resistors into banks.
Connective elements:
Enable the wiring or connection of the Power Resistor to the electrical circuit.
Protective elements:
Enclosing boxes or coatings that protect Power Resistors against environmental
conditions and accidental contact.
Wirewound Resistor:
A wirewound resistor element is generally made by winding a metal wire around a
ceramic, plastic, or fiberglass core. The ends of the wire are soldered or welded
to two caps, attached to the ends of the core. Corrosion resistant alloy wire is
used or alternatively the assembly is protected with a layer of paint, molded
plastic, or an enamel coating baked at high temperature. For higher power
wirewound resistors, either a ceramic outer case or an aluminium outer case on
top of an insulating layer is used. The aluminium cased types are designed to be
attached to a heatsink to dissipate the heat; the rated power is dependant on
being used with a suitable heatsink, e.g., a 50 W power rated resistor will
overheat at around one fifth of the power dissipation if not used with a heatsink.
Because wirewound resistors are coils they have more inductance than other types
of resistor, although this property can be minimized by winding the wire in
sections with alternately reversed direction.
Edgewound Resistor:
An edgewound element is made by edgewise wounding a metal strip in a spiral form
on tubular ceramic insulators. A rod is placed inside to support the resistor
elements when placed between end-pans to form the resistor assembly. The cilinder
thus formed can vary in diameter from 2 to 8 inches and in lenght from 10 to 40
inches. Their current range is between 10 to 100 continuous amps. They are very
compact and shock proof.
Punched Grid Resistor:
Generally used for low resistance and high current grid resistors used to be made
of cast iron but are now being made with punched steel sheet in the form of a grid
with a hole at each end for mounting. The eyes are usually ground and copper-
plated to insure good contact between grids. The grids are stacked on insulated
steel rods. Mica washers are inserted between the grids for insulation. The rods
are then usually mounted between steel end frames in standard sizes. To obtain
better electrical connection the grids are welded together.This type of resistor
element can handle over 300 continuous amps. The resistance obtainable with a
single grid varies from about 0.005 ohm to 0.125 ohm for the ones with smaller
cross sections.
Ribbon Resistor:
Its construction is similar to grid resistors but instead of a multiplicity of
grids it uses one continuous strip of steel bended in a zig-zag pattern and held
together by insulated rods passing through eyes punched at equal intervals
throughout the lenght of the strip. Mica washers are also used for insulation and
the rods are also mounted between steel end frames in standard sizes. This type
of resistor can handle over 600 continuous amps.
Bushing:
An insulating lining for an aperture through which a conductor passes. Often made
with porcelain, resins or polymers.
Current transformer:
A type of instrument transformer that provides a secondary current that is
proportional to its primary current. Used to measure safely large currents often
in the presence of large voltages.
Potential transformer:
A type of instrument transformer that provides a secundary voltage that is
proportional to its primary voltage. Used to measure safely large voltages, often
in the presence of large currents.
Zig-zag transformer:
A transformer with primary windings but no secondary winding. Can be used to
provide a neutral reference point to transformers or generators with secondary in
Delta.
Eyebolt:
A screw with a loop in one end. Eyebolts are usually attached to the top cover of
heavy enclosures to facilitate their lifting by cranes or similar equipment.
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