negatively earthed
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From: france
negatively earthed
hello folks
is a correct statement that whenever the POH says that the electrical system is negatively earthed it means that the negative pole of the battery is grounded to the airframe - i guess a wire stretching from the post to the airframe -, so that as electricity is the flow of electrons from the negative to the positive post, electricity will travel from the negative post to the positive one, then to a bus bar from where it is made available to the component. the singular component is then grounded to the airframe to cmplete the circuit, allowing the electrons to use the chassis as a connection allowing the electrons to flow back to the negative post?
how does this apply to the DC generator - alternator with slip rings?
when diodes are used instead of slip rings, how do they revert the polarity turning it into DC?
many thanks
Baobab72
is a correct statement that whenever the POH says that the electrical system is negatively earthed it means that the negative pole of the battery is grounded to the airframe - i guess a wire stretching from the post to the airframe -, so that as electricity is the flow of electrons from the negative to the positive post, electricity will travel from the negative post to the positive one, then to a bus bar from where it is made available to the component. the singular component is then grounded to the airframe to cmplete the circuit, allowing the electrons to use the chassis as a connection allowing the electrons to flow back to the negative post?
how does this apply to the DC generator - alternator with slip rings?
when diodes are used instead of slip rings, how do they revert the polarity turning it into DC?
many thanks
Baobab72
Joined: Jul 2007
Posts: 186
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From: UK
When studying the molecular nature of electricity it is necessary to consider the true direction of electron flow negative to positive.
For all electrical applications the direction of flow can be considered in either direction, but in most diagrams we use conventional flow positive to negative. i.e bus bar, consumer, earth.
Current flow is not often regarded as important it is when it stops that is the problem and requires action.
When a current flow is induced using magnets and coils the generator will always be spinning around. The phase of this induced voltage will alternate between a negative and positive polarity each 360 degree revolution. 90 degrees being the peak positive voltage. Zero and 180 degrees being zero volts. 270 degrees being peak negative voltage.
When rectifying this produced wave the diode or bank of diodes will only allow the voltage to go one way and not the other like a non return valve. This produces a Direct one way current or rectified current.
Hope this gets you started.
B
For all electrical applications the direction of flow can be considered in either direction, but in most diagrams we use conventional flow positive to negative. i.e bus bar, consumer, earth.
Current flow is not often regarded as important it is when it stops that is the problem and requires action.
When a current flow is induced using magnets and coils the generator will always be spinning around. The phase of this induced voltage will alternate between a negative and positive polarity each 360 degree revolution. 90 degrees being the peak positive voltage. Zero and 180 degrees being zero volts. 270 degrees being peak negative voltage.
When rectifying this produced wave the diode or bank of diodes will only allow the voltage to go one way and not the other like a non return valve. This produces a Direct one way current or rectified current.
Hope this gets you started.
B
Thread Starter
Joined: Jul 2010
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From: france
negative grounded
Many thnaks for your reply
if i undertsand that correctly, the flow of electrons goes from the positive post to the bus bar from where it is distributed to the consumers and then to the negative "post" which, grounding the negative post to the airframe, will be the airframe itself, and then? does it go back to the power source? if the consumer is turned on, is the current utilized by the gizmo or does it still need to be negative grounded to allow the flow of electrons through them? what when the item is off - not energized?
and how does that relate to the alternator-generator? Do you still need to ground every item to initiate the flow of electrons regardless the consumer is turned on or off?
one last question: when diodes are used in lieu of slip rings, are they coupled to a capacitor to make up for the time when the polarity reverses itself?
many thanks
Baobab72
if i undertsand that correctly, the flow of electrons goes from the positive post to the bus bar from where it is distributed to the consumers and then to the negative "post" which, grounding the negative post to the airframe, will be the airframe itself, and then? does it go back to the power source? if the consumer is turned on, is the current utilized by the gizmo or does it still need to be negative grounded to allow the flow of electrons through them? what when the item is off - not energized?
and how does that relate to the alternator-generator? Do you still need to ground every item to initiate the flow of electrons regardless the consumer is turned on or off?
one last question: when diodes are used in lieu of slip rings, are they coupled to a capacitor to make up for the time when the polarity reverses itself?
many thanks
Baobab72
Joined: Apr 2008
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From: back of beyond
I think you still don't have it boabab.
