motors
...ase induction motor by connecting consecutive coils in opposing pairs to the three phases of the power supply. Many single-phase induction motors use capacitors to simulate the three-phase effect. The rotor is made of coils wound on iron armature which is all part of an axle. In the AC motor there is no need to connect the rotor to a power supply. The magnetic field of the rotor is instead induced by the eddy currents produced by the magnetic field of the stator. So the magnetic field in the stator is induced by eddy currents and exerts torque in the direction of the rotational magnetic. The rotor coils can be just single copper bars in the surface of the armature. The bars are connected at the ends by a ring or disc of copper which allows current to flow in a loop between the bars. The main disadvantage of an AC motor is the difficulty to change the speed. Most AC motors run at a set speed of 3000 revolutions per minute. Generators: The DC generator is the simplest form of generator. If you apply a physical force to a DC motor it will produce a DC current from the commutator. So a DC generator can be very simple. As with a DC motor the generator consists of a stator which is the field structure the armature or rotor, the coils which are made up of many turns of copper wire and wrapped around the iron armature, the slip ring commutator consisting of a number of metal bars separated by narrow gaps filled with insulating material, and the conductive brushes that take the direct current from the commutator. The operation of the DC generator is almost as simple as the operation of the DC motor. The armature is rotated by a physical force. This means the coils constantly cut the magnetic flux and eddy currents of the stator. Because of this there is an emf induced because of the eddy currents that flow between the poles of the magnet. This creates a flow of electrons in the coils. This electrical current then travels through the coils back to the commutator, the commutator then delivers the charge to the conductive brushes as a direct current. Ac generators are different to a typical AC motor. To the left is a diagram of a simple AC generator. There are the field structure magnets, the armature/rotor, contacting brushes and the slip rings. The slip rings allow the armature to rotate freely while staying constantly connected to a single connection from the armature. The electric current is once again produced by the emf created when the coils on the armature cut the flux line between the north and south poles of the magnetic field. The fundamental difference between AC and DC generators apart from the different type of final current is the way in which the charge is taken from the generator. In a DC generator the electrical current is removed through the split ring commutator which supplies a direct current. In an AC generator the current is removed through the slip rings each individual coil is connected to one of these slip rings. Once the motor has turned 180 degrees the current will be changed the coils that where delivering + charge will now deliver – charge. However they are still connected to the same slip ring hence the alternating current is produced. Current can also be taken from the Ac generator by reversing the rolls of stator and rotor and withdrawing the current from the stator. This is done by putting permanent magnets in the rotor and coils in the stator. Apart from these few small differences the DC and AC generator share very few differences. Back emf: Emf or electromagnetic force plays a big part in a electrical motors. Without emf the AC motor would not work. However Emf also applies a resistance to electrical motors. This effect is called back emf. The back emf directly opposes the motion of the motor and opposes the current that flows through the coils. The back emf provides a safe guard against high currents burning out the motor and regulates the speed of the motor. Without back emf an electrical motor would continue to go faster and faster indefinitely. As the speed of the motor increases the back emf is also increased. Industry study John Charlier, Country Energy Advantages and Disadvantages: The advantages and disadvantages of AC and DC generators relate directly to two things that is DC generators use a split-ring commutator, while AC generators use slip rings. One of the major disadvantages of the DC generator is wear. This occurs particularly at the commutator. The commutator bars can wear down due to heavy spring pressure from the brushes. The insulating material between the bars then sticks out preventing the brushes from making proper contact with the bars, reducing the efficiency of the generator. Brushes of the dc generator constantly remain in contact with the commutator, this means as the rotor spins they strike the edge of each bar in the commutator. This wears the brushes meaning they often need to be replaced. This wear can cause efficiency, and maintenance problems in the generator. The slip rings in an AC generator have smooth surfaces. This allows the brushes to remain continuously in contact with the slip ring surface. Therefore brushes in an AC generator do not wear nearly as fast as in a DC generator. An AC generator therefore requires less maintenance and is more reliable than a DC generator. This leads to more use as commercial generators and also in cars (the alternator). Another major advantage of the AC generator is that it is possible to actual draw the power from the stator rather then from the rotor. Many large scale generators are of this structure. The generator at copeton dam however was a brush design. The disadvantage of withdrawing current from the rotors is the heaviness of the coils and the stress this places on the bearings of the rotor. This means less maintenance required for the ac generator once again. Another advantage of AC generators is that they can easily be designed to produce three-phase electricity. This means AC generators are ideal for generating electricity on a large scale. The main advantage of a DC generator is that it can produce a much smoother current. This is achieved arranging many coils in a regular pattern around the armature. The brushes then can be designed to make contact with the bars of the commutator at exactly the same time as the coils produce the highest voltage. This means a voltage that stays at an average measurement rather than changing between zero and the maximum. The more coils, the smoother the output. This constant current is specially needed for use with equipment that requires a very steady voltage rather than a varying voltage. This constant voltage can’t be achieved with an AC generator. Energy Losses: The resistive energy losses in power lines are caused predominately by heat and sound. Heat being the larger cause of lost energy. Heat is generated in transmission lines because of the resistance of the wires. The resistance per kilometer is actually relatively small. Power stations are often in remote places. This means power lines can take electrical current for hundreds of kilometers over these great distances the resistance becomes queit significant. The power loss in power lines is given by P = I2 R. The power lost is the square of the current times the resistance. This means the higher the more power lost. To keep energy losses economical electricity is transmitted at the highest allowable voltage, with the lowest practical current. The voltage in power lines close to power stations will be very high. Closer to the consumer, voltages are lower. This can happen because since distances...