Monthly Archives: July 2016

AC Motors and Energy Transformations

Describe the main features of an AC motor

The AC induction generator used in large-scale power stations has a very similar structure to an AC induction motor. However, in an AC induction generator, the rotor is an electromagnet powered by a separate DC circuit, and the stator consists of 6 coils. A source of torque is used to rotate the electromagnet at 50 revolutions per second, which causes AC electricity to be generated in the field coils.

There are three types of AC motors- standard AC motors, universal motors and AC induction motors, and they each work differently. A standard AC motor is essentially identical to an AC generator, with a stator providing a magnetic field, a rotor that current is passed through, and slip rings connecting the rotor to a circuit. In addition, an AC motor usually has a fan to keep the rotor cool, a ferromagnetic core in the rotor to strengthen the magnetic field and it runs at 50 revolutions per second, the same as the frequency of AC power oscillation (50Hz). A universal motor is similar to a DC motor. It can operate on an AC or DC supply. Power is fed in, and runs through electromagnetic stators before entering a commutator. Each brush is connected to a wire that comprises one of the field coils, and is also connected to one end of a circuit. With a DC source, the commutator switches the current and the motor operates. With an AC source, although the direction of current being fed into the commutator is varying, the same variations are fed into the field coils, with the net effect that AC oscillation is cancelled out and the motor runs.

AC induction motors are entirely different. Induction motors have a rotor that is not connected to a power source- instead changing flux is used to induce a current in the rotor. This means that there is very low friction as the rotor is not actually in contact with the rest of the motor, and it also means there is very little wear and tear. AC induction motors have a more complicated stator with several field coil pairs. There are a total of 6 field coils, and each opposite pair is fed one phase of triple-phase AC power. This sets up a rotating magnetic field inside the stator. The rotor of an induction motor is generally similar to a squirrel cage (the type that allows pets to run endlessly), with two end rings and aluminium or copper bars linking the end rings to form a cylindrical shape. This cylinder is encased in a laminated iron armature so that the magnetic field passing through the rotor cage is intensified. As the field rotates, it induces current in the bars of the squirrel cage. This creates a force in the same direction as the rotation of the magnetic field, from Lenz’s Law. The squirrel cage then rotates, ‘chasing’ the changing magnetic field.

Remember- AC motors have a stator, rotor and slip rings. They also use an iron core and usually a fan. A universal motor uses a commutator and has a magnetic field generated using field coils. AC induction motors have a stator with 3 pairs of field coils (for a total of 6), and a “squirrel cage” rotor.

Perform an investigation to demonstrate the principle of an AC induction motor

An AC induction motor relies on the principle that a moving magnetic field induces a current in the rotor with a direction that, according to Lenz’s law, causes the rotor to spin in the same direction as the magnetic field. We demonstrated this principle by using a thin aluminium disk suspended by a string from a clamp on a retort stand so that the disk was free to rotate. To demonstrate the principle of an AC induction motor, we moved a strong ceramic magnet in circles around the circumference of the disk. The induced eddy currents caused the disk to rotate in the same direction as the magnet, thereby demonstrating the principle.

Remember- An aluminium disk was suspended by a string and rotated by moving a magnet.

Gather, process and analyse information to identify some of the energy trans- fers and transformations involving the conversion of electrical energy into more useful forms in the home and industry

Electricity is simply an easy way to transmit energy from point to point which enables energy to be collected and transmitted on a large scale. The advantage of electricity is not only that it is relatively easy to transport, but also that it is easy to convert it into other forms. In light bulbs, electrical energy is converted into light energy. In the home, it is also converted into heat in devices such as heaters and toasters, and into sound through speakers. In the industry electricity is most often converted into kinetic energy which drives machinery used in the production of goods. So generally electricity is converted into kinetic energy or electromagnetic radiation in the house and in industry.

Remember- All electrical devices convert electrical energy into other forms.

Transformers

Describe the purpose of transformers in electrical circuits

A transformer is designed to change the voltage of electricity. Its purpose is to either step-up (raise) or step-down (lower) the voltage that is fed into it.

Remember- Transformers change the voltage of electricity.

Compare step-up and step-down transformers

Step-up and step-down transformers are almost identical. Both have an identical structure, with primary and secondary coils and an iron core. In both transformers, the number of turns in each of the coil varies, with one coil having more turns than the other. In a step-up transformer, the secondary coil has more turns than the primary coil. This results in a higher voltage output. In a step-down transformer, the secondary coil has less turns than the primary coil. This results in a lower voltage output. So essentially the difference between a step-up and a step-down transformer is whether the primary coil has more or less turns than the secondary coil.

Remember- Step-up transformers increase voltage, and step-down transformers decrease voltage.

Identify the relationship between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage

The ratio between the ratio of the number of turns in the primary and secondary coils and the ratio of primary to secondary voltage is identical, according to the formula \frac{V_p}{V_s} = \frac{n_p}{n_s}

Gather, analyse and use available evidence to discuss how difficulties of heating caused by eddy currents in transformers may be overcome

Heating due to eddy currents isn’t the only form of energy loss in transformers. Current in the coils causes them to heat up, increasing resistance. This heating is countered by the use of coolant to keep the coils conducting efficiently.
The founding principle of the transformer is the induction of current in the secondary coil because the secondary coil experiences changing flux. However, the iron core of the transformer also experiences changing flux, which induces eddy currents in the core, heating it up. There are two ways in which this can be addressed:
Firstly, instead of iron, ferrites, complex oxides of iron and other metals can be used. Ferrites are good at transmitting flux but poor at conducting electricity, so eddy currents and heating are minimised.

