Certification Assistance for Canadian Navigators
Canadian Ocean Navigator
Electrotech Notes and Questions
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Transport Canada has ask us to advise users of this webpage to keep in mind that these questions are not the exact questions found in their exams. Martin's Marine Engineering Page - www.dieselduck.net is not affiliated with Transport Canada and these questions have been gathered from various sources.
Thank you Walt P. for submitting this material. Minor editing by Martin Leduc
Sorry, sketches not shown,
What is a Resistor.
Resistance is the opposition to he flow of current by any conductor.
A resistor is any device with a known resistance, measured in ohms. Designed to have any particular value of resistance.
Tolerance - Actual resistance of tolerance for a 10 ohm resistor rated at 5%+/- would be 9.5 ohm to 10.5 ohms.
There are the fixed type of resistors, single value, which remains constant in value.
Wire wound resistors - (wwr) nickel chromium wire wound around a ceramic rod or core (frame) this wire has a much greater resistance than copper wire per unit length, therefore can be much shorter and the resistor smaller. Nickel Chromium wire also has a much lower temperature co efficient of resistance than copper wire, and the heat generated can be easily dissipated 1 ohm to 100k ohms.
Carbon composite resistors - (ccr) graphite or soft carbon composite.
Other types of resistors have a thin film of conductive carbon, metal, or metal oxide deposited on a ceramic or glass rod. A spiral grove cut in the film during production establishes the Resistive Value.
The Power Rating of Resistors; how many watts of heat it can dissipate before sustaining damage.
Variable Resistors - sliding arm makes contact with stationary resistance element.
Mutual Inductance - When a magnetic field in motion, cutting the turns of a coil, induces an (EMF) electromotive field, and if close enough to cut the turns of a second coil, another corresponding EMF will be induced .
The unit of Mutual Inductance is a slightly modified version of the Henry.
Henry - a current charging at a rate of 1 amp. per second in one coil, producing an EMF of 1 volt to be induced in a second coil, then the two coils have a mutual inductance of one Henry.
Transformers utilize this principle; two coils wound on a common core of magnetic material, such that there is a good ?coupling ? (large mutual inductance between the coils). The EMF induced in the second is proportional to the primary coil, as the number of turns in the primary coil to the number of turns of the secondary coil.
This is often referred to as the turns of voltage ratio and expressed:
Np = Ep
The transformer is used to raise or lower the voltage in various applications
Step up or Step down transformers.
Unit Of Capacitance:
The Farad, after Faraday, is the charge in coulombs stored in a capacitor, with an EMF of 1 volt applied.
A coulomb is the amount of charge represented by 1 amp. flowing for one second.
Capacitors have values expressed in ( Uf ) micro farads and ( Pf ) pico farads.
A farad, is the capacitance of a capacitor that, when given a charge of 1 coulomb, produces a p.d. of one volt between its poles.
Capacitance is the ability to store an electric charge
C = Q (charge ) C = EA ( area of plates )
V c ( distance between plates )
E ( electric field strength ) = 9.D = V V/meter = potential gradient
Energy stored in a capacitor = ? CV
When opening or closing a switch in an RL circuit, current rises, falls to zero, decays.
However, steady value shows after a finite number of seconds.
T = L in a RL circuit
1 T Time in seconds required for the current in an RL circuit to rise to 0.632 times its steady value V Conversely T = 63.2% of the final value. R
Current increases exponentially to near maximum (99 % +) to five time constants.
What is a Time Constant:
Time constant ( time in seconds ) = CR C= capacitance R = series resistance in ohms
When a supply voltage is applied to an RC circuit, as the capacitor charges, the voltage across the resistor falls and so does the current.
T is the time the voltage across the capacitor would take to reach the supply voltage if it entered to rise at its original rate and represent the time needed to charge the capacitor 63.2% of the source voltage..
At each time constant interval, the capacitor raises its voltage to 63.2% of what is left to charge.
After 5 time constants the capacitor is considered to be fully charged. The TC is a significant indicator of the speed at which RC circuits respond to applied voltage.
Series and Parallel Circuits:
Only one path for current to flow
Current ( I ) is the same in all parts of the circuit
Total Resistance of the circuit ( sum of all resistors ) RT = R1 + R2 + R3
Total Voltage ( V ) sum of the voltages across each resistor VT V1 + V2 + V3
One or more components connected across the same voltage source.
Voltage is the same for all parts of the circuit
Current for each branch is the resistance divided by the voltage. I2 = V R2 = V
Total Resistance RT = 1 + 1 + 1 + 1
R1 R2 R3 R4
How DC Current is Generated.
By rotating loops of wire in a magnetic field.
A simple DC generator consists of an armature coil with a single turn of wire placed in a magnetic field, which when rotated, set up a voltage. The commutators are designed such that the armature will always be cutting across the magnetic field in the same direction. When it makes contact with the brushes, the brushes have the same polarity and the segments of the commutator are insulated from each other.
Each brush changes contact from the segment to the other each time the loops passes through a position perpendicular to the axis of the magnets ( zero voltage ) Output voltage is a series of pulses varying from zero to maximum.