When you turn on a device, electrons flow in this order:
- from the negative post of the battery to the airframe
- along the airframe to the earthing post on the device
- through the device making it work
- along the wire to the switch
- through the switch, breakers, etc
- back to the positive terminal of the battery.
When the switch is off, no electrons will flow (even though the switch is on the "wrong side" of the device) as the circuit is not complete.
As you can see it is much easier to think of "positive electricity" doing the same route in reverse, as Beeline suggested.
Regarding your last question, it's a bit confusing: slip rings are not used "in lieu" of diodes, rather diodes have to be used with slip rings in an alternator (a generator, which has a commutator, provides direct current). The diodes are connected in a full bridge arrangement so that when the current reverses itself, the connection is also reversed so that the alternator produces current of the right polarity throughout each revolution.
Even if a half bridge was used, a capacitor would not be required - the battery serves the same purpose of "filling in" those parts of the cycle when the alternator is not producing useful power. In fact, running an alternator without the battery connected can cause serious damage as very high peak voltages will be produced.
When you turn on a device, electrons flow in this order:
- from the negative post of the battery to the airframe
- along the airframe to the earthing post on the device
- through the device making it work
- along the wire to the switch
- through the switch, breakers, etc
- back to the positive terminal of the battery.
When the switch is off, no electrons will flow (even though the switch is on the "wrong side" of the device) as the circuit is not complete.
As you can see it is much easier to think of "positive electricity" doing the same route in reverse, as Beeline suggested.
Regarding your last question, it's a bit confusing: slip rings are not used "in lieu" of diodes, rather diodes have to be used with slip rings in an alternator (a generator, which has a commutator, provides direct current). The diodes are connected in a full bridge arrangement so that when the current reverses itself, the connection is also reversed so that the alternator produces current of the right polarity throughout each revolution.
Even if a half bridge was used, a capacitor would not be required - the battery serves the same purpose of "filling in" those parts of the cycle when the alternator is not producing useful power. In fact, running an alternator without the battery connected can cause serious damage as very high peak voltages will be produced.
Joined: Jun 2001
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From: Australia
An alternator doesn't have just one loop in the rotor. It has many. There is no need to fill in the gaps between voltage peaks, because there is a rotor peak in voltage every few degrees of rotation. (I don't know exactly how many rotors, or how many degrees of rotation.)
If you did want to "fill in the gaps", yes you would use resistors & capacitors. They were called R-C circuits when I went to school...
If you did want to "fill in the gaps", yes you would use resistors & capacitors. They were called R-C circuits when I went to school...
Thread Starter
Joined: Jul 2010
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From: france
electricity flow
many thanks for your explanation, they all were very clear with a special note for the post from fuzzy about the electron flow - i had been looking for such a clear answer for a long time.
would you be please so kind to indicate the flow pattern from the alternator? i guess the power supplied by the alternator goes to the bus bar through the switch to the gizmo to the grounding wire and then through the airframe where?
many thanks
baobab72
would you be please so kind to indicate the flow pattern from the alternator? i guess the power supplied by the alternator goes to the bus bar through the switch to the gizmo to the grounding wire and then through the airframe where?
many thanks
baobab72
Joined: Jul 2007
Posts: 186
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From: UK
Just to add DC commutator ripple can be reduced by a capacitor shunted across a say reduce interference on a radio receiver.
Very simply if the aircraft we are talking is large then the main alternator would be a generator providing an Alternating Current 115volts or 200volts (depending on wiring) at a constant frequency of 400Hz.
The provider of the D.C current is either the battery or the AC mentioned above converted to DC through a Transformer Rectifier Unit (TRU) this provides 28vdc to many of the control circuits amongst other devices.
The main feeder cables run into the main bus bars and through various relays to the consumers. These relays are usually pulled down by switches or solenoids etc by the control DC volts.