Alternatively, the iron core can be sliced into thin layers and then put back together with insulation between each layer. This process, known as lamination, breaks up large eddy currents and minimises them because currents can only form in each of the lamina. This means smaller eddy currents and therefore less heating. The laminations must not be in the same plane as the coils- instead they must “slice” this plane as thinly as possible to minimise eddy current formation.

 

figure 1

Remember- Laminations cut the plane of the coils to break up eddy currents.

Perform an investigation to model the structure of a transformer to demon- strate how secondary voltage is produced

In this experiment, we had a primary coil producing a changing magnetic field which was used to induce a current in a secondary coil. The setup consisted of a hollow coil that slid into the middle of a larger hollow coil. An iron core consisting of a solid iron rod fitted into the middle of the smaller coil. An AC power supply was connected to the primary coil, and a galvanometer was connected to the secondary coil. When we passed AC current through the large coil, the galvanometer detected current in the secondary coil, showing that induction was taking place. The iron core intensified the induction- when we removed the core the induced current dropped greatly in strength. This is because the iron core directs the magnetic field from the primary coil into the secondary coil, thereby increasing efficiency.

figure 2
Remember- The primary coil induces a current in a secondary coil, the voltage is changed because they each have a different number of turns, and the iron core intensifies the effect.

Explain the role of transformers in electricity substations

Transformers are used in substations to step-up and step-down electrical energy for long distance transmission. At the generator, a step-up transformer at a substation raises the output voltage from 23kV to 330kV. This minimises losses during long distance transmission by reducing the current flowing through transmission wires. In substations located in urban areas, step-down transformers are used to reduce the voltage for transmission within cities or suburbs. So transformers convert voltages in substations to reduce losses when transmitting electricity.

Remember- Transformers are used in substations to change the voltage of electricity to minimise transmission losses.

Gather and analyse secondary information to discuss the need for transformers in the transfer of electrical energy from a power station to its point of use

Power losses in the transmission of electricity are largely caused by heating in transmission wires. The energy consumed is equal to I^2R(since the energy lost is the same as the power “used” by the wire, from P = I^2R), so it can be seen that power loss is dependant on the current flowing through the wire, as well as the wire’s resistance. This heating is a huge problem, because it results in less energy reaching the point of use. However, transformers can be used to raise the voltage of electricity, and thereby reduce current. This dramatically reduces the power consumed by transmission wires, and thereby reduces wasted energy. Using transformers in the transfer of electrical energy from a power station to its point of use provides massive efficiency gains, reducing the fuel consumed by a power plant and reducing the price of electricity.

Also, different devices require different voltages- computers and incandescent lights require much lower voltages than TV’s and fluorescent lamps (which can require up to 10000V). Transformers are required to ensure each device is supplied with an appropriate voltage. However, high voltage power lines are subject to arcing and so need to be separated, as do substations which can be extremely dangerous for people nearby. Although transformers have dramatically increased the efficiency of electricity transmission, they have also produced safety concerns and have required specialised infrastructure to keep people safe.

Remember- Transformers are required for electricity transmission to reduce otherwise prohibitive losses.

Explain why voltage transformations are related to conservation of energy

According to conservation of energy, energy cannot be created nor destroyed, only transformed. Electrical energy is expressed as P , measured in watts. P = IV . Conservation of energy means that the energy in the secondary coil must equal the energy in the primary coil, so that P_p = P_s. This means I_p V_p = I_s V_s. Therefore, when voltage is changed in a transformer, the current then must also change so that P remains constant, according to conservation of energy. So when voltage is stepped up, current is reduced, and vice versa.

Remember- Voltage transformations are related to conservation of energy because the total power on either side of a transformer is the same.

Discuss why some electrical appliances in the home that are connected to the mains domestic power supply use a transformer

Many electrical appliances are designed to run on low DC voltages. This is particularly true of any device which uses a battery, because batteries are only capable of providing low DC voltages. The reason batteries are used is to provide portability- so that devices such as laptops and mobile phones can be moved around. Also, some circuits particularly those in computers, only function at low DC voltages- otherwise they would overheat and burn out. In these electrical appliances, a transformer is required, in addition with a rectifier, to convert household mains domestic power (240V AC) into low voltage DC for use in the appliance.

Remember- Home devices with transformers usually have low-voltage chips, or can run on batteries as well.

Discuss the impact of the development of transformers on society

The direct impact of transformers was to make AC power a viable solution. By allowing large scale generation from outside urban areas, the uptake of electricity was rapid. Therefore the impacts of transformers on society are the same as the impact of AC power, because the key impact of transformers was to provide efficient AC power distribution.
Transformers have had a significant impact upon society. The main reason AC power was successful over DC power was because the voltage could be changed to minimise transmission losses and dramatically slash losses in the electricity grid. These voltages changes were only made possible by the development of the transformer. It has resulted in the wide uptake of electricity and it helped lower the cost of electricity, making it accessible to almost everyone in economic terms. However, the widespread introduction of electricity made many unskilled jobs redundant and increased unemployment levels, which was detrimental to many people. Also, widespread demand for electricity led to large scale use of fossil fuels such as coal to power the generators, which has resulted in a great deal of atmospheric pollution in the form of sulfur and nitrogen oxides, as well as increased carbon dioxide levels which contribute to global warming. So transformers have had a huge impact on society, from bringing electricity into the reach of the broad public to indirectly causing environmental damage and social problems.

Remember- Transformers have led to the large scale uptake of electricity.