B3 Electricity ch79-84
Strength of flux or field around and electromagnet is proportional to
Current flow in coil, number of turns in coil, and permeability of the core.
The basic unit of electrical current, watt, is calculated by
Current times Volts
Maximum voltage is produced when the conductor is cutting the magnetic lines of force at
90 degrees
The right hand rule for generators is used to
Determine direction of induced current flow
A conductors resistance is
Directly proportional to its length
How does a coiled conductor affect magnetic field around the conductor
Each coil reinforces the magnetic field.
The flow of electrons in a uniform direction is called
Electrical current flow
Electromotive force. E
Electrical pressure or force that causes current flow. Measured in volts. V. Can be produced by battery, generator, or photocells.
Unlike poles attract and
Lines of force complement each other
Magnetic flux can be said to be
Lines of force outside a magnet, flowing from N to S.
Under normal circumstances, the heat produced by the electrical wires of a circuit is very small due to the
Low resistance of the conductor
A megger consists of
Hand operated DC generator, providing 500 Volts DC or higher. Instrument has two coils sound on a light frame and rotate around a core. Current from generator supplies coils, and mechanism is connected to external circuit. Resistance of conductor is equivalent to unknown resistance.
Open circuit
Has a switch that breaks flow of power.
Suspended magnet will always
Have the north pole facing north magnetic pole, same with south pole and magnetic pole
Insulation test may need
Higher voltage to force small current through a high resistance. Electronics has lower resistance thus less voltage required and a multimeter can be used.
An ammeter must have
Low resistance so reduction of current flow will be negligible. Ammeter must be connected in series with all equipment being tested. This may mean multiple points in a parallel circuit.
Iron and steel may be magnetized by rubbing metal with a
Magnet
The space or zone around a magnet through which lines of force travel is termed
Magnetic field
Megohmmeter, or megger, measures
Millions of ohms. It tests insulation breakdown on conductors.
Voltage measurement
Most common type of verification of electricity. The meter is intended to be connected across a voltage source, or electrical load that has a potential difference across it.
Permeability
Property that determines a materials ability to magnetize. Iron and steel are permeable
The battery of an electrical circuit
Provides the electromotive force
Electrical switch
Mechanical device used to open or close electrical circuits.
With an increase in temperature, most conductors tend to increase in
Resistance
How might a solenoid be powered by an electromagnet
Switch closes circuit and energy flows into electromagnet. It draws an iron core up, compressing a spring which was holding a valve shut
In an equivalent circuit developed from a multi-resistor series circuit
The sum of voltage drops across the original resistances will be equal to voltage drop of the equivalent circuit
if the direction of the magnetic field is reversed, the force on a _________ carrying conductor placed within that field will be reversed.
Voltage
A commutator
continuously reverses connections between the loop by only allowing one side to connect at a time, causing current to flow in one direction only. Also known as a rectifier.
Positive terminal of the ammeter is always connected to the
positive terminal of the voltage source
Fleming's left hand rule considers motor action to determine direction of the
force
If the current in a current carrying conductor in a magnetic field is reversed, the direction of the force tending to move the conductor will
reverse
The opposition, a device or material offers to the flow of electric current, is known as
A resistance
A rheostat is also known as
A variable resistor
Transformer cooling oil has
A very high specific heat
High voltage
Above 750V. Will cause arcing through the air over small distances to a grounded body. Installations must be locked.
Back electromotive force
According to the right hand rule for generators, an electromotive force will be generated and will oppose the applied emf force driving the motor.
Since power factor = actual power/ apparent power and apparent power is E times I then
Actual = EI x power factor
Actual power and apparent power
Actual power is the power doing actual work. Apparent power is the sum of actual power and wattless power.
Paralleling alternators
Aka synchronizing. Connecting to equipment in parallel. Must be same voltage. Phase sequence must be same as system. Must be in phase with system. Alternator frequency and system frequency must be the same.
Shell type transformer
All of primary and secondary is wound on the centre leg. The complete winding is surrounded by a shell of iron.
Loading on any system should be balanced between
All the live conductors
What does a slip ring accomplish on an AC generator
Along with brushes, slip rings connect generator loop to an external circuit. One side of loop is always connected to same side of the circuit causing alternating current
Synchronous motors have construction identical to
Alternators
Generators producing AC current are called
Alternators. Large alternators are much more common than DC.
