Converting my 7740 from 12v to 9v

[zephyr1934]

17 hours ago, Alexland said:

Unsure if it's a coincidence but th 9v motor on this 7740 has now started to play up.

I was running it with 1 carriage and 4 wagons and it started running very slowly and then eventually stopped with just the lights working.

It's now intermittently going slugish (even with just the loco top on and lights disconnected) and sometimes stops but occasionally going fast before slowing and stopping again.

It's not the track as I don't have this problem running my other 9v motors.

Has the motor failed, is there any way to fix it and could it have been caused by the 12v lights and/or cable conversion?

Ugh! I would suspect that the motor has gone bad. Keep in mind that 4 wagons with 4 12v wheel sets each is a heavy load for a 9v motor. So you probably want to keep your run time down with this configuration. Even with two 9v motors it would still be a heavy load.

I know that I had problems with one superchief locomotive and 5 santa fe cars with a single 9v motor, and the 9v wheels have a lot less friction than the 12v wheels. You might want to convert to roller bearing wheels to GREATLY reduce your friction.

You can probably get a feel for pulling heavy trains by monitoring the temp of the motor over time. This would be a one time calibration for a given type of train, e.g., 4 cars, one loco and one motor. Run the train as you like it, then about every five min stop and feel the motor, when it starts getting warm to the touch is when you should give it a rest. Note how much time has passed since you started and this is probably about how long you want to run without a break to let the motor cool down. (disclaimer- while this is what I would do, I make no guarantees that it is actually safe for your motors, so proceed at your own risk)

At any rate, I have had brand new 9v motors burn out after two days of running at a show even though I would run at most 2 hr on 2 hr off.

 

In terms of motor repair, here's some good links

https://www.eurobricks.com/forum/index.php?/forums/topic/83667-fixing-a-9v-lego-train-motor-with-a-pf-lego-motor/

 

https://www.eurobricks.com/forum/index.php?/forums/topic/102241-repairing-worn-out-contacts-on-legacy-9v-train-motors/

 

1 hour ago, Toastie said:

Next: Thermistors do naturally age. As all 9V train motors are of historical value, so are the thermistors inside. The longer the motor was operated in his history, the less reliable the thermistor works. Aging means that the chemical substance responding to heat by increasing its electrical resistance has degraded over time. This degradation process is driven mostly by "time x current" going through it.

Interesting, thanks for the info.

[Alexland]

Thanks Thorsten and Zephyr,

So if the 12v lights are wired in parallel they should have no impact on the load on the 9v motor and this fault was probably triggered by pulling too many things with high friction 12v wheels as the 9v motor is less powerful. I have spare 9v wheels so it would probably make sense to swap them out if they put less load on the motors and enable them to last longer. I never run the motors for long probably around 5mins at a time and not at top speed as I don't like them derailing and getting the bricks all scratched up.

Would the thermister just cut-off the power completely when the motor is running too hot or would there be the gradual deline in performance that I have seen? It could well be temperature related as the motor can stop, not work for a bit then start up again fine for a while before gradually deterioriating again even when the motor isn't under much load. It's unclear if the thermister is doing it's job and protecting the motor or defective. It wouldn't be a problem if it only happened under excess load. It's been a couple of days since I last ran it so I might have another try tonight keeping the load to 3 wagons with 9v wheels to see how it runs. Maybe the problems I was seeing was because even under light loads it hadnt fully cooled down from the high loads...

Thanks again,

Alex

 

[Toastie]

21 minutes ago, Alexland said:

Maybe the problems I was seeing was because even under light loads it hadnt fully cooled down from the high loads...

Good afternoon Alex,

could all very well be:

• The overload protection is working fine and indeed the load was so high, that the motor got really hot and then you waited not enough time. Cooling down time may be well over 10s of minutes and even then the motor may still be warm and heat up to the trip point when using it again even with no or small load.
• The thermistor may behave erratically when going bad. It can slow down the motor by limiting the current going through - but not entirely - it could also just trip. This depends quite a bit on age and may be not well predictable, see document below for normal operation behavior.
• In principle, the thermistor should "trip" = cutting the power off. The moment its temperature is below the tripping point it should reapply power. With a certain hysteresis and not "so well" defined on/off temperatures that is. This explains why the motor, once its temperature is reaching the tripping point of the thermistor will turn off, a short period of time later turn on again but when then under any load right away trips again, as the temperature was just going down a bit during that time. And even turning on under low load again, it would readily trip again as well because the motor generates heat even with no load (but much less - but enough though to trip again). The idea of this overload protection type is to immediately turn off the power and then wait, see above .
• Vishay has a nice little document explaining how they work: https://www.vishay.com/docs/29008/ptcappli.pdf

So when the motor has really cooled down and you turn it on with no load: Does it run smoothly? Then you could gradually increase load (just the motor, run it for 10 min, let it cool down, then only the loco, same procedure, then apply one coach etc ...) and see what happens. That way you will find out whether the thermistor is faulty. Once the thermistor trips, wait for half an hour or so to do more experiments - or put it in the fridge wrapped in some kitchen paper (no joke - did that for my "experiments" before just getting them out and in the trash ...).

