HobbyKing SK3s

[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.