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The 4 - 20 mA signal is an extremely popular signal specification in instruments.  The 4 to 20 mA signal provides "live zero" function.  i.e. in the event of a wire break, the signal drops to zero mA.  This can be used to detect wire break or sensor failure.  This is a crucial advantage of the 4 to 20 mA format.

  Being a Current signal, it is also less susceptible to external interference.


The 4 to 20 mA format works by varying the resistance to a constant source voltage.  The sensor consists of a transistor which regulates the current passing through it in accordance with the measured value.  Sensors which use this format can be categorized into active and passive transducers.  Active transducers are devices which can provide the system voltage as well as regulate the current.  Passive devices require an external voltage and only regulate the current.

Another advantage of this 4 to 20 mA format is that it can be easily converted into a voltage signal by means of a resistor. (1 - 5 volts for a 250 ohm resistor).

List of Electrical Software

ETAP
Popular design software used across the industry for a wide range of design applications  in areas such as power management, substation automation, load shedding, etc..  Demo Version Available.
www.etap.com

AutoCAD
Popular Software for Electrical Design.  Offers Trial Version. 
http://usa.autodesk.com/autocad-electrical/

Elecdes
Electrical Design Software with a free demo version. Suitable for Electrical Panel Design, Plant Instrumentation, Wiring Diagram, Plant Raceway Design, Cable Routing,
http://www.elecdes.com/

PCschematic
CAD software with free Trial version.
www.pcschematic.com

SmartDraw
Electrical Software for designing electronic Circuits, Automotive Wiring, Circuit Schematics and designing digital circuits.
www.smartdraw.com

Electrical Estimating Software
Software for Estimating Electrical projects. Contains online demo.
www.electricalestimatingsoftware.com

EasyPower
Software for Arc Flash Analysis and short circuit Calculations, Relay setting coordination and Equipment Sizing.  Offers Trial Version.
http://www.easypower.com/

Schneider Electric CAD
Software for designing electrical installations in industrial and tertiary buildings.  Trial version available. 
http://www.soft.schneider-electric.com/

MTOE is the acronym for Million tonnes of Oil Equivalent.  It is a unit to quantify the amount of energy which is released by the burning of a million tonnes of crude oil.  

Different fuels have different MTOE values.  The OECD defines one tonne of energy equivalent for crude oil to be 11,630 kWh.  That is, 11630 units can be produced from one tonne of crude oil. 

MTOE is important as it helps one understand how many units of electricity can be from an given fuel.  This depends on the calorific value of the fuel and the efficiency of the generating process. 

Hydroelectricity can also be quantified in terms of MTOE (million tonnes of Energy Equivalent).  When we say that Brazil has 89.6 tonnes of oil equivalent in hydroelectricity.  It means that the power generated by the total hydroelectric resources and infrastructure is equal to the power which could be generated by 89.6 tonnes of oil.

MTOE is a unit which helps compare the potential and contribution of different sources of power such as hydroelectricty, wind electricity, etc. 

When a dc coil connected to a contactor or a solenoid is energized, it draws a high current. This high current is necessary to generate enough magnetism to pull the plunger or the contactor. This is known as the pull in current. 

After the the contactor has been pulled in, less current will be sufficient to hold the coil. This is called the holding current. 

Hence, to limit the current after the contactor has operated, a series resistance is connected. This is known as the economy resistor. The economy resistor enables the design of contactors which can draw heavy initial current and less holding current (resulting in lower power consumption during normal operation). 

In the case of AC coils this is not required as the current is limited after the initial inrush due to the back-emf.

The chief losses in a transformer are the losses that occur in the core.  These are the the eddy current loss and the hysteresis losses.  About 1 to 4 percent of the power which passes through a transformer is lost due to these losses.

The losses in the distribution transformers constitute nearly 20% of the total losses in the distribution system. Since transformers are online continually, the no load losses of the transformers is constant throughout the day regardless of the load. 

The Amorphous Metal Transformer is fast emerging as an efficient alternative to the conventional transformer.  The Amorphous Metal transformer has a core which is made of ferromagnetic materials such as Iron or Cobalt in a glass former such as phosphorous, silicon or boron.

Metglas, as this substance is known, has high susceptibility, low coercivity and high resistance.  The low coercivity reduces the hysteresis losses while the high resistance greatly reduces the eddy current losses.

By using Amorphous Metal Transformers, it is estimated that many millions of units of electricity can be saved.  Amorphous Metal Transformers are widely used in developing economies such as India and China in an effort to bring down the distribution losses.  

Some of the other advantages of Amorphous Metal Transformers are the lower operating temperature, higher overloading capability, slower ageing of the winding insulation and better performance when subjected to harmonics.