Induced voltage depends on:
speed of rotation
flux density ( strength of magnetic field )
length of conductor in that field
Heat Produce by an Electric Current; a wire carrying an electric current contains electrons moving against resistance. When collisions take place, the atoms vibrate and heat is generated
I? x R = WATTS
Some conductors are designed to minimize heat loss, while others are maximize the heat effect. ( heating elements )
The greater the current the greater the heat. The greater the loss of kinetic energy, and loss of velocity, the greater the work done, expressed in heat ( watts )
A Thermostat, basically a temperature operated switch, comprising of a temperature sensitive element, generating a mechanical force or electrical signal and a regulator 9 could be an electrical switch or fluid valve or comparable device.
e.g.. Bourdon Tube
bi metal strip. ( fire detection head )
Characteristics of a Series Wound Motor:
A series wound motor has its field windings in series with the armature winding so that the current flows through both ( field windings have heavy wire turns to ensure no resistance )
The armature of a DC motor produces its own magnetic field, which reacts with the magnetic flux of the field coils producing rotary motion.
The conductors on the armature cut the lines of force of their own field consequent a back EMF to be induce in the armature opposite to the current flow in the windings.
On starting: no back EMF so a large current in the armature and field, a starting torque..
As speed increases, back EMF increases, as current decreases you have a decrease in torque.
An increase in load, decreases the speed, decreasing back EMF, causing an increase in current, which again increases the torque.
A good useful ability to respond to varying load, and high start torque, make this motor ideal for windlasses.
EMF Electro Motive Force
When the armature of a motor is rotating, a voltage is induced in the windings as they cut across the magnetic field. The induced voltage is in opposite direction to the supply voltage and known as back EMF
When the motor is stationary, there is no back Emf and armature current is a result of supply voltage and winding resistance.
When the armature speed increases, back EMF increases until a balance is achieved, the difference between applied voltage and back EMF provides enough current to drive the motor.
Motor load increases, more armature current is needed, and it must slow down. Greater difference between supply voltage and back EMF, an increase in current and torque.
Conversely, a smaller load, an increase in rotation causing a smaller difference between back EMF and supply voltage, causing smaller ( decrease ) current and torque.
Definitions and Symbols:
Inductance; ( L ) Measure in Henry?s, and only occurs with A/C current.
The ability of a conductor to produce voltage in itself when the current changes
Capacitance; ( Farads ) The property in a capacitor that permits the storage of electrically separate charges when partial differences exists between conductors.
The property of an electrical circuit that enables it to store electric energy by means of an electrostatic
field and release it later ( batteries )
Impedance; ( Z ) The total opposition a circuit offers to the flow of an alternating current, it includes resistance and reactance and is measured in ohms. It is the product of the combined value of resistance and reactance.
Relative Permeability; The measure of how much a given material allows a path for magnetic flux relative to ( = 1.0 )
Permeability is the number of flux lines per square meter set up by 1 ampere- turn per meter of the magnetic circuit = flux density in lines/meter squared
Weber; a unit of magnetic flux ( Wb ) -- 1Wb = 1 x 108 magnetic lines
A magnetic line of force is termed a Maxwell and is so small that 1 x 108 x 1 Maxwell is deemed as one Weber.
The strength of the magnetic field ( flux ) density ) is measured in Weber/square meter.
Atoms, Electrons and Ions, in electrical theory.
Matter is anything having mass and occupying space. Its composed of particles called Atoms.
Matter is in 2 groups; elements and compounds.
Atoms (elements) are composed of subatomic particles of electrons, (negative charge) orbiting about a nucleus made up of positive charged protons, and neutral neutrons.
In its natural state the atom has an equal amount of protons and electrons > and is electrically neutral<, and is stable.
If enough external energy is provided, electrons will be lost from orbit.
Free electrons is what causes electric current.
The atom becomes positive charged; positive ion grants free electrons, (negative charged) to gain or loose electrons (ionization). Ionization is very significant in the transmission properties of various radio signals.
Electrical theory hinges upon the concept of metallic conduction which assures that the free electron are what makes the electrical current possible. The potential difference between unlike charges enables the force to be exerted on other charges. ( work ).
Definitions and Symbols.
Reluctance ( ) Ampere/meter/turns
The resistance of a material to the passage of lines of magnetic flux
= ( )ampere/turns , ampere per Weber
Permeability; A measure of how well electrical lines of force are established in a die electric>
The symbol units of farad/
Ratio of the flux density to the electrical field elasticity in the die electric.
Resistively ( p ) Ohm
1/ resistance of a semiconductor, 2/ resistance of a sample material having specific dimensions, depends directly on the length of the conductor and inversely on its cross section area and the material used.
R = l ? p
Magnetic Induction ( ) Units Wb/m? Magnetic Induction
( magnetic flux density)
Number of magnetic flux lines passing perpendicular through an area.
Area in m?
Resistance and its Relationship with ( Mho).
It is the unit of conductance. Defined as the opposite of resistance.
A measure of the ease of which electric current passes through a conductor.
Conductance uses the symbol G. and G = I
Ohms law may be expressed in terms of conductance I = GV
The Mho is the Imperial unit for conductance, in the more modern SYSTEM INTERNATIONAL ( SI ). it is the Siemend ( ?S?)
Power and Work and their Relationship ( in units ) ( P )
Work is the magnitude of force multiplied by the distance through which that force is applied.
Work done is ( force ? distance ) measured in Newton's or Joules
Power is the rate at which work is done ( p ) . Measured in watts ( W ) defined as watts = joules/meter/second. or newtons/meter/seconds.
A capacitor with a dielectric between its plates, will have a greater charge per unit of potential difference than an identical vacuum capacitor.
The reason is due to the permeability of the dielectric, a measure of how easily the dielectric will permit the establishment of flux lines with the dielectric. The greater its value the higher the charge on the plates of the capacitor.
