AC/DC difference
01-17-2019, 12:45 PM
#1
Gets Weekends Off
Thread Starter
Joined APC: Sep 2018
Position: E135 FO
Posts: 295
AC/DC difference
Hey folks, I'm preparing for an interview and I know this will be one of the questions for sure.
I'd like to hear different answers to It, because all I find is too technical so far, and I don't know how much I should extend on my answer.
Any volunteers?
I'd like to hear different answers to It, because all I find is too technical so far, and I don't know how much I should extend on my answer.
Any volunteers?
01-17-2019, 01:55 PM
#2
Gets Weekends Off
Joined APC: Oct 2008
Position: JAFO- First Observer
Posts: 997
From Wiki:
AC is easier to produce with the aircraft engines, that can act as AC generators. The engines have a rotating shaft that it is easily equipped with magnetic dipoles all around the shaft to produce electric current in an AC Generator.
Then, depending on the instrument, the AC current is either used directly or, by the use of converters, in DC form as not only it is easier to produce, it is also easier to convert. So electronic devices that uses 12, 5 or 3V may contain their own voltage converter (though this is less and less true with modern efficient Transformer-Rectifier Units / TRU’s (switch-mode converters). If DC conversion was as much efficient as AC conversion, we wouldn't see high-voltage lines. Last but not least, it's easier to switch, because the current is null twice per cycle. DC must be switched at full current, which is expensive and the wiring weighs alot more.
AC is easier to produce with the aircraft engines, that can act as AC generators. The engines have a rotating shaft that it is easily equipped with magnetic dipoles all around the shaft to produce electric current in an AC Generator.
Then, depending on the instrument, the AC current is either used directly or, by the use of converters, in DC form as not only it is easier to produce, it is also easier to convert. So electronic devices that uses 12, 5 or 3V may contain their own voltage converter (though this is less and less true with modern efficient Transformer-Rectifier Units / TRU’s (switch-mode converters). If DC conversion was as much efficient as AC conversion, we wouldn't see high-voltage lines. Last but not least, it's easier to switch, because the current is null twice per cycle. DC must be switched at full current, which is expensive and the wiring weighs alot more.
01-17-2019, 03:01 PM
#5
Disinterested Third Party
Joined APC: Jun 2012
Posts: 6,009
You are a pilot and don't know the difference between AC and DC electricity?
Think of electricity like a garden hose. Voltage is the difference in pressure between the source feeding the hose, and the place the hose goes or attaches; greater pressure at one end forces water through the hose: that's voltage. It's potential, or a differential between the electrical source and the other end.
The flow of the water is amperage. Low water flow, not much water used. High flow, a lot of water gets used. Same for electricity. The danger from electricity is tied to amperage, much like the difference between getting hit with a garden hose and a fire hose. High amperage hurts or kills. When you're having an electrical problem and are trying to conserve electricity, you're reducing your amperage draw, which is a bit like turning down the nozzle on the hose so not so much escapes. You've got less electrical sources available or a limited supply, so don't throw it all away: shut off non-essential electrical, per the QRH/checklist.
DC electricity flows one direction. AC electricity reverses direction; the number of times it reverses direction in a given time period (per second) is the frequency, so when you read about 115v 60 hz electricity, it's 115 volts AC (VAC), and reverses direction 60 times per second. It's a bit more complicated than that, but if you remember that, you'll have the idea.
All batteries produce DC electricity. All alternators and generators produce AC electricity.
To get DC electricity from a generator, it must be "rectified," which you can think of as being "filtered." When we say the electricity changes direction, what we're really saying is that it changes polarity: positive to negative.
You're familiar with a battery which has a positive terminal and a negative terminal. Nearly all manuals and texts refer to electricity flowing from positive to negative, though this isn't true; electricity flows from a negative charge (abundance of electrons) to a positive charge (abundance of protons, or shortage of electrons. again, oversimplified, but enough.
All generators produce AC electricity; they rely on spinning a magnet and a wire winding; the nature of doing this means that the electricity changes polarity during the rotation of the generator. A rectifier is used to make a DC generator; it has a one-way "filter" called a diode that admits electricity in only one direction; it can also use other means to prevent the electricity that's delivered from the generator from changing direction.
An AC generator delivers the natural product of the generator: AC electricity.
An inverter is used to convert DC electricity to AC electricity.
