HobbyKing SK3s

[bearing]

Specific Km: 0.44

Could that be true? it's more than the C80100. This 0,84 kg motor is going to have better efficiency at 5kW than the 1,84kg C80100?

 

[Miles]

That's why I think there must be a mistake over the resistance......

Maybe for the 300 euro Hacker, which is 70 grams heavier, but.....
http://www.hacker-motor-shop.com/e-vendo.php?shop=hacker_e&SessionId=&a=article&ProdNr=15727608&t=3&c=32&p=32

 

[adrian_sm]

Hi All,


after reading through the biggest thread with over 70 pages, I got stuck to the idea of starting a friction drive conversion of my old bikes, too. Great job you did up to now ... wonderful forum

I would like to ask the specialists among you :

Has anybody already gained some experience, or even measurements, with the Turnigy SK3-6374, especially with the KV149 version ?

HK's technical data is somehow confusing (1265 g weight in the product list, 840g in the product details page), and also the 21mOhm resistance looks very low, compared to anything we've ever seen on stock 6374 motors of most cheap CN-brands.

If all published data is true, this looks the most suitable motor ever for the outrunner friction drive, having lots of reserves even for steep hills.


Many thanks in advance for any information about these new motors.

Georg

I don't have any of the 6374's but I bought three of the SK3-6364-190kV motors when they first came out. I have been using these on my friction drive, and they seem like quite nice little motors. The came in a very pretty box, with good padding etc. Good for protection, and making it feel like you got you moneys worth, but increases shipping weight and cost I guess.
View attachment 4

Outer diameter is 59mm not 63mm, as the name suggests.
Mounting holes are on 44mm PCD
The shaft circlip sits proud of the mounting face of the motor.

8mm shaft, not a 10mm shaft that you see on some other motors in this class.
No load current was 2.0,2.1 & 3.0 across the three. Not sure why one was higher than the others.
They also have the internal "fan" as part of the skirt bearing support, and some cooling slots in the motor can itself.
View attachment 2

I have been happily running them at 500-1000w on my friction drive without any issues. The only downside in my application is the location of the cooling slots in the can. As they sort of line up with where the edge of the tire tread would be. So this may allow road debris to enter the motor via these slots. But then again maybe not.


I had planned a lot of back to back tests against other motors, but this takes quite a bit of time to get useful data, but haven't had the time to do this properly yet. One of the reasons I haven't done this is becasue I ahd planned to use the temperate data logging of my Brain Box to collect all the data from a temp sensor mounted in one of these motors. But the small skirt bearing, with the "fan" support out to the can does not give much room left to get the sensor and wires passed. So I haven't bothered. yet.

The acoustics of the motor are a bit different too, they have a higher pitch sound to them, than the 6374-200kv that has been my bench mark motor.

Here are the frequency spectrum for the two motors, note that the SK3 has that peak at around 3-4000Hz, where as the EMP6374-200kv is a lot flatter.
Note: these motors were mounted to a Commuter Booster at the time, which changes the sound profile a bit, and I dind't take care of how close the microphone was, so pressure levels should not really be compared.


All in all, they seem really good. No complaints.

- Adrian

 

[full-throttle]

Adrian,

Would you be able to confirm the 28mOhm winding resistance as stated by HK?

 

[adrian_sm]

Not easily. I don't have the tools for mOhm measurements. DMM won't do it. Don't have a CC bench supply.

Guess I could put one phase in series while I charge a battery.
- measure current
- measure voltage drop

What is the best way to measure it?

Feel free to borrow a motor from me and measure it yourself.

 

[full-throttle]

Guess I could put one phase in series while I charge a battery.
- measure current
- measure voltage drop

What is the best way to measure it?

4-wire method is the best, you have just described it. As long as the current is steady and the coil doesn't heat up too much. 1~2A is enough to get a good estimate.

Since the motor is delta-terminated phase resistance is 1.5x of what you'd measure. Do you need to account for that Miles?

 

[Miles]

It's the phase to phase resistance that you need.

 

[full-throttle]

Of course hence the sqrt[3] conversion from star to delta..

 

[georg2410]

Hi Miles, Adrian and others


thanks for the awful lots of explanation / information and pictures. I've already calculated / inhaled the physics behind all this, now I'm in the motor selection phase, but there's so many motors which "looks" suitable, so the choice is not easy.