A vacuum has the least value of permeability, 8.85 ? 10.0 -12 and is assigned the value of one ( 1 )
All other materials have a relative permeability greater than that of a vacuum, and 0?0 The greater the plate area the greater the flux density.
Ratio of Flux Density = D/E
Coulombs Law of Force.
a) positive value of the magnitude of the force between 2 charges.
b) negative value of the magnitude of the force between 2 charges.
Coulombs Law, the force of electrostatic attraction or repulsion is directly proportional to the product of the two charges and inversely proportional to the square of the distance between them
F = M1 ? M2
F= magnitude of force
M=strenght of two poles
?= permeability of medium poles are in
d= distance between poles
a) is caused by a deficiency of electrons, indicates repulsive force between like charges.
b) caused by a surplus of electrons; attractive force between unlike charges.
How A/C is rectified to produce a D/C in a Generator,
and the position of the brushes at the instant the conductor passes a zero field position between the Poles.
A D/C can be achieved by passing the current through a rectifier. In its simplest form this consists of a switch which modifies the circuit to allow only / to flow through the load. It is automatic and commonly called a ( semiconductor; junction diodes ) offering little forward resistance, but a very high resistance in the reverse direction
Rectification removed the negative side ( half ) of the A/C wave leaving unidirectional D/C.
D/C is produced in an A/C generator by a system of SPLIT RINGS and BRUSHES so it will deliver D/C to an external circuit, termed a commutator. Each ? of the commutator is connected to opposite ends of the rotating coil and the electrical connection to the external circuit are made via spring loaded carbon brushes bearing on a separate surface.
When the coil is horizontal, the induced EMF is maximum and when the coils are rotated and the brushes are in the central part of the commutator strips the EMF falls off as the coils approach the vertical position.
Every ? cycle, the connection between the coils and external circuit will be reversed. The brushes will be spaced between the 2 ? ?s of the commutator at zero field position between the 2 poles.
A 1mF Capacitor is charged to 100 volts and a 2mF capacitor is charged to 200 volts. The two capacitors are then connected in parallel, plate to plate ( + Ve to + Ve ). 1) find resulting potential difference ( voltage ) and (2) the charge on each plate. (3) how much energy is lost by making the connection.
Find the resistance of a Copper Bar 3 meters long having a diameter of ? cm Resistivety of copper is 1.724 ? 10-8 ohm meters.
R= L ?= 1.724 ? 10-8 L= 3m A=
Electrical Potential and Electrical Intensity, Define and show that Newton's per coulomb is equivalent to Volts per meter
Electrical Potential of a point is the work done as a positive charge to bring it from a zero potential, to that point. Its measurement is in Volts.
PD. = joules
Electrical intensity at a point, is the force acting on a unit positive charge at that point, measured in Newton?s per Coulombs
Newton's per Coulombs = Newton's = Newton's Meters
Coulombs Coulombs Meters
= Joules = Newton Volts
Coulombs Meters Coulombs Meters
= Volts = Volts per meter
When a Glass Rod and silk is rubbed together, each is found capable of attracting small light objects
Explain the following; Why does this not occur using metals?, What is meant by a positive charge?
In a balance Atom, there is an equal # of protons ( + ) and electrons ( - ). If an imbalance occurs. electrons go from one atom to another, the former becomes a net + charge, ( more protons than electrons ) and the latter to have a negative - charge ( more electrons than protons )
Glass and Silk are both insulators ( non conductors ). When their surfaces are rubbed together, there occurs a transfer of free electrons from the atoms of the glass to the atoms of the silk.
Glass becomes + charged by a surplus of protons, and the silk - charged by a surplus of electrons. They will both be capable of attracting small neutral objects.
Non conductors retain their charges while metals, being conductors, dissipates charges quickly. ( the effect is not possible ).
A positive charge in this theory denotes an atom with a net loss of electrons (-) and a surplus of protons (+).
What is a Capacitor. Illustrate its principle. What is meant by the term dielectric strength of a capacitor material.??
A Capacitor is a simple device, its function being the short term storage of an electrical charge.
It is composed of two metal plates, separated by a layer of insulating material called the Dielectric;
( usually plastic or ceramics ). Wires connected to the plates, one is + the other is designated -, and are connected to ( e.g.. a battery, ) to complete the circuit.
When first connected, a current will flow due to the PD. across the plates. The flow of the electrons from the plates to the terminals will slow as the PD. is decreased due to +/- charges beginning to behave with O.PD.. An electrical field is established by this electron movement with the dielectric. It can be reclaimed by discharging of the capacitor.
The storage of energy in a capacitor is achieved by deforming the electron orbits in the atoms of the dielectric; an external energy is required. ( the charging current )
Dielectric Strength refers to the maximum voltage that can be applied for a specific thickness of dielectric material with out rupturing or damaging the dielectric; and is expressed as Volts per MM.
Why Series Wound Motors are more suited than other types of DC motors for winches and windlasses.
A series wound motor has its field windings in series with the armature windings so that the armature current flows through both. On starting, there is no back EMF, ( no magnetic field strength ), so a large current in the armature and field is developed, and a large initial torque is created.
As the speed of the motor increases, back EMF, increases, which in turn decreases current and torque. If the load increases the speed decreases and back EMF which allows greater current to flow and a surge in available torque.