A rectifier converts AC to DC electricity
The speed at which a generator turns will determine the output frequency. Because engines turn generators and engine speed varies, many aircraft utilize a type of automatic transmission between the engine and generator, a "constant speed drive." It has it's own fluid. It has the ability to disconnect the generator if the generator has a problem and can't be controlled, often called a CSD Disconnect. These typically are a one-shot deal and must be disconnected above a certain speed, and can only be reconnected on the ground.
A generator uses a regulator that adjusts a small control frequency to change voltage output; the regulator is the voltage regulator and it adjusts field current: think of it as a small trickle that tells the bigger output flow what to do.
AC electricity is used primarily because it has less voltage drop over longer distances of wire, and can use smaller, lighter wires.
DC electricity suffers "line drop" or a loss of voltage, the longer and/or smaller the wire.
DC voltage is found in the battery and hot battery busses, which often function as emergency busses. Fire protection, for example, is typically DC, because it's got to be available when nothing else is: if the battery is the last thing available, one still has to be able to fight fire. Detection and warning is often AC, but it depends on the system.
Electricity is useless without a circuit; it requires a source of electricity, from which it comes, and to which it returns (a battery, for example, between the negative and positive terminals), as well as a means to carry the electricity (a wire, for example), and it needs a load (lightbulb, motor, etc). In some cases, such as heating element, the load is simply a "resistor," or something that resists the flow of electricity, and heats up; an incandescent lightbulb works the same way, giving off heat and light.
That's more than you need to explain, but enough for you to get the idea.
Explain that DC goes one direction, AC goes both. DC is in the battery, all generators produce AC, and can be rectified to be DC generators. Explain that DC can be turned into AC with an inverter. Both AC and DC are used in aircraft.
Electrical sources are the battery, engine driven generators, APU generator, and external power (GPU). An air driven generator is available on some aircraft to use slipstream airflow for emergency power (a "ram air turbine," or RAT). Additional electrical sources which change electricity, but don't produce it, are rectifiers (AC to DC) and inverters (DC to AC).
That should cover most of which you would be asked. If they ask why AC, it's largely because it allows the use of lighter wiring throughout the aircraft because it doesn't have the same drop over distance, and can use smaller wires.
Think of electricity like a garden hose. Voltage is the difference in pressure between the source feeding the hose, and the place the hose goes or attaches; greater pressure at one end forces water through the hose: that's voltage. It's potential, or a differential between the electrical source and the other end.
The flow of the water is amperage. Low water flow, not much water used. High flow, a lot of water gets used. Same for electricity. The danger from electricity is tied to amperage, much like the difference between getting hit with a garden hose and a fire hose. High amperage hurts or kills. When you're having an electrical problem and are trying to conserve electricity, you're reducing your amperage draw, which is a bit like turning down the nozzle on the hose so not so much escapes. You've got less electrical sources available or a limited supply, so don't throw it all away: shut off non-essential electrical, per the QRH/checklist.
DC electricity flows one direction. AC electricity reverses direction; the number of times it reverses direction in a given time period (per second) is the frequency, so when you read about 115v 60 hz electricity, it's 115 volts AC (VAC), and reverses direction 60 times per second. It's a bit more complicated than that, but if you remember that, you'll have the idea.
All batteries produce DC electricity. All alternators and generators produce AC electricity.
To get DC electricity from a generator, it must be "rectified," which you can think of as being "filtered." When we say the electricity changes direction, what we're really saying is that it changes polarity: positive to negative.
You're familiar with a battery which has a positive terminal and a negative terminal. Nearly all manuals and texts refer to electricity flowing from positive to negative, though this isn't true; electricity flows from a negative charge (abundance of electrons) to a positive charge (abundance of protons, or shortage of electrons. again, oversimplified, but enough.
All generators produce AC electricity; they rely on spinning a magnet and a wire winding; the nature of doing this means that the electricity changes polarity during the rotation of the generator. A rectifier is used to make a DC generator; it has a one-way "filter" called a diode that admits electricity in only one direction; it can also use other means to prevent the electricity that's delivered from the generator from changing direction.
An AC generator delivers the natural product of the generator: AC electricity.
An inverter is used to convert DC electricity to AC electricity.
A rectifier converts AC to DC electricity
The speed at which a generator turns will determine the output frequency. Because engines turn generators and engine speed varies, many aircraft utilize a type of automatic transmission between the engine and generator, a "constant speed drive." It has it's own fluid. It has the ability to disconnect the generator if the generator has a problem and can't be controlled, often called a CSD Disconnect. These typically are a one-shot deal and must be disconnected above a certain speed, and can only be reconnected on the ground.
