Electricity PREP TEST QUESTIONS WITH VERIFIED ANSWERS

Electricity PREP TEST QUESTIONS WITH VERIFIED ANSWERS The size of the conductor is proportional to the amount of the current. True or False? - ANSWER True. What happens when a conductor moves relative to a magnetic field and it passes through the lines of magnetic flux? - ANSWER A voltage is induced in the conductor. What is Electro-magnetic Induction? - ANSWER As long as there is movement between a conductor and a magnetic field, an induced voltage is created. Describe what the magnetic lines of flux do when AC current passes through a conductor. - ANSWER - Field sets up around the conductor - Field alternates with the current, building up in one direction then collapsing into the conductor and building up in the opposite direction What is Lenz's Law? - ANSWER - The law of electro magnetic induction - " When a magnetic flux changes and by its change is responsible for inducing a voltage, the direction of the voltage is such that it tends to oppose the action producing a flux change. " What is ' Self Inductance ' ? - ANSWER - An opposing voltage which combines with the resistance of the coil to give a total resistance Magnetic field build with... - ANSWER Each turn of wire around the coil Role of self-inductance in a DC circuit? - ANSWER Self inductance delays the build up of current to its maximum value, determined by the value of the applied voltage and the resistance of the coil. In a DC circuit, does self-inductance remain a factor even after the current is at its maximum value? - ANSWER No, self inductance is no longer a factor. If the circuit opens however, then self inductance again becomes a factor. Role of self-inductance in an AC circuit? - ANSWER Continually changing current makes it so self inductance is always a factor in limiting current through the coil. What is the PRINCIPLE on which a transformer operates? - ANSWER Mutual Inductance: When magnetic flux produced by one coil cuts the conductors of a second coil, a voltage is induced in the second coil. What would define a well designed transformer? - ANSWER If the flux produced by the Primary cuts every turn of both the primary and secondary winding, thereby inducing the same voltage in every turn. Step up Transformer? - ANSWER The number of windings in the secondary is greater than that of the primary inducing a greater voltage in the secondary. Step down Transformer? - ANSWER The number of windings in the secondary is less than that of the primary inducing a lower voltage in the secondary. What is equally proportional in a transformer? - ANSWER Voltage is proportional to the number of turns in each winding. Through normal load conditions, how does the flux of a transformer virtually remain constant? - ANSWER - Secondary current flows produce a magnetic flux which opposes and reduces primary flux - Secondary flux reduces the self-inductance in the primary allowing more primary current to flow, which brings the main flux back to its original value 2 types of single-phase transformers? - ANSWER Core type and Shell type. Each type winds the low voltage conductor nearest the iron core first. Core Type transformer? - ANSWER - Primary and secondary windings are split into two equal parts with one half of each wound on each of the 2 legs of the transformer. Shell Type transformer? - ANSWER All of the primary and secondary is wound on the centre leg. The entire winding is then surrounded by a shell of iron. " EpIp = EsIs " - What does this equation determine? - ANSWER This means that the primary and secondary power factors are pretty much the same (losses are negligible) A three-phase transformer is similar to what type of single-phase transformer? - ANSWER The Shell type transformer, except the windings are wound on each of the three legs. What is meant by "pancake" or "sandwich" windings? - ANSWER The High voltage windings cover the low voltage windings in a three phase transformer. In a three phase system, what device could replace a three phase transformer? - ANSWER - A bank of single phase transformers - Single phase banks are more expensive and less efficient than 3 phase transformers, but provide a certain convenie