Electricity Final

the capacitance value

- the higher the capacitance, the lower the capacitive reactance = more change is stored & released - the lower the capacitance, the higher the capacitive reactance

autotransformer

- used only one coil for primary & secondary - uses taps to produce different ratios & voltages

RC timing circuit

- uses a resistor & capacitor connected in series in DC circuit to create an electronic timer

DC capacitor characteristics

1. a capacitor charges to a voltage that's equal to the source voltage 2. a charged capacitor acts like an open & will not allow current to flow in the circuit

an ohmmeter can be used to:

1. check for opens in coils - if a coil shows infinity, the winding is open & the transformer is bad 2. check for shorts between primary & secondary coils 3. check for coils shorted to the core

3 types of voltage divider networks

1. firm voltage divider 2. still voltage divider 3. loaded voltage divider

4 steps to troubleshoot a circuit to find a short

1. look for visible signs of damage 2. disconnect power supply 3. connect an ohmmeter across main line 4. disconnect each branch one at a time

5 factors determine the amount of inductive reactance

1. number of turns in a coil - more turns = greater inductance reactance 2. diameter of the coil - larger diameter = greater inductance reactance 3. length of coil - longer coil = greater inductance reactance 4. type of core - iron core produces greater inductance reactance than air core 5. frequency of alternating current - higher frequency = greater inductance reactance

3 components of transformers

1. primary coil 2. secondary coil 3. core

types of variable resistors

1. rheoslat - resistor used in high-power circuits 2. potentiometer - resistor used in low-power circuits

multimeter

a multi-purpose device that can measure voltage, current and resistance

a common input device

a switch

Underwriter's Laboratories (UL)

- independent product safety testing & certification organization

International Electrotechnical commission (IEC)

- standards for many types of electrical equipment

circuit breaker

a switch that automatically stops the flow of electricity through a circuit to prevent damage if current becomes too strong

center tap

a tap that splits a secondary in half

a short circuit occurs when...

there is a direct path with little or no resistance created between pos/neg ends of the power supply

power supply

used to modify the power supplied from the power company a constant CURRENT supply provides the same current level regardless of the load. a constant VOLTAGE power supplies a constant voltage output, regardless of the load.

control transformer

used to reduce voltage from main power line to a lower voltage that operates a machine's electrical control system

analog multimeters

used when there are very fast changes in a reading cuz the needle responds faster than digital

filler

using a capacitor to smooth out the changes in voltage

capacitive reactance in AC circuits

voltage & current are continually changing (alternating), therefore, capacitor is constantly charging & discharging

principle of mutual inductance

when 2 electrical coils are placed near each other, AC electrical current flowing in one coil induces an AC voltage in the other

selector switch

you rotate a knob

isolation trasnformer

- has primary & secondary windings that are physically & electrically isolated - magnetically coupled - transformer is a filler between primary & secondary

Advantages of a transformer

- improves efficiency of the transfer of energy from one coil to another - a primary coil creates a magnetic field that's concentrated by the core - voltage induced in secondary coil can be used to drive a load

The Wye-Configured system

- has one end of each of the phase windings connected together

National Electrical Code (NEC)

- contains many codes that deal with electrical systems & equipment - the accepted guide in USA for safety installing & maintaining electrical systems & equipment

primary coil

-an electrical wire wrapped around the core -transformer INPUT

secondary coil

-an electrical wire wrapped around the core -transformer OUTPUT

inductor

-created by winding an electrical wire into a coil -purpose is to resist changes in current flow -referred to as chokes

Polarized or Electrolytic capacitors

-markings for polarity of leads - neg marks with minus -pos will always be longer -provide the most capacitance in smallest space & least cost

Non-polarized capacitors

-not polarity sensitive -doesnt matter what they're connected to -wide variety of tolerances

fuses

-used in applications where a problem rarely occurs -a low cost device that must be replaced each time a short circuit occurs

Variable capacitors

-values can be adjusted by turning a knob -trimmer capacitors are small - adjust by screwdriver

Types of Disconnect Devices

1. Fusible Disconnect - has a fuse for each phase & a manually operated switch that'll disconnect all phases at once 2. Circuit breaker - must be able to open all phases when it trips 3. in-rush current - a motor draws more current at startup than when it is operating at its rated full-load speed 4. Time-delay current - a thermal element that'll melt & open after an excessive current is present past its time-delay.

