Airframe Electrical Systems Mid-term Exam

Define Electricity Answer- the flow of electrons What are the 5 basic units of electricity? For each give what is measures? Answer- 1. Coulomb- quantity of electrons 2. Amp- current 3. Volt- electrical pressure 4. Ohm- opposition 5. Watt- Power dissipated Give the strict definition of a coulomb Answer- 6.28 x 10^18 electrons Give the strict definition of an amp Answer- amount of coulombs passing a point in 1 second Give the strict definition of a volt Answer- the electrical pressure it takes to move 1 amp through 1 ohm. Give the strict definition of a ohm Answer- the opposition that allows 1 amp to be moved by 1 volt. Give the strict definition of a watt Answer- the power dissipated by 1 amp moved through 1 ohm by volt What are the two basic methods of increasing the strength of the flux produced by an electrical current? Answer- 1. Increase current (amps) 2. increase the flux density What are the 3 elements any complete circuit? Answer- 1. Source 2. Conductor 3. Load State Ohm's Law Answer- "The current in a circuit is directly proportional to the voltage and is indirectly proportional to the opposition of circuit." Give Ohm's Law mathematically (define all variables and give units) Answer- E= I/R or I=E/R I=current in amps E= Voltage in volts R= Resistance in ohms What is the purpose of a solenoid and a relay? Answer- They use a small amount of current to control a large amount of current What is the difference between a solenoid and relay? Answer- Relay- has a unmovable core Solenoid- has a movable core What is the principle is AC generated? Answer- Electromagnet induction- a conductor is moved through a flux field in such a way that it cuts through lines of flux, and a current is induced into conductor through the constant movement of flux lines. Define frequency Answer- the number of cycles per second Define period Answer- how many seconds 1 cycle requires What is the mathematical relationship between frequency and period? Answer- F=1/P P=1/F Discuss the difference between inductance and inductive reactance. Answer- Inductance- opposition to change to current Inductive reactance- opposition to current caused by inductance Discuss the difference between total opposition to current flow in AC and DC Answer- DC- total opposition to current flow is just the total resistance, measured in ohms AC- combination of total resistance, total inductive reactance, total capactive reactance, called impedance Define a battery Answer- a device composed of two or more cells in which chemical energy is converted into electrical energy. Explain how batteries create voltage Answer- 1. each cell of a battery has two dissimilar metals called electrodes and there is a solution separating called electrolyte. 2. One electrode is forced to give off a electron (becoming positive and known as the anode) and the other receives it (becoming negative and known as the cathode). That difference in charge creates the voltage. Give 3 purpose of the primary battery in an aircraft application Answer- 1. Ground OPS (Engine start-ups) 2. To absorb spikes of voltage from generators or alternator. 3. A "back up" voltage for emergency What are the 2 batteries most widely used in aircraft application? Are they primary, or secondary cells? Answer- Lead-acid Ni-Cd they are Secondary Explain the differences in the testing of lead-acid battery, and a Ni-Cd battery (include all applicable methods of testing for each type of battery) Answer- Lead-Acid: *Hydrometer test- checks specific gravity level of electrolyte. *Load test- compares open and closed cell voltage *High rate discharge test: checks capacity of battery NiCd: *High rate discharge test Which battery terminal must always be disconnected first and reconnected last? Answer- Ground (negative) Define a generator Answer- devices that converts mechanical enegry into electrical enegry by (electro)magnetic induction through the use of a stationary flux field. Define an alternator Answer- a device which changes mechanical energy into electrical energy by (electro)magnetic induction through the use of a rotating flux field Name and explain the basis of the principle of operation of both generators or alternators. Answer- Electromagnetic induction- conductor is passed through flux and voltage is induced How do you determine the polarity of potential of generator and alternator? Answer- LEFT hand rule: i. Thumb- direction of conductor motion ii. Index finger- flux directed (point at south) iii. Middle finger- direction of current flow (negative) What are the 4 major components of a generator? Answer- 1. Field assembly 2. Armature 3. Brush assembly 4. Structural parts. What is the purpose of the field assembly of a generator? Answer- provides flux lines so the conductor can cut them What is the purpose of the armature of a generator? Answer- its the conductor that cuts the lines of flux which the voltage induced to produce current What is the purpose of the brush assembly in a generator? Answer- to transfer the system current from the commutator to the posts of the generator housing What is the purpose of the commutator in a generator? Answer- rectifies AC to DC What is the purpose of the structural parts of a generator? Answer- supports the field and armature How can you tell if a generator is a DC or AC? Answer- DC- uses a commutator to rectify AC- does not use a commutator Explain the complete operation of a DC generator.