28mm OD Low kv brushless motors

[Monahan Steam Models]

Spent a number of hours searching for some low kv rated 28mm brushless Outrunner motors today. Mainly I am after motors capable of spinning lower rpm’s for directly driven prop shafts.

All the typical airplane and drone websites had listings for brushless Outrunner in this diameter but most of these motors still had kv ratings in the 1000’s. Lowest I could find was about 700kv in a 28mm OD.

By dumb luck I stumbled upon some random listings on amazon for some 28mm 350kv and 500kv brushless Outrunner motors. Here’s a couple links.

https://www.amazon.com/gp/aw/d/B07XQ6XLST/ref=sspa_mw_detail_4?ie=UTF8&psc=1&pd_rd_i=B07XQ6XLSTp13NParams

https://www.amazon.com/Waterproof-brushless-Motor-Underwater-Floating/dp/B08DKLKZB2/ref=mp_s_a_1_1?dchild=1&keywords=rov+2838&qid=1620152935&sr=8-1

https://www.amazon.com/gp/aw/d/B0831NDTZH/ref=psdcmw_2234129011_t3_B0831NZ3S3

It appears these same motors are used in a number of thruster pod products available from a number of different sellers.

I picked up a couple to test them out. Posting because I figured this might help someone else out.

Nick

[Bob Gato]

Nice find! That will help a lot of people out-especially me...Sometimes I think that they are just outrunner motors run underwater-which they do well anyway. Its really nice that they are so low of a KV rating and small diameter size-that's very rare.

I would also like to see the stats for maybe a 15mm outrunner (usually in the 1500KV-2000KV range) run slow and under load but in the wet-I bet it would survive due to the constant cooling of being underwater? I'll put that test on my list for someday...

[Monahan Steam Models]

I’m expecting these motors are probably just regular Outrunners without the cooling slots machined into the housings (which I’m not sure I like that). They claim the bearings are anti corrosive and that the motors can run in salt water.

As you mentioned, finding small diameter low kv motors is rare and that’s is why I grabbed a couple of the 500kv and 350kv ones to test out. Down side they don’t already have a shaft but that’s not a big deal. A bolt on output shaft can be made easily. Should see the 500kv motors show up Thursday and the 350kv motors maybe arrive sometime next week. Will post here after I test them each.

[Sinksalot]

I tend to buy my 300kv, 400kv and 500kv brush-less motors from Europe. As its hard to find them in the USA. When I needed a 28mm diameter 400k motor, I ended up buying a Graupner brushless motor model 7281. There hard to find as the company stop making them when closed and then reopened as a new company with less items for sale. But there out there yet. I think the shaft was 3.17mm.

[Monahan Steam Models]

Well the 500kv I ordered last week to test were supposed to be here last Thursday. Still waiting for them to arrive.....

The 350kv motors arrived today. I tried removing the outrunner can to get a look at the internals but after loosening the set screw I still could not pull it off. Thought I should do some testing instead first before risking any damage. What I could see through the threaded holes on the face of the outer can was the winding looked to be sealed in a potting compound. Will try again to open one up to confirm this later.

Okay so I set one 350kv motor up with it attached to BLHeli 32 ESC. Power was sent to the ESC from a bench top power supply set at 12 volts. 12v multiplied by 350kv should yield an top RPM of 4,200.

It did not during the first tests. I was seeing a top rpm of maybe 300.

After trying multiple different settings in the ESC software with not much of a change, I finally turned of the default setting in the software for low rpm protection. Once this was changed, the motor really woke up and the top rpm’s at 12 volts is about 4,200 now. The range of rpms from 0 to 4,200 are very smooth and the motor puts out a tremendous amount of torque throughout this range. Very little amps were drawn when I was physically trying to stall the motor by applying pressure with my fingers to the spinning can. Sorry not very scientific.

So far I like what I’ve seen with the 350kv motors and I think these will be what I use in the type VIIC project.

Nick

[Bob Gato]

I am glad that you are doing your own testing and checking-you can't believe what's written in these ads-I don't have to say but IPX1 and waterproof should never be in the same sentence! ..it's obviously a mistake and the motor appears to be pretty water resistant. (This was from the first link above)

[Monahan Steam Models]

LOL, I noticed the IPX1 written too before I decided to pick a couple up to test.

And you’re correct, never believe anything written in these ads. I find so many errors in them constantly. Especially when multiple sellers are offering the same product. I usually try to trace the product back to the manufacturer and see what data on the product they offer.

With the little bit of testing I’ve done so far, the motors seem good enough. I still want to open them up and take a look at the windings and bearings.

[Bob Gato]

Nick..curious did they come with the external slide on blades?

[Monahan Steam Models]

The motors I ordered did not come with the slide on prop but other sellers offer the motors with the props included.