The Consumer of either the AC or DC supply will be earthed to a location on the airframe. They are usually grouped together in various locations in bars or terminal blocks, hopefully in a place where they do not corrode. Invariably they are not and do corrode and reap havoc lol..
Don't know if this helps but good luck.
Very simply if the aircraft we are talking is large then the main alternator would be a generator providing an Alternating Current 115volts or 200volts (depending on wiring) at a constant frequency of 400Hz.
The provider of the D.C current is either the battery or the AC mentioned above converted to DC through a Transformer Rectifier Unit (TRU) this provides 28vdc to many of the control circuits amongst other devices.
The main feeder cables run into the main bus bars and through various relays to the consumers. These relays are usually pulled down by switches or solenoids etc by the control DC volts.
The Consumer of either the AC or DC supply will be earthed to a location on the airframe. They are usually grouped together in various locations in bars or terminal blocks, hopefully in a place where they do not corrode. Invariably they are not and do corrode and reap havoc lol..
Don't know if this helps but good luck.
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From: Falling off the end of the thread
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From: UK
Don't worry about the actual current flow, (which is electrons going from negative to positive). Think of every electrical user as a simple circuit consisting of a battery, two wires and the electrical device: (conventional) current flows from the positive terminal of the battery, through the wire to the device, then from the device to the negative terminal of the battery completing a circuit. In actual fact, there will be switches and fuses or circuit breakers between the battery positive and the user, allowing control and protection of each user.
A negative earth system simply means that instead of there being a return wire from the device to the negative terminal of the battery, the aircraft (or car's) metal chassis is used as that return wire. This reduces weight and complexity. Each user therefore has a positive connection from a cable, but its negative connects to the chassis somewhere nearby. The negative terminal of the battery is also connected to the chassis, so the returning current from all users can return to the battery.
An alternator usually has 3 coils, and each coil produces a sinusoidally varying voltage: starting at zero, increasing to a max positive voltage, then reducing to zero, before increasing to a max negative voltage and back to zero again. This is one cycle. The cycles from all three coils drawn on a graph of voltage against rotational angle will overlap, 120 degrees apart. This is effectively 3 phase. However, each coil is fed through diodes which switch its output onto the positive or negative terminals of the alternator as its output changes polarity. The alternator must always be connected across a battery, so the result is a fixed DC voltage with a ripple voltage superimposed on it. (This ripple DC can sometimes be heard on the intercom or radio as a tone varying with engine speed)
Users such as motors and heaters are fine with ripple DC, but others, such as radios and navigation equipment require even smoother DC, so have smoothing circuits built inside them. These vary in complexity according to the equipment's requirments, but a capacitor connected across the positive and negative will have a smoothing function. It would need to be 250 microfarads or greater to have a useful effect. Resistors will not smooth ripple DC at all, but chokes, and more complicated voltage regulators will.
Hope this helps.
A negative earth system simply means that instead of there being a return wire from the device to the negative terminal of the battery, the aircraft (or car's) metal chassis is used as that return wire. This reduces weight and complexity. Each user therefore has a positive connection from a cable, but its negative connects to the chassis somewhere nearby. The negative terminal of the battery is also connected to the chassis, so the returning current from all users can return to the battery.
An alternator usually has 3 coils, and each coil produces a sinusoidally varying voltage: starting at zero, increasing to a max positive voltage, then reducing to zero, before increasing to a max negative voltage and back to zero again. This is one cycle. The cycles from all three coils drawn on a graph of voltage against rotational angle will overlap, 120 degrees apart. This is effectively 3 phase. However, each coil is fed through diodes which switch its output onto the positive or negative terminals of the alternator as its output changes polarity. The alternator must always be connected across a battery, so the result is a fixed DC voltage with a ripple voltage superimposed on it. (This ripple DC can sometimes be heard on the intercom or radio as a tone varying with engine speed)
Users such as motors and heaters are fine with ripple DC, but others, such as radios and navigation equipment require even smoother DC, so have smoothing circuits built inside them. These vary in complexity according to the equipment's requirments, but a capacitor connected across the positive and negative will have a smoothing function. It would need to be 250 microfarads or greater to have a useful effect. Resistors will not smooth ripple DC at all, but chokes, and more complicated voltage regulators will.