4 applications of ohm's law

1. troubleshooting circuits - used to diagnose problems by comparing the actual voltage and current to the theoretical values 2. calculating power dissipation - used to determine how much electrical power is needed to supply a circuit 3. size component - used to size components and wire in a circuit 4. create multi-voltage levels

indu tance

a characteristic of electromagnetic devices that oppose any changes in current flow

open circuit

a circuit that has a break in it, preventing electrons from flowing

closed circuit

a circuit with complete path for current to flow

distribution transformer

a common application of a center tap

neutral

a conductor that provides a common return path to the source for the conductors that carry current in a system

core

a ferromagnetic material that's capable of conducting a magnetic field.

common

a known reference point

a common output device

a load

voltage

a measure of the difference in electric potential between 2 points in space, a material, or an electric circuit

transformer

an electrical device that converts AC electricity from one voltage level to another

rectifiler

an electrical device that converts AC to DC

relay

an electrical switch operated by an electromagnet

tap

an extra lead attached to a secondary coil

magnetic field

an invisible force created around a wire that electrons flow through

Normally closed (NC)

are closed until acted upon by operator current is able to flow

Normally open (NO)

are open until acted upon by operator current flow is prevented

digital multimeters (DMM)

automatically detect polarity (pos/neg)

knife switch

consists of a lever (operator) that conducts current and one or more sets of contacts

series-parallel/combination circuit

contains elements of both series and parallel circuits - components that are connected in SERIES have the same CURRENT flowing through them -components that are connected in PARALLEL have the same VOLTAGE across them.

in-rush current

critical value to consider when picking a transformer for a machine with motors

rated full-load current

current that a motor draws when it's fully loaded & operating at its rated speed

when current is (increasing / decreasing), the induced voltage tries to keep current constant by using the energy stored in the electromagnetic field while it is collapsing.

decreasing

continuity

describes a situation in which there is a continuous or complete path for current to flow in a circuit - does not represent zero resistance

ammeter

device that measures electrical current

ohmmeter

device that measures the resistance between 2 points in a circuit - the rest leads are placed in parallel with the component to be measured

voltmeter

device that measures the voltage

electromagnets

devices that use this force to produce some type of output

two types of electrical currents

direct current alternating current

Occupational Safety & Health Administration (OSHA)

establishes & enforces all rules concerning the health & safety of workers

alternating current

flow alternately in one direction and then in the other

direct current

flow in only one direction

electricity

flow of electrons in a conductor

Delta-configured system

has each phase connected end-to-end to form a loop

parallel circuit

has more then one path for current to flow the same amount of voltage is available for each branches of the circuit

series circuit

has only one path for current to travel total voltage supply is divided among the loads

when current is (increasing / decreasing), the induced voltage slows the current change and the electromagnetic field builds.

increasing

info needed to SIZE a transformer

input voltage available output voltage available output current required, both in-rush & steady state

equipment grounding

involves connecting all metal parts of an electrical system that don't normally carry current to earth ground

Tagout

involves placing a tag on the power source that warns others not to restore power

voltage divider network

is a combination circuit causes the load is connected in parallel with one part of the circuit

5 output devices

lamps resistors buzzers solenoids motors

2 basic categories of switch operations

manually - activated by a person automatically - activated by a machine

ampacity

max current carrying capacity of a conductor

ways to determine if a transformer fails

measure input and output voltages check transformer with an ohmmeter

resistance

measure of a components ability to resist the flow of current in a circuit

power

measure of the energy consumed by a circuit

ground

negative side of power supply

bastard voltage

not a reliable source of voltage because the voltage drops drastically when a load is applied

ground fault

occurs when one of the phases becomes grounded - will cause power to shut down

2 components of a manual switch

operator - causes switch to activate contacts - will open/close the circuit when operator is activated

4 basic components of an electrical circuit

power supply output device input device conductor

Lockout

purpose of blocking the energy flow from a power sources to a piece of equipment & assuming that is remains blocked

pushbutton switch

pushing the operator causes the contacts to be opened or closed switches with a spring return are momentary swithes

electrical schematic diagram

represents the components in a circuit and how they're connected

National Electrical Manufacturer's Association (NEMA)

standards for a wide range of motor control equiptment

force

strength or energy as an attribute of physical action or movement

Kirchoff's Current Law (parallel)

the amount of current flowing from the power supply is equal to the current flowing back to the power supply.

electromagnetism

the creation of a magnetic field by the flow of electric current

Capacitor effect in DC circuits

the current in DC circuit is blocked cuz the capacitor charges to the level of source voltage and current can't flow through dielectric.

voltage drop

the difference in voltage referenced to ground between 2 terminals of the component.

frequency of the current

the higher the frequency, the lower the capacitive reactance = number of times per sec that charge is stored & released

capacitance

the measure of a device's ability to store electrical energy

inductance

the resistance to changes in current flow