passenger1955 Can someone please help me understand why the motor must be electrically isolated from both the frame and the track? I understand it is a requirement for DCC, but I had a friend ask me why this is the case and I didn't have a good technical explanation. Thanks.
Can someone please help me understand why the motor must be electrically isolated from both the frame and the track? I understand it is a requirement for DCC, but I had a friend ask me why this is the case and I didn't have a good technical explanation. Thanks.
The simple answer is there can be no other path for motor power to flow. It has to return to the decoder. If the motor is connected to the track, either by the frame or wheels, current has another path to follow and you will destroy the decoder. In DCC, one rail is Positive and the other is held to "zero", and this rapidly alternates, so you have a return path to if the motor isn't isolated.
Rail A is hot while Rail B is the return, then Rail B becomes hot and Rail A is the return to the booster. So if your decoder's output connects to a rail, the current can return to the booster anytime that rail is held to "zero", which is many times a second. Also, you have a lot of (uncontrolled) current being supplied to the decoder's output many times a second from the booster.
gregc So it would make sense to connect the ground, if there is one, to the motor frame when there is a plastic housing.
So it would make sense to connect the ground, if there is one, to the motor frame when there is a plastic housing.
I certainly think so in the case of power tools. And, since the motor frame is always metal, as are the bearings, chuck, and any installed bits, that would preclude using a "double insulated" tool that doesn't have a three-prong plug.
Of course, I use battery powered tools a LOT. Can't do much about them.
Ed
BMMECNYC I dont have an example off the top of my head, but riddle me this, what is the purpose of providing an electrical path around the UL listed transformer against the manufacturer's recommendations?
I dont have an example off the top of my head, but riddle me this, what is the purpose of providing an electrical path around the UL listed transformer against the manufacturer's recommendations?
I think you'd have to ask someone who wanted to do that. I don't, particularly.
In theory, the transformer that steps down the 120VAC to 0-22VAC or whatever you are using should be disigned to preclude the introduction of household current to the output of the transformer.
In theory, yes. Perhaps a short between the primary and the secondary?
Would running your vacuum over its own power cord and shredding the insulation, resulting in 120VAC contacting the ground wire in the vaccuum cord, running back to the outlet, through you because you happened to touch the track, while standing in a puddle of water which reaches to the household piping or some other path to ground that is shorter than the electrical box?
If, as you state, the 120VAC contacts the ground wire, the breaker will trip. I can see a possibility of two ground paths for the fault current, as you describe: one through the standard ground system and one, at least partially, through "me". Before the breaker trips, there will be current both through the ground system and through me. The extent of either would be deduced by comparing impedances. A properly installed ground system should have a very low impedance.
Fyi the GFCI you connect your drill to does not shut off the power to the wire you drilled into, please never forget that. There have been deaths that resulted from assuming that the tool was de-energized when in fact it was still energized by the 480 or 4160VAC (cant remember which it was) wire it was sticking into on the other side of a concrete wall (they were not using an electric drill stop).
As I said, I am very fond of grounded power tools on construction sites.
Please note that I am not advocating for grounding any part of the low voltage DCC system to building ground. I believe it extremely unlikely that the rail system would ever go live. Not to mention, you can only ground-for-safety ONE rail. What dya do if the OTHER ONE goes live?
I was attempting to correct some errors related to line voltage building wiring.
passenger1955I didn't have a good technical explanation.
Here it is...
Hello all,
Here's the long answer...
A Direct Current motor is like a water wheel.
Water is drawn or pushed over the wheel which turns the spindle in the center of the wheel to do work- -milling, running saws, turning a turbine, etc.
If the water cannot flow; either by cutting off the water supply or not allowing the water to exit, the device cannot turn.
Electrons are like water; they need a path from - to + to do work.
All modern locomotives are equipped with DC motors. There was a time that some systems used AC; Alternating Current motors, but that's a whole other can 'O worms!
Manufacturers use the locomotives wheels as pickups to move the electrons from the negative side of the track through the motor and then back to the positive side.
