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.
Simply put, the DCC decoder can’t control trhe power to the motor if it’s getting power dirctly from the tracks…
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.
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?
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.
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.
Rich
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 scre
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.
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.
Ed
In my experience, there does not seem to be much, if any, misunderstanding between the terms “ground” and “common”. What gets confusing to many is the use of the terms “common” and “neutral”.
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
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.
–Randy
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
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.
CG
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.
On a DC layout, you’re controlling the track, not the engines. Power goes from the power pack to the track, and any engine on that section of track will respond to it. In DCC, you’re controlling each engine separately. There’s a constant 14V AC power on the track. Pickups connected to the engine’s wheels transfer that AC power to the decoder. The decoder converts the power to DC, and sends DC power to the DC motor in the engine. If you tell the decoder you want the engine to move forward at 50% full power, the decoder applies 50% of the maximum available DC power to the motor, with polarity adjusted so the engine moves forward.
None of that would work if you the motor was also getting 14V of AC power directly from the track. So the motor has to be isolated from the track, and since many older engines were designed for the motor to pick up power directly from the metal frame, the motor has to be insulated from the frame for that to happen.
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.
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
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 po
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.
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.
Ed