Study material for SSE/JE examination by RRB on Electrical Engineering Part I

Last updated on October 23rd, 2014 at 03:46 am

Effect of Current

Current is having wide range of effect in doing work namely heating, motoring, illumination, etc. Flow of electric current not only produces resistive effect but also electro-magnetic and electro-static effect. When electron is static it produces electro-static field around it and when moving, it produces electro-magnetic field around it. Electro-static and electro-magnetic field results in properties of inductance and capacitance and equipment called Inductor and Capacitor. Some important effects are explained here

Heating effect of resistance also called Joule heating

When current passes through a resistance, it results in heating and power consumed during heating is given by V*I,  where voltage is the potential difference across the conductor i.e. V= I*R and I is the current through it. If we substitute the value of voltage drop across the resistor in V*I , we get the heat produced  across the resistance equal to I²*R. This is also called Joule’s first law. The unit of power is Watt and energy is given as watt.sec.

Watt*sec. or  1 w-sec = 1 Joule and 4.2 Joules = 1 cal.

This is an important equation one should remember for conversion of electrical to mechanical and heat energy. 1 kW heater working for 10 secs. will produce heat to raise the temperature of 1 litre of water in 10 secs. by 1⁰C. Another important point to note that heating effect is irrespective of direction of current. In alternate current, the effect of heating is same in both the positive and negative cycle.

The electric resistance changes with the rise in temperature and is directly proportional to initial resistance, rise in temperature and temperature coefficient of resistance and given by the formula δR=R_t-R₀=αR₀*t . Knowledge of temperature coefficient is important for selection of material for different applications and to provide feature for compensation of expansion which normally keeps on taking place with varying ambient temperature.

Temperature Coefficient at 20⁰C (x10^-4) for different general purpose engineering material is Al(40), Cu(39), Iron(65), Nichrome(1.5), Mercury (8.9) and Carbon (-5).The value of temperature coefficient is not constant but different at different temperature. At 50⁰C, the value of coefficient of temperature for copper reduces from 39×10^-4 to 35×10^-4.    The temperature coefficient of carbon is negative, meaning thereby its resistance reduces at higher temperature.

Illumination

The lamp is the most important discovery by Thomas Edision for illuminating the space and is also  the most important contribution of Electrical Engineering to mankind. Lamps provides illumination and increases effective hours of activity.  It is also an important factor for mental and physical well being, mood elevation, safety and security.

Luminous Flux ( lumen ), Illuminace ( lumen/m2 or Lux ) and energy efficiency ( lumen/watt ) are some of the important measurement units  used in specifying Illumination Engineering. Wattage/Room is the total power rating of the lamps provided in a work place and calculated as:

[Lux level required*Area of room] /[Lumen/watt] Note: This is factored by  utilization and  maintenance factor, shape of the room and working plane.

For illumination design, following factors are considered

Economy:   Price of lamp, Electricity cost and Lumen/watt),

Quality:   Color Rendering Index i.e. preferred  light which is close to natural light. A CRI of sunlight is 100

Usage:   Reading, Leisure, Machine Shop, safety, restaurant

Location:   Street, Railway Yard, Railway Platform Covered-Uncovered Area, Circulating area around Railway    Station,   Concourse Area at the  entrance of Railway Station, Office, Residential lighting, Control Office,

Maintainability: Loss of illumination due to dust on the outer cover, possibility of moisture, dust and  insects entering into the luminary, life of the lamp including its accessories

Lux level is decided by the activity level, the nature of work, safety and security of human beings.It should be noted that this is the highest level of illuminance   to serve the purpose when activity level is maximum. This is not required to be sustained throughout the switched ‘on’ time. Therefore, there shall be two or three Switching circuit  built in the design for switching on and off depending upon the activity level either manually and automatically. Thus usage and efficiency determines the energy conservation measures to avoid any wastage when high capacities are built in.

