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268 Cards in this Set
- Front
- Back
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(Dc generator) For a conductor being rotated through a magnetic field, how can the direction of current be determined? |
Using Flemings right hand rule |
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If a loop of wire is is being rotated in a magnetic field (Dc generator), when will the max amount of EMF be induced in that conductor? |
When the conductor is at right angles to the magnetic field |
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How is the max output voltage of a (Dc) generator calculated? |
E=Blv |
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How is the instantaneous output voltage of a DC generator calculated? |
e= Emax sin 0 |
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What are the functions of a commutator? |
To transfer the induced loop current to the external circuit and to periodically switch the external circuit to keep the current flowing in the same direction through the load |
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How is a more constant output obtained from a simple Dc generator? |
By increasing the amount of armature loops |
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What is meant by the term pole pitch? |
The angle between each main pole of the opposite polarity |
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State the main characteristics of a lap wound armature? (Dc gen) |
The number of parallel paths for current always equals the number of brushes and poles Terminal voltage is equal to the EMF induced in one path. The delivered current is equal to the sum of the parallel paths It is used on large, heavy current machines |
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State the main characteristic of a wave wound armature? |
The type of machine has two parralel paths with each coil in the path adding their respective induced voltages together using only two brushes irrespective of the number of poles. This type is used in smaller machines and more common on aircraft generators |
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What is armature reaction? |
The distortion of the magnetic field caused by the armature field |
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Define the term the GNA of a generator? |
Geometric neutral axis Point where the flux across the air gap is uniform and a line drawn vertically at a point midway between the poles will pass through the middle of each line of force. |
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Define the term MNA of a generator? |
The line joining the two points at which no EMF is induced in a coil i.e the line along which the brushes are placed to obtain good communication (magnetic neutral axis) |
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How is the MNA affected by an increase in armature current? |
Magnitude of the magnetic field surrounding the armature winding will increase. This will increase the strength of the armature field which causes more distortion to the main field. This will cause the MNA to move around by a greater angle of lead |
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How is armature reaction reduced? |
Fitting compensating windings in the pile faces. By connecting these winding in series the armature field can be neutralised |
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What is meant by the term reactive sparking? |
Inability of an armature winding to change current direction quickly enough as the winding fired through the point of commutation abs is influenced by the next main pole. The current then jumped as a spark from the commutator segment to the brush |
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How is reactive sparking reduced? |
By using interpoles. |
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Why is the polarity of a generator Interpol the same as the next main pole in the direction of rotation? |
To change the direction of induced current in the armature coil prior to the armature coil moving under the influence of the main Pole |
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What areas cause copper and iron losses in a DC generator? |
Copper losses are caused by the armature, interpole, compensating and field windings Iron losses are caused by hysteresis and eddy current losses |
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Name the main component parts of a DC generator |
Pole pieces with field windings Armature Commutator Carbon brushes |
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What are the main characteristics of a self-excited series wound DC generator? |
An increase in voltage output for increasing load current. The voltage output is at zero for no load current |
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What are the main characteristics of a self excited shunt wound DC generator? |
A decreasing output voltage for increasing load current. The voltage output is at maximum for no load current |
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What effect would there be on a self excited shunt wound generator if a load was connected to the output during initial rotation of the generator? |
Only a small voltage is induced. Therefore armature current is small and insufficient to assist initial excitation. Generator fails to excite |
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What does the term flashing the field mean? |
Momentarily passing current through the field from the positive terminal to the negative terminal to reestablish the following loss of residual magnetism |
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What self excited cumulative wound generator gives a full load voltage that equals its no load voltage? |
Level compound |
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What self excited cumulative wound generator gives a full load voltage that exceeds its no load voltage? |
Over compound |
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What happens if an element/atom loses an electron? |
It becomes a positive ion/positively charged |
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What is the definition of coulombs law? |
A lwa stating like charges repel and opposite charges attract, with a force proportional to the product of the charges and inversely proportional to the square of the distance between them |
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Define static electricity |
Imbalance of electric charges in a material. Charge remains until it is able to move away by means of electric current or discharge |
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What is the seebeck effect? |