Ohm
Amount of resistance that will allow 1 ampere of current to flow through it when an electromotive force of 1V is applied.
Clamp-on meters are used to measure
Amperage
What part of a DC machine is built up of laminated steel plates mounted on a shaft. The coils are connected to a commutator mounted to one end of the shaft
Armature
3 phase AC motor operation principle
As the magnetic field rotates, it cuts the conductor of the squirrel cage rotors inducing current
Smallest unit of a pure substance
Atom
Like charges repel, and unlike charges
Attract
Ohms law
I = E/R. I is current in amps. E is electromotive force in Volts. R is resistance in Ohms
Synchronous motors are used to
Improve the power factor of a power system
Voltmeter is placed
In parallel with the circuit
DC generators were almost exclusively during early years of electrical industry. Currently used
In process where DC power is needed. Example portable welding machines and DC power to elevators
Ammeters are connected
In series with the circuit
What is the main way we can increase voltage of DC generator
Increase magnetic field since speed of conductor and number of conductors are predetermined once unit is built.
Considering conductor numbers and generator speed is constant, how is DC generator electromotive force increased
Increase magnetic flux via field regulator or rheostat.
Motor nameplate Type
Indicates enclosure. Open; atmosphere air is drawn in and forced over engine. Totally enclosed: cooling air is supplied or recirculated and cord by heat exchange. Externally vented. Explosion proof
When magnetic flux lines travel and meet in opposing directions, the result can be
Motor Action
When magnetic flux produced by one coil induces an EMF in a second coil, it's called
Mutual inductance
Shunt wound DC generator
Nearly constant output voltage at all loads because shunt field winding is connected to the output terminals from the armature, so the strength of the magnetic field is fairly constant
Ammeters are connected with the negative or black lead connected to the
Negative side of the circuit
Voltages ratings are considered
Nominal. +- 5% variance.
Wound- rotor or slip ring induction motor
Normal stator. Rotor is wound with coils of many turns connected to slip rings mounted on rotor shaft.
Advantage of synchronous motor
Not a lagging power factor, which decreases facility power factor and leads to energy surcharges. It produces a leading power
Distance of approach to a high voltage system
Not less than 2 metres
Low power factor is due to
Number of induction motors and other inductive equipment connected to the AC distribution system.
Step up transformer vs step down transformer
Number of secondary turns are greater than primary vs secondary lesser than primary
Because primary and secondary power factors are almost the same, formula for power factor
Output volt amperes equals input volt amperes
Series wound DC generator
Output voltage rises sharply from minimum load to full load. Armature current, generated to supply the load, flows through the series field. As the load increases, field strength increases and output voltage increases
Current losses due to copper resistance are calculated by
P = I² R
Utility companies have a surcharge to their customers based on
Peak demand
In general, percentage frequency drop is proportional to
Percentage motor speed drop.
Motor nameplate Service Factor
Permissible overload. Often 15% for short periods. Nameplate power times service factor. Max allowable temperature may be reduced.
When synchronizing an alternator, voltage, phase, and frequency must be the same, as well as
Phase sequence
When you change the scale on an ammeter you are
Physically changing which size of shunt resistance you are using in parallel with the circuit
Slip speed
Rotor will try and increase speed to catch up to the field. It will never get there and that difference is called slip speed. It produces necessary voltage, therefore current and torque for load on the motor
Three phase transformer
Same construction as single phase, low voltage closest to iron and high voltage over them.
2 types of 3 phase transformer windings
Sandwich or pancake. And concentric
The three main types of DC generator are
Self excited, shunt, and compound
When AC action causes a counter current in a coil, it is termed
Self inductance
How is output controlled on slip ring induction motor
Resistance can be added to the circuit via slip rings. Increased resistance decreases motor output. This also results in decreased efficiency.
Internal multiplier
Result of Mechanical workings of the meter. Displayed on face of meter.
Depending on design, compound wound DC generator can have 1 of 3 voltage to load characteristics
Rising output voltage vs load. Constant output voltage. A decreasing
Most practical build of alternators have
Rotating field and stationary conductor.
Three phase AC motor advantages
Self starting. Smaller dimension per power output. Better power factors and higher efficiencies
The load current has no effect on the field excitation of a
Separately excited generator
If you were required to select a DC motor needing constant speed regulation, which type and why
Shunt motor since it has nearly constant speed between no load and full load.