Energy conservation of Electric Lamp

• T5 lamp with fitting replaces T8 or T12. Be careful  Lumen/watt  of different make also varies. Incandescent is highly energy inefficient and replaced from all locations.
• CFL replaces the incandescent lamp using the same holder.
• Electronic choke consumes 4w less as compared to electromagnetic choke. There are issues of reliability of electronic choke that need to address in right earnest.
• Metal halide and sodium vapor lamp takes more time to start around 3-4minutes. It is more suitable for illumination of Railway station, circulating area and yard.  Avoid use of sodium vapor lamp for low height mounting at platform/yard as it may give conflict signal to loco pilot.
• LED is long lasting, durable, cool (3.4 Btu/hr), mercury free, energy efficient. LED lamp is costly but still economically suited for lower wattage application in conjunction with battery (torch light, emergency light, night light, solar panel led light, focus lights, etc.). There is a caution about the reliability of driver unit and life of accessories not matching with the life of LED lamp. Large scale usage shall begin with 24×7 hrs air conditioned dust control area like control office, ticket counters etc.

Magnetic effect of current

Magnetic field is produced either by magnetic material or flow of charge. Magnetic field is denoted by magnitude and direction and is  vector quantity. It was first demonstrated by the Biot–Savart,  deriving a relation between the magnetic field generated by an electric current. It relates the magnetic field to the magnitude, direction, length, and proximity of the electric current. The Biot–Savart law is used for computing the resultant magnetic field B at position r generated by a steady current I.

Ampere’s Circuital Law

Ampere’s circuital law talks about electro-magnetic effect whereas gauss’s law in similar way discusses electro-statics field of charge. This law states that

” The line integral of resultant magnetic field along a closed plane curve is equal to μ₀ time the total current crossing the area bounded by the closed curve provided the electric field inside the loop remains constant” Thus,  this law is also used to calculate the magnetic field due to any given current distribution.

It is denoted by term H and B, both expressing in magnetic field with units of amp/meter and tesla. In a simple way to express what is H and B, than H is magnetic field and B is what is induced in a magnetic material and is given as B = µ₀*µ_r *H where µ₀ is called the permeability of free space equal to 4π*10^-7 Henry/meter and µ_r relative permeability. Depending upon the value of relative permeability of material, the magnetic material are classified as Ferro-magnetic- very high µ_r, Para-magnetic – µ_r slightly higher than 1 and dia-magnetic – less than unity.

Magnetic Materials

Dia-magentic material: Diamagnetic materials create an inducted magnetic field in a direction opposite to an externally applied magnetic field, and are repelled by the applied magnetic field. Diamagnetic materials are Bismuth, Mercury, Silver, Lead, Copper and water. Super contductors are perfect diamagnets.

Ferro-magnetic material: These materials get magnetised in the presence of magnetic field and retains it for a longer period. Ferro-magnetic material are Cobalt, Nickel, Iron, Ferro-magnetism is very important in industry and modern technology, and is the basis for many electrical and electro-mechanical devices such as electromagnets, electric motors, generators, transformers and magnetic storage such as hard discs.

Para-magnetic materials: Paramagnetic materials have a small, positive susceptibility to magnetic fields. These materials are slightly attracted by a magnetic field and the material does not retain the magnetic properties when the external field is removed. Paramagnetic materials are Tungsten, Aluminium, Magnesium etc.

How Current, Magnetic field and Motion interacts?

The development of a relation between current, magnetic field and motion is the foundation for motoring action. A current carrying conductor in a magnetic field experience a force and given by

Where F is the force, B is the magnetic field in the material,  is current through the conductor,  is the length of the conductor and  is the angle between magnetic field and the current.

Fleming’s left-hand rule (for motors), and Fleming’s right-hand-rule (for generators) are a pair of visual mnemonics. These were displayed  by John Fleming, in the late 19th century, as a simple way of working out the direction of motion in an electric, or the direction of electric current in an electric generator.

When current flows in a wire, and an external magnetic field is applied across that flow, the wire experiences a force perpendicular both to that field and to the direction of the current. A left hand can be held, as shown in the illustration, to represent three mutually orthogonal axes on the thumb, first finger and middle finger. Each finger is then assigned to a quantity (mechanical force, magnetic field and electric current). The right and left hand are used for generators and motors respectively.

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