A temperature difference between two dissimilar electrical conductors or semiconductors produces a voltage difference between the two materials |
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What is a thermocouple? |
Electrical device consisting of two dissimilar electrical conductors forming electrical junctions at differing temperatures. A thermocouple a temperature dependant voltage as a result of the thermoelectric effect and is used to measure temperature |
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How do electrons flow in a cell? |
They are negatively charged so are attracted to the positive end of a battery and repelled by the negative end. When the cell is connected they flow from negative to positive |
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What would connecting two batteries in parralel do? |
Same voltage. Increase amps per hour (AH) |
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How do you calculate terminal voltage? |
V=emf-IR EMF =voltage difference I=CURRENT R=INTERNAL RESISTANCE Rearranged equations I=EMF/Rload+r
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How does conductor length and thickness affect resistance? |
Length doubled = resistance doubled Thickness doubled = resistance halved |
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What does a gold band on a resistor mean? |
Multiply by 0.1 if its the third band +/- 5% tolerance if its the fourth band |
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Using letter and digit codes for resistors, what would 4K7 mean? |
4.7kohms The letter is essentially a decimal point |
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Using Flemings right hand rule, which finger indicates magnetic field (N to S) ? |
Index |
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How do you calculate power? |
IxV RxI^2 V^2/R |
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What is the maximum power transfer theorem? |
Load resistance when equal to thevenin resistance (circuit resistance) results in the greatest power dissipation |
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If a piece of insulating material is inserted to fill the space between the plates of a capacitar what will happen? |
Capacitance will increase |
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What is the right hand grip/grasp rule? |
Where a vector must be assigned to the rotation of a body, magnetic field, or a fluid. Thumb points in direction of North Pole, fingers in direction of conventional current (or vice Versa e.g thumb points from pos to neg in conventional current) |
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What is permeability? |
Ratio of flux density to magnetic field strength at any point |
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What are diamagnetic materials? |
Repelled by a magnetic field. An applied megnetic field creates an induced magnetic field in them in the opposite direction (opposite to ferromagnetic materials) |
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Describe a series wound DC motor |
Field and armature windings connected in series and with power supply. Current in armature and field windings therefore the same. Can draw a large current when starting allowing field strength to build up quickly and gives the motor advantages of: High starting torque Good acceleration with rapid build of back EMF induced in the armature to limit current flow through the motor Good for heavy loads |
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400hz = 0.0025 for 1 cycle so 0.01 = |
4 cycles |
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What sort of wave form is a tranigular wave? |
Linear waveform (sharp a distinct lines. No curves) |
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In a 3 phase power system, what is the phase difference? |
120° |
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How do you space the currents in a 3 phase system to balance out the load? |
120°apart |
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What are line and phase voltages? |
Phase voltage is measured between the phase line and the neutral line. Lone voltage is measured between two phase lines and is the phase sum of the voltages induced in the two phase windings involved |
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Transform 20v and 5A to 4V and..? |
25A |
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What is ac motor slip? |
A rotor will always lag the rotating field whether the motor is loaded or not. This is called slip. Usually expressed as a percentage of the synchronous speed(speed of rotating field) Slip =(synchronous speed-rotor speed/synchronous speed) x100% |
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What sort or circuit is an acceptor circuit? |
Series circuit. As a series resonance circuit only functions on resonant frequency, this type of circuit is also known as an Acceptor Circuit because at resonance, the impedance of the circuit is at its minimum so easily accepts the current whose frequency is equal to its resonant frequency |
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In what equipment is a photon radiated when an electron leaves a hole? |
LED |
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What is the max number of electrons in shell N of an atom? |
32 |
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An element with fewer than 4 electrons in their valency shell are.... Conductors |
Good |
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A good electrical insulator is a materials with electrons that..? |
Are tightly bound to their parent atoms |
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Germanium and silicone have how many electrons in their outer shell? |
4 |
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An atom with a deficiency in electrons has? |
Low resistance |
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Hydrogen atoms consist of a? |
Proton and electron only |
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Faradays law states... |
The magnitude of the EMF is directly proportional to the rate of change of flux |
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The term that describes the combined resistive forces in an ac circuit is? |
Impedance |
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EMF corresponds to what in a magnetic circuit? |
MMF |
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What is the si unit of work? |
Kilogram metres - force |
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A coulomb is... |
One ampere per second |
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A volt can be considered to be a.... |
Unit of electrical pressure |
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The basis for transformer operation in the use of alternating current is mutual...? |
Inductance |