A generator uses a regulator that adjusts a small control frequency to change voltage output; the regulator is the voltage regulator and it adjusts field current: think of it as a small trickle that tells the bigger output flow what to do.
AC electricity is used primarily because it has less voltage drop over longer distances of wire, and can use smaller, lighter wires.
DC electricity suffers "line drop" or a loss of voltage, the longer and/or smaller the wire.
DC voltage is found in the battery and hot battery busses, which often function as emergency busses. Fire protection, for example, is typically DC, because it's got to be available when nothing else is: if the battery is the last thing available, one still has to be able to fight fire. Detection and warning is often AC, but it depends on the system.
Electricity is useless without a circuit; it requires a source of electricity, from which it comes, and to which it returns (a battery, for example, between the negative and positive terminals), as well as a means to carry the electricity (a wire, for example), and it needs a load (lightbulb, motor, etc). In some cases, such as heating element, the load is simply a "resistor," or something that resists the flow of electricity, and heats up; an incandescent lightbulb works the same way, giving off heat and light.
That's more than you need to explain, but enough for you to get the idea.
Explain that DC goes one direction, AC goes both. DC is in the battery, all generators produce AC, and can be rectified to be DC generators. Explain that DC can be turned into AC with an inverter. Both AC and DC are used in aircraft.
Electrical sources are the battery, engine driven generators, APU generator, and external power (GPU). An air driven generator is available on some aircraft to use slipstream airflow for emergency power (a "ram air turbine," or RAT). Additional electrical sources which change electricity, but don't produce it, are rectifiers (AC to DC) and inverters (DC to AC).
That should cover most of which you would be asked. If they ask why AC, it's largely because it allows the use of lighter wiring throughout the aircraft because it doesn't have the same drop over distance, and can use smaller wires.
01-17-2019, 04:33 PM
#7
China Visa Applicant
Joined APC: Oct 2006
Position: Midfield downwind
Posts: 1,919
Originally Posted by Erj135dude
Did I say I don't know the difference? I just asked for help to phrase it correctly, that's all.
Thanks for the explanation.
Thanks for the explanation.
"AC lies, DC dies" is the phrase you're looking for.
01-17-2019, 05:39 PM
#8
Line Holder
Joined APC: Feb 2015
Posts: 44
Originally Posted by JohnBurke
You are a pilot and don't know the difference between AC and DC electricity?
Think of electricity like a garden hose. Voltage is the difference in pressure between the source feeding the hose, and the place the hose goes or attaches; greater pressure at one end forces water through the hose: that's voltage. It's potential, or a differential between the electrical source and the other end.
The flow of the water is amperage. Low water flow, not much water used. High flow, a lot of water gets used. Same for electricity. The danger from electricity is tied to amperage, much like the difference between getting hit with a garden hose and a fire hose. High amperage hurts or kills. When you're having an electrical problem and are trying to conserve electricity, you're reducing your amperage draw, which is a bit like turning down the nozzle on the hose so not so much escapes. You've got less electrical sources available or a limited supply, so don't throw it all away: shut off non-essential electrical, per the QRH/checklist.
DC electricity flows one direction. AC electricity reverses direction; the number of times it reverses direction in a given time period (per second) is the frequency, so when you read about 115v 60 hz electricity, it's 115 volts AC (VAC), and reverses direction 60 times per second. It's a bit more complicated than that, but if you remember that, you'll have the idea.
All batteries produce DC electricity. All alternators and generators produce AC electricity.
To get DC electricity from a generator, it must be "rectified," which you can think of as being "filtered." When we say the electricity changes direction, what we're really saying is that it changes polarity: positive to negative.
You're familiar with a battery which has a positive terminal and a negative terminal. Nearly all manuals and texts refer to electricity flowing from positive to negative, though this isn't true; electricity flows from a negative charge (abundance of electrons) to a positive charge (abundance of protons, or shortage of electrons. again, oversimplified, but enough.
All generators produce AC electricity; they rely on spinning a magnet and a wire winding; the nature of doing this means that the electricity changes polarity during the rotation of the generator. A rectifier is used to make a DC generator; it has a one-way "filter" called a diode that admits electricity in only one direction; it can also use other means to prevent the electricity that's delivered from the generator from changing direction.
An AC generator delivers the natural product of the generator: AC electricity.
An inverter is used to convert DC electricity to AC electricity.