Most frequently used types of generators
Shunt, compound, and separately excited DC generators
How does the revolving disk operate
Similar to an induction motor, driven by a rotating magnetic field proportional to the current used and potential applied. AC in the voltage and current coils produces an alternating magnetic flux at the poles which induces current in the disk. Torque produced on the disk is proportional to the product of the Eddy currents in the disk and the flux from the load current coils. Thus the torque is proportional to the power used by the load.
To transfer power to or from the rotor of alternators or some AC motors
Slip rings are employed
Coils or windings on a DC machine armature are placed in _____ cut in the face of the armature
Slots
Eddy currents
Small currents induced in the core material by the alternating flux. Generate heat and waste energy. Solved by laminating the core.
Commutator should be checked for wear regarding
Sparking or pitting. Brushes should not be chipped or worn and move easily in their holders with a spring tension of about 15 kPa
3 factors that control EMF developed by DC generator
Speed conductors cut magnetic lines of force. Number of conductors cutting magnetic lines of force. Strength of magnetic flux.
Alternator voltage control
Speed has to be constant to maintain frequency and the number of conductors is fixed, voltage is controlled by varying DC current to rotating field. Can be manual via rheostat or electronic control.
Motor nameplate R/min and HP or kW
Speed, and power delivered.
Most common type of rotor used in induction motors
Squirrel cage. Heavy copper or aluminum bars fitted into slots in the rotor iron. Bars are shorted together at each end by a ring of same material
Alternator construction
Stator is frame mounted. 1800 r/min rotor is mounted on a steel shaft. 3600 r/min rotor is on a cylindrical casing. Slip rings of brass or steel. Brush gear with carbon brushes connect DC excitation to rotor windings through the slip rings
In series, total resistance equals
Sum of all resistances
An AC motor which will drop out of synchronism and simply trip when overloaded is
Synchronous motor
3 types of AC motors
Synchronous. Induction. Wound rotor
What is used to synchronize phase and frequency of alternators
Synchroscope. Needs expert training from an electrical technician.
Why is high voltage not often used as a utilization voltage
System sensitivity to arc tracking is directly related to debris and moisture.
When using a megger to test for insulation leaks in a three phase motor
Test for grounds to each lead as well lead to lead. Join leads together to zero meter.
Resistance measurement
Tests conductivity of a material, or an insulator test. A megohmeter is used for higher voltage systems.
In order to control the starting current drawn by large AC motors
They are started on reduced voltage
Explain the cause of back electromotive force
When a DC motor is running, the armature conductors are cutting magnetic lines of force between field poles. EMF is developed in the armature conductors of the motor which oppose the EMF applied to the armature
Lenz's law
When a magnetic flux changes and induces a voltage, it's direction opposes the flux change. The total E induced in the coil by Lenz's law, opposes the voltage applied to the coil and is known as self inductance.
How is flux of a transformer virtually constant through all load conditions
When load connects to secondary of a transformer, it interferes with the primary, reducing the counter voltage in primary, allowing more primary current to flow, thus returning main flux to former value.
Magnetic hysteresis
When magnetic domains are forced to re-align, they put up a resistance. After magnetising current is gone, some magnetism is left in core material. As current flow reverses, some energy is used to cancel the residual magnetism.
Mutual inductance
When magnetic flux produced by one coil cuts the conductors of a second coil, a voltage is induced in the second coil. This is the principle upon which the transformer operates
kVa rating of a transformer is governed primarily by
safe working temperature of the insulating materials used.
A conductor supplied with current while in a magnetic field will
tend to move at right angles to the flux lines
Transformer coolants may be
toxic. For this and other reasons, never work on a transformer alone.
K-type transformer
traps harmonics on the primary windings. It directs harmonics to the primary side of the transformer where they create less problems.
Never clamp ammeter to
two wires at the same time.
Clamp-on ammeter
uses basic transformer principle to measure amount of current flow through a conductor.
The total circuit resistance is decreased
when a circuit is in parallel
Motor nameplate Duty
Continuous or intermittent
Advantage of slip ring motor over squirrel cage motor
Controlled starting current and torque.
Current
Flow of electrons in a uniform direction. Neg to pos.
Three major losses that occur in a transformer are iron losses, copper losses, and
Flux leakage
Power company may penalize for power factors under
0.8
Transformer inspection
1-2 times per year. Clean of oil and dust, inside and out. Check oil for acidity and clarity. Check for hot or loose connections in and out.