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What is the si unit of magnetic flux density? |
Tesla |
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The opposition by a coil to the flow of alternating current is called..? |
Inductive reactance |
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A shunt is used with? |
An ammeter |
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What is joules per second? |
Power |
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In conventional flow, the left hand rule applies to? |
Motors |
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EMF is in induced when a conductor is cut by magnetic lines of force. Who's law is this? |
Faradays |
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The property of a material to accept lines of flux is called? |
Permeability |
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MMF can be calculated how? |
IxN |
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How would you find the direction of electron flow in an armature located in a magnetic field? |
Flemings left hand rule |
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Si unit of conductance? |
Siemen |
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What is the formula for conductance? |
1/R |
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XL is the symbol of? |
Inductive reactance |
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If one volt is induced onto a circuit with a current varying at 1 amp per second, the circuit has... |
1 Henry |
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Why are thermocouple harnesses made from the same materials as the thermocouple? |
So mini junctions aren't made |
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A light/heat sensitive cell is a..? |
Transducer |
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The max output voltage from a single loop generator is when the loop is...? |
Cutting maximum lines of magnetic flux |
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Two 10v, 20ah batteries are connected in parallel and connected across a 10ohm load. How long can they supply until voltage decay begins? |
40 hrs |
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A zinc-carbon battery life depends upon...? |
The amount of zinc |
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What should you do when checking the SG of the electrolyte in a lead acid battery? |
Check all the cells |
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The PD at the terminals of an open circuit battery with a small internal resistance will be what? |
Same as the EMF |
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An accumulation of hydrogen on the plates of a battery is known as? |
Polarization |
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What part of a battery is covered in hydrogen during polarisation? |
Anode |
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The method of getting the voltage of a standard aircraft lead acid battery is by checking...? |
The voltage off load |
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Two batteries 12v and 40ah each are in series. What is the total capacity? |
24v 40ah |
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No load voltage will be greater than on load voltage when the battery has what? |
Low internal resistance |
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Conventional current flow inside a battery is from? |
Anode to cathode |
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What is the build up on the cathode called in a voltaic cell? |
Sulphation |
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In a mercury cell, what is the steel casing? |
The positive terminal |
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If a nicad battery was charging, the electrolyte would...? |
Rise |
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Electrolyte in a nicad battery is...? |
Potassium hydroxide |
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When the temperature of the electrolyte in a battery increases, the SG...? |
Goes down |
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In a thermocouple where is volts measured? |
Cold junction |
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What effect does hydrogen have in a battery cell? |
Polarisation |
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Formation of white crystals of potassium carbonate on a properly serviced nicad battery indicates? |
Over charged |
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Can a primary cell be recharged? |
No |
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When a battery is connected to the aircraft, which terminal should you connect first? |
Positive |
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A 10v battery supplies a resistive load of 10ohms for 1 minute. What is the work done? |
600j |
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A galvanometer measures? |
Milliamps |
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In a circuit containimg three resistors connected in paralel (of equal value), if one goes open circuit, how does current change in the other 2? |
It stays the same |
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20 amps flows for 20 seconds. How many coulombs have flowed? |
400 |
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If electrical supplies taken from a bus bar are in paralel, isolating some of the services would..? |
Reduce the current consumption from the bus bar |
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In a balanced Wheatstone bridge, across the centre of the bridge there is...? |
No current flow |
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What are potentiometers varying by doing what... |
Voltage by changing its resistance |
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Formula for resistance in series is...? |
R1 +R2 etc =Rt |
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What is the formula for resistors in parallel? |
1/RT =1/R1 +1/R2 etc |
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The total resistance in a parallel circuit is always...? |
Smaller than the lowest resistor |
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What happens to voltage in all parts of a parallel circuit? |
It's the same |
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How does temperature affect resistance? |
Increased temperature increases resistance |
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Watts is based on what time frame? |
Seconds |
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In a power circuit what is the purpose of an inductor? |
Dampen power surges |
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The voltage rating of a capacitor is..? |
