[WIP] Super Fast Speedcar - current top speed: 32.4km/h

[jam8280]

51 minutes ago, TechnicSummse said:

You are right... the battery-light should turn off then.

Hmm.. but thats really strange then... maybe your motor is used to much, and the thermistor is kind of damaged? Because as wrote earlier, even with 2:1 geared claas wheels i couldnt trigger the motors protection. And this is really hard work to push ~900g with 2:1 upgeared claas wheel. Did you try another motor?

At wich temperatures did you do your runs? 30°C and more?

By the way... the protection should recover really fast (~10-15 seconds max)

Some kind of malfunction is a possibility, as I bought all 5 of my Buggy motors used, with no idea of their prior histories. When I overheat any of them to the point of shutdown, though, full recovery takes a lot longer than 15 sec, even at room temp or in the fridge. (Not a word to my wife!)

Three of the 5 motors are installed in the red and yellow cars I showed earlier, and all behave pretty much the same. Brown-outs are not infrequent after several runs on hot rough asphalt at 30+°C air temperatures -- especially uphill. But on an air-conditioned gym floor, the 2 motors in the yellow car will run at full throttle till the PF LiPo dies.

Keeping motor thermistors as cool as possible for official top-speed runs might be an argument for push starts, as current draws (and therefore thermistor heating rates) are highest when accelerating from a standing start (effectively, a stall). Though I have yet to try them, I see push starts as perfectly legit when top speed is the only goal.

Edited August 19, 2017 by jam8280

[TechnicSummse]

@jam8280

The car in the fridge made my day :D

Hmm maybe its a normal behaviour... i never tried mine at temperatures over ~28°C. But allway on flat asphalt.

I think others could help here?

[jam8280]

One more practical point about Buggy motor thermistors: Current flow is determined only by the voltage applied and the temperature of the thermistor itself. How that temperature is reached doesn't matter.

On a hot day last summer, I absent-mindedly put the yellow car (2 Buggy motors) in the trunk of my car to show off its speed to a friend. When I got there 10 minutes later, the yellow car wouldn't budge. Both motors had shut down just from being in the hot trunk! Once the car had cooled off at home, it ran just fine.

Since a thermistor throttles current somewhat gradually as its temperature rises, a car with warm Buggy motors for any reason could appear to work when it's actually running at reduced current despite being at full voltage.

Edited August 19, 2017 by jam8280

[mocbuild101]

19 hours ago, jam8280 said:

Has anyone run into thermistor heating inside the Buggy motors?

No, not me, the only time I had protection kick in was when I tried to use a V1 PF receiver to power a buggy motor - but nothing happened to the buggy motors, they keep working whatever I do to them!

9 hours ago, jam8280 said:

For those who haven't had the pleasure, when I've tripped the overcurrent protection in PF LiPos in my boats, the battery lights always went out, and the batteries wouldn't turn back on until they'd been recharged. Others have reported the same behavior. But none of that happened here.

That's strange... The light is suppose to stay on when the protection kicks in.

 

But that does remind me of something that happened to my PF lipo a couple of years ago...

Spoiler

Not long after I got my battery - and before I got the correct charger for it - I was trying to find something to charge it up with. I had done some research on what other people had used instead of the official (and very expensive) 10v Lego charger, and had found that it could handle anything up to 12v with 750mAh current. So I found a cheap 12v 700mAh AC-DC adapter with the correct plug on it, and tried charging the battery...

It didn't work.

So I put that aside for a while, and decided to just use up the charge I already had in the battery while I continued searching for a charger. A few days later, I had built something that used the battery. I pressed the ON button, and nothing happened - the light didn't even come on! So I tried again. Nothing!

'Oh no!' I thought, 'I've broken it!'

I tried heaps of things to try to get it to work again, but nothing worked. So I put it in my cupboard, and just forgot about it...

 

A few months later, I found the battery in my cupboard (I had completely forgotten why I'd put it there), and just put it with all my other battery boxes. Not long after, I again made something with the battery, and tried using it.

'Oh... that's right - it's broken' I remembered.

Angry at myself for breaking such an expensive Lego part, I shoved the vehicle away from me. To my amazement, it started moving!!!!!!!

The current generated by the motor had fixed the broken battery! 'I fixed it! I fixed it!' I thought to myself.

To this day I have never had any other problems with my lipo battery, and I now use it for everything.

 

It just goes to show that it's best to use the charger that was designed for it - and not a $2 "wall wart"