It is this ability to respond appropriately to differential loads combined with an initial Hi-Start torque that makes it ideal for winches and windlasses. It has one disadvantage: the motor races without a load, this can be easily controlled by means of a variable resistance, to vary the current to prevent racing.
A shunt motor, ( armature and field windings in parallel ) are least suited, low start torque and little change in speed under load.
A Compound motor (part series and part parallel ) are middle of the road.
Why the Armature Core of a DC motor is constructed of thin annealed sheets of steel, VS a solid block.?
When a metal rotor is moving in a magnetic field, an internal EMF is set up as explained in Faraday?s
The EMF sets up eddy currents in the iron of the rotor = power loss due to heat and also produces back EMF. To reduce this effect, the core of the armature is laminated and insulating material is placed between the sheets to reduce conductivity and the eddy currents are limited to small loops equal to the thickness of the laminations are insulated with special VARNISHES. Resistitory of the steel is increased by allowing up to 4% friction.
OHMMETER, what is it used for ?, can it measure large resistance's?.
An ohmmeter is a device that measures resistance's in a circuit or component . I consists of a DC Current Meter movement, low voltage power source, one or more current limiting resistors and a pair of flexible leads. ( pointed probes ) which allows detection of open circuits or shorts.
Before use the probes are held together at the pointed ends and the meter adjusted to ZERO. When the probes are held together on either side of the component being measured the circuit is complete and resistance to current is flow will register on the meter scale. The higher the resistance, the less the current flow and the less the scale reading. When the probes are apart, ( isolated ) no current flows - scale has an infinite reading.
A standard series Ohm meter is ideally suited for measuring midrange resistance. At higher resistance's, the reading becomes increasingly inaccurate. An internal resistance adjustment helps, but some ohm meter requires large voltage to accurate measure very large resistance's. A special type of ohm meter called a MEGGER is used. It has a hand driven DC generator called a magneto. If the shaft is rotated above a set value, the output of the generator is set at a fixed value, e.g.., 500 volts / 1000 volts ).
Measures resistance in MEG ohms. This devise is commonly found in the installation of heavy conductors for power transmission lines, heavy electrical machinery, transformers ect.
Explanation, and function of a Potentiometer, and a Rheostat.
Both are variable resistors....
Resistive element usually made of resistance wire.
has one fixed and one variable contact.
usually connected in series with a circuit.
controls current, not voltage.
often used to control very high current, e.g.., motor or lamp loads.
resistive element usually made of carbon composition.
two fixed terminals, and one variable contact.
used to vary voltage values across a circuit.
small ones can be used as rheostats by ignoring the contact at one end.
used for control of voltage in amplifiers, radios, tv?s, and other electrical devises where variation of voltage is required.
Both employ a standing arm which makes contact with a stationary resistance element. As the arm rotates, its contact point on the S.R element changes, the resistance changing in tandem.
10 batteries; each with an EMF of 2 volts, additional resistance of 0.05 ohm connected in series are to be charged with a current of 8 amp. Find energy stored in 15 minutes, and amount of heat wasted.
Total Resistance ( RT) = 10 ? 0.05 ohm = 0.5 ohm
Voltage needed to overcome R = I ? RT
V = IR = 8A ? 0.5 ohm
= 4 volts
V required to overcome
EMF of the batteries = 10v ? 2v = 20 volts
Required charging voltage = 20v + 4 volts = 24 volts
Energy stored in 15 minutes = V ? I ? T ( seconds )
= 20v ? 8A ? 15 ? 60 = 144,000 joules
energy lost as heat = V ? I ? T (seconds )
= 4v ? 8A ? 15 ? 60 = 28,000 joules
Energy = power ) watts ) ? time.
P = V ? I
= I?R = V?/R
How much resistance must be placed in series with a 60 volt, 50 ohm lamp to operate it from a 120 volt line? How many meters of nichrome wire ( dia. 1mm ) are required to make a limiting resistor?
Sketch and Describe one type of coil measuring device
Basic Moving Coil GALVANOMETER
The current to be measured passes through the coil which produces an magnetic field. This moves the pointer in proportion to the load. The spring provides a torque opposite to the deflective torque, and when balanced the needle indicates a value based on the current. ( scale is calibrated )
Damping is used to steady the reading - called eddy current dampening. The current cuts the lines of force and due to LENZ's law an EMF is generated in it. This current generating its own magnetic field and opposes the one producing it ( rotating of the coil ) net effect steady the pointer. When the pointer is at rest there will be no EMF generated by the coil movement and the accuracy unimpaired. Used to detect small currents. Minor modifications ( series, parallel connected resistance with the galvanometer movement ) can be used as a voltmeter, amp meter, ohm meter.
Shunt, Series, and Compound winding for a DC Motor: Advantages and Disadvantages of each.
Armature and field windings joined in parallel across the power supply. They have relative constant speed and torque. Useful in operations requiring these features, e.g.., cooling fans, different typed of pumps ect. They have low starting torque.
In parallel, so some current flows in each arm ( Arm + field windings ) divided between the two. Increase load, the armature slows down ... Induced EMF decreases and more current flows in the armature windings. But less current is left to flow in the Field windings - so the product of the current is managed = constant torque.
Armature and field windings are joined in series. They have high staring torque, speed varies with load. Races if unloaded. Inherently poor speed control. which is solved with variable resistors.
Good for electrical vehicles, winches, windlass and starter motors, and are connected to load via gears verses belts to prevents racing.
