Electrotech Notes and Questions.docx

Electrotech Notes and Questions

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
Ns Es


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 

c d

2
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
R


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:

Series Circuit:
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


Parallel Circuit:
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 
R2 I2


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
ampere-turn/meter

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
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.


= D 
E 


Resistively ( p ) Ohm
m, m?

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 
A


Magnetic Induction ( ) Units Wb/m? Magnetic Induction
( magnetic flux density)

Number of magnetic flux lines passing perpendicular through an area.

= Weber
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 
R

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.

Capacitors

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 
? d?

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 
Coulombs


Electrical intensity at a point, is the force acting on a unit positive charge at that point, measured in Newton?s per Coulombs

E= Newton's 
Coulombs


Newton's per Coulombs = Newton's = Newton's Meters 
Coulombs Coulombs Meters


= Joules = Newton Volts 
Coulombs Meters Coulombs Meters 


= Volts = Volts per meter
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
1th.,law.
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....

Rheostat 
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.


Potentiometer

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.


Shunt;
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.

Series:
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 

H= F 
M 

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 
H

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 f...