To achieve this, wires are used to transmit the electrons from the energized wheels to the poles on the motor; - or +.
In this scenario the motor is considered isolated.
Note: it is possible to use wipers; small strips of conductive metal, directly contacting the rails, rather than transmitting electrons through the wheels to the motor.
The wheels on the pickup trucks are isolated from one side (rail) to the other either physically or electronically.
This electron flow turns the motor which turns the gears to power the wheels and propel it down the track.
Some DC locomotive manufacturers chose to use the metal frame as a pathway for the electrons to flow rather than wires.
Wipers were fitted from one side of the pickup trucks to the metal frame, thus energizing the entire frame either - or +. One pole of the motor was in direct contact with the energized frame.
The other side of the pickup trucks were fitted with wipers that energized a strip of copper alloy, typically attached to the top of the gear tower, that directly connected to the opposite pole of the motor.
This last example is a non-isolated motor. One pole of the motor makes a direct, physical, connection to the energized frame.
In a DCC locomotive a DC motor is still used.
The difference; in an EXTREEMLY simplified explanation, is that with a DCC system the power and command packets (that control the output functions of the decoder) are combined and move through one side of the track simultaneously.
The other rail provides the outgoing pathway necessary for electron flow, along with packet termination at the command station.
The decoder picks up the combined power/signal and splits it into motor power and function power; lights, sound, etc, through the decoder itself.
Because the motor receives it's power; - & +, from the decoder, not the track; as in DC, it must be isolated from the power/signal from the track.
If the motor is not isolated from the power/signal from the track the power will feed-back through the motor leads and toast the decoder, thus releasing the "magic smoke" that makes electronic components function.
"Uhh...I didn’t know it was 'impossible' I just made it work...sorry"
7j43kThis is true if you have one BB locomotive. If you have TWO BB locomotives and you MU them, you have a 50% chance of shorting the rails through the couplers and coupler boxes IF there is not an insulator intervening. Perhaps BB frames have been modified to forestall this problem. Ed
Ed,
The new blue box or "RTR" this appears to be the case. The ones I have are equipped with plastic coulper boxes, but it would not hurt to check for continuity between your coupler and wheels before testing this statement, as I have only three RTR locomotives (albeit they are different models and have widely separated dates of assembly).
Right, the Zephyr manuals don't mention connecting tot he house ground. But they walked it all back in the new DCS240 manual. Quote:
4.5 Ground (GR) Terminal
The terminal marked ground provides electrical safety features and an RF ground reference for minimum radiated noise. This should be the ONLY point of any DCS240 installation that is connected to the AC safety ground pin provided on most 3 pin 110V AC power sockets. Grounding the system is a sensible safety precaution that should not be ignored.
There is an extensive discussion on the 'grounding' and common failure modes in an article that members put together on the Digitrax yahoo group, which is pointless to link here because if you aren't a member you can't access the files section. This has been discussed ad naseum on the Digitrax forum, in fact it seems to come up every few months, and you get a whole lot of misinformation and finally this document gets pointed to - a document that they reviewed with AJ at one of the NMRA shows and received his blessing for on exactly how and what to wire to 'ground'. The updated manuals for the DCS100 and DB150, and obviously the Zephyr Xtra, all walked back on the idea of connecting that terminal to the house ground. Now in the new DCS240 manual they once again are saying to connect it to house ground. That in fact is what spurred the latest 100+ message argument over how to hook things up.
You want an example? The rail boltage on a Digitrax booster is realtive to that ground terminal - it is really the common for the output H bridge. In fact one of the recommended ways to measure track voltage on a Digitrax system is to measure DC between Rail A and 'ground' and Rail B and 'ground'. Each should be (assuming nothing's failed, and no one has address 00 selected with a speed which will introduce a DC offset) equal, and added together is the total track voltage. It's no so much a failure mode as it is making the whole railroad possible at or close to house ground so if you happen to touch a live wire (maybe for the lighting system) and the rail at the same time - bad news. Without the house ground connected to the booster, the rails are floating - like using a battery powered meter to probe a live circuit, or better yet, a handheld battery power oscilloscope - a AC mains powered oscilloscope has the ground side of the probe directly connected to the ac house ground, and you can easily blow up a scope probing the wrong thing. Much harder to blow up an isolated batter powered scope.