Yes, I also had the 6364 in mind, since you mentioned it already in another thread. And even the Emax/Epower GT6354 sourced by GiantCod/UK looks good, from the specs


Also, you mentioned EMP's 6374 in this and other thread, but your pictures of that motor (in thread Custom Full Suspension Flatbar Road bike build) look different from EMP's C6374 and N6374 now found on *bay.
Could you tell me which EMP 6374 you got, and when and where you bought it from ?


Thanks

 

[adrian_sm]

The 6374-200kv used be sold through hobbyking, and was the motor I did most of my testing with. But Hobbyking don't seem to even list it on their site anymore. Others have bought them from LeaderHobby. Looks like the C6374s on ebay like this one

The link is in my post above, but here it is again.
http://www.leaderhobby.com/product.asp?ID=9394001224160

Don't over think the motor selection too much. The main things are get the KV right, and get the heaviest one you can find. This ensure the motor will perform it's task efficiently, and handle any waste heat well.

So:
1) decide on what speed you want to get assistance up to
2) select a motor kv, that still gives you no load speed above your max assistance speed when your batteries are hitting LVC.
3) get the biggest heaviest motor that will fit your build, as this will handle the heat more
4) get a motor with a skirt bearing if you plan on using the motor can as the drive surface, and want >500w power
5) only pick the smaller motors if you have a reason too. ie bigger one doesn't fit.
6) It is not tragic if you get it wrong. It is only $60-100. The rest of your build will cost a shite load more.

Oh and I tend to like to stick to <=6s LiPo. This means you can stick with the cheaper controllers, easier battery pack build & balancing, and you can still get motors of the right kv to give good assist speeds.

- Adrian

 

[adrian_sm]

Guess I could put one phase in series while I charge a battery.
- measure current
- measure voltage drop

What is the best way to measure it?

4-wire method is the best, you have just described it. As long as the current is steady and the coil doesn't heat up too much. 1~2A is enough to get a good estimate.

Since the motor is delta-terminated phase resistance is 1.5x of what you'd measure. Do you need to account for that Miles?

Hope you only wanted ball park figures.

Motor under test:
Turnigy_Aerodrive_SK3_6364_190kv_Brushless_Outrunner_Motor

Method:
I just charged a battery with two of the motors phase connectors used to complete the circuit, and measured the voltage drop across the phase connectors.

Results:
1.0 Amps => 0.030 Volts
2.0 Amps => 0.059 Volts
3.0 Amps => 0.087 Volts

 

[Miles]

That seems close enough to the specification given. Assuming the Kv is actually 190 rpm/V that gives a Km of 0.29 So, a specific Km of 0.42

 

[adrian_sm]

Don't have a tacho on hand, but the previous FFT acoustic analysis was performed on a 5s 20.03V pack @ full throttle, and appears to show a nice peak about where you would expect it at ~3800Hz.

Now I just have to get my head around what Km actually means for me.

 

[georg2410]

That seems close enough to the specification given. Assuming the Kv is actually 190 rpm/V that gives a Km of 0.29 So, a specific Km of 0.42

Yeah. That sounds great so far. If the specs seem to be true, I hope this is also valid for the about 21mOhms of the larger SK-6374 149KV.

My target application is:

Friction driven Bike, limited electronically to German speed limitation (about 30kph) only, ignoring the max. power limit of 250W (because this limit seems senseless, especially since the law only refers to a normative continuous motor rating, which is up to the manufacturer anyway - who can seriously believe that a Hub-w/gear from Bafang or Golden which is labeled "250W" is only able to continuously handle 250W at higher speeds ??)

Skirt bearing of course, in order not to kill the motor by the friction drive - method. The larger no-load current is not really important for me, since it should be mainly a hill climbing support for my GF's bike.

System weight about 100 kgs.

Max. slope of hills to be climbed 10-12% with about 6 to 10kph, therefore the wish for KV 149 and not higher.

=> Torque/Current limited electronically to 4Nm / 62A by either ESC or by self-designed separate switched current limiter in front of the simplest cheap 100A ESC.

If needed, (a later stage), lowest speed support by rotary hall-encoder with round magnet on the axis of the motor, with the new single-chip Infineon digital hall encoder with 3-sensor emulation output (if they finally become available )

 

[Miles]

That all seems to add up:

 

[full-throttle]

Hope you only wanted ball park figures.

1.0 Amps => 0.030 Volts
2.0 Amps => 0.059 Volts
3.0 Amps => 0.087 Volts

Thanks, that's close enough to 29mOhm claimed

Nice motors

 

[adrian_sm]

Oh, and the no load current was 2.04-2.1Amps at 24.4V.