You can also refer to Philo's motor page and measure the current that the motor draws when under load - this way you know exactly what is going on. Provided you have a multimeter. Just measure the current going into the rail system.

Best
Thorsten

[zephyr1934]

1 hour ago, Alexland said:

I never run the motors for long probably around 5mins at a time and not at top speed as I don't like them derailing and getting the bricks all scratched up.

If the motor was able to pull the train without stopping I can't imagine much damage would have been done to the motor in 5 min. I did have one train stall at a show in such a way that the motor cooked itself (full power but not going anywhere for several minutes before I noticed) but I don't think you had that problem. So if you are having this problem after only running for 5 min I would think it is the thermistor rather than the load of the train.

[Alexland]

Thanks again both of you,

I tried the motor again tonight and under the load of the loco only (lights disconnected) it was working fine but as soon as I added 1 carriage it started going sluggish and stopping again within seconds.

I then cut off the tabs on the bottom and opened it up carefully on a table and to my suprise there wasn't a thermistor inbetween the grippers on the motor contacts. The motor had been in perfect cosmetic condition with perfect tabs so I don't see how that is possible unless it was omitted by Lego (or their subcontractor) themselves when it was manufactured? I put it back together and again it was acting slugishly and stopping under small loads.

I then took it apart again on the table and removed the wheels, gears and motor (looking for a thermistor that might have fallen inside) and found a small thin metal disc around 0.5cm diameter that didn't appear to be serving any puropose underneath the motor. I removed it and put the unit back together again. Then trying the unit again for the past 20 mins it has been perfect in either direction with 12v lights attached and pulling 1 carriage and 2 wagons at various speeds.

So it seems to be fixed but I have no idea why? How could the thermistor be missing? What was the small thin metal disc for?

Thanks,

Alex

Edited October 29, 2020 by Alexland

[Toastie]

56 minutes ago, Alexland said:

So it seems to be fixed but I have no idea why? How could the thermistor be missing? What was the small thin metal disc for

Photographs would help a lot.
Now, was this disc just "lying" there? Or did you accidentally remove any press-fit "conductors"? I kinda remember such a disc as well. People here may know better. I threw it out and actually soldered the wires going from the pickups to the motor to make sure. This was only because I could not figure out what it was for.

More research to be done - and I guess my memory needs to be propped up as well. This is all 10 years ago ...

The thing is though: It works!!! That is very good news!!!

I suspect that there are disc-type thermistors. Pressed in somewhere. Need to check. Just conspiracy theory here - but isn't that hip nowadays? Just kidding.

Best
Thorsten

Edit 1: So here we go: Hit one on Google: Disk type thermistors: https://media.digikey.com/pdf/Data Sheets/Panasonic Resistors Thermistors PDFs/ERT-D..therm.pdf

This is not what we are looking for, but I'll update here 

Edit 2: They seem to be all over the place:

Well, I guess the next step is how this disc was sitting between the power feed and the motor being able to interrupt current. Then we know. Provided you wnat to figure out.
Next step: None: Just be aware of this. Motor may burn out when operated like crazy. But then: Simply get a new one for a couple of bucks.
Better: Reduce friction, use two 9V motors ...

Mission accomplished. It is 5 days to election ... and I am very very nervous ...

 

 

 

Edited October 29, 2020 by Toastie

[Alexland]

Hi,

I have photos but the forum is restricting my upload to 4KB presumably because the picture at the top of the thread used up 94KB of my 100KB allowance.

Yes the disc looked like the one in you photo just much thinner with a pair of lines on one edge (a circle only has one edge...) maybe it was some kind of metal thermistor and moved when opening up the case?

I'm not planning on opening it up again and this will be the motor I use with my young kids making it very clear they shouldn't pull more than 3 carriages or 4 wagons at once. ;-)

I will leave the 12v light bricks and cables in the 7740 as it's a bit messy and sub optimal converting them to 9v parts.

Thanks for the help everyone,

Alex

Edited October 30, 2020 by Alexland

[zephyr1934]

5 hours ago, Alexland said:

I have photos but the forum is restricting my upload to 4KB presumably because the picture at the top of the thread used up 94KB of my 100KB allowance.

That 100k is for things like your profile photo, conversations, etc. You should host all of your photos for threads on Flickr, brickshelf, or similar. Please do share your photos of the surgery

[duifkelego]

On 10/21/2020 at 11:35 AM, Alexland said:

Also does anyone know the spec of the 12v wire to buy more new? It's impressive how well it has lasted over 40 years compared the the 9v wire which tends to go flaky.

 

On 10/21/2020 at 5:37 PM, Andy Glascott said:

I don't, but I use speaker wire on my 12v layout and it works fine. I've used several different brands/types over the years with no problems. 

Took a while to get an answer but I don't want to withhold it.

I've found some grey double wire at conrad. But I think the color is off by just  a little.

https://www.conrad.nl/p/l2185-gr-draad-2-x-014-mm-grijs-5-m-1398102

Although most of the grey wire holds out for very long I've already seen some decay, they get hard a brittle. Soi I've used the conrad wire to replace them.