High voltages are stepped down to _______ Volts before the voltmeter can be used to measure the amount
120
A 6 pole alternator producing 60 Hz power must turn at
1200 rpm
Alternator frequency
2 pole rotor runs at 3600 r/min to produce 60 Hz. 4 pole rotor runs at 1800 r/min for 60 Hz.
Low voltage
30V to 750V. Serious risk to life. Will power reasonable power loads
Replace bearings at first sign of issue. Fill grease bearing to
40 percent capacity if under 75mm and 60 percent for larger
High amperages are stepped down to __________ amps before the ammeter can be used to measure the amount
5
Single phase AC motors are normally found in ratings not exceeding
500 watts output. Some types need special starting windings, such as universal series and repulsion motor
Three-phase transformers have
6 windings
AC electricity with a frequency of 60 Hz will have its polarity alternate at
60 cycles per second
A clamp-on ammeter measures current flow by using
A basic transformer principle
Why is carbon used for brushes
Because it is softer than the copper commutator bars, wears smooth, and has a low coefficient of friction.
Extra low voltage class
Below 30V. Doorbell, buzzers, flashlight. Tend to suffer from voltage drop.
What transfers power to or from the armature
Brush gear. Carbon brushes mounted in insulated holders with springs and imbedded copper wire for connection to machine terminals.
Advantages of alternator with rotating field
Brushes and slip rings only carry excitation current. Size of rotating mass is reduced. Only one pair of slip rings is needed for a three phase alternator. It is easier to insulate output leads, especially with high voltages.
Riser diagrams
By means of single lines and simplified symbols, the distribution of electrical systems in a multistoried structure
Synchronous motor connection
Can be used to correct power factor. Under exciting will draw reactive power, while over exciting will give reactive power.
Capacitive reactance will
Cancel inductive reactance. Usually aim for .95 as extra cost of reaching unity is not worth the cost.
Many AC circuits contain
Capacitance or inductance. Current will either be leading, or lagging behind the voltage
The yoke that supports the main components of the DC machine is frequently made of
Cast steel or iron
Some of the hazards of transformer cooling oil can be compensated for by adding
Chlorinated hydrocarbons
Neutral wire
Circuit conductor that normally carries current, and is connected to ground at the service.
Three phase alternator
Coil windings are connected in three different groups, one for each phase. Identical magnitude but displaced by 120 degrees
DC welding generators use
Compound generator where a high voltage is required to start an arc, and after that low voltage, high current is needed.
Separately excited
Constant output voltage at all loads. Load current has no effect in field excitation, since field current is supplied by a separate source.
Shunt type DC motor
Constant speed
Advantage and disadvantage of using 3 single phase bank to replace a 3 phase transformer
Convenience. If a coil breaks on a 3 phase, it stops the system. When one of the single coils breaks, system can run at 58% power. Disadvantages are efficiency and cost.
Single phase transformers
Core or shell. Both types have low voltage coil wound nearest the iron core, high voltage wound over the low.
Motor starting devices for DC motors and some AC motors are designed to limit the starting ________ to a safe value.
Current
In a well designed transformer, assume that flux produced by primary voltage
Cuts every turn of primary and secondary windings
A DC generator will also run as a
DC motor depending on connection.
The rotating field of an alternator is obtained by exciting the windings on the rotor with
DC power
Synchronous motors
DC supply to rotor. AC supply connected to stator windings. Unlike induction, DC rotor field locks in to rotating field of stator causing synchronous rotation from no load to full.
How does an ohmmeter work
De-energized circuit. Ohmmeter has a fixed resistance. A battery in the unit produces a current which flows through circuit and meter's resistance. R1 and R2.
Motor nameplate Frame numbers
Designate a specific set of external dimensions and particular HP at a certain speed.
Dust coating increases temperature which will
Deteriorate insulation and reduce operating life of equipment.
The voltage induced in the secondary windings of the transformer is ______ the turns ratio of the transformer
Directly proportional to
DC machine, because of its commutator, is particularly susceptible to
Dirt, grease, oil, moisture, and corrosive atmospheres.
Why does an open circuit not always mean no power
Downstream conductors may store enough energy, or capacitance to pose a threat to life. Lines must be grounded before work commences.
Open motor subtypes
Drip proof: water or solid particles from vertical to 15 degrees. Splash proof: same but up to 100 degrees. Weather protected: minimize entrance of rain, snow, other particles. Waterproof: totally enclosed against water. Cooling tower fans use this.