Max voltage that can be constantly applied |
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A capacitor with double the area and double the dielectric thickness will have...? |
The same capacitance |
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Where is a electrolytic capacitor used? |
Polarised input |
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Three 12 farad capacitors in series total what capacitance? |
4 farads |
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The time constant of a capacitor is the time for..? |
EMF to reach 63.2% of maximum |
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How do you get the total capacitance from capacitors in parallel? |
Add them up |
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When two capacitors connected in series, the charges stored on each is..? |
The same |
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Formula for charge is..? |
Coulombs (capacitance) x volts |
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Dielectric constant of a capacitor is a measurement of? |
Electrostatic energy storing capacity of the capacitor dielectric |
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A capacitor rating is? |
Max continuous voltage it can take |
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Why is a electrolytic capacitor used? |
Small size for large capacity |
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What sort of capacitor is used for heavy and light loads? |
Electrolytic |
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What is the formula for capacitance of a capacitor? |
C=K(dielectric constant) x area/distance |
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Dielectric strength is measured in? |
Farads per meter |
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Below a certain temperature, ferromagnetic materials can be...? |
Magnetised |
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Glass is an Example of a... |
Diamagnetic material |
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To find the North Pole on an electromagnet, use the..? |
Right hand clasp rule |
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Lines of magnetic flux do what to each other? |
Repel |
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Why is iron used in an electromagnet? |
It demagnetises easily |
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What is the unit of flux? |
Weber |
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Lines of flux pass from...? |
North to south |
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What is the symbol for flux density? |
B |
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A paramagnetic material has a relative permeability of? |
Greater than unity |
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What sort of Magnets have high reluctance and high coercive force? |
Permanent Magnets |
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Why is a soft iron core used in an electro magnet? |
Has high permeability and low coercivity |
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A non magnetic meter has no permeability? T or f |
True |
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What is the effect of inserting an iron core into a current carrying coil? |
Flux density increases |
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Flux density increases linearly with? |
Magnetic flux |
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Vibration in a Magnet causes flux to...? |
Decrease |
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Permeability can be found by dividing flux density by..? |
MMF |
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A material exposed to heat will be harder /easier to magnetise? |
Harder |
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Of cobalt has a permeability of 4800, it is a...? |
Ferromagnetic |
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What is permeability.? |
Ability to of a circuit to produce magnetic flux under the influence of a MMF |
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How do you calculate inductance in series? |
1/Lt=1/L1+1/L2 |
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How do you calculate inductance in parallel? |
Lt=(L1xL2) /(L1+L2) |
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What is the time constant of an inductor? |
L/R |
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If the rate of change of current is halved, mutual inductance will..? |
Stay the same |
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Why are iron cores of induction coils laminated? |
Reduce effects of eddy currents |
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An induced current in a coil opposes..? |
The EMF producing it |
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1 volt is produced when a current of 1 amp per second is varied in a coil. This is a definition of..? |
1 henry |
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In a DC motor, the stators magnetic field, due to armature reaction, moves...? |
Opposite direction of Rotation of the armature |
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Decreasing the field current in a shunt motor will do what to speed and torque? |
Increase speed and torque |
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To calculate generator output, you need to know..? |
Armature speed and field strength |
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A starter generator has a high/low resistance series field and a high/low resistance shunt field? |
Low High |
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How do you change the rotation of a DC electric motor? |
Reverse the electrical connections to either the field or armature windings |
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Aircraft engine starter motors are generally which type? |
Series |
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What will happen if a heavy mechanical load is removed from a series motor? |
Speed will increase and the armature current will decrease |
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In a generator, what could loose brush springs cause? |
Sparking |
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Few turns of thick wire are used on the field of a...? |
Series wound DC motor |
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Generator brushes are usually made of? |
Carbon |
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As engine speed increases, current consumed by a DC starter motor will..? |
Decrease |
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How are generators rated? |
Watts at rated voltage |
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What polarity do interpoles take in a DC motor? |
