Three Chapter 7

Transformers

• Most electrical energy is generated far from the place where it is needed.
  • hydro electricity is generated near fast moving or falling water.
  • nuclear and thermal electricity is generated away from cities due to the hazards.
• Whenever electrical energy moves through a conductor, heat is produced (super conductors are an exception).
• This heat is a loss of energy. It represents energy that is useless.
• Example 1:
  • A 10 kW power plant transmits electrical energy at 200 V in a wire with resistance of 1W. Calculate the power lost due to the resistance of the wire.

• This power loss tells us that 2500 J of energy are lost due to heat every second. It amounts to 25% of what was produced (10 kW).
• Example 2:
  • A 10 kW power plant transmits electrical energy at 2000 V in a wire with resistance of 1W. Calculate the power lost due to the resistance of the wire.

• This time, for the same power plant, and the same transmission line, the power loss due to heat is 25 W or only 0.25% of the total production.
• Conclusion - From these two examples, it is obvious that it is more efficient to transmit electrical energy at higher electrical potential (Voltage).
  • Most power stations produce potentials around 10 kV. This voltage is too low for transmission of the electricity (because there will be too much energy loss), and too high for use in homes.
  • Therefore we need a device to step up and step down electric potential.
• Transformer - A device used to step up and step down electric potential
  • Transformers only work with alternating current
  • consists of a core of soft iron and two separate coils of wire.

• Primary Coil - The coil in a transformer that has current supplied to it
• Secondary coil - The coil in a transformer that has current induced in it
• The primary coil produces a magnetic field which changes with time. The changes in magnetic field induce an alternating current in the secondary coil.
• By varying the number of turns of wire in the two coils, the potential is stepped up or down.
• Step up transformer - A transformer with more windings in the secondary coil than the primary coil.
  • increases the voltage output from the secondary coil.
  • decreases the current output from the secondary coil.
• Step down transformer - A transformer with more windings in the primary coil than the secondary coil.
  • Decreases the voltage output from the secondary coil.
  • increases the current output from the secondary coil.
• The relationship between voltage (electric potential), current, and number of turns is:

• The N's are for number of turns, V's for voltage, and I's for current. 1 is for primary and 2 is for secondary.
• To reduce energy loss, the transformer should use
  • Copper coils of low resistance.
  • A core of high-permeability, soft ferromagnetic materials. (These reduce energy required to change the magnetic field in the core.
  • The core should have a shape and size to ensure a large amount of induction between the coils.
• In a well designed transformer, the power in the primary coil should be approximately equal to the power in the secondary coil and power loss (DP) approximately equal to zero.

Explaning Transformer Operation

• The cause of induced current in a conductor is relative motion between the conductor and a magnetic field.
• As a conductor comes closer to a magnet, the magnetic field gets stronger. This strengthening is opposed by the induction of a current in the conductor.
• With a transformer, alternating current in the primary coil causes a changing magnetic field in the iron core and at the secondary coil.
• These changes involve the field building up (strengthening) and collapsing (weakening).
• To the conductor in the secondary coil, this strengthening and weakening is no different than the conductor moving closer and further away from the coil.
• Therefore, there is an induced current in the secondary coil which opposes the action (strengthening and weakening) of the inducing field.
• The field strength at the secondary coil produced by the primary coil depends upon
  • the number of turns of wire in the primary coil.
  • permeability of the core.
  • primary current.
• The potential difference in the secondary coil is dependant upon.
  • the number of turns in the secondary coil.
  • strength of inducing field.
  • rate of change of the inducing field.
• If the primary current did not change, then the magnetic field would be constant at the secondary coil and therefore there is no change that must be opposed by an induced current. That is, no induced current forms at the secondary coil. This is why transformers only work with alternating current.
• The reasons we use alternating current and not direct current are:
  • Electrical energy must be transmitted at high voltage to reduce energy loss
  • High voltages can be reached with transformers and then reduced again to usable levels.
  • Transformers only operate on alternating current.

Distribution of Electrical Energy

• Electrical energy often must be transmitted over long distances, therefore it is
  • Stepped up to high voltage near the generator
  • kept at high potential through out the network.
• Most electrical devices use 60 Hz alternating current at 120 V with a few at 240 V (like a stove).
• 60 Hz means electrons will be driven in one direction and then the other 60 times in a second
  • They spend 1/120 s travelling one way and 1/120 s travelling the other.
  • at the time when they change direction, the current is momentarily zero
  • This high rate is needed so that lights do not appear to flicker when the current is zero at electron reversal.
• Modern systems generate electricity at about 20 kV
  • It is stepped up to 230 kV or 500 kV or even 765 kV by a transformer.
  • Once the electricity reaches the city, district transformers reduce this to 115 kV.
  • Power moves to local transformers which step it down to 44 kV or 22.6 kV.
  • Power moves to transformer substations in each neighbourhood to step down to 4 kV.
  • Power now moves to neighbourhood transformers which step it down to 240 V.
  • Three wires run from the neighbourhood transformer to each house.
    • one from each end to provide 240 V
    • one from the middle to provide 120 V

January 17, 2014