Disconnecting an alternator
Driving torque of prime mover of the alternator removed till alternator is supplying zero current. Next open main disconnect. Output voltage is reduced to 0 and DC excitation switch is opened.
Methods of transformer cooling
Dry type: fan forced cooling. Core in an oil-filled tank: better cooling and insulating but comes with combustion issues, and higher cost and maintenance.
Wattless power and wattless current
Due to leading or lagging current, some current will surge back and forth in circuit, creating heat and not completing any useful work.
Copper losses
Due to resistance of copper wires in the winding. Solution: larger wire but drawbacks are cost, bulk, and weight.
Why is manipulation of AC voltage and current levels handy
Easier to transmit high voltage at low current, then transform it where it needs to be used. Size of conductor is proportional to current. Savings in Copper
Motor nameplate Insulation
Electrical insulation of stator and rotor windings. Class A, B, F, H. Tag on motor usually indicates max rise above ambient temperature.
In order to produce the strength of magnetic flux required in larger DC machines
Electro magnets are used
Instead of a single loop, coils are laid in slots in the armature
Ends of coils are brought to one end of armature and connected to commutator bars or segments.
Ep is 500 Volts and Np is 20 turns and Ns is 5 turns
Es is 125 Volts
The main components of a DC frame are the yoke or frame that support _________ , and the rotor which is called an armature
Field poles
In large DC machines, electromagnets needed to produce strength of magnetic field needed are
Field poles. Built up layers of silicon steel plates called laminations
How does a 3 phase squirrel cage induction motor rotate
Field windings on a three phase motor are displaced by 120 degrees, matching the phase difference of the three phases. This produces a rotating magnetic field which pulls the rotor around with it.
Poles
Form rotating field of a motor.
Ep/Es=Np/Ns
Formula for voltage. Basically primary voltage over secondary voltage equals primary windings over secondary windings. Hence, Es= Ep x Ns/Np
The number of cycles per unit of time, measured in Hz, is known as
Frequency
Schematic or elementary diagrams
Graphical symbols showing functions, sequence, and electrical relationships of a specific circuit relationship
Grounding conductor
Green or bare. Never used as part of the circuit. No Volts between neutral and ground since the neutral is grounded at service.
A phenomena produced by electric switchgear that can result in excessively high voltages in transformer are known as
Harmonic distortion
Demand meters
Have an extra face. A red pointer shows current load. Black pointer stays at high load mark until reset. Used by the utility company to ensure ability to meet peak demand.
Step down transformers
Have larger diameter wire in secondary windings
Describe transformer breathing and its issue
Heating and cooling pushes and pulls air from the transformer. This breathing causes moisture to be taken in and is a common issue for breakdowns. Filters may be used and need to be checked.
Series type DC motor
High speed at low load, low speed at high load.
Overcorrection beyond unity power causes
High voltage and poor voltage regulation, along with increasing the line current. Overcorrection cam eliminate any savings attained by proper correction
For high voltage, a warning sign stating
High voltage must be conspicuous.
Before being worked on, a high voltage circuit must be
Interconnected and grounded
Overall multiplier
Internal times external.
Self-inductance
Is an opposition additional to that provided by the resistance of the coil. In DC circuits, it delays the buildup of current to its maximum value determined by the value of applied voltage and coil resistance. Once the current reaches its steady max value, self inductance is no longer a factor, until the circuit is opened, and will try and delay the current collapse.
Strength of magnetic field produced by a shunt wound generator
Is fairly constant
Why is the rheostat in the shunt field of a DC generator called a regulator
It controls current flow through the shunt field. A low current in the shunt field produces a weak magnetic field and v.v. This proportionally affects generator output voltage.
When using an electrical meter to check a circuit is safe to work on, it is important to verify the proper scale and current is being used because
It may appear no voltage or current is present. Electrical wiring may be damaged by fire or overheating. The user will be subjected to danger.
What if synchronous motor is overloaded
It will drop out of synch, producing heavy currents and will trip circuit protection.
Power system losses in the distribution system are equal to
I²R. Power system losses increase according to the square of the current flow.
Clamp-on ammeter readings may be inaccurate if
Jaws are dirty or misaligned
How do you calibrate an ohmmeter
Join text leads together and adjust dial to 0.
Advantages and disadvantages of auto transformer
Leads to savings in copper, but are limited to small ratios of transformation.
