CornUponCob

Specific question about m motor capacitors

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Since this is my first post I'll start by saying I've been a life-long fan of lego technic. I'm 29, and I recently got back into the hobby and I'm building a 16 unimog-wheel scale mobile construction crane with at least a 4, and probably 5 section telescopic boom (but it will only go straight up and down. A boom that large made of lego can't work in cantilever. . . at least not work well.

Anyway, I have I think 22 m motors from various sets, and I've started testing the motors. Turns out I got a bad motor. I opened it up and the problem is an internal short in the capacitor. If I squeeze it or apply any pressure to it it shorts and the motor stops spinning (and the LED on the battery pack turns off indicating high-current draw protection).

I'm handy with a soldering iron and I have small spare capacitors laying around, so I thought about replacing it. I also thought about trying to get a replacement from Lego (either through the internet or at a store). I was curious as to the RMA process through lego.

Secondly, I thought about not replacing the capacitor at all, and just taking it out of the circuit. This made me wonder if this would give me IR V2 comparability if I did this to all of my m motors. I understand the importance of a capacitor for noise reduction, which doesn't seem important here since we're using infrared and not RF. I read this post on TechnicBricks a while ago:

"The raw motor currently used in the LPF Medium motor internally contains a relatively large capacitor (1 uF) across each of the 3 motor coils. This is done by the motor manufacturer for noise reduction.

We knew that this would be an issue with the new CMOS motor driver (DRV8833). Since it can source a very high inrush current the over current protection will kick in sooner (typically after 2,25 us with a current exceeding 3,3 A). At start up the motor driver will first charge the input capacitance. With 2 or more LPF Medium motors at the same output this can trigger the over current protection. It will repeatedly try to start the motors and you will only hear a singing noise.

For many reasons we have been searching for a higher quality solution for the LPF Medium motor and we now have an approved new raw motor. It has better quality commutation and only 1nF across the terminal. An updated LPF Medium motor will be released during 2013."

All of my M motors are definitely not the new versions, and I'm not sure I'll ever buy any more M motors (because I have enough) and the L motor is about to be purchasable by itself. I guess my question is regarding the above quote is are there 3 additional capacitors inside the motor itself? The capacitor that I can see when I remove the shroud is definitely in-line with the main power wires. It does not cross the poles. If I remove that capacitor will it negatively affect performance, and will it give me IF V2 compatibility?

While I'm at it, here's a picture of my build station.

IMG_2195-XL.jpg

And more random pictures of my build so far. Really just prototyping, although the wheel carrier design is finalized so I went ahead and built all 16 of them.

http://sammorganphoto.smugmug.com/gallery/28096733_2sPxG6#!i=2374027084&k=HnHn3Pm

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Just saw the pictures on your link...that's a lot of bucket you have there :cry_happy:

You really spend tons of money for your MOC! :thumbup:

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Just saw the pictures on your link...that's a lot of bucket you have there :cry_happy:

You really spend tons of money for your MOC! :thumbup:

Yeah that's from 7x 8069 and 4x 8043, and then one more I got from some guy on craigslist. At some point in the future I'll build a giant bucket wheel excavator (I have a collection of tracks that's growing as well). I know that bucket wheels don't use buckets that wide, but I want to make a "fatty" anyway. Building things exactly to scale is not hugely important to me.

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What you see outside of motor core is not a capacitor, but a current-limiting thermal fuse. You should not remove it, as it protects the motor from burning if it is stalled for a long time. Your best bet is to use LEGO Service, but if you want to do it yourself, the device is a Bourns MF-R050. This has nothing to do with the capacitors that cause problem with PF receiver V2, these capacitors are inside the motor core itself.

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What you see outside of motor core is not a capacitor, but a current-limiting thermal fuse. You should not remove it, as it protects the motor from burning if it is stalled for a long time. Your best bet is to use LEGO Service, but if you want to do it yourself, the device is a Bourns MF-R050. This has nothing to do with the capacitors that cause problem with PF receiver V2, these capacitors are inside the motor core itself.

Extremely helpful, thanks. I had just assumed it was another capacitor but was then confused since that article did not mention one outside of the motor housing.

So if I'm the kind of guy who's generally pretty responsible with not over-loading a motor, or not running it for too long if I know it's in a high load situation, I guess I could take those out without any ill effects.

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Do it at your own risks :wink: But if you do, then you must short the device, since it is in series with the motor. BTW, it'a always possible that you have a defective component (eg. broken wire), but I would first try to check the solders, as it's the most probable failure!!!

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Well you learn something every day. Not sure if I'm 100% correct or not, but the fault seemed to be in one of the other two wires that goes into the motor that are not used. Let's call them 1 and 4 out the outside. 1 of them was shorting with the metal motor case when pressed just so. I thought it was the capacitor (later turned out to be not a capacitor at all, but a thermal fuse). I have seen a number of button (I think that's the name) capacitors like that go bad so I figured "press on this, motor stops working - must be this".

The worlds smallest piece of electrical tape fixed the issue.

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I have found LEGO RMA to be great (At least the edu side) I placed an order for the NXT kits, as soon as they came out. I got a batch with bad capacitors in the display boards. The NXT bricks would work, but the display had a random pattern on it. By three years after I purchased them 18 of the 24 had broken. I called up support and they were replaced no questions asked.

Total investment 10 dollars in shipping the bad ones back.

Great service, but that is what you would expect.

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Wow, that's some spiffy crane. But, why do you have the wheels carriers set up the way they are? It would be simpler to make a solid axle, unless you want all axles steered.

And, by sheer coincidence, I am also building an 8 axle mobile crane, also with a four section boom. Also, your wheels sets look very much like parts I designed for a Trial Truck.

Oh look, 300th post! Nope, only 290. :blush:

Edited by Saberwing40k

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Wow, that's some spiffy crane. But, why do you have the wheels carriers set up the way they are? It would be simpler to make a solid axle, unless you want all axles steered.

And, by sheer coincidence, I am also building an 8 axle mobile crane, also with a four section boom. Also, your wheels sets look very much like parts I designed for a Trial Truck.

Oh look, 300th post! Nope, only 290. :blush:

I wanted all wheels to be driven, steered, and independently suspended. I found out today though that I won't have very impressive steering if every wheel is driven (the range will be too small). Only the center 4 axles of the vehicles will be driven. The model will still have 2 steering modes (and maybe 3, but unlikely). It will be able to crab steer.

The reason I set up the wheel carriers the way I did was to get the turning axis of each wheel as close as possible to the center of the wheel. I thought long and hard about dual link virtual pivots but decided it wasn't sturdy enough for a vehicle of this size. The wheels would have been very hard to turn if it had "unimog style" steering (pivot point far from the center of the wheel), so the turn table system was what I decided on. The portal axles also carry additional gear reduction because I want a center drive shaft in the vehicle instead of a motor for each axle. The motors will be geared UP before going through the transmission -> multiplexer -> bevel style differential -> telescoping axle -> portal axle. None of those components can handle high torque, so hence the need for heavy gear reduction after the differential.

Ohhh yeah, and I think the steering of the wheels will look better this way. I don't like to see a lot of forward/backward motion in a wheel as it turns.

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Is there any way of testing the power of an m-motor or are they in general all the same?

Performance is very constant between motors of the same kind...

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