Anyway, this has drifeted far afield of the OP's topic of loco chassis wiring, all because I made a comment that 'ground' and 'common' are not interchangeable but ARE so used commonly in the US. No, any sort of EE or electrical professional would not confuse the two, but the term 'earth' is not commonly used in the US, It's VERY common to see someone write "the motor is grounded to the frame" or "cars used to be positive ground, now they are almost all negative ground" - how can it be ground when 4 rubber insulators isolate the car chassis from the earth? There's a reason an electronic schematic will often use a different symbol to mean a true earth ground, and the low side common is not referred to as 'ground'. I stand by my statement that this is less likely to be confused in Commonwealth countries where the term "earth" is used to be a true earth ground. A common return run between various pieces of equipment is never referred to as "earth" unless it actually IS connected to earth. Just some of the sloppiness that has crept into US English - not that any other variation of the English language is immune to such things.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
7j43kNo, it does not. If the neutral connection is broken, whether or not there is a grounding system, the related equipment will simply fail to work. If it was properly installed. There is no particular safety hazard for operating personnel.
i'm not suggesting that anyone should interchangably use the neutral or ground connection. The motor of a drill, for example, should be connected to the live and neutral connection, not the ground. The ground is for protection and should be connected to the case or the motor frame.
7j43kHowever, should you have a new-fashioned drill that's got a lot of plastic and insulation and IS NOT grounded, the bit and the chuck, at a minimum, will become live and stay live.
greg - Philadelphia & Reading / Reading
7j43k I was hoping for a bit more specificity in answer to my first request. Such as "an example". Then I was hoping for a response to my next two requests. Ed
I was hoping for a bit more specificity in answer to my first request. Such as "an example". Then I was hoping for a response to my next two requests.
The scenario would probably involve faulty house wiring, or where the third prong that your layout is connected to ends up being the least path of resistance.
But we are now kind of way off topic.
I dont know if the Op has gotten his answer yet, but:
decoder outputs are not designed to receive track power from the dcc system. The electronic circuitry on the board will literally get fried (I have some fried decoder pictures that I could dig up if you would like).
gregc the ground connection on a 3 prong plus is a 2nd path to gound that is often connected to metal enclosures. It provides protection should somehow the neutral connection within the enclosure is broken.
the ground connection on a 3 prong plus is a 2nd path to gound that is often connected to metal enclosures. It provides protection should somehow the neutral connection within the enclosure is broken.
No, it does not. If the neutral connection is broken, whether or not there is a grounding system, the related equipment will simply fail to work. If it was properly installed. There is no particular safety hazard for operating personnel.
The point of a grounding system is to connect all normally non-energized metallic parts of an electrical system to ground so that, should one of those parts become energized, there will be a return path to ground WHICH WILL TRIP THE BREAKER/FUSE and disconnect the power. If there is no grounding system, that metallic part will become energized and a person would likely get a shock IF that person were grounded.
You make an interesting point about metallic tools on construction sites. Let's suppose you're using a drill and drill into a live wire. IF you have one of those old fashioned metallic tools and it's grounded, the breaker will trip.
However, should you have a new-fashioned drill that's got a lot of plastic and insulation and IS NOT grounded, the bit and the chuck, at a minimum, will become live and stay live. You may not even know it's live if the wire you hit is "in there". So then what happens if you touch the bit and you ARE grounded? Sadness and/or pain, I would say.
This is why all my 120 V. power tools on a construction site are three prong.
Of course, GFCI's are also there to do their job. But I am kind of a fan of redundant safety measures. Up to the point of their becoming irritating.