The same current flows through both the Field windings and the Armature. The torque is proportional to the square of the current. When the load increases, the motor slows down, induced current decreases, extra current is drawn from the source. Since torque is proportional to the square of the current the extra current produces a surge of torque and High starting torque and speed varies with load.
COMPOUND: Field windings and armature connected partly in series and partly in parallel.
Middle of the road - fair start torque
- not as high start torque as series
- speed control not as good as shunt
Series connected provides High Start torque, and the speed is controlled by the shunt connection.
Commonly used for lifts, presses, starting compressors, ect..
Magnetic Field Intensity & Magnetic Flux Density; What information does each quantity provide.
The strength or Intensity of the Magnetic Field at a point is defined as: The force that would be exerted on a unit N Pole placed at that point. The unit is known as a OERSTED and represent the intensity of a magnetic field in which a unit magnetic pole experiences a force of one dyne... If a pole strength M experiences a force of one dyne at a point in a magnetic field, the intensity at that point
Can also be used to measure the electromagnetic force produced in a coil when current flows through it
H= Amp turns
length of coil
The relationship between H & B is called permeability = B
Magnetic Flux density at a point can be represented in magnitude and direction by a vector called the flux density or magnetic induction ( B ) Where magnitude of B determines the density of the magnetic field called field lines ( flux density ) at a point. Where lines are dense B?s. If the flux density is constant in magnitude and direction, the field is uniform. The units of B are TESLAS , which are Webers/M squared
Flux is used to measure EMF. Induced EMF is proportional to the rate of change of magnetic flux linking a coil
Weber = the flux linking one turn of a coil inducing an EMF of one volt as it is decreased to zero in one second at a uniform rate.
39) What are the Active Components of a Lead Acid Cell when in the Charged/Discharged Condition.
In a lead peroxide cell a spongy porous lead plate ( negative electrode) and a lead peroxide (positive electrode) are immersed in a electrolyte. Due to chemical reaction, the spongy lead accumulates a layer of negative charge as its gradually dissolves in the electrolyte. The atoms leaving the spongy lead are + charged and attract to the - charged atoms of the electrolyte while repelling the + charged ions of the electrolyte surrounding the lead peroxide plate.
This causes electrons to be removed from the lead peroxide leaving it with a positive charge. If a load is placed between the terminals the forces of attraction and repulsion will cause the free electrons to move towards the + charged lead peroxide and be given up for external use in the circuit. Some of the acid of the electrolyte combines with the active material on the plates and Lead Sulfate is formed.
A positive charge is fed in through the negative terminal and the acid which was absorbed ( during discharge ) by the plates is returned to the electrolyte. The active material is charged back to its original state ( charged condition ) Spongy lead and lead peroxide and the electrolyte to its original state of strength.
The EMF of the cell remains pretty constant during the charge and discharge phases of the cycle. Can measure it with a hydrometer: the greater the Specific Gravity of the water the higher the charge.
The active material react with the gases produced -- oxygen at the anode; hydrogen at the cathode.
a) + plate(s) of Lead Peroxide ( PbO2)
b) - plate(s) of spongy lead ( Pb )
c) Electrolyte of Sulfuric Acid ( H2SO4 )
d) both plates become Lead Sulfate ( PbSO4 )
The Specific Gravity of the cell at these times:
Charged 1.33 Discharged 1.11
2.1 volts 1.8 volts
During charging: well ventilated, ignition free atmosphere.
Hydrogen and Oxygen are being produced and the gas so formed id both poisonous and explosive.
Heat is also produced and inspection for nearby flammable?s and damage to cells.
Watch the acid level, too little may damage the plates of the cell and weaken the electrolyte ( only distilled water should be added.
On ships, good idea to have an insulated floor, fans, intentionally safe wiring and a lock on the door.
Describe ( diagram ) how electric current is Produced on a conductor by the Thermal Method and where is it used.
A thermocouple is a device using the thermoelectric ( Seebeck effect ) and consists of a closed circuit with a dissimilar metallic conductors, the junction between the different metals are maintained at different temperatures. An electric current flows.
The voltage output is proportional to the temperature difference. So may be connected to a suitable voltmeter calibrated to give direct reading of the hot junction temperature. Useful for measuring furnace and flue gas temperature.
A large number of them could be connected to produce a useful source of power temperature detectors inside large generators, motors, electric pyrometers.
Can also be used to measure small or low frequency AC or DC currents in conjunction with a Milliameter.
Particularly active metals with readily give up their electrons or accept electrons (e.g.. copper & zinc ) an electron path is developed and intensifies with heat and current flows.
Resistively of metal increased with heat. Electrons will flow from hot to cold.
Draw a Wheatstone Bridge Circuit and explain.
A circuit containing 4 resistance arms ( one an unknown value ) A battery connected across one diagonal and a galvanometer across the other allows the determination of resistance ( Rx ). Since the resistance in the arms is variable, it is adjusted until it is in balance ( potential across are the same )
R3 + R1 = R2 + Rx . A zero reading results, no current flows and no deflection of the galvanometer.... The value of the variable resistor ( R3 ) is equal to the unknown resistance
When the source is AC and the bridge contains capacitors or inductors, it can be used to measure capacitance and inductance in a similar manner.
The Armature of a DC Generator has a resistance of 0.4 Ohm The terminal potential difference of the Generator is 120 Volts with no load, and 115 Volts under full load. How much power is delivered at full load?.
Effective Voltage + terminal Voltage - counter EMF ( back ) 120 - 115 = 5 volts
Armature current ( I ) V ( effective voltage )
R ( Armature Resistance) 5v/0.4 Ohm = 12.5 Amp.
