RPM and rollout question/decision help? - RC-Monster Forums

simplechamp · **RPM and rollout question/decision help? -** 04.27.2010, 04:43 PM

Here's the deal: I have a Neu 1512/1.5Y/F (1850kv) that I was going to run in my buggy. On 4S it would be at about 27k RPM and I would gear for 40mph with a rollout of 1.6. Another guy offered to trade me my Neu for his Castle/Neu 2650kv. The 2650kv would be at about 39k RPM on 4S, I would need to put the 52T spur on my Slipperential and I could gear for 40mph with a rollout of 1.1

What would you do? I definitely need to stick with 4S. Is it better to have the higher kv higher RPM with a lower rollout, or lower kv lower RPM with a higher rollout?

jhautz · 04.27.2010, 05:08 PM

Depends on what you are going to do with it IMO.

For a race setup I would go for the 1850kv Neu motor. Should be a very nice smooth driveable setup on the track.

For bashing I would go for the 2650. A little more "fun" with some spare power on tap for goofing off.

simplechamp · 04.27.2010, 05:36 PM

I'm a basher, so sounds like the 2650kv might be the way to go. Even though it's higher kv I like that the rollout will be lower (even if that means I have to buy the 52T Slipperential spur)

bdebde · 04.27.2010, 09:45 PM

I like more rpm for a basher. My bashers run about 40,000 + rpm, for racing, more like 30,000 is better

nuz69 · 04.28.2010, 04:44 AM

No.
Both motors have same power, but lower RPM = better efficiency and longer life time (bearings).
More over the neu is lighter. You can compensate the lower RPM with higher gearing and bash at exactly the same top speed and cooler temps.

===> keep the neu 1512 1.5Y

simplechamp · 08:02 AM

So basically if it takes a set amount of power to make a vehicle go it's max speed (e.g. 750W to go 40mph) it comes down to what motor will be most efficient. And from what you are saying the 1850kv motor running at 27k RPM will be higher efficiency than the 2650kv motor at 39k RPM which should translate into the 1850kv running cooler?

It just seemed to me the gearing would kind of even things out. Even if the 2650kv is less efficient, the gearing would be lower, so it wouldn't be working as hard. And even though the 1850kv would be more efficient it would be geared higher, so it would be working harder. I mean, how much efficiency difference are we talking here? a fraction of a percent? 5-10 percent? I have no idea. Is it really enough to make a noticeable difference, or will the difference in gearing cancel it out?

nuz69 · 08:18 AM

No!
The both motor can handle the same power, that mean geared for the same speed, they will consume EXACTLY the same amout of current (amps).
You can put a higher pinion on the 1850kV BECAUSE it provide more torque for the SAME CURRENT than the 2650kV. Both motors will work exactly as hard.
BUT, the 1850kV will have less RPM, meaning less losses from bearing drag (that mean better efficiency) AND higher bearing lifetimes.
The 1850kV is lighter too, mean less weight in the car and a little less current consumption.
Other than that, the lower kV possess more controlability due to the higher self inductancy of the winding, Although the punch control can help to get some controlability...

VintageMA · 04.28.2010, 09:06 AM

nuz69 - I don't agree with your arguements.

- Both motors are 1512 motors and will weigh the same.
- Neu motor are actually more efficient up around the 40K range. It is the cheaper motors which are more efficient in the lower RPM ranges primarily because they are, well, cheaper.

Listen to what Jeff is saying - he knows his stuff.

IMO if you use the 1850kv motor you should really be going up to 5S to bring the RPMs closer to 35K. If you go with the 1850kv motor with 4S and taller gearing you are going to pull many more amps on acceleration than you would with the 2650 with shorter gearing. The 2650 motor will just have an easier time spooling up.

I would say run the 2650 with 20-30% punch and you'll be pretty happy.

nuz69 · 09:24 AM

IMO you are wrong,
The torque per amp is higher when the kV is lower, meaning for the same acceleration force, both motor will consume exactly same current. Its all about energy, you need it to accelerate your truck, and it comes from your battery depend only of voltage and current ==> same voltage and acceleration ==> same current.

Neu 1512 1.5 : 10.23 oz
Castle/neu 1512 1Y : 13.1 oz

It appears that the Castle/neu has a little less efficiency than the real american neu.

