ELECTRICAL 2 Flashcards
What is the formula for power in watts in terms of voltage and current? … Don’t forget to modify it for an AC current! … Can you modify it for a three-phase motor too?
P = V * I
P = V * I * pf
P = V * I * pf * the square root of 3
Direct current systems are more often used for what kind of application?
Low-voltage
What is a solenoid?
A wire wound spirally around an iron core to produce a magnetic field and that is used as an electromagnetic switch
What is the most basic form of power generation?
A single – phase alternator
What do transformers do?
They change the voltage of an AC circuit to a higher or lower value
A coil or winding with more turns will have a higher what?
Voltage
What are transformers called when they increase the voltage? And when they decrease it?
Step up and step down transformers
Transformers don’t waste a lot of energy but what they do waste turns into…?
Heat
Ventilate a transformer. Because if a large one overheats it could…
Explode
The primary winding on a transformer is used for _____ and the secondary winding is used for _____.
Input… Output
How do electric heaters work?
They are the same as heating coils and furnaces and hairdryers.
A length of stainless steel wire forms a coil. This wire is simply a resistance to the current, which generates heat.
Are electric heaters efficient?
They are super efficient, but unfortunately electric power plants are only about 30% efficient… So in a broad sense, no.
Three – way switches can be controlled from how many points?
Two
What are DC motors used for?
Small-scale applications and for elevators which need continuous and smooth acceleration
Larger motors are usually what kind of motor?
Three – phase induction motors
All motor should be protected against ____ _____, which shut off the power when any part of the motor or housing gets too hot?
Thermal relays
What does a capacitor do?
Improves the power factor in a circuit
In residential construction outlets should be no farther than __ feet apart.
12
What is a panelboard?
A set of fuses or circuit breakers that control the circuit loading in a building. A central distribution point for branch circuits for a building, floor, or part of the floor.
Many circuit that carries a motor should have the rated load multiplied by___ in order to determine the wire size.
And .25. Motors should run on no greater than 80% of the capacity of the circuit.
Which is the safest type of conduit and what is it made out of? … What is a cheaper alternative?
Conduit, made out of schedule 40 plumbing pipe
Cheaper alternative with thinner walls is intermediate metallic conduit
What is the thinnest of conduits?
Electrical metal tubing EMT
“This conduit is so thin we need an EMT!!”
Can you use “flex”? … Also called Greenfield. … Okay what is it actually called? And where can’t you install it?
Flexible metal conduit… Can’t be installed underground
What is the long name for BX cable?
Similar to “flex”, it is called interlocked armored cable. It is a prewrapped set of wires in an interlocking metal spiral armor.
In small residential construction or where you don’t need to use conduit, what can you get away with using?
Romex!!! Plastic-sheathed wire.
Don’t embed it in concrete obviously!!! And don’t use it in commercial garages.
How much of a percentage of voltage drop is allowed in lighting circuits? Answer good supporting motors?
You’re only allowed to drop 3% in a voltage in a lighting circuit.
5% for a motor circuit
What is a short circuit?
When 2 conductors that are next to each other lose so much insulation that a current flows directly between them. With little resistance a very high current may result, causing the wires to get very hot.
… Or any situation where the current is flowing where it shouldn’t.
What are fuses?
Shutoff (safety) devices. Little cartridges that get hot enough to melt (and shut off the circuit ) when the current flow is too high.
What are circuit breakers?
Devices that automatically disconnect a circuit when the current is excessive. Unlike a fuse they can be reset.
What is a typical item to ground wires to?
Steel cold water pipe in the plumbing system, which should provide a direct path into an under the ground
What is the service drop?
This is all of the electrical services arriving onto the site… Wires for the mainline, transformer, a meter and a disconnect search
Enlarge your buildings meters measure not only consumption but also ____ ______.
Peak demand … Because dramatic peaks require the utility company to build more power generating capacity to meet that peak, which then sits idle. This is an efficient and expensive.
What do you get from the utility company if you have a dramatically higher peak demand?
Demand surcharge
Minimum distance from transformer to meter
150 feet