Hope this helps.
Last edited by Uplinker; 22nd May 2012 at 22:09.
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From: England
I agree. Best forget which way the electrons actually flow and just assume current allways flows from positive to negative. It's only really necessary to understand electron flow if you are actually designing transistors or some other kind of semiconductor device. Not usually even needed to design circuits that use them.
Generally negative earth system means that the -ve terminal of the power source (battery or DC supply) is connected to the chassis or earth terminal.
Most system use negative earth these days. Can't remember last time I saw a positive earth system. Probably was in a 1970's car?
If you come across a positive earth system be very careful. Things like connecting up an oscilloscope or other test equipment to diagnose a problem need greater care to avoid accidentally shorting out the power supply. For example it's not allways safe to connect the oscilloscope earth probe clip to a voltage rail marked "0V".
Generally negative earth system means that the -ve terminal of the power source (battery or DC supply) is connected to the chassis or earth terminal.
Most system use negative earth these days. Can't remember last time I saw a positive earth system. Probably was in a 1970's car?
If you come across a positive earth system be very careful. Things like connecting up an oscilloscope or other test equipment to diagnose a problem need greater care to avoid accidentally shorting out the power supply. For example it's not allways safe to connect the oscilloscope earth probe clip to a voltage rail marked "0V".
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From: London UK
Most electric railways use negative earth, with a straightforward power supply through an overhead wire or third rail, returning through the regular rails and the ground. However the London Underground, uniquely, uses two separate power rails, a positive which is outside the track and a negative which is in the centre. You can look for this the next time you are on the Tube. Furthermore the voltage is 630v DC. This is not all in the positive rail; it is +420v in the positive outside rail, and -210v in the negative middle rail, net potential difference is 630v. There are reasons for this fully insulated system, to do with the iron lining of tube tunnels, and the avoidance of corrosion by electrolysis due to ground currents.
Notably some lines of the Underground continue on the National rail system out in the open, who do not do it this way. They still provide the middle rail specially for the Underground trains, but it is at 0 earth potential, and just bonded to the ground, while the outside power rail is at +630v DC. The Underground train sees the potential difference exactly the same. Trains run across the transition between the two at the boundary without any issue. The National railway trains which share these tracks just run with their normal +630v in the outside rail, the current returning through the wheels and tracks/ground as elsewhere on the system not shared with the Underground.
Last edited by WHBM; 22nd May 2012 at 17:22.
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From: Seattle
WHBM's examples illustrate the difference between an earthed (grounded) system and one that uses an earth return.
Most electrical systems have one point tied to earth/ground/frame for the purpose of safety. This keeps system voltages within design limits with respect to surrounding structure to protect personnel, allow for fault detection equipment to operate, limit certain types of electromagnetic interference and prevent insulation breakdown. In some systems, the grounding connection is made through a resistance to limit fault currents. However, many of these systems provide a dedicated return conductor (neutral) to complete the circuit. So, under normal circumstances, no current returns through the earth or frame.
In some cases (automotive and aircraft as examples), the earth or frame is actually utilized as the return conductor. This is done to save weight and cost, but it has a few down sides. Corrosion due to electrolysis, the difficulty in detecting certain types of faults and the larger loop area of the circuit (resulting in higher inductance, voltage drop and EMI problems) being a few of them.
Most electrical systems have one point tied to earth/ground/frame for the purpose of safety. This keeps system voltages within design limits with respect to surrounding structure to protect personnel, allow for fault detection equipment to operate, limit certain types of electromagnetic interference and prevent insulation breakdown. In some systems, the grounding connection is made through a resistance to limit fault currents. However, many of these systems provide a dedicated return conductor (neutral) to complete the circuit. So, under normal circumstances, no current returns through the earth or frame.
In some cases (automotive and aircraft as examples), the earth or frame is actually utilized as the return conductor. This is done to save weight and cost, but it has a few down sides. Corrosion due to electrolysis, the difficulty in detecting certain types of faults and the larger loop area of the circuit (resulting in higher inductance, voltage drop and EMI problems) being a few of them.
