"What it says on the tin" N20s / Far from tacky tacho

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MySolderIsOlder
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"What it says on the tin" N20s / Far from tacky tacho

Post by MySolderIsOlder »

Firstly a plea - has anyone found a *reliable* source of N20 motors on AliExpress that they're willing to share links to? Ideally motors that are at least vaguely close to the advertised speed...

Secondly - a tip that might be useful to other newbies. Some months back, I ordered a few batches of what were supposed to be 6v 300rpm N20 motors from 3-4 different suppliers on AliExpress for ridiculously low prices. On testing some, it immediately became apparent that not only were they not the expected speed - but also that within one batch there were completely different types of gearbox. That got me started on trying to measure the actual speed of each one. Tried various methods: attaching a wheel and trying to count the revs over 10 secs, filming on the phone and playing back in slow-mo, even downloaded a variable strobe light app for the smartphone to try and manually sync the rotation frequency. None of these methods worked terribly well and all were very time consuming. Then I found this gizmo on Amazon for ten pounds:
Image
Called an optical tachometer. It comes with some strips of self-adhesive highly reflective tape - you snip a bit off, stick it to a wheel, then run the motor and point the laser at it. Within a second or two it establishes a stable readout of the RPM. Genius!
Using a freshly charged 6v battery pack and a Heath-Robinson clamping arrangement I went through all of my stash N20s and discovered that several of the "300rpm" motors were actually 72rpm, some were 160rpm, most were around 200rpm, a couple were 520rpm - and one was 2000rpm! Repeated some of the tests as a control and got identical readings each time, so despite its budget price, the gizmo seems reliable. Since I've yet to find a genuinely permanent marker that works on smooth metal, I also scratched the speed onto each motor with a carbide tip, just so I won;t need to repeat the exercise.

Having measured all my spare brushed motors, I also tested the tacho on the current work in progress, "PolycarbonAnt", which is going to be my first vertical spinner (weapon is a 80mm diameter 6mm thick polycarbonate disk with 2 machine screws threaded into opposite edges). With the weapon disk mounted on its MultiStar Elite brushless motor, driven by a 12A Afro on 3s lipo, I was able to clock it at a slightly alarming 17,500rpm before any vibrations started to get develop. Only a short test (can't believe how quickly it runs down a 180mah lipo) but seems to confirm both that the tacho works fine at higher speeds too - and also that my cunning plan for a well-balanced diy weapon disk is gonna work! So hopefully PolycarbonAnt, and his orthogonal cousin LexAnt may yet make an appearance at AWS 58, assuming I can find something stronger than the M2.5 HT screws, whose socket heads both sheared off during the first impact test...
Stuart (Tony's dad)
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Kyro
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Re: "What it says on the tin" N20s / Far from tacky tacho

Post by Kyro »

i got my n20's from ebay... i cannot say whether they are the quoted rpm, however they are as fast as i would expect and they are all (3 different orders) the same speed and gearing...

all 3 of my 4wd bots running the same motors run at the same speed and theres no pulling to one side from faster or slower motors... the seller i buy from has a few choices in motor speed... 100, 1000 and 1200 rpm at 12v (a weird jump from 100 to 1000) they are £4 something each so not the cheapest, but they are reliable as i have not had one break on me yet...

https://www.ebay.co.uk/itm/DC6V-12V-N20 ... 4728545005

i dont know if this helps at all but they are from the uk and as such, arrive quickly...

im willing to upload a video so you can see the speed if you like


Luke
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MySolderIsOlder
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Re: "What it says on the tin" N20s / Far from tacky tacho

Post by MySolderIsOlder »

Thanks Luke - will give some of those a try.
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voorsk
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Re: "What it says on the tin" N20s / Far from tacky tacho

Post by voorsk »

The only ones I bought from ebay were rubbish, so I never did that again. The rest came from either hobbyking or pimoroni, and were all as advertised.

As they all look about the same, I labelled my motors using little bits of sticky label (the stuff for sticking printed addresses to envelopes) with the gear ratio written on them.
Team BLEEP Suspicious Houmous / Sprouting Potato / Fermented Melon / Black Carrot
MySolderIsOlder
Posts: 190
Joined: Mon Jul 30, 2018 1:22 pm
Location: Kent

Re: "What it says on the tin" N20s / Far from tacky tacho

Post by MySolderIsOlder »

Just to follow up - I ordered four 12V 1000rpm N20s from that eBay link that Luke recommended. Arrived within a week. Tested them with the tachometer last night and results were pretty conclusive - running on a freshly charged 6v NiMH battery, all four were within a couple of revs of 510 RPM. They also ran nice and smoothly with no grating noises (unlike several of the ones I've had from AliExpress). So thanks again for the recommendation - will have to order up a few more of those!
Stuart (Tony's dad)
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Kyro
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Re: "What it says on the tin" N20s / Far from tacky tacho

Post by Kyro »

glad i could help, also good to learn that my n20's are roughly the rpm expected... i knew they were all equal and decently fast, but i had nothing to measure the exact rpm, so thanks for posting results.
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