Argess

Mattnin: Congratulations.....as far as I am concerned, you've proven the concept (with your overlay graph)......Higher KV (less turns) makes less torque at low speeds than a lower KV (more turns) motor.

I noticed your 7.5 motor speed ended before your torque zeroed out. Using the rpm at max power, it should wind up to about 41,000 rpm.

Using the Torque equation (as a function of N and w) I did for brushed motors, I find about a 7% error using the equation between the max rpms of the two motors. Not great, but I suppose anything under 10% isn't bad when you apply theory to a electro-mechanical device (and not all losses are accounted for).

Using Novak's KV rating of 5800 for the 7.5 and 3300 for the 13.5, I get max rpms of 43500 and 24750. That reduces my torque equation error to about 4%.

Unfortunately, the Torque equation is out by a whopping 25% at zero rpms. I suspect the windings between the two motors may not use the same gauge wire in the turns. Or it may be differences in the rotors, although both appear to use the same rotor, P/N 5908.

I think you mentioned you didn't have a way to measure the current yet, but if you have access to a milliohmeter, could you measure the winding resistance of each motor please?


Edit: Looks like they do use different gauge wire in their stators...at least sometimes:

Novak: "The Velociti Light series is very similar to the original Velociti series but is considered “light� because the wire used to wind the motor is a smaller gauge. Though this may seem like a small change, the ramifications are exciting. The smaller gauge allows for the same top speed, but with slower acceleration and softer braking power."

mattnin

Argess, I do have a current measurement now. That is how I'm getting efficiency. If there is any test you are interested in, let me know and I will publish the #'s

Here is my novak 13.5 motor, measured at 0 deg boost and 30 deg boost. You can compare my motor to my friends Novak 13.5 that I posted earlier. Notice his revs to almost 1k rpms higher but mine has more torque. I believe the only difference is that he has used his more, but they were purchased at around the same time.

Access

If you deal with electromagnets, at some point things saturate, you can't create a stronger magnetic field by increasing the current. Where this point is depends on the core meteriel used, etc.

"slower acceleration and softer braking power" - in other words, they intentionally designed the motor to have less torque.

Access

Earlier you did one test at 5V, can you graph the same novak motor run on the dyno at 5V and then at 7V? To show how a higher voltage affects the system.

Argess

Mattnin: I'd like to know the peak current at w=0 for both the 7.5 and 13.5 Novak motors. If the peak current occurs a "micro-rpm" or so past zero, that's OK......should be close enough. Thank-you.

Access: Could be a saturation problem, but I have to assume "normal" operation for now.

mattnin

Access, I know what test you are referring to. That was the test that John Stranahan did. I did not run that test. I merely posted his results here.

Argess, the peak current I seen with the all the motors is over 100A at first pull of the throttle. The sentry does not record higher than that, so I am sorry I can't get a more accurate number.

ScottStaypuff

I don't have much to add except that I look at two things to compare brushless motors. Efficiency and the number of watts they put out. If two motors were otherwise equal (other than kV rating) I would go with the lower kV rating. My mindset is it will probably be a little more reliable and run cooler with lower kV. Unfortunately not all manufacturers publish these figures and even if they do are they "apples to apples"?

Argess

Mattnin: That's OK...I'm pretty certain, my math is out by just the wire gauge difference between motors. It worked fine for the brushed motors' torque curves and the top end of the brushless, but as the intial low-end brushless torque is definately a function of R (total winding resistance), I feel that's where the error is from.

I used R = r x N where "r" is the resistance per unit length (of one turn). As I assumed that "r" was the same for both motors, it cancelled out on all my ratio comparisons of T as a function of N and w. I probably need to put "r" back in if the wire gauge is different. Oh well.....

Again, very good work...or as they say "Yah dun gud!"

Access

Does the ESC you are using have an adjustment for 'boost'? 'fixed boost' isn't really an optimal thing to do for brushless motors, I know the castle ESCs I use let you set it via a 'forward advance' parameter and this way you can get the best of both worlds. ie. they are not going to use '30 degrees boost' at the lower RPM range where it actually hurts you. 90% efficiency and 75% efficiency is a pretty big difference, that's roughly 2.5x the motor heat which is mainly what restricts gearing over a 5-minute race. I know in stock 17.5 class where LiPo is now allowed, racers are gearing 'through the roof', ie. 35/67.

I also did see a post in the other board that brought up a useful notion that you should account for the rotational inertia of the motor shaft itself in addition to the flywheel. Is it significant relative to the flywheel?

mattnin

I am using an RS Pro. I assume the 30 degrees of boost happens at all RPMs. It definitely makes the motor run warmer. After the dyno run, the motor was already at 144F. Yea, this dyno makes it draw a lot of current, but it got the motor to spin to almost 30k rpms!