Transformers require
Less maintenance than most electrical equipment
In a series wound DC generator
Load current passes through the field windings
Terminal and connection diagrams
Locations and markings of each terminal and the interconnection that is required.
Induction motors at low loads cause
Low power factors. Operating costs can be reduced by sizing motor to meet its demands.
Generator
Machine that converts mechanical energy into electrical energy
Shunt wound DC motor are common where speed must remain constant, regardless of load such as on
Machinist lathe
Motor speed is inversely proportional to
Magnetic field strength.
Compound type DC motor
May be designed to incorporate characteristics of both series and shunt motors
Commutator of a DC machine is made of copper bars mounted in
Micanite
Whenever voltmeters are connected to a direct current the
Positive lead must be connected to the positive point of the circuit
An instrument transformer used to measure voltage is called a
Potential transformer
Actual power/apparent power
Power factor.
Core type transformer
Primary and secondary windings are split into two equal parts, half primary and secondary on each of the transformer legs
Batteries
Produce a steady DC voltage
Apparent power is the power
Provided by the utility
How to read a meter
Read right to left, notate numbers same. Meter often has a much lower capacity than operation being read.
A power meter
Reads only a small percentage of the actual load.
Kilowatt hour meters
Register amount of power used in a period of time. They consist of a small motor whose speed is proportional to the power consumed by the equipment being supplied.
Connection or wiring diagrams
Relative physical arrangement of components, including ask wiring and connections
The unit for opposition to current flow is
The ohm
External multiplier
The product of the factors that bring operation voltage and current down to meter values.
Permanent magnets are used for
The smallest DC motors and generators
Why do DC generators need a commutator
To connect the generator loop to the external circuits through brushes. It also continuously reverses connections between loop and external circuit so current only flows in one direction.
Brush gear and commutator
Transfers power to or from armature, depending on generator or motor
Watt meters measure
True watts
Because series motors operate well either on AC, or DC power, they are often called
Universal motors
How is magnetic hysteresis dealt with
Using low retentivity core material such as silicon steel.
Modern electronics has made frequency changing equipment affordable to supply an AC motor with
Variable frequency power to smoothly control motor speed. Important in pumps and fans rather than using control valves and dampers.
Components of kWh meter. Same as wattmeter
Voltage coil. Current coil. A compensating coil. A revolving disk. A registering mechanism
DC current equals
Voltage times current. P = EI
Why should secondaries of current transformers never be opened in active circuits
because to drop the current, the voltage goes up proportionally and can be dangerous to life.
Harmonic distortions
causes the load to heat up, resulting in excessively high voltage and currents being produced, leading to short circuits, capacitor explosions, reduced transformer life, deficiencies in performance, insulation stress and fuse and circuit breakers burning out.
Auto transformer
has part of its winding common to both primary and secondary and only has one winding.
What might be the cause of high current draw
Heavy work on motor like pumping large amounts of water. A bearing starting to seize.
In Fleming's left hand rule, index finger represents
Motor action flux direction. AKA direction of magnetic field
Resistance
Opposition to current flow
Reluctance
Opposition to establishing a magnetic field. Depends directly on permeability.
Electrical power formulas
P=IE P= I² R P=E²/R
Doubling diameter of wire will
Quarter its resistance
The property of resistance to conducting magnetic lines of force is
Reluctance
Current flow is inversely proportional to the
Resistance
In AC generation, one side of the loop is
always connected to the same side of the external circuit. This means it crosses the magnetic field in both directions over one rotation, creating current flow in both directions in the circuit.
When taking a current reading
always start on the highest possible scale and work lower to prevent damage to the meter. No cycling on and off of motor while taking reading unless scale is set to highest.
Unit of measure of electrical current flow is
ampere. A. Quantity of electrons past a given point in a given time
High current causes
fuses to blow and circuit breakers to trip. When this happens, a current or load test should be done.
When current flows through a substance
heat is produced
magnetite is also known as
lodestone
Current flow is
proportional to the applied voltage
Electrical work
the installation, alteration, repair, or maintenance of an electrical system designed to provide heat, light, or power in or on buildings, structures or premises.
When 1 ampere of current flows through a resistance by the application of 1 volt, the resistance is
1 ohm
Series circuits characteristics
1. Current is the same in all parts of the circuit. 2. Each resistance opposes current, total resistance is sum of all resistances. 3. Total voltage drop across the circuit is equal to sum of individual voltage drops across the resistances. 4. Any break in circuit stops the current because there is only one path.
