Generators and Transformers

Generating Current By moving a magnet in and out of a coil of wire an alternating current (a.c.) is being generated

Alternating Current

Generator/Dynamo A generator works by rotating a coil in a magnetic field (or rotating a magnet in a coil). A commutator produces alternating current (a.c.)

AC Generator Most of our electricity comes from electric generators. Simply put it is a coil of wire spun inside a magnet and an A.C current is produced

DC Generator The easiest way to make an A.C. generator D.C. is to add a commutator. How does a commutator work?

True or False Write down the true statements and write corrected versions of the false statements where possible

Transformers

The National Grid TRANSFORMERS!!!!!! The National Grid is the system used for distributing electrical energy around the country. The cable might have up to a hundred miles from the power station. Cables that long have very high resistance (in the example below it is about 50Ω), they heat up as the current flow through them which wastes energy. At first, the National Grid was very inefficient. The scientists came up with the solution …… TRANSFORMERS!!!!!! The power station produces electricity at a voltage of 25,000 Volts and a current of 100 Amps. Output power = 2.5 MW Due to the high current, the cables will heat up and most of the electrical energy will be lost as heat. Lost power = I2R = 0.5 MW The total energy reaching homes and factories is only a 2.0 MW

But since P = I x V, reducing the current means increasing the voltage The National Grid The solution is to reduce the current in the power lines in order to reduce the dissipated power. But since P = I x V, reducing the current means increasing the voltage 230 Volts 132,000 Volts 25,000 Volts

But since P = I x V, reducing the current means increasing the voltage The National Grid The solution is to reduce the current in the power lines in order to reduce the dissipated power. But since P = I x V, reducing the current means increasing the voltage The step up transformer steps up the voltage to 132,000 Volts (by a factor of 5.28). This causes the current to drop (by a factor of 5.28) to only 19Amps . Lost power = I2R = 0.02 MW very little heating effect The power station produces electricity at a voltage of 25,000 Volts and a current of 100 Amps. input power = 2.5 MW The step down transformer steps down the voltage to a safe 230 Volts. The current varies according to the appliances used.

The Simplest Transformer Iron core The alternating voltage in the primary coil magnetises the iron core in alternating directions. (magnetic effect of current) This is just like moving a magnet in and out of the secondary coil. There will be an induced voltage in the secondary coil (electromagnetic induction). a.c. output Secondary coil a.c. input Primary coil

The Simplest Transformer Iron core The alternating voltage in the primary coil magnetises the iron core in alternating directions. (magnetic effect of current) This is just like moving a magnet in and out of the secondary coil. There will be an induced voltage in the secondary coil (electromagnetic induction). a.c. output Secondary coil a.c. input Primary coil

The Simplest Transformer Iron core The alternating voltage in the primary coil magnetises the iron core in alternating directions. (magnetic effect of current) This is just like moving a magnet in and out of the secondary coil. There will be an induced voltage in the secondary coil (electromagnetic induction). a.c. output Secondary coil a.c. input Primary coil

The Simplest Transformer The output voltage produced in the secondary coil depends on the number of turns in the coil. A higher number of turns in the secondary coil induces a higher voltage than the input voltage. A lower number of turns in the secondary coil induces a lower voltage than the input voltage Iron core secondary coil a.c. output voltage a.c. input voltage Primary coil

A conventional transformer consists of three main parts.

Symbol

Transformers A changing current in the primary coil produces a changing magnetic field in the core. This changing magnetic field induces a changing current in the secondary coil. A.C. Current A.C. Current Magnetic field

Transformers Represented diagramatically: Primary Coil Core Secondary Coil Alternating EMF Load 2. This causes alternating magnetic field in the core 3.This causes alternating EMF at terminals of secondary coil 1.Alternating EMF causes alternating current in primary coil 4. This causes alternating current in load. .

Changing Voltages Vp/Vs = Np/Ns The ratio of the number of turns on the coils equals the ratio of the voltages # of turns = Ns # of turns = Np Vp/Vs = Np/Ns

Stepping down transformer Primary coil secondary coil a.c. output a.c. input Iron core Stepping up transformer Primary coil secondary coil a.c. output a.c. input Iron core

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Step Up or Step Down?

Copy the table and fill in the blanks Primary turns Np Secondary turns Ns Primary voltage Vp Secondary voltage Vs Step up or step down 100 20 3 400 10 000 10 50 240 12 5 000 33 000 11 000

Solutions Primary turns Np Secondary turns Ns Primary voltage Vp Secondary voltage Vs Step up or step down 100 20 15 3 down 400 10 000 10 50 240 12 5 000 33 000 11 000

Solutions Primary turns Np Secondary turns Ns Primary voltage Vp Secondary voltage Vs Step up or step down 100 20 15 3 down 400 10 000 10 250 up 50 240 12 5 000 33 000 11 000

Solutions Primary turns Np Secondary turns Ns Primary voltage Vp Secondary voltage Vs Step up or step down 100 20 15 3 down 400 10 000 10 250 up 1 000 50 240 12 5 000 33 000 11 000

Solutions Primary turns Np Secondary turns Ns Primary voltage Vp Secondary voltage Vs Step up or step down 100 20 15 3 down 400 10 000 10 250 up 1 000 50 240 12 15 000 5 000 33 000 11 000

Question

Practical Transformers Most practical transformers have the two coils on top of one another rather than side by side The core is laminated to reduce eddy currents and made of soft iron to allow easy magnetic reversals Often transformers have multiple outputs allowing many voltages to be tapped off Transformers are very efficient but larger ones require cooling (usually with oil) hence we can often assume that: Power in = power out or Vin Iin= Vout Iout or VPIP=VSIS