In real world experience, I am not really confident boost helps a whole lot. All I noticed was a runaway heat problem. Maybe I was geared too tall to get it to really work for me. I had my new hacker 13.5 get to 190F in 5 mins once and scared the crap outta me because it was a new motor. Now I just run 0 deg boost and I am fast enough anyway. I'm not going to be running any better because I use 30 deg boost. I suppose in a touring car where less torque is needed and more power, maybe it is a good idea, but in stock truck, I like more torque and cooler temps IMO.

John has measured the rotor moment of inertia, and it brought the power rating of the Hacker from 215W to 216W. It is a small difference and probably below the accuracy threshold, nevertheless it should be included. On the newest spreadsheet, there is a box where we can enter it in for the rotor.

I like this Sentry dyno so much, I'm probably just going to be selling my Fantom brushed dyno now. This Sentry dyno is doing everything I want, and I am not limited in doing things like I was with the Fantom. It is a shame too, because I managed to get to get both flywheels, and two available keys for the windows software.

Access

Have you had a chance to do a partial-throttle test, to see if this improves the awful efficiency at lower RPMs? Or a comparison voltage test (same motor, different voltages) to see how this affects the test?

30K RPMs is good for a stock motor, but with the efficiency down that low, it's going to have to be geared much lower to keep it from burning up in a race. 30 degrees may well be overdoing it, you may want to try something like 10 and see if that is more manageable. But I think you'll find 'no boost' is the best way to go, just letting the ESC 'do its thing' and gearing up if you need more speed.

mattnin

Ahh, I forgot about doing that test. I haven't done anything with the dyno past week since my laptop broke so I couldn't test anything at the track, but I should be able to do this test this weekend [sm=thumbs_up.gif]

Access

http://www.rctech.net/forum/electric...al-thread.html

There is some really good testing going on here.

http://www.rctech.net/forum/attachme...4-20-09001.jpg

http://www.rctech.net/forum/attachme...4-16-09002.jpg

http://www.rctech.net/forum/attachme...2s-lipo001.jpg

http://www.rctech.net/forum/attachme...4-16-09003.jpg

Not sure what to make of it yet though. Rctech is a racing board so people there tend to look more at getting the most from a class-limited, power-limited motor. I tend to look at these tests from more of a general design perspective where I am trying to choose the best components for a given vehicle setup.

These are 'full throttle' dyno runs only, and the ESC has a 'soft start' to counter what would otherwise be a massive startup current.

Just looking at those 4 runs, I guess you could say that lower KV does make for more starting torque, ie. torque in the 1000RPM, 2000RPM ranges. The 3.5 motor starts around 250 N*mm while the 13.5 starts around 400 N*mm. But by 10,000RPM, all the motors have roughly the same torque, in the ballpark of 220N*mm to 260 N*mm.

savagecommander

if the esc has a soft start function, i think that throws any meaningful comparison right out the window

savagecommander

either way, torque is based on field saturation, and a motor with more winds has a more saturated core, therefore more torque...

mattnin

I'll be getting back my dyno middle of next week with a new upgraded amp sensor, so I'll be able to do that test

Matt

coreyfro

Hey yall,

Sorry for digging up an old post, but I am deeply interested in this debate.

I have seen a lot of information here without the consideration of simple mechanical properties.

Am I correct when Ipropose that a rotor of X wieght, moving a rate if y RPM's has a power output of z regardless of the magnetic influences that got the rotor to that point?

So, I am suggesting that it is true that a 20kv motor at 10v has equal properties to a 10kv motor at 20v...thus this whole discussion is not about which number of windings produces more torque, but which configuration of windings does so more efficiently?

Because I am sure that, at a point, more windings ceases to be more efficient.

david7697

Power=Torque*AngularVelocity
That is all...

suzonka

My Feedback: (7)

motors can be talked about for days but here is the bottom line , If you had 2 planes the same model same battery's and ESC and props, yet one thing in difference the motors . I have found out with the right combination of everything put together and it works well beyond your needs ,and u get and good amount of flying time out of the entire system , then stick with that on a same weight air frame and you'll be fine.

phmaximus

one big point everyone is forgetting.... Torque is dependant on load.....

chuck a watt meter on a rc car and compare the differences from free wheeling to doing a burnout..... that in it self blows out some of the arguments here

Just because it has the potential it doesn't mean it is

phmaximus

ID love to get one of these.......

phmaximus