RC-Monster Forums - medusa 70mm 2000kv wooooott!!!

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Quote:

Originally Posted by E-Revonut (Post 271464)

If your real car says max RPMs are 8000 are you going to run it at 8000RPMs everytime you get behind the wheel? Most like to leave some headroom for safety. Also remember that in General most of us are running a lot of WOT in our RCs unlike a 1:1 where a tranny shifts and you cruise at maybe 1/4-1/3 the redline RPMs.

right...!!but run a motor on 6 cell like the 2000kv 70mm does not seem that your always in the red line.....50 000rpm at 25 volt its away from the 60 rpm rated....and my motor was only 138f...

High RPM and high load causes issues. Try the 2000Kv with 4S lipos and see how it does. If it's too slow for you, just get a bigger pinion. I run my BPP truggy with a 1577Kv motor on 5S (29175rpm) most of the time, but to show off I use 6S (35009rpm). Now my other truggy I run 8S with a 1100Kv (32560rpm) motor and it has proven to be my most efficient and powerful set up yet due to the characteristics of a lower wind motor offering higher torque (Shaft bending torque to e exact).

Here is your setup as I understand it right?

Quote:

Differential Ratio: 2.8461538461538463
Transmission Ratio: 1.8333333333333333
Other Ratio: 1
Spur Tooth Count: 68
Pinion Tooth Count: 16
Total Voltage: 21
Motor KV: 2000
Tire Diameter (inches): 7
Total Ratio: 22.17628 : 1
Total Motor Speed: 42000 RPM
Vehicle Speed: 39.44 mph (63.36 km/h)
Effective KV Value: 2000

Give or take a few mph for tire size, you are at a 22:1 FDR. That's really really low if I've got the gears right. Proper gear should be 15-17:1 I would say. I had my 1100kv on 8S geared 13:1 before and was faster than 40mph on 28K rpms in a Revo (w/ smaller tires of course.) This is a 60mph setup, not a 40.

The reason the motor heated up so much was b/c of the low gear. Efficiency is quite low w/ low motor loads, plus the rpms and voltage will be higher than what is listed. This is evidenced by the 160F motor temp at 5F ambient. Inside that little furnace the magnet was prolly 200F+.

Now as far as Medusa's specs, they also list the motor as 1100W. Your setup will easily pull twice that (say 120A at 21V+2500W.) Remember these are plane motors. You can do high rpms in a fast moving plane that draws a fairly steady amount of power. Not the peaky power cars do. 2500W->300W---> 2300W--->brake-->2500W--100W, etc all in a few secs. Now a medusa can prolly handle it, but everything else needs to be setup just right as well or its asking for trouble.

IDK and don't care what TRX lists for gearing. The above is true. Now the inside gets very hot, high rpms...boom goes the magnet. You can see it came right off the shaft. Now maybe it was just faulty anyway, and had nothing to do with the setup. I had a Neu that did the same w/o warning and never got above 120F. The mag isn't wrapped, so it smacks the coils and you have one fubared motor.

If thats the speed you want I would drop voltage or kvs. Its like having a 500hp supercharged v8 and leaving it in 2nd gear bc you don't want to go too fast while you are flooring it. It just isn't gonna last

Quote:

Originally Posted by Finnster (Post 271545)

Here is your setup as I understand it right?

Give or take a few mph for tire size, you are at a 22:1 FDR. That's really really low if I've got the gears right. Proper gear should be 15-17:1 I would say. I had my 1100kv on 8S geared 13:1 before and was faster than 40mph on 28K rpms in a Revo (w/ smaller tires of course.) This is a 60mph setup, not a 40.

The reason the motor heated up so much was b/c of the low gear. Efficiency is quite low w/ low motor loads, plus the rpms and voltage will be higher than what is listed. This is evidenced by the 160F motor temp at 5F ambient. Inside that little furnace the magnet was prolly 200F+.