Same as the next main pole behind in the direction of rotation |
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To increase the speed of a shunt motor, a resistance is placed where? |
In series with the field |
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What is a method for restoring generator field residual magnetism? |
Flash the field's |
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On a combined DC starter generator system, when is the voltage regulator connected to the shunt field? |
After the start cycle is completed |
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In a wave wound generator with 4 poles, what would be the minimum number of brushes be? |
2 |
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Why does the current reduce after starting a motor? |
Back EMF |
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How are interpoles connected? |
Series with the armature |
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What are the windings embedded in the pole faces of a DC generator? |
Compensating windings |
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A 6 pole wave wound generator has how many brushes? |
2 |
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A DC generator armature has 6 poles and is lap wound, the number of brushes required are? |
6 |
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What's the difference between armature reaction MNA movement in a DC generator and a motor? |
Motor against the direction of rotation Generator with the direction of rotation |
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What is a stabilising winding used for in a generator system? |
Prevent voltage over shoot |
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What part of a DC motor would you find the poles? |
Stator |
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How can commutator ripple be reduced? |
Ripple filter |
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Armature reaction is? |
MMF opposing rotation |
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Describe the speed in a shunt motor |
Constant speed |
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What is the main disadvantage of a starter generator? |
Will not supply output at low rpm |
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In a motor, when is back EMF greatest? |
At operating speed |
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What type of motor is used in a linear actuator? |
Series |
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In the shunt generator, the field is supplied by the... |
Armature |
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The series and shunt fields assist each other in what sort of generator? |
Cumulative compound wound generator |
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Compound DC generators must have an equalising or load sharing loop when...? |
Operated in paralel |
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Which uses less current, DC or AC motor? |
Dc |
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In a shunt wound direct current motor, the torque is..? |
Proportional to armature current |
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Convention requires What voltage is the reference phase in a symmetrical 3 phase system? |
Red voltage |
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With ac generator output, how do you calculate average voltage? |
0.63 x peak voltage |
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What does a rectifier do? |
Converts AC to DC |
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The ratio between apparent power and true power is? |
Power factor |
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How do you calculate true power in an ac circuit? |
Volts x amps x power factor |
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Transmitting at a high voltage with a low power loss is an advantage of...? |
Ac |
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The peak factor for a sine wave is...? |
1.414 |
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What is the power factor of an ac circuit? |
Cosine of the phase angle |
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The form factor of an ac waveform can be found by...? |
Rms value divided by the average value |
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A differentiator has a time constant which is...? |
Short |
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If the frequency is increased in an AC circuit of pure resistance, the effect is..? |
Nothing |
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Impedance of an AC circuit is measured in? |
Ohms |
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What is phase angle? |
Difference in degrees of rotation between 2 alternating voltages or currents |
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In a resonant parallel circuit, current is..? |
Minimum |
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What is the power factor in a purely resistive circuit? |
Equal to 1 |
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What's the relationship between voltage and current in an ac circuit containing resistance and inductance. |
Current lags voltage by up to 90° |
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A low frequency supply may damage what? |
Inductive components |
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In a capacitive circuit, if the frequency is increased, current....? |
Increases |
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The impedance of a series tuned circuit at resonance is..? |
Minimum |
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A parallel circuit at resonance has its impedance at..? |
Maximum |
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What is the formula for impedance in a series circuit? |
Root of R2 + (XL-XC) x2 |
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In a parallel circuit, if the capacitance is increased by 4 times, what happens to the impedance of the capacitor? |
Decreases by 75% |
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If the frequency to a capacitor increases, the current through it..? |
Is zero |
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A circuit containing a resistor, capacitor and inductor where resonant frequency is met, what will the impedance of the circuit be? |
Resistance across the resistor |
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How many turns does the secondary winding of a 2:1 step up transformer have? |
Less than primary |
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How do you reduce eddy currents in a transformer? |
Reduce thickness of laminations in the magnetic core |