Power = V x I = 115V x 12.5 Amp = 1437.5 watts or 1.4375 Kw
What is meant by MUTUAL or SELF INDUCTANCE.
As Ac in a conductor goes through one complete cycle, the magnetic field around the conductor builds up and then collapses. It then does this in the opposite direction. When the magnetic field builds up from collapse ( zero ), the lines of force, flux lines. expand from the center and outward and ? cut? there the conductor and EMF is generated independently of the supply power. This is the case when going from max. to zero again ( collapse ).. A change in current also causes a change in the magnetic field which induces an EMF in a conductor = SELF INDUCTION.
A coil has a self inductance of one HENRY if a current, charging at a rate of 1 amp/sec. induces an EMF of 1 volt.
If the flux lines of one coil were to cut the windings of a nearby coil, a voltage would be induced in that coil. Amount of voltage depends on distance between the two coils. Also the higher the number of turns in the secondary coil the higher the induced voltage ( EMF ). This secondary EMF will also induce EMF in the primary coil as well
Four EMF?s are present;
1) primary applied
2) primary EMF
3) secondary EMF
4) secondary EMF induced in the primary coil
Two coils have a mutual inductance of one Henry, if a current charging them at a rate of 1 amp per second induces an EMF of one volt in the secondary coil.
Using Faraday?s Law of Electromagnetic Inductance the voltage ratio of the primary and secondary windings in an ideal ( no loss ) transformer is the same as the ratio of the turns in the secondary windings.
Whenever a conductor cuts the lines of force an EMF will be generated in the conductor.
The value of the induced EMF depends on the rate at which these lines of force are being cut, and the number of lines being cut.
An Ideal Transformer work by providing an alternating output voltage. Magnetic coupling is employed to transfer EMF from primary to secondary coils.
If secondary windings have more turns than the primary, the voltage across the secondary will be proportionately higher than across the primary. In an ideal transformer the power is constant. When the voltage is stepped up the current is stepped down and visa versa.
Does a similar relationship between the current hold when a pure resistive load is connected to the secondary windings?
With a purely resistive load V1 and V2 have the same phase so:
I2 = N1 and power = V x I
When a motor is running, what effect does LEN Z's Law have on the current?
Lenz?s Law: An electric current flowing through a conductor produces magnetic lines or flux which surround the conductor. A change in the current produces a proportional change in the flux encircling the conductor. This change in the EMF in the circuit which opposes the change in the current. THIS IS LEN Z's LAW.
When a DC supply is applied to the field and armature windings. A magnetic field if produced and current will flow in the armature conductors situated in the field. A force is exerted on the conductors as the armature rotates. ... An EMF is induced and its direction opposes the flow of the current.
The back EMF , the effect of friction produces a torque opposing the rotation, and this increases with speed. Eventually friction torque = density torque and motor revelations at constant speed with constant change.
TORQUE = flux x I ( amperes )
The same current flows through both the field windings and the armature.
The same current flows through both the field windings and the armature. Torque is proportional to the square of the Current. When the load increases, the motor slows down, induced current decreases, extra current is drawn from the source. Since torque is proportional to the square of the current, the extra current produces a surge of torque; hence high starting torque, and the speed varies with load.
Field windings and armature is connected partly in series and partly in parallel.
Middle of the Road: Fair starting torque as series.- not as high a starting torque as series - speed control not as good as shunt.
Series connected parallel high start torque and the speed is controlled with the shunt connection. Commonly used for lifts, presses, starting compressors.
Describe the chemical method of producing EMF
Using a primary voltage cell that is an electrolyte cell containing an electrolyte and electrodes of different conductive materials, an electrochemical EMF is generated between the electrodes. When the electrodes are joined with a load, a current flows and chemical charges take place which is effectively irreversible. Cells are wet or dry depending on whether the electrolyte is a liquid or a paste
The electrolyte is sulfur acid in older models. Later ones as above were replaced with Copper Sulfide and zinc sulfate separated by a porous partition which raised the internal resistance.
Water actuated primary cells have been developed, stored dry, their immersion in water, salt, or fresh,
to actuate silver/chloride, magnesium cell . Lead oxide, magnesium cell + type..e.g.., PFD cells.
48) An iron ring is closely wound with an current carrying conductor. The ring is now cut to form an air gap. Explain why the existence of an air gap increases the reluctance of the magnetic circuit?
Reluctance is the opposition to the production of the flux and corresponds to electrical resistance. Reluctance is inversely proportional to Permeability. Iron has high permeability and low ?R? Air has low permeability and high ?R?
The air gap is the air space between the poles of a magnet. Since air has high ?R? , the size of the gap affects the value of ?R? The shorter the gap, the higher the field in the gap. The larger the gap, the larger the magnetic couple and the more area to spread out the lines of force and increase Reluctance, called Fringing flux ???
The permeability of iron is 1000, air is 1. That means 1000 times as many ampere turns are needed to achieve a given flux density in a pole of the same length in air.
Introduction of an air gap will greatly contribute to the reluctance of the iron .
RT = R air gap + R iron.
If the Flux Density is to be maintained and the number of turns to remain constant, how would the current change?.
49) Name 3 effects of electrical current used when measuring Electrical Quantities. Which are used directly or indirectly in measurements for detection of current?.
2) Heating > Effects
The most widely used is magnetic as electrical fields always coexist with a proportionate magnetic field.
