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.