Does anyone have any input as to why Internal and External Cables on DC Traction Motors will overheat? Could this be the result of a locomotive at rest going directly to full power? Is it the result of aggressive dynamic braking? Are there any safeguards on the locomotive that cannot be overridden to prevent such extreme conditions? Any thoughts would be appreciated.
Randy Stahl, in particular, will have much to say on this subject.
I wouldn't think the connecting cables themselves would heat up before parts of the motor structure [for either transient or sustained overloading], unless there was some damage (e.g. corrosion) to the conductors inside or the connections. A number of overload conditions, and of course some varieties of short, can make a DC motor heat up, or draw excessive current that results in ohmic heating. But I'd expect windings in the rotor or stator of the motor itself, or the brushes and commutator surface, to be the critical areas where failure becomes evident -- not the external connection.
In the early to mid-'70s we could still find things like very ancient high-hood Alco switchers being used on PC, moving what could be long cuts of cars on bad track. Not unusual to hear bad snarling and snapping sounds from underneath, and big chunks of glowing something falling to the ties -- those could only have been pieces of brush. But they kept running them.
We had a bad batch of traction motor cables a few years ago. They were always blowing out. They've must have switched suppliers, or gotten better. It doesn't happen as much anymore for us.
When it did happen, it was mostly under heavy loads - pulling cars up and out of industries. Lots of amps + slow speeds.
The opinions expressed here represent my own and not those of my employer, any other railroad, company, or person.
I'm having trouble posting pictures on here to show what I'm referring to. The lugs themselves are not always damaged, but the cable stranding is overheated or even melted. I was wondering what conditions can cause extended amounts of overload to the traction motor circuit and are there any safeguards that can overridden to cause these overload conditions? FYI, the failures are only in the cables, not the lugs or the field coils.
High amperage for extended periods of time can also damage traction motor cables on AC-traction locomotives.
I have been trying to add some photos but no success. The failures I'm referring to are mainly related to the cable/stranding and not the lugs or the field coils. Almost as if a large surge or just extended overcurrent is taking place.
Overheated stranding on Internal Cable
Overheated External Cable
What type of locomotive?
My knee jerk diagnosis is that they are disregarding short time ratings.
Brian614 I have been trying to add some photos but no success. The failures I'm referring to are mainly related to the cable/stranding and not the lugs or the field coils. Almost as if a large surge or just extended overcurrent is taking place.
Instructions are right in the General Discussion (Model Railroader) forum.
Rich
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.
Brian614The lugs themselves are not always damaged, but the cable stranding is overheated or even melted.
Specifically where within the cable structure is the overheating occurring?
How is the join between the lugs/ferrules and the stranded conductor accomplished? Is the conductor end 'tinned' and then compressed so that power through the lug goes equally into all the strands? I'd be tempted to look at potential flux or RoHS issues if so.
Brian614Could this be the result of a locomotive at rest going directly to full power? Is it the result of aggressive dynamic braking?
Again, I'm looking to see what Randy thinks some of these causes might be.
Most modern locomotives restrict the rate at which power is applied to the traction motors, in some cases (notoriously GEs) through both the engine governor and the traction alternator/generator excitation, so the engine may not achieve full load until 30 seconds or more has passed.
In dynamic braking, I don't see how the connecting cable, which is designed NOT to be the major resistance in the configuration, would be the thing that heats to destruction even with failure of the DB circuitry to regulate field excitation etc. in the motors. I would suspect too 'aggressive' DB would cause wheelslide on an affected axle before toasting correctly made, correctly dimensioned cables.
Overheated Internal Cable
My nontechnical understanding of the situation is that the thermal protection circuits (which reduce horsepower when a traction motor is at risk of overheating) on DC-traction locomotives are generally adequate to prevent damage to the power cables. However the thermal limits on AC traction motors are so high that they can produce, for extended periods of time, levels of tractive effort that draw enough amperage to cause thermal damage to their power cables. In order to reduce that risk, there is a software-imposed limit on the magnitude of the rotational torque (which translates into tractive effort) that an AC traction motor can produce; and efforts can be made to prevent a given locomotive from remaining, for an extended period of time, in an assignment that will require it frequently to produce levels of tractive effort that approach the rotational torque limit.
The Locomotives are F40PH with AR10 Alternator and D78 Traction Motors.
Brian614 Overheated Internal Cable
I see visible corrosion on the cable strands going into the ferrule, and it does not appear to me that the strands were tinned at all before the crimping was done.
Can you supply a zoomed-in closeup of what appears to be the end of an internal cable in the shot of the external cable? It looks as if the conductor in that cable is not aligned correctly with the ferrule.
Can you explain what looks like electrical tape above and below the exposed section of the external cable, and why the strands appear to be so messed up and corroded? Did the insulation 'burn through' unobserved and the locomotive continue to operate in wet weather conditions?
Sometimes mechanical damage to the cable will damage or compromise stranded conductors with no clear visible evidence 'outside' on the insulation or jacket. This then leads to decreased effective cross section of copper, and perhaps increased arcing or sparking across the damaged section, both of which will likely cause cumulative additional damage inside, eventually manifesting in melted insulation.
The cable/strands are tinned from the factory. What you see in the internal cable picture is where the cable temperature got so hot, it melted the tinning which made it corrode. This corrosion then acted as an insulator making a higher resistance/higher heat in the cable assembly. I'm curious as to what actions would cause an increase in the current to the traction motors which would eventually lead to this type of failure.
The insulation you are referring to on the external cable was rubber tape covered with shrink tubing but due to the excessive heat, the insulation melted/burned up. The crimped lug was in mechanical tolerance but it is the highest resistance point in the circuit which failed first. The coils were not shorted and there was no other damage to the traction motor other than these burned up cables.
I thank all of you for your input!!
This is exactly correct. There is no electrical overloading issue. The issue would be in the clamoing of the cable to keep them immobile. The condition of the external traction motor cable is common and caused by either incorrect clamping and insulator boot application to the carbody or just old age. Same with the internal cables. Over time clamps will loosen and allow cables to move around too much breaking strands inside the cable.
Randy
Brian614I'm curious as to what actions would cause an increase in the current to the traction motors which would eventually lead to this type of failure.
It's the increased resistance across the point(s) of failure in the cable that's producing the difficulty. You'll want to measure this with a megger (Randy can explain why). Remember that a traction motor is a wattage load (P=EI) but the voltage drop across a high resistance (here, in the failing cable) makes the motor draw higher current to compensate -- current which passes through all parts of the circuit including the failure zone. If the resulting ohmic heating from the higher current (here P in watts developed across the resistor = I-squared R) causes yet further damage, you'll see the results of overheating, but only in the higher resistance zone. The motor does not see this heating; only the effect of the higher current through its resistances/impedance, which is why your motors aren't burning out too. Just for grins, what's the actual voltage measured at the motor when loaded, compared to developed (or nominal) voltage at the traction alternator?
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