Now as far as Medusa's specs, they also list the motor as 1100W. Your setup will easily pull twice that (say 120A at 21V+2500W.) Remember these are plane motors. You can do high rpms in a fast moving plane that draws a fairly steady amount of power. Not the peaky power cars do. 2500W->300W---> 2300W--->brake-->2500W--100W, etc all in a few secs. Now a medusa can prolly handle it, but everything else needs to be setup just right as well or its asking for trouble.

IDK and don't care what TRX lists for gearing. The above is true. Now the inside gets very hot, high rpms...boom goes the magnet. You can see it came right off the shaft. Now maybe it was just faulty anyway, and had nothing to do with the setup. I had a Neu that did the same w/o warning and never got above 120F. The mag isn't wrapped, so it smacks the coils and you have one fubared motor.

If thats the speed you want I would drop voltage or kvs. Its like having a 500hp supercharged v8 and leaving it in 2nd gear bc you don't want to go too fast while you are flooring it. It just isn't gonna last

your quite bit right....as i read i was undergeared again....!!???but i dont want to make some top speed....i just want to race on my snow track without issue....so 42000 rpm its ti high for this motor....???on 6 cell i have a good run time...low temps ...motor(neu),esc and lipo...8 cell its to heavy,,,

IMO, 42k rpm isn't really that high on a quality motor. The good motors are rated up to 50-60k rpm. Of course we shouldn't run them near that high, but 42k is only 70% of the 60k max, so it should be just fine. And true, the efficiency of an unloaded (or very lightly loaded motor) isn't great, but in a vehicle geared for anything over 14mph, it should be more than enough load. Besides, that 42k rpm is probably using the unloaded kv, so it will drop the more current you pull.

It's one thing if you were doing speed runs only hitting WOT all the time because it was geared so low, but on a track, the throttle is mostly somewhere in between slow and WOT.

next time i will try to gear up....so its the first time that ive seen a motor that ive proble to set-up correctly....

42K is not high per se, as said it would be fine in a plane app where your constant power pull is within specs. However we do use rpm as proxy for power, where the rule is (was) 28-32K was great for trackable levels of power in a 1/8th truggy or Revo. This yielded about 1500-1800W. Ie a 1Y on 4S, geared properly for ~40mph.

Take that same 1Y and spin to 42K, ie 6S. Geared the same that's more like 60mph. Amp draw is about same, but voltage increases by 50%. So does power then.
So you try to tone down the power/speed by gearing down, but now you are unloading the motor. I wish Lehner still had the graphs up, but they had very detailed data on load vs input/output power. Even for high end motors that reach 90%+ efficiency, at low loads, eff dropped to as bad as 50%, quite remarkably. Overloading the motor does not produce such drops.

So even if you are not producing that much power, the heat losses can still be terrible due to low eff. Try slow spd driving, sand (or snow) driving or driving w/ a loose slipper where the motor is lowly loaded. They will heat up the motor faster than high spd runs. You are far better off just dropping the cell count than the tooth count to get the desired speeds. If you want more runtime, use larger cells, not more of them (if you leave the motor the same.) Much has been made of the gearing, but the driving environment also is a factor. If you have the Revo all covered w/ snow, revving to the max and just kinda plowing thru snow drifts w/ big snowy rooster tails , then yea, that's a real challenge for any motor to stand up to.

Quote:

Originally Posted by e-mike (Post 271771)

next time i will try to gear up....so its the first time that ive seen a motor that ive proble to set-up correctly....

I see that you used the lowest timing for the medusa. As far as I know 4 pole motors aren't efficient at that kind of timing and they heat up quite a bit. Try increasing the timing. Head over to e-savage.com and look out for Kostaktinos_mt. He is very knowledgeable when it comes to motor efficiency.

Your motor was probably spinning too fast. There is a range of rpms which the motor will operate at its max efficiency. Check with medusa and gear it accordingly.

I am very interested in getting a 70mm medusa too. Do drop me a note on how the motor performs when u get your motor back. :D

I'd run 8-15 degrees of timing on a 4 pole motor. Start off with 8 and work your way up to where you like it.

I'm loving my 70mm, geared 18/46 in my Hyper 7 after a run the motor is around 90-100 and my ESC is about 20 degrees warmer than ambient. I've got a 24T sitting here that I'll try on it next.