(5) Answer- 1. Engine starts and turns armature 2. a flux field is produced by field assembly pads and the armature cuts through it producing a voltage/current. 3. the voltage/current flows through armature to commutator (at this point AC) 4. The voltage/current is rectified into DC by the commutator (brushes) at the end of armature. 5. Extends to outside circuit and is routed through field(excitation current) Name the 3 types of windings (armature/field connections) of a generator Answer- Series Shunt Compound What is the advantage and disadvantage of a series wound on a generator/alternator? Answer- ADV- High voltage output DISADV- Hard to control that output What is the advantage and disadvantage of a shunt wound on a generator? Answer- ADV- Easier to control output DISADV- low voltage output What is the advantage and disadvantage of a compound wound on a generator? Answer- ADV- Easy to control output and high voltage output DISADV- heavy and complex What are the 3 minimum controls necessary for any generator? Answer- 1. Current limiters 2. Voltage regulators 3. Reverse current- rely What is one thing that generators have that alternators do not have? Answer- alternators do not provide their own initial excitation and residual magnetism What are the 4 components of a DC alternator? Answer- 1.Rotor 2. Stator 3. rectifiers 4. Structural parts. What is the purpose of the rotor in an alternator? Answer- A rotating flux field that provides flux lines for the stator to cut. What is the purpose of the stator? Answer- Cuts the flux field provided by the rotor inducing a voltage by electromagnetic induction What is the purpose of the rectifiers? Answer- converts AC to DC What is the purpose of structural parts? Answer- supports the stator and rotor How can you tell if an alternator is a DC or an AC? Answer- DC- used rectifiers AC- does not use rectifiers What are the 2 types of stator windings? Answer- Delta (triangle) Wye (Y) Explain the compete operation of a DC alternator (4) Answer- 1. Engine starts and turns rotor 2. A flux field is produced by rotor and the stator winding cut through it, producing a voltage/current. 3. The voltage/current flows from the stator, to the rectifiers where it is rectified into DC from AC. 4. Extends to outside circuit. What are the 2 minimum controls necessary for any alternator? Answer- Current limiters Voltage regulators Why is a reverse current rely not needed for alternators? Answer- the type of rectifier used. (diodes) What is the advantages of an alternator over a generator? (5) Answer- 1. Physical size- because the flux producing poles are closer together, the lines of flux are going to be much denser, (more efficient) 2. Has more steady output at low PRM because there are more poles, and the poles are closer together. 3. Since the rotating body is smaller, the alternator can be turned at a grater rate. So, capable of a greater output. 4. The brushes can be smaller, and they lost linger, because they carry less current. 5. Solid state rectifier, so there is no need for a RCCR What is the disadvantage if an alternator over a generator? Answer- is no residual magnetism, so the alternator is not self-exciting Define a motor Answer- a device that converts electrical energy into mechanical energy Name and explain the principle which is the basis of operation of a DC motor. Answer- Electromagnetic repulsion- when a conductor with a current passed through it is passed through a flux field, it is repelled out of that flux field. How do you determine the direction that the conductor will repelled? Answer- RIGHT hang rule: a. Index- flux direction (south pole) b. Middle- electron flow (POSITIVE) c. Thumb- conductor motion What are the 4 major components of a DC motor? Answer- 1. Field 2. Armature 3. Brush assembly 4. Structural components How is the direction of a rotation of a DC motor changed/reversed? Answer- reversing the polarity of the flux field What are the 2 methods of reversing the direction of a DC motor? Answer- 1. Use a SPDT switch, to route current through one of the two windings 2. Use a DPDT switch to reverse current in a single field and keeping the current in the armature the same. How is the speed of rotation of a DC motor controlled? Answer- control the strength of the flux field What are the 3 types of windings (armature/field connections) of a DC motor? Answer- series shunt compound What is the advantage and disadvantage of a series wounded motor? Answer- ADV- has higher torque abilities DISADV- low speed control What is the advantage and disadvantage of a shunt wounded motor? Answer- ADV- Good speed control DISADV- Low torque abilities What is the advantage and disadvantage of a compound wounded motor? Answer- ADV- high torque abilities and good speed control DISADV- heavy and complex What are the 3 types of AC motors? and give a typical use for each. Answer- Universal- vacuum cleaners, drills, floor buffers Induction- tachometer Sychronous- flaps, landing gear, compressors