A rectifier converts AC to DC electricity
The speed at which a generator turns will determine the output frequency. Because engines turn generators and engine speed varies, many aircraft utilize a type of automatic transmission between the engine and generator, a "constant speed drive." It has it's own fluid. It has the ability to disconnect the generator if the generator has a problem and can't be controlled, often called a CSD Disconnect. These typically are a one-shot deal and must be disconnected above a certain speed, and can only be reconnected on the ground.
A generator uses a regulator that adjusts a small control frequency to change voltage output; the regulator is the voltage regulator and it adjusts field current: think of it as a small trickle that tells the bigger output flow what to do.
AC electricity is used primarily because it has less voltage drop over longer distances of wire, and can use smaller, lighter wires.
DC electricity suffers "line drop" or a loss of voltage, the longer and/or smaller the wire.
DC voltage is found in the battery and hot battery busses, which often function as emergency busses. Fire protection, for example, is typically DC, because it's got to be available when nothing else is: if the battery is the last thing available, one still has to be able to fight fire. Detection and warning is often AC, but it depends on the system.
Electricity is useless without a circuit; it requires a source of electricity, from which it comes, and to which it returns (a battery, for example, between the negative and positive terminals), as well as a means to carry the electricity (a wire, for example), and it needs a load (lightbulb, motor, etc). In some cases, such as heating element, the load is simply a "resistor," or something that resists the flow of electricity, and heats up; an incandescent lightbulb works the same way, giving off heat and light.
That's more than you need to explain, but enough for you to get the idea.
Explain that DC goes one direction, AC goes both. DC is in the battery, all generators produce AC, and can be rectified to be DC generators. Explain that DC can be turned into AC with an inverter. Both AC and DC are used in aircraft.
Electrical sources are the battery, engine driven generators, APU generator, and external power (GPU). An air driven generator is available on some aircraft to use slipstream airflow for emergency power (a "ram air turbine," or RAT). Additional electrical sources which change electricity, but don't produce it, are rectifiers (AC to DC) and inverters (DC to AC).
That should cover most of which you would be asked. If they ask why AC, it's largely because it allows the use of lighter wiring throughout the aircraft because it doesn't have the same drop over distance, and can use smaller wires.
Think of electricity like a garden hose. Voltage is the difference in pressure between the source feeding the hose, and the place the hose goes or attaches; greater pressure at one end forces water through the hose: that's voltage. It's potential, or a differential between the electrical source and the other end.
The flow of the water is amperage. Low water flow, not much water used. High flow, a lot of water gets used. Same for electricity. The danger from electricity is tied to amperage, much like the difference between getting hit with a garden hose and a fire hose. High amperage hurts or kills. When you're having an electrical problem and are trying to conserve electricity, you're reducing your amperage draw, which is a bit like turning down the nozzle on the hose so not so much escapes. You've got less electrical sources available or a limited supply, so don't throw it all away: shut off non-essential electrical, per the QRH/checklist.
DC electricity flows one direction. AC electricity reverses direction; the number of times it reverses direction in a given time period (per second) is the frequency, so when you read about 115v 60 hz electricity, it's 115 volts AC (VAC), and reverses direction 60 times per second. It's a bit more complicated than that, but if you remember that, you'll have the idea.
All batteries produce DC electricity. All alternators and generators produce AC electricity.
To get DC electricity from a generator, it must be "rectified," which you can think of as being "filtered." When we say the electricity changes direction, what we're really saying is that it changes polarity: positive to negative.
You're familiar with a battery which has a positive terminal and a negative terminal. Nearly all manuals and texts refer to electricity flowing from positive to negative, though this isn't true; electricity flows from a negative charge (abundance of electrons) to a positive charge (abundance of protons, or shortage of electrons. again, oversimplified, but enough.
All generators produce AC electricity; they rely on spinning a magnet and a wire winding; the nature of doing this means that the electricity changes polarity during the rotation of the generator. A rectifier is used to make a DC generator; it has a one-way "filter" called a diode that admits electricity in only one direction; it can also use other means to prevent the electricity that's delivered from the generator from changing direction.
An AC generator delivers the natural product of the generator: AC electricity.
An inverter is used to convert DC electricity to AC electricity.
A rectifier converts AC to DC electricity
The speed at which a generator turns will determine the output frequency. Because engines turn generators and engine speed varies, many aircraft utilize a type of automatic transmission between the engine and generator, a "constant speed drive." It has it's own fluid. It has the ability to disconnect the generator if the generator has a problem and can't be controlled, often called a CSD Disconnect. These typically are a one-shot deal and must be disconnected above a certain speed, and can only be reconnected on the ground.