Name the Rule, and Explain how to indicate direction of motion of induced current in the conductor, when the direction of motion of the current and magnetic fields are known.
FLEMINGS RIGHT HAND RULE: Conventional magnetic field direction is from N to S poles, and since the conductor moves in a particular direction, the direction of the induced EMF can be deduced.
Fleming used the thumb, 1th., finger, ( index ) and 2nd., finger of the right hand and put them a right angles to each other. By placing the thumb and first finger to the known quantity, the third finger,
( Induced EMF ) can be noted.
50) Advantages of Revolving Field Generator over Stationary Field.
Describe the Revolving Type.
- less costly. direct connection to external circuits
-Problems of insulating are simplified, no slip rings needed
- Higher Voltages possible from stationary armature to switch board
- use of sliding contact rings and brushes.
- centrifugal force on the armature is eliminated.
Revolving Field type A/C Generator ( most heavy service units are this type )
Armature Coils are permanently mounted around the inner circumference of the generator housing, while field coils and their pole pieces are mounted onto a shaft and rotate within the stationary armature. Relative motion of the magnetic flux lines then induce the voltage.
* Higher rotational speeds are possible.
Two Types of Revolving Field>
1) Salient Pole: poles are bolted or riveted to a central cast iron bus or spider, field coils ( exited by D/C )
are fitted over the poles. usually Large diameter and short axial length.
2) Turbo Alternator: The rotating magnetic field is a cylindrical steel forging, on the outer circumference on which slots are milled axially. A field winding, ( flat copper strips ) is laid in the slots and keyed in position.
51) An Alternator is run as a motor: what is it called?., Outstanding feature, and usefulness??..
Its Called A Synchronous Motor: A/C applied to the stator, D/C applied to the rotor.
AC current causes a magnetic field to be set up around the rotor and being energized by DC, acts like a bar magnet. If a rotor so energized is rotated a a speed such that the relative speed between rotor and rotating magnetic field is low ) nearly equal ) then the poles of the rotating magnet line up and rotate in step with the opposite poles of the rotating field around the stator and ? turn a load.
A 2 Pole Stator?s Speed is 3600 RPM for 60 Hz. The Synchronous Motor is a constant speed devise and its speed depends on the Frequency of the Ac motor supply, and the number of poles.
It has a very accurate speed and therein lies the usefulness. e.g.. timing devices, no load full load......
52) No Load Current of a Transformer ( Also called the exiting Current ) What 2 components make up this current.
If the secondary winding of a transformer is left open, the primary current is low, and termed no load current.
It produces the magnetic flux and supply the hysteres effect and eddy current losses in the core.
1) magnetic current component, large applied primary voltage by 90 deg.
2) Core loss component - always in phase with the primary voltage.
53) Comment on the construction of a Three Phase Arrangement.
Advantages.. What phase difference between each of the set of windings, What 2 ways are there of connecting the phases and what difference in line voltage does the 2 ways have.
Single Phase: generators coils ( armature ) connected in series.
Two Phase: coils are wired so that the generator has 3 separate output voltages. Separated 90 degrees
in phase, two loops are at right angles, to each leg, with their own slip rings.
Three Phase: 3 equal spaced windings, 3 output voltages, 120 degrees out of phase loops are 60 degrees apart.
Advantages: Greater efficiency, greater total output with same mechanical power, smoother steady power ( current ) and greater flexibility of application. the 3 separate outputs may be used for different circuits, while the 2 phases used in conjunction can be used to run A/C Motors.
There are 2 ways of connecting the 3 armature windings to the load....
Delta: ( mesh system) the 2 windings are connected in series to form a closed circuit, and they connect to the load at the junction of the 3 windings, this gives increase in current, but no increase in Voltage.
The line voltage is the same as the voltage in the windings.
The current however. is square root of 3 multiplied by current ( 1.73 times ) the current in any one phase of the winding. Total true power derived is 1.73 times the power of any one line.
VL = VP line voltage = phase voltage
IL = 1.73 x IP
b) 3 conductors are joined to a common spring at the end, and the other end connected to its own spring. The central ( neutral ) line is common to all. One of the leads is connected together the other three leads are brought out and connected to the load.
VL = VP x 1.73 IL = IP
Advantages: easy to ground the connection from the neutral point so less chance of shock. By connecting the neutral point and one other lead, you connect a single phase load to the system.
In the Delta type the line V = 1.73 x phase V with the Voltage between any two of the leads ( line V )
= Voltage generated in one winding.
3 phase insert
54) Explain with a diagram how a Galvanometer may be put into a circuit so as to measure current and voltage respectively.
As an ammeter a low resistor is out in parallel with the galvanometer so only a small current passes through.
As a voltmeter, a high resistance is put in series with the galvanometer and the two combined put in parallel across the load, where potential drop at being measured.
55) Explain Back EMF in a motor and how it occurs. what Law Governs the direction of this effect.?
The armature of a DC motor produces its own magnetic field which reacts with the magnetic field coils to produce rotary motion. The conductors on the rotating armature cut the lines of force of their own field and consequently a counter or back EMF is induced opposite to the direction of the current flow in the armature windings.
LEN Z's LAW: the direction of the induced EMF is such that it opposes the effects producing it.
A motor will pass more current when allowed to run freely. The input voltage remains constant, the current output is diminished by the production of back EMF which opposes that of the driving EMF.
The current flowing through the armature depends on, and is controlled by the back EMF.