7j43k rrinker ...connecting the low voltage side of the system directly to earth ground means in the event of a wiring fault, you could be putting mains potential on the rails. I'm not following this. Please give an example of such a wiring fault. And please explain how that would put "mains potential on the rails" (120V). And please explain how NOT grounding the low voltage side would preclude putting "mains potential on the rails". Thanks, Ed
rrinker ...connecting the low voltage side of the system directly to earth ground means in the event of a wiring fault, you could be putting mains potential on the rails.
...connecting the low voltage side of the system directly to earth ground means in the event of a wiring fault, you could be putting mains potential on the rails.
I'm not following this. Please give an example of such a wiring fault. And please explain how that would put "mains potential on the rails" (120V). And please explain how NOT grounding the low voltage side would preclude putting "mains potential on the rails".
Thanks,
I would venture to guess any other device connected to 120VAC in your home with an electrical fault that results in 120VAC in places other than where it should be.
Hope this helps.
gregc CentralGulf Digitrax Most important is to be sure the 110V input voltage is safely isolated from the low voltage secondary of the transformer. A properly installed safety ground will ensure that the layout cannot become a shock hazard if there is mis-wiring or insulation failure. Since us modelers often have exposed electronics, this suggests what the green ground wire on a 3 prong plug cable can be connected to to protect us from loose live wires. It suggests keeping the 110V side of any wiring hidden behind a plastic box or metal enclosure with the ground connected to the enclosure. It also suggests that we make sure any 110V outlets are properly wired - both the neutral and ground wires back to the circuit breaker box.
CentralGulf Digitrax Most important is to be sure the 110V input voltage is safely isolated from the low voltage secondary of the transformer. A properly installed safety ground will ensure that the layout cannot become a shock hazard if there is mis-wiring or insulation failure.
Since us modelers often have exposed electronics, this suggests what the green ground wire on a 3 prong plug cable can be connected to to protect us from loose live wires. It suggests keeping the 110V side of any wiring hidden behind a plastic box or metal enclosure with the ground connected to the enclosure.
It also suggests that we make sure any 110V outlets are properly wired - both the neutral and ground wires back to the circuit breaker box.
Exactly. They are calling for properly grounded AC power to the layout, not for grounded "ground" terminals on their command stations.
CG
at least in my house, the bus bar in the circuit box is literally connected to ground through a wire connected to a water pipe. As the figure below shows, the bare copper wire or neutral is connected to the bus bar, as well as the neutral spade. Only one terminal on a 3 prong plug (below) is live and is the black wire connected through a circuit breaker to the power lines from the street.
the ground connection on a 3 prong plus is a 2nd path to gound that is often connected to metal enclosures. It provides protection should somehow the neutral connection within the enclosure is broken. Should this happen, any live connection within the enclosure touching the case would have a direct path to ground instead of thru a person. This was probably more important when portable power tools were made of metal and dropped on construction sites.
CentralGulfMost important is to be sure the 110V input voltage is safely isolated from the low voltage secondary of the transformer. A properly installed safety ground will ensure that the layout cannot become a shock hazard if there is mis-wiring or insulation failure.
Why does the motor have to be isolated?Short answer: Because otherwise there is a good chance you will burn out the decoder.Long answer: See short answer.
Disclaimer: This post may contain humor, sarcasm, and/or flatulence.
Michael Mornard
Bringing the North Woods to South Dakota!
I do not find any instructions on grounding the "ground" terminal in my DCS51's instructions.
On the Digitrax web site, there is this general advice, which seems to be applicable to all command stations.
Most important is to be sure the 110V input voltage is safely isolated from the low voltage secondary of the transformer. A properly installed safety ground will ensure that the layout cannot become a shock hazard if there is mis-wiring or insulation failure.
That statement is ambiguous, I can't tell for certain if they are refering to a properly grounded AC receptacle, or grounding the "ground" terminal on the command station, but I believe it is the former.