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A transformer has 4500 secondary turns and 750 primary turns. It's turn ratio is..? |
1:6 |
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What is the point of coercivity? |
When magnetic flux is zero even though a magnetising force is being applied |
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How can a primary winding of a 3-phase transformer be wound? |
Either delta or star wound |
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In a transformer core loss is 200w and copper loss is 220w at no load. What is the core loss at full load? |
200w |
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Transformer copper loss on full load is 220w. on half load the loss will be? |
55watts |
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How can you tell if transformer windings are shorted together? |
Transformer will be hot in normal operation |
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A transformer should always have its secondary winding...? |
Open circuit or on load |
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In transformer operation, hysterisis losses are...? |
Energy wasted in reversing core magnetization |
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What causes a transformer to be noisy? |
Air gap in the core |
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What causes copper losses in a transformer? |
Resistance in the windings |
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If a transformer is designed to operate at 60hz and is connected across a 50 hz supply, the primary windings would...? |
Draw more current |
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A current transformer when disconnected from a circuit must be..? |
Short circuited |
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At transformer rated at a frequency of 200hz may only be used at what frequency? |
200hz or above |
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An autotransformer windings are...? |
Connected |
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What voltage can the neutral line carry in a star transformer? |
Zero |
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Line voltage in a WYE wound transformer is? |
Root 3 x phase voltage |
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Magnetic flows induced into the core of a transformer are...? |
Eddy current losses |
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A 3-phase star delta wound transformer gives what degree phase shift? |
30° |
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Flux losses in a transformer can be prevented by....? |
Winding the primary on top of the secondary |
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What would you use for the core in a generator current transformer? |
Air core |
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What sort of transformer would be used to step up in an aircraft? |
Laminated core |
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A high pass filter will allow frequencies of....? |
Above a certain value to pass |
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What does a band stop filter do? |
Stops a narrow range of frequencies |
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A Capacitor in series and inductor in Parallel make what kind of filter? |
High pass |
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A circuit with good selectivity will have...? |
A narrow bandwidth |
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What will a capacitor in series and inductor in Parallel create? |
High pass filter |
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In a delta connected generator, what's the relation between line current /voltage and phase current/voltage? |
Line volts =phase volts Line current = root 3 x phase current |
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Ac generators are rated in..? |
KVA |
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Frequency output of a 4 pole generator is 400hz. At what rpm Is it? |
12000rpm |
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Voltage induced in the stator of an ac generator is produced by? |
Rotating magnetic field |
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How are the three voltages of a three phase generator connected? |
Independently |
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How far are the windings electrically in a 2 phase generator? |
90° |
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Where are the output windings on an ac generator? |
Stator |
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An advantage of having two Potentials available is of what wound type generator? |
Star wound |
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What are permanent Magnets for in an ac generator? |
Provide initial excitation |
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Variable frequency output from ac generators is used for? |
Supplying non inductive loads |
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What's the potential between the neutral point and earth in a 3 phsse generator with balanced load? |
Zero volts |
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A conventional aircraft generator will be wound how..? |
Star wound |
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Where is the output from a rotating armature ac generator taken from? |
Rotor |
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A generator with more than two phases is known as...? |
Polyphase |
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What are stability windings in a 3 phase ac generator used to sense? |
Over volt |
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In a delta connected generator, the line current is equal to. |
1.7x phase current |
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Shaded poles in an alternating current motor are intended to facilitate...? |
Starting |
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In a 3 phase motor, if 1 phase is lost, what speed will it run? |
The same |
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How would you change the direction of a 3 phase induction motor? |
Swap two of the stator connections |
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What is a capacitor in a single phase ac motor for? |
Provide a phase shift |
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What's more efficient? Inductance motor, or a capacitance motor? |
Capacitance motor |
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Slip on a synchronous motor on start is..? |
100% |
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A shaded pole motor has shaded poles to create a..? |
Rotating field |
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What sort of motor would a fuel pump require? |
Continuously rated |
