Saturday, January 24, 2009
Nameplate Information
NEMA Standard
MG 1 requires the following information on the nameplate, as applicable:
Manufacturer’s type and frame designation (TYPE and FRAME)
Horsepower output (HP)
Shaft rotation direction (ROT)
Maximum ambient temperature for which motor is designed (MAX AMB)
Insulation system designation (INSUL CLASS)
RPM at rated load (RPM)
Frequency (HZ)
Number of phases (PH)
Rated load current (AMPS)
Voltage (VOLTS)
Rating or duty (RATING)
Locked rotor current (L.R. AMPS)
Service factor (SF)
Words “THERMALLY PROTECTED” if motor is protected as specified in MG 1
Words “OVER TEMP. PROT.” followed by a type number identifying protection type for motors rated above 1 hp equipped with overtemperature devices or systems
Cutaway View of a synchronous AC generator
Schematic diagram and cross-section view of a typical 500 MW Synchronous Generator and its 2400 kw dc exciter. The dc exciting current Ix (6000A) flows through the commutator and two skip-rings.
the dc control current Ic from the pilot exciter permits variable field control on of the main exciter,which in turn controls Ix.
Total rewinding GE Rotor Generator 180 MW 3000 RPM
Wednesday, January 14, 2009
Jointing Theory
The ideal joint achieves a balanced match with the electrical, chemical, thermal, and mechanical characteristics of its associated cable. In actual practice, it is not always economically feasible to obtain a perfect match. A close match is certainly one of the objectives.
The splicing or joining of two pieces of cable together can best be visualized as two terminations connected together. The most important deviation, from a theoretical view, between joints and terminations is that joints are more nearly extensions of the cable.
The splicing or joining of two pieces of cable together can best be visualized as two terminations connected together. The most important deviation, from a theoretical view, between joints and terminations is that joints are more nearly extensions of the cable.
TERMINATION THEORY
A termination is a way of preparing the end of a cable to provide adequate electrical and mechanical properties. A discussion of the dielectric field at a cable termination serves as an excellent introduction to this subject.
SHIELDING OF POWER CABLES
Shielding of an electric power cable is accomplished by surrounding the assembly or insulation with a grounded, conducting medium. This confines the dielectric field to the inside of this shield. Two distinct types of shields are used:
metallic and nonmetallic.
The purposes of the insulation shield are to:
1. Obtain symmetrical radial stress distribution with the insulation.
2. Eliminate tangential and longitudinal stresses on the surface of the insulation.
3. Exclude from the dielectric field those materials such as braids, tapes, and fillers that are not intended as insulation.
4. Protect the cables from induced or direct aver-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to attenuate surge potentials
metallic and nonmetallic.
The purposes of the insulation shield are to:
1. Obtain symmetrical radial stress distribution with the insulation.
2. Eliminate tangential and longitudinal stresses on the surface of the insulation.
3. Exclude from the dielectric field those materials such as braids, tapes, and fillers that are not intended as insulation.
4. Protect the cables from induced or direct aver-voltages. Shields do this by making the surge impedance uniform along the length of the cable and by helping to attenuate surge potentials
Skin Effect
In ac circuits, the current density is greater near the outer surface of the conductor. The current tends to crowd toward the outer surface.
This is called skin effect. A longitudinal element of the conductor near the center of the axis is surrounded by more lines of magnetic force than near the rim. This results in an increase in inductance toward the center. The decreased area of conductance causes an apparent increase in resistance. At 60 hertz, the phenomenon is negligible in copper sizes of #2 AWG and smaller and aluminum sizes #1/0 AWG and smaller. As conductor sizes increase, the effect becomes more significant.
This is called skin effect. A longitudinal element of the conductor near the center of the axis is surrounded by more lines of magnetic force than near the rim. This results in an increase in inductance toward the center. The decreased area of conductance causes an apparent increase in resistance. At 60 hertz, the phenomenon is negligible in copper sizes of #2 AWG and smaller and aluminum sizes #1/0 AWG and smaller. As conductor sizes increase, the effect becomes more significant.
INSULATING MATERIALS FOR CABLES
Electrical insulation materials are employed over the metallic conductors of
underground cables at all voltage ratings. Polymeric materials are employed as
the insulation, but the nature of the polymer may vary with the voltage class.
POLYMER TYPE PROPERTY >>Low Density Polyethylene Low dielectric losses Moisture sensitive under voltage stress
Crosslinked polyethylene>> Slightly higher losses vs. PE Ages better than PE
EPR / EPDM>>Higher losses vs. XLPE or PE More flexible than XLPE or PE Requires inorganic filler
PVC >>Must contain plasticizer for flexibility
Higher losses
underground cables at all voltage ratings. Polymeric materials are employed as
the insulation, but the nature of the polymer may vary with the voltage class.
POLYMER TYPE PROPERTY >>Low Density Polyethylene Low dielectric losses Moisture sensitive under voltage stress
Crosslinked polyethylene>> Slightly higher losses vs. PE Ages better than PE
EPR / EPDM>>Higher losses vs. XLPE or PE More flexible than XLPE or PE Requires inorganic filler
PVC >>Must contain plasticizer for flexibility
Higher losses
Proximity Effect
In closely spaced ac conductors, there is a tendency for the current to shift to the portion of the conductor that is away from the other conductors of that cable.
This is called proximity effect. The flux linking the conductor current in one conductor is distorted by the current in a nearby conductor which in turn causes a distortion of the cross-sectional current distribution.
Since skin and proximity effects are cumbersome to calculate, tables have been established to give these values for common modes of operation
Thursday, January 8, 2009
Wednesday, January 7, 2009
.jpg)
Subscribe to:
Posts (Atom)