Electrical Flashcards
What are Atoms made up of?
Protons +
Neutrons (Neutral)
Electrons -
What holds electrons in orbit?
Force of attraction between opposite electric charges
Atoms contain equal numbers of _____ and _____
Protons and electrons (electrically neutral)
Free electrons forced to drift in the same general direction is called _____ ______
Electric Current
Elements that make available a large number of free-electrons allow relatively large currents to flow
Conductors
Elements that make available relatively few free-electrons allow only incredibly small currents to flow
Insulators
What three things does electric current produce?
Heat, magnetic, chemical
If molecule in a metal can be aligned so that their individual magnetic fields all act in the same direction what does it become
A magnet (ferrous metals)
What makes a good temporary magnet?
Iron, silicon steel (used in larger motors, relays, solenoids)
What makes a good permanent magnet?
Steel, steel alloys w/ nickel and cobalt (simple electric motors, energy meters)
Law of magnetism
Like poles repel, and unlike poles attract
How can you increase a magnetic field?
Iron core inserted into the conductor coil
What behaves like a strong bar magnet that can be turned on or off?
Electromagnetism
The path which an electric current takes is called?
A circuit
What is current flow measured in?
Amps
What is potential energy measured in?
Volts (energy source)
What is resistance measured in?
Ohms
This uses electrical energy to do useful work
Load (motor, lights, heaters etc.)
These function as a remote controlled switches within control circuits
Relay
Types of circuits
Series: Same current through each load
Parallel: Same potential difference (voltage) across each load (most common)
Series Parallel: Combination of both
At a given temperature, the current flowing through a conductor is directly proportional to the voltage across that conductor
Ohm’s Law (Increasing the voltage across a conductor increases the current flowing through it)
The resistance of a conductor is independent of _____ and ______
Current and Voltage
An objects electrical resistance depends on what?
The resistivity of the material it is made from
As the temperature of a conductor increases, the value of resistivity _____
increases
As the temperature of insulators increase, resistivity ______
decreases
Power is measured in _____
watts
Electric Current in one direction only
DC (battery)
Electric Current back and forth continuously
AC (generators)
Transformers cannot work with this
DC
These increase and decrease alternating voltages with very little energy loss
Transformers (distribute electricity on a large scale)
If a secondary winding has fewer turns than the primary winding
Step-down transformer
If the secondary winding has more turn than the primary winding
Step-up transformer
What do homes and small workshops use
Single-phase AC 240V and 120V supply
Larger building and factories use
Three-phase AC (three separate armature windings displaced by 120 degrees)
Three phase windings are connected in a ___
wye (or star)
Star point most common
Delta/Mesh also used
Advantages of three phase induction motors
Smaller, simpler
Three phase induction motors are also called
Squirrel cage motors
What does interchanging two of the three lines to the motor do
Change direction of rotation
What are circuit protection devices connected in series with a load
Fuses (element made of an alloy such as silver-tin)
The greater the value of a fault current, the faster the fuse operates. What is this called?
Inverse time characteristic
What can high-voltage fuses do?
extinguish arcs using sand-filled cylinders or spring loaded elements
These are switching devices that can interrupt fault currents
Circuit breakers
Name the two types of Circuit breakers
Oil circuit breakers
Air-blast circuit breakers
A heavy duty, electromagnetically controlled switch (contactor) which opens and closes the circuit to the motor
Motor controller
These monitor the load current
Temperature sensors (built into motor controller)
What electrical charges don’t move what is it called
Static electricity
The interactive relationship between electric charge and magnetic flux
Electromagnetism
The ease which a material can be magnetized by a solenoid is a measure of it’s ______
permeability
A material that is neither a good conductor nor a good insulator
Semiconductor (germanium and silicon most common)
This is similar to pressure in hydraulic
Voltage
This is the rate of flow of electrons (current flow)
Amps
1 horsepower =
746 watts
This is similar to a flow restriction in hydraulics
Resistance
If the area of a conductor is doubled, the resistance is ______
Halved
If the length of a conductor is doubled, the resistance ______
Doubles
Standard power line frequency
60 hertz
Math Formulas
E = I x R P = E x I
Series circuit rules
Total resistance (ohms) is equal sum of each individual resistance (Rt = R1 + R2 + R3). More loads added means resistance increases.
Current flow (Amps) is the same throughout
Parallel circuit rules
Total resistance (ohms) is always less than the smallest resistor in the circuit.
Total current flow (amps) is equal to the sum of the current flow through each load.
Total Resistance is Rt = 1/((1/R1)+(1/R2)+(1/R3)
Volts is the same throughout circuit
Which switch does not provide overload protection
Drum switch
List two manual motor starters
Toggle switch (max 1hp @230V) Push button starter (5hp at 230V)
These can safely interrupt the in-rush current or locked-rotor current of an electric motor
Horsepower rated contacts
Types of switches
SPST - Single Pole Single Throw SPDT - Single Pole Double Throw DPST - Double Pole Single Throw DPDT - Double Pole Double Throw Any more than 2 would be 3PST etc.
What guards against overloads motor starters use these protective devices
Overload relays (Low melting alloy and bimetallic)
This is often called a solder pot relay
Low melting alloy relay (normally closed)
How do you reset a Bimetallic strip relay
Allow it to cool, reset by turning handle to the off position
(Push button type push to the off position)
What do you need to prevent automatic startup after a power failure
Low voltage protection (manual starters do NOT provide this)
An overload relay has 2 basic parts
A heater element
A normally closed auxiliary contact
Who requires that each line of a three phase motor has OLR protection?
Canadian Electrical code (CEC)
Steps for resetting an OLR
Determine the cause of the overload
Wait for the OLR to cool down
Reset the OLR
3 types of rectification
Half wave (cuts bottom of AC wave) Full Wave (inverts bottom AC wave to top, output still spikes) Rectification with filters (Flattens out spikes, nearly flat wave)
When an area of lighting suddenly dims and stays that way it is called
Brown-out (could be plant wide low voltage)
A Typical two-wire plus ground circuit has the following colour coding
Hot wire is Black
Neutral is White
Ground wire is natural copper or green
How far should you be from power lines until you know the voltage?
22ft (7m)
Electric arc temps typically reach
3500 C
Switches over this horsepower are most vulnerable to arcing
50hp
Hot energized connections to a disconnect switch or circuit breaker are called
Line Lugs
These prevent electrical equipment from operating out of sequence
Electrical interlocks
Max AC volt range on a Multimeter
750 V
These are used to test insulation of motor windings
Megohmmeter (meggar) Don’t test low voltage circuits
This is the allowable current carrying capacity of a wire or cable assembly
Ampacity
Wire sizes smallest to biggest
40 gauge (smallest), 0000 (biggest)
The magnetic field around a magnet is called
magnetic flux
The number of cycles an alternating current generates per second is called _______
Frequency
This is similar to a hydraulic accumulator
Capacitance
A phase on the sine wave is measured in
degrees of rotation
