Atlas/Tomix vacuum car problems

[peteski]

Unlike a locomotive which if stalls at full throttle will just spin its wheels, if something jams the impeller with full voltage applied, it will actually lock the rotor and will fry the motor's windings after few minutes.

Judging by the photo showing the traces the rectifier is in fact used to power the motor.

As for the breaker's trip current ratings they could be determined from some online searches.  But if you'll just use the working component from your board then there is no need to find out its specs.  I just looked up the official designation: These M&M-shaped things are called PTC circuit protectors or resettable fuses.

[peteski]

I asked AI and this looks to be reasonable answer:

The PTC resettable fuse marked 60 025 is a commonly used, radial-leaded Positive Temperature Coefficient (PTC) device designed for overcurrent protection. It is typically manufactured by companies like Bourns (e.g., MF-R025), Littelfuse (e.g., 60R025XU), or Fuzetec (e.g., FRX025-60F).

Here are the specifications for this component:
Marking Breakdown:
60: Represents the maximum operating voltage (60 Volts DC).
025: Represents the holding current (0.25 Amperes or 250mA).

[Maletrain]

Peteski beat me to the post.  I was also looking, but without "AI", to find the current limitation this devise imposes.  For N scale, 250 mA seems like a good limit.

I will put a new one in the Atlas unit before I give it back to the N scale DC guy at the club.

Thanks all for the help.  (It's what makes TRW great!)

[Maletrain]

Thinking about this DCC conversion a bit more, I am wondering if it is worth keeping a current limiting device in the circuit due to the locked rotor potential.

Where it is located now is in the way of the usual decoder install, but I could easily move it to a location beneath the board.

All of the install videos I have watched just eliminate it, and I am not reading about people burning out their DCC vacuum car motors.  So, is it worth keeping it?

Opinions?

[peteski]

Thinking about this DCC conversion a bit more, I am wondering if it is worth keeping a current limiting device in the circuit due to the locked rotor potential.

Where it is located now is in the way of the usual decoder install, but I could easily move it to a location beneath the board.

All of the install videos I have watched just eliminate it, and I am not reading about people burning out their DCC vacuum car motors.  So, is it worth keeping it?

Opinions?

With DCC I soul install it in the motor circuit (now it is in the track power circuit).  As for not keeping it - why not? It is a very cheap insurance. Like insurance - good to have, even if you will never need it.

[Maletrain]

Anything I should know about installing a PTC?  Can I trim the wires?  Does it need to be sitting in an open space such as the rather isolated open location in these vacuum cars?

[peteski]

Anything I should know about installing a PTC?  Can I trim the wires?  Does it need to be sitting in an open space such as the rather isolated open location in these vacuum cars?

Yes, you can trim the leads like the one which is installed on the Tomix board.  PTC device does have some resistance (IIRC around 1 ohm) in normal operating state, so it might get slightly warm while operating for prolonged time at the holding current (250mA). Since they are thermal devices they will get hot when they experience overload (in this case 500mA for several seconds) after which they create open circuit.  Just treat it like the designers treated the PTC they installed in that car and you should be safe.  You will be basically emulating what the Tomix designers did when using that PTC device.

[Maletrain]

OK, That is what I was suspecting, because the PTC in those vacuum cars is located well away from the other electronics, in what looks almost like a chimney.

I will try to leave it where it is, instead of replacing it with the decoder (like all of the decoder installation videos do for this car).

That means putting the decoder under the pcb board, after removing the cap and rectifier.  I think I can make the DZ126 fit there.

The only remaining issue is whether to put the PTC on the input or the output to the decoder. 

If on the decoder input, the PTC would shut off all current flow and reset after about 20 seconds, so the decoder would be shut down and then repowered and repeat the over-current/PTC-trip sequence.

If on the output, then the decoder remains powered and trying to pass current through the PTC, which would probably prevent the PTC from resetting once the motor rotor is unlocked.  But, to unlock the motor rotor, I would probably be taking the car off the track to get something out of the fan impeller, so the decoder would probably be powered off anyway to clear the blockage at the impeller.  So, it would be able to reset so long as it took me at least 20 seconds to clear the rotor.

So, I am thinking that the better place for the PTC is between the decoder and the motor - does that make sense?

I am also thinking that I want to turn BEMF off for this decoder application.  Does that make sense?

[peteski]

So, I am thinking that the better place for the PTC is between the decoder and the motor - does that make sense?

I am also thinking that I want to turn BEMF off for this decoder application.  Does that make sense?

That is what I already mentioned in reply #19.

Yeah, BEMF would not be useful in this example since the motor is under constant load from the impeller.