The electromagnetic efficiency depends a lot of the load, not of the RPM : between 25k and 40k efficiency is the same. But mechnical losses are higher at 40k.
==> For the same top speed and same voltage, the 1850kV will always provide better results for in matter of temps and runtimes...
==> I would keep the neu.
Who is jeff ? ^^
Can someone qualified confirm or negate ?

VintageMA · 04.28.2010, 09:34 AM

Quote:

Originally Posted by nuz69

Neu 1512 1.5 : 10.23 oz
Castle/neu 1512 1Y : 13.1 oz

Hmmm... fair enough - should have looked up the specs instead of making an assumption.

Quote:

Originally Posted by nuz69

It appears that the Castle/neu has a little less efficiency than the real american neu.

Previous research and threads from Patrick have shown that the Castle Neu's will run about 10* hotter than the hand-built Neu's, IIRC.

Quote:

Originally Posted by nuz69

The electromagnetic efficiency depends a lot of the load, not of the RPM : between 25k and 40k efficiency is the same. But mechnical losses are higher at 40k.
==> For the same top speed and same voltage, the 1850kV will always provide better results for in matter of temps and runtimes...
==> I would keep the neu.

What is your source of this information?
In my own personal experience, many years of testing and many $$$ spent on about 15-20 different Neu motors has shown me that 35-40K is the sweetest spot for these motors. Heli guys run them up in the 50k rpm range - which I have found in surface gets them hot way fast.

The truth of the matter is that 4S is not the sweet spot for heat and efficiency - 6S and higher is. More voltage = less current for the same power and that equals less heat. But if 4S is a requirement - go with the 2650 motor. Going with the 1850 and trying to gear for 40mph is going to create a lot of heat.

Just my opinion -

Quote:

Originally Posted by nuz69

Who is jeff ? ^^

jhautz

nuz69 · 10:12 AM

I took this from my personnal experience. In europe we compared different configurations such as tekin buggy 1900kV and CC/neu 2650kV, on track. Both setup geared for the same top speed, the 2650kV ran a little hoter than the 1900kV, and the motors are theoritically capable of handling the same power, ~ 1200Watts.
And its logical, as you said, power is volt x amp, the same acceleration and voltage you have, the same current you will drop and so the same heat, end of story no ?
But it's true I don't have your kind of experience... Just wait someone else to confirm this all kV thing...

simplechamp · 04.28.2010, 10:30 AM

Lots of info flying around! I can definitely see both sides of the argument.

Nuz69: in your first response you wrote "lower RPM = better efficiency" but then you go on to say "The electromagnetic efficiency depends a lot of the load, not of the RPM" which seems to be contradicting.

I can understand the lower kv higher voltage setups drawing less current, but I'm not so sure about higher kv lower gearing versus lower kv higher gearin when both are at the same voltage. Wish I had an EagleTree and both motors so I could see for myself.

Anyway, back to the original idea of if I should keep my motor or trade, I ended up declining because the guy was outside the US. So regardless of what is better I'm keeping the 1850kv for now.

nuz69 · 10:37 AM

I meaned lower RPM = better efficiency of your bearings. It's drag efficiency, contrary to electromagnetic efficiency which depends a lot of the timing and load (current) you pull in your motor.
An eagle-tree on both setup would maybe close this debate ^^

simplechamp · 10:52 AM

The way I see it the higher kv setup would have the smaller load (lower gearing) and the lower kv setup would have the larger load (higher gearing). So I understand the lower kv motor might have more torque, but it also has a bigger load applied. The higher kv motor has less torque, but it has less load too. If you look at the rollout numbers the lower kv motor would be doing an extra 36% more "work" per revolution than the higher kv motor. For each revolution it's turning the tires further by 36% so I'm not sure if the extra torque will really do much besides cancel this out, if it will even do that.

It will take a given amount of power to make the vehicle go a certain speed (for example 1000W to go 40mph) and the battery voltage is staying at 14.8V for both motors, then each motor should draw 68A to run the vehicle at 40mph. That logic seems to say that it shouldn't make much difference which motor is used.

nuz69 · 10:52 AM

Exactly, the difference will be on the bearings AND the better efficiency of the real neu AND the higher self inductancy of the widdings of the lower kV (which result in a better controlability), what I am saying from the beginning ^^