56) An Inductor of 0/17 H and a resistance of 50 Ohms are connected in series across a 110 Volt, 60 hertz line.
Find Inductive reactance....
c) total current
d) voltage drop across resistor and coil
e) Power used
57) What is meant by Armature Reactance?
a) how does it affect the motor,
b) how is the effect counteracted?
When a motor?s armature is supplied whit a current, a magnetic flux is build up around the conductors of the armature windings.
:. 2 magnetic fields in the space between the pole pieces: the main magnetic field, and the field around the armature and combine to give a RESULTANT FIELD. This field has been distorted from the original main field and is termed Armature Reaction. The distortion being produced by the rotation of the Armature field.
The neutral plane of the resultant field is shifted from the GEOMETRIC NEUTRAL plane in operation, the amount of shift determined by the extent of the armature reaction and is longer with a higher currents. The results in less efficiency in power produced by the motor.
Counteraction when motor is to be run at a constant speed and direction, involves repositioning the brushes, to the resultant neutral plane to increase commutation. But if varying speeds and direction, and loads are to be used, then inter poles are used to handle the variable nature of armature reaction. The windings are connected in series with the armature windings and a current is set up around them. They correct the current sent to the armature coils and :. decrease the distribution of the flux field, keeping the neutral axis close to the geometric and :. increases the efficiency of communitation with a minimum of sparking. This effect is proportional to the armature current and :. is variable
The Armature is also called a ROTOR..
58) A voltage of 1oo volt; f: 1 x 104 Hz is applied to a circuit having an R of 20 Ohms in parallel with an inductance of 3 x 10-6 Henrys. Find current in each of the current branches ( legs ) and the angle of the lag.
*** In a circuit with xL, current always lags voltage.....
59) Explain INDUCTIVE and CAPACITIVE REACTANCE . State their effects.
a) Inductive Reactance is the opposition to primary EMF A/C current by back EMF and is measured in Ohms: Symbol = XL and increases with frequency of the current, and causes current to lag behind the applied voltage. Current is caused to be lowered .
XL = 2 ? pie ? frequency ? L = Ohms f + frequency in Hz, L = inductance in H.
Capacitive Reactance is a special type of resistance by a capacitor opposing the passage of A/C . Measured in Ohms, Symbol = XC. XC decreases as the f frequency of the A/C increases. Causes current lead voltage by the applied EMF. Causes current to increase
XC = 1
2 pie f C = Ohms
Since they have opposite effects; they are both present in a current, they will tend to cancel each other out, and this is known as RESONANCE.
60) When is an electric circuit said to be in Resonance, and how is the Phenomenon used?.
A circuit is said to in resonance when the Inductive reactance equals the Capacitive Reactance.
These types of circuits are used in electronics to provide frequency selectivity :. called tuned circuits, they are used for impedance measuring, bypass factors and oscillators. Resonant circuits are used extensively in marine electronics, in Radar, Tele-comunications ( radios) ect. depth sounders.
At resonance , impedance ( Z ) = Resistance ( R ) :. Current is maximum and phase angle is Zero. This is used some navigation systems..
A resonant circuit is tuned to and from the resonant f the current increases or decreases :. providing the f selectivity one is ?????
21) a PD. of 200 Volt is connected across 2 capacitors in series, c1 = 2?F, and c2 = 6?F. What is the resulting charges on each Capacitor and the PD., across it?. What is the total energy of the combination?.
Total Capacitance = 1 = 1 + 1 = 1 + 1 = 6
CT C1 C2 2 6 4 ?F
CT = 6/4 = 1.5 ?F 1 + !
2( 10-6 ) 6 ( 10-6 )
QT ( charge ) = CV = 1.5( 10-6 ) ? 200 V = 3 ? 10-4 coulombs
Inductivety: V1 = QT = 3 ? 10-4 = 150 volt
C 2 ? 10-6
Total energy = ? CV2 = ? ( 2 ? 10-6 ) ? 1502 + ? ( 6 ? 10-6 ) 502
= 0.0225 + 0.0075 = 0.03 joules..
25) Analyze the following circuit diagram and calculate the current and voltage for the resistors with KIRCHOFF?s LAW
Kirchoff?s Law (voltage ) The voltage applied to a closed circuit = sum of the voltage drop across the resistance's in the circuit must algebraically equal zero ? 0 ?.
Current Law the sum of the currents entering the circuit = sum of the current leaving a junction. the common point is called a NODE.
Sum I = I in - I out = zero ?0?.
37) In the diagram below, what power is delivered to the motor, the current being 5 AMP.
If the motor delivers 1KW with a speed of 1200 RPM, what torque is delivered?.
P= VI I = 5 amps. V at motor = VT - R1 - R2 -RG
= 230 - ( 5 ? 2 ) - ( 5 ? 2 ) - ( 5 ? 1.5 ) = 202.5 volts
230 - 10 - 10 - 7.5
Power = 2 pie ? RPM ? torque :. torque = Power ? 60
2 pie ? RPM = 1000 ? 60
2 pie ? 1200
= 7.958 Newton/Meters
47) An A/C of 120 volts with a f of 1000 Hz is imposed on a circuit having a resonance of 10 Ohms in parallel with a coil of 400?H. Find the current, the phase angle between I and V and solve which leads the other
36) In the diagram, a potentiometer of 120 Ohms is connected across a 240 volt D/C supply. A current of 2 amp is required in the 40 ohm coil. Find the position of tapping point ?B? by finding the ?R? of ?AB?.
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