One only has to look on the front (or back, depending on the model) of any Digitrax command station/booster to see a misuse of the term. The terminal is labaled GROUND. It's NOT. It's the circuit common - all downstream boosters and a few other items all need to be interconnected via a wire run between this terminal on each device (explained int he manual) - this terminal should NOT be connected to earth ground (not explained in the manual - in fact it tells you do do the opposite). Very wrong. The input is isolated, connecting the low voltage side of the system directly to earth ground means in the event of a wiring fault, you could be putting mains potential on the rails. Connecting the "ground" terminal to earth ground via a 10 meg resistor IS recommended as an anti-static measure - there's a reason anti static wrist straps and mats don't connect directly to an earth ground but instead use a resistor - connecting directly to ground would be like you stnading barefoot on a concrete floor - you now ARE the ground and accidently touching a hot lead would be very dangerous. With 10 Mohm plus body resistance, you're reasonably safe as the current will be less than a microamp. Just the 10 meg resistor alone limits 120V to 1.2 microamps.
7j43k The "US" did not mix up the terms "ground" and "common". "Americans" may have. The National Electrical Code clearly describes and names the elements of 240/120 Volt distribution systems.
The "US" did not mix up the terms "ground" and "common". "Americans" may have. The National Electrical Code clearly describes and names the elements of 240/120 Volt distribution systems.
Household wiring is based on AC power. Power is provided through the "hot" wire (usually the black or red coated copper wire), and the "neutral" wire (always white or gray) completes the circuit back to the service box which is "ground" to the earth. The "neutral" wire is often referred to as "common" which is not necessarily wrong, but electrical codes always refer to the return wire as "neutral".
The neutral wires connect to a common metal bar in the service box whereas the hot wires are connected to individual circuit breakers inside the service box. Incidentally, neutral wires are never "interrupted" by switches within a circuit. Neutral wires are only connected to outlets or light fixtures, but their path back to the metal bar in the service panel are never interrupted by wall switches.
In electrical systems in which wiring is enclosed inside of metal conduit, it is the metal conduit that provides the "ground" path back to the service panel. In electrical systems in which wiring is simply contained inside of plastic cables instead of metal conduit, a "ground" wire (which is always green) from each outlet or light fixture must be connected to a metal box which is grounded or else the ground wire from the outlet or light fixture must be connected to the green wire from the plastic cable to provide a continuous, uninterrupted path back to the service box.
Rich
Alton Junction
rrinker Just what is "ground" in a DCC system varioes from manufacturer to manufacturer. Some system do indeed alternate around a common point (in other countries outside the US they are a bit more clear on this - but 'ground' and 'common; are NOT the same thing, necessarily. The US mixed up useage of these terms interchangably is wrong and potentially dangerous. Ground should ONLY mean earth ground, like in your mains electrical system, and common is a circuit common which might be connected to earth ground but may be something that is and should remain completely iusolated)
Just what is "ground" in a DCC system varioes from manufacturer to manufacturer. Some system do indeed alternate around a common point (in other countries outside the US they are a bit more clear on this - but 'ground' and 'common; are NOT the same thing, necessarily. The US mixed up useage of these terms interchangably is wrong and potentially dangerous. Ground should ONLY mean earth ground, like in your mains electrical system, and common is a circuit common which might be connected to earth ground but may be something that is and should remain completely iusolated)
The "US" did not mix up the terms "ground" and "common". "Americans" may have. The National Electrical Code clearly describes and names the elements of 240/120 Volt distribution systems. Unfortunately, it is quite easy to misinterpret the useages and the terms. And that happens too frequently. Professionals who work in the field have no business making such errors.
In localities that have a 240 Volt system, misinterpretation becomes less likely. And a 240 Volt shock becomes more likely.
It is also ok if the wheels of one side power the loco frame - most Athearn BB locos are like this. As long as the motor brushes are isolated from the frame, there will be no problem if the frame is actually live with track power.
This is true if you have one BB locomotive. If you have TWO BB locomotives and you MU them, you have a 50% chance of shorting the rails through the couplers and coupler boxes IF there is not an insulator intervening. Perhaps BB frames have been modified to forestall this problem.