[mmagliaro]

Something I think you might want to check...
While those PTC device do sometimes go flakey and open below their rated current, you should make sure that's what the problem really is.

Run it on DC on the track, with a new Polyswitch in there (the PTC thing) and make sure it doesn't shut down again.  Don't just test it on the bench.  It is possible that the motor is behaving badly and actually is drawing over 250 mA.  And bear in mind, those PTC devices typically won't shut down until you get about 50% over their rated current.  That 250mA is the "hold" current.  At 2x the current (500 mA) it will typically trip immediately.  But up to 50% over (which would be 375 mA in this case), they can hold for a long time (even a minute).  So if the PTC is bad, that would explain a lot.  But if it's not, that motor is cooking.

I say this because I also own one of those track cleaning cars, and the motor in them is kind-a cheap.  A few times over the years I've owned it, when I would take it out to use it, it would need a kick in the pants to get it spinning, and it would draw a lot of current for the first minute or two after being stored for a while.  This smells of carbon accumulation or faulty brushes.   Therefore, I wouldn't be surprised if that motor is drawing way over its rated current and the PTC is saving it.

I typically pull mine with a couple of very old 1st gen Kato F3's, which were geared low.  On DC, you can get the track voltage high enough to get good vacuum suction without having the train flying like a slot car.

[Maletrain]

Thanks, Max!  You saved me from making a mistake.

I put the Atlas vacuum on the tracks with an RRampMeter inline, and saw that the track current was indeed 250MA at the 7.5 volt level where the PTC would barely not trip.  With that device jumpered out, the vacuum draws 310mA at 12 volts.  For comparison, the new Tomix vacuum that I am going to convert to DCC draws only 200mA at 12 volts.

This brings up the question about what to do with the Atlas version - and maybe also the Tomix version, once it has some hours of service and some years of non-service storage.

Should I just leave the jumper in-place on the Atlas version, so there is no locked rotor protection?  The next higher PTC trip value is 500mA.  I stalled the motor and the current went well above 1 amp the first couple of times I did it.  Later, it seemed to not go above 80mA when stalled, but still worked as before when not stalled.  Not sure what was happening there, so I quit stalling it.    Blocked rotor condition was only powered for a few seconds every time - just enough time for the RRampMeter to settle to a reading.  I was using a MRC Tech 7 DC power throttle, so maybe it was doing something unexpected in the later locked rotor tests?

Anyway, it seems that going to a 050 PTC would provide both full power running and locked rotor protection.  Does that sound like the best solution?

[peteski]

Max did bring up a good point.  As for the cause of higher current draw, I think most likely culprit is carbon dust (from the brushes) packed in the commutator gaps.  Since it is conductive it basically like additional resistor in parallel with the rotor winding.  I say "most likely" because other than that there are just enameled copper wires conducting electricity and those do not often short out.

If there was access to the commutator, the brush dust could be cleaned out, bringing the motor to optimal condition.

[Lemosteam]

After reading this I recalled a product discussion on the Tomix unit- ater reading there may be some things to consider from this thread:

https://www.therailwire.net/forum/index.php?topic=45039.msg582029#msg582029

[Maletrain]

Thanks, Lemosteam. I read the thread , now that you posted the link.

That thread has some interesting operational observations.
 
But, it mistakes the "yellow M&M thing" for an MOV, but it really is a PTC.

We do seem to be getting more clarity about the design and its performance.  So, I'll call that progress.

I'll take a look at the Atlas motor again later today, to see if I can find any reason for its greater current draw, compared to the Tomix motor.  But, visually, it looks almost new.  I think it was probably run on the DCC layout, with a voltage of about 14VDCC, and quickly failed, so was handed to the DC guy, who couldn't get it to run.  But, who knows - the plastic link between the "Off/On/CL" knob on the top and the electrical contacts below was broken, too.

As we discuss this circuit design, remember that there is also that "CL" switch position, which seems to have something to do with engaging or not engaging that electrolytic cap.  (I have not tried to trace that current path, mainly because I plan to never use it.)

[peteski]

As we discuss this circuit design, remember that there is also that "CL" switch position, which seems to have something to do with engaging or not engaging that electrolytic cap.  (I have not tried to trace that current path, mainly because I plan to never use it.)

Without knowing for certain the wiper arrangement on the rotating part of the switch and and where the contacts end up in each position I can't tell how the cap gets engaged into the circuit.  My guess is that in CL position it gets connected in parallel with the motor, but with such a small capacitance (4.7 µF) it will not make much difference in smoothie the rectified DC with the motor consuming relatively large current