How many electrons do good conductors have
1. More than 3 electrons make poor conductors. With only 1 electron, it is not tightly bound to the nucleus.
A voltmeter must offer a high resistance to current flow because
1. Only a very small current, sufficient to cause full scale movement of the moving coil and pointer, is permitted when full scale voltage is applied. 2. The voltmeter must not drain current leading to reduced reading.
Parallel circuits have the following characteristics
1. Total electric current is equal to sum of currents in each branch. 2. The reciprocal of the total equivalent resistance is equal to the sum of the reciprocals of individual resistances. 3. Supply of voltage is applied equally to each branch of parallel circuit. 4. If there is a break in any of the parallel branches, current continues to flow in remaining. 5. The current through each resistance is found using Ohm's law. L1=E/Rq
A standard dry cell has a potential difference, or an electromotive force of
1.5 Volts
parallel resistance formula
1/p = 1/R1 + 1/ R2 + 1/ R3
Minimum voltage generated by a single loop DC generator occurs when the conductor is cutting the magnetic lines of force at
180 degrees, or parallel with the magnetic force lines.
When an electric current flows in a conductor,
A circular magnetic field is set up around the conductor.
If a few turns of wire are wrapped around a core of soft iron and a current is of electricity passed through the wire, you will have
An electro-magnet
Michael Faraday discovered in 1831 that
An induced voltage is produced in a conductor when the conductor is moved across a magnetic field
2 types of electrical metres
Analog and digital
The flow of electrons in a uniform direction from atom to atom through a circuit, is known as
Current
The total number of lines of force per square metre in a magnetic field is
Called the magnetic flux
Atom is composed of
Centre mass of protons and neutrons, with orbiting electrons
Rules for megohmmeter tests
Circuit or equipment must be de-energized. Know exactly what components are in circuit tested. Discharge capacitors before and after test. Do not hold contact or bare test leads when meter is being cranked.
Major difference between an AC and DC generator, is that a DC generator has a
Commutator
Two methods of finding line current in a parallel circuit
Find equivalent parallel resistance and determine via voltage and Ohm's law. Or, sum of separate branch currents.
Many diagrams are based on conventional current flow and show
Flow from positive to negative.
If a conductor is held stationary and a magnetic field is moved across it, it forms what is called
Generator action
In order to take a reading, you must know
If power is AC or DC
How to increase the magnitude of voltage produced
Increase speed at which magnetic lines are cut. Increase strength of magnetic field. Increasing the number of conductors cutting magnetic field with conductors connected in series.
Increase in temp cause resistance in conductors to
Increase. A few substances, like carbon, decrease resistance with temp increase.
Current measurement
Indicates the amount of power a device is using. Current draw is a function of load on the motor.
Metals with magnetic properties
Iron is most. Steel, cobalt, and nickel
Lines flow from
N to S on the outside of magnet and S to N inside.
Electrons move from
Negative to positive. The term electron flow is more common in electronics. They travel from one atom to the next.
When atoms of different elements combine they form
New compounds
Define Right Hand Rule for direction of magnetic field
Point your thumb in direction of current flow. Fingers will wrap around conductor in direction of magnetic field. Also picturing a screw and the threads represent magnetic field flow.
Watt is an electrical unit of measure for
Power
Relationship of Mechanical power and electrical power
Power is rate of doing work. Mechanical power units are watts. Joule/second Electrical power units are watts. Amps x volts
The purpose of having upward movement on a switch handle is to
Prevent gravity from throwing the switch
How does the Right Hand Rule determine polarity of a coil or electromagnet
Same as direction of flux around current carrying conductor. If your right fingers were around coils of wire, thumb will point to north pole of magnet
Single pole vs double or triple pole
The number of leads a switch contacts when thrown.
Electrical power, like power, is
The rate of doing work. Measured in W, kW, MW
Right hand rule for generators
Thumb - direction of conductor motion. Index - direction of magnetic field. Middle - direction of current flow
Fleming's Left hand rule for motors
Thumb - direction of force Index - magnetic field Second - direction of Current
Never connect an ohmmeter
To an energized circuit
A conductor's resistance is inversely proportional
To its cross-sectional area. Cross-sectional area of a wire increases proportional to the square of its diameter. Thus, conductor's resistance is also inversely proportional to the square of its diameter.