Just what is "ground" in a DCC system varioes from manufacturer to manufacturer. Some system do indeed alternate around a common point (in other countries outside the US they are a bit more clear on this - but 'ground' and 'common; are NOT the same thing, necessarily. The US mixed up useage of these terms interchangably is wrong and potentially dangerous. Ground should ONLY mean earth ground, like in your mains electrical system, and common is a circuit common which might be connected to earth ground but may be something that is and should remain completely iusolated), and others will drive both rails above the common point. The end result is the same thing to the rails, but this can lead to issues connecting two brands of boosters together - which is why there are boosters with optically isolated inputs, so that these different potentials do not mix and fry the input circuitry of one of the boosters.
As for loco isolation, a good example is the Proto 2000 Alco S1 switcher. Out of the box, the motor is isolated from the rails, but one brush IS connected tot he frame. BTW, in a permag motor, it's actually the BRUSHES that need to be isolated, not necessaily the motor frame. If the end cap holding the brushes is plastic, it makes no difference if the motor frame is connected to the frame as long as neither brush is wired to the motor frame. Anyway, this loco is just fine,a nd will work with a decoder plugged in to the procided 8 pin connector - right up until you derail at a turnout or other trackwork than can push the truck fully as far as it can turn - which is until a wheel touches the frame. Now you have track power going through the wheel into the frame into the motor side of the decoder - and for a brief moment of time, your loco will smoke like a genuine Alco. The fix is fairly simple, you need to run a new orange wire from the lower motor brush right to the decoder. There is a small tab on the brush which rubs against the frame, and the stock orange wire screws to the frame and goes to the decoder. The tab can be clipped off the bottom brush cap, and a new orange wire soldered on. A layer of Kapton tape will prevent any electrical contact in the bottom of the frame.
It is also ok if the wheels of one side power the loco frame - most Athearn BB locos are like this. As long as the motor brushes are isolated from the frame, there will be no problem if the frame is actually live with track power. In the case of a derailment with this sort of loco, either it will push a wheel into the frame that is already the same polarity as the frame, or it will push a wheel from the other side into the frame which will cause a dead short across the rails and trip the breaker.
Any voltage, DCC, a form of AC or a DC voltage feeding the decoder ouput will fry the decoder output circuit. The slightest connection will do it.
In all the forums I belong to, that has happened to some.
Also, there is no ground in DC. There is plus, minus, common. Minus is usually called common.
In decoders, DCC, a form of AC into the decoder and plus, minus voltage to operate the decoder with plus as the common.
A little Google searching will show all this very clearly.
Below is a link I found some years ago. Store it and do lots of reading.
http://www.members.optusnet.com.au/nswmn2/DCC.htm
Technically, ground is the cold water pipe in your house which connects to all the outlets.
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
in DCC, the rails are alternating polarity. This doesn't mean that one is at ground and the other is alternately connected to a positive or negative voltage. It means that when the one rail is connected to the positive supply the other is connected to the negative (or ground) and visa versa, when the one rail is connected to the negative, the other is connected to the positive supply. I believe an H-bridge circuit is used to do this.
I hope you can see that connecting one side of the motor to either rail (there isn't a common rail), that it would be complicated to provide a voltage on the other motor terminal such that there is a constant DC voltage across the motor.
The simpler solution is rectify the rail voltage to provide a constant DC voltage to the decoder. The decoder also uses an H-bridge to control the polarity of the voltage to the motor using the rectified DC so that it can drive the motor forward and in reverse.
Common rail wiring is discouraged with DCC.
http://www.dccwiki.com/No_Common_Rail_Wiring
And just so I get it, why is it a problem if the ground is connected to the common rail (and the positive lead is electrically isolated)? Is this because the user might flip the direction button reversing polarity? Or is there a problem beyond that?
Technically, the motor really only has to be isolated from the track. It doesn't have to be isolated from the frame if the frame is isolated from the track; however, if it is not and you have a derailment where the frame comes in contact with the track, then the motor will no longer be isolated from the track and you will likely smoke the decoder.
Simply put, the DCC decoder can't control trhe power to the motor if it's getting power dirctly from the tracks.....