A generator uses a regulator that adjusts a small control frequency to change voltage output; the regulator is the voltage regulator and it adjusts field current: think of it as a small trickle that tells the bigger output flow what to do.
AC electricity is used primarily because it has less voltage drop over longer distances of wire, and can use smaller, lighter wires.
DC electricity suffers "line drop" or a loss of voltage, the longer and/or smaller the wire.
DC voltage is found in the battery and hot battery busses, which often function as emergency busses. Fire protection, for example, is typically DC, because it's got to be available when nothing else is: if the battery is the last thing available, one still has to be able to fight fire. Detection and warning is often AC, but it depends on the system.
Electricity is useless without a circuit; it requires a source of electricity, from which it comes, and to which it returns (a battery, for example, between the negative and positive terminals), as well as a means to carry the electricity (a wire, for example), and it needs a load (lightbulb, motor, etc). In some cases, such as heating element, the load is simply a "resistor," or something that resists the flow of electricity, and heats up; an incandescent lightbulb works the same way, giving off heat and light.
That's more than you need to explain, but enough for you to get the idea.
Explain that DC goes one direction, AC goes both. DC is in the battery, all generators produce AC, and can be rectified to be DC generators. Explain that DC can be turned into AC with an inverter. Both AC and DC are used in aircraft.
Electrical sources are the battery, engine driven generators, APU generator, and external power (GPU). An air driven generator is available on some aircraft to use slipstream airflow for emergency power (a "ram air turbine," or RAT). Additional electrical sources which change electricity, but don't produce it, are rectifiers (AC to DC) and inverters (DC to AC).
That should cover most of which you would be asked. If they ask why AC, it's largely because it allows the use of lighter wiring throughout the aircraft because it doesn't have the same drop over distance, and can use smaller wires.
01-17-2019, 06:28 PM
#9
Gets Weekends Off
Joined APC: Nov 2005
Position: Formerly Avantair
Posts: 197
01-17-2019, 08:05 PM
#10
Prime Minister/Moderator
Joined APC: Jan 2006
Position: Engines Turn Or People Swim
Posts: 39,261
If you can understand and articulate this, that should suffice for an interview...
Direct Current (one way flow) is used in aircraft where batteries are involved, since they only do DC.
Alternating Current (Sloshes back and forth) is what is naturally produced by alternators/generators, and allows for more power density in motors and generators (weight savings), and lighter wires. Higher frequency AC is smoother, and better for precision electronics (hence 400Hz on aircraft vice 60 Hz at home).
You can convert back and forth if needed.
AC => DC: Transformer/Rectifier
DC => AC: Inverter
A motor-generator can also couple AC and DC systems with power flow possible in either direction, with instant, seamless reversal available.
Turbine airplanes typically have equipment to both supply some AC systems from battery DC, and all DC systems from the normal AC power system (to avoid depleting, and charge, the batteries).
Higher voltage is more efficient, especially over longer wire runs, but as voltage increases, insulation weight increases and much above 200V you need to start spacing the wires apart to avoid inductive interference (or arcing).
Motorcycles might use 6V (short wire runs). Cars use 12V, modern airplanes use 28VDC. AC will probably be somewhere between 100V and 400V.
Direct Current (one way flow) is used in aircraft where batteries are involved, since they only do DC.
Alternating Current (Sloshes back and forth) is what is naturally produced by alternators/generators, and allows for more power density in motors and generators (weight savings), and lighter wires. Higher frequency AC is smoother, and better for precision electronics (hence 400Hz on aircraft vice 60 Hz at home).
You can convert back and forth if needed.
AC => DC: Transformer/Rectifier
DC => AC: Inverter
A motor-generator can also couple AC and DC systems with power flow possible in either direction, with instant, seamless reversal available.
Turbine airplanes typically have equipment to both supply some AC systems from battery DC, and all DC systems from the normal AC power system (to avoid depleting, and charge, the batteries).
Higher voltage is more efficient, especially over longer wire runs, but as voltage increases, insulation weight increases and much above 200V you need to start spacing the wires apart to avoid inductive interference (or arcing).
Motorcycles might use 6V (short wire runs). Cars use 12V, modern airplanes use 28VDC. AC will probably be somewhere between 100V and 400V.
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