looking for a thread

Originally Posted by hoovhartid

This one?:
http://www.rc-monster.com/forum/showthread.php?t=12697

yes sir that is the one Hoov

[lutach]

Lower Kv motors have more torque and require less AMPs to make the same power as a higher Kv. Simply put it this way:

8S lipos 29.6V X 60A = 1776W (That was the AMP spike I saw in my 8S set up with a Neu 1515/2Y 1100Kv)

6S lipos 22.2V X 80A = 1776W

4S lipos 14.8V X 120A = 1776W

Amps is what will cause most of the heat and the lower the amp plus the torque of a lower Kv motor will give you a cooler running set up. I can also run smaller capacity packs ranging from 2200mAh - 3700mAh 8-10S lipos. he other factors will be gearing, weight, drivign style and smoothness of drive train.

[sleebus.jones]

Originally Posted by lutach

Lower Kv motors have more torque and require less AMPs to make the same power as a higher Kv. Simply put it this way:

8S lipos 29.6V X 60A = 1776W (That was the AMP spike I saw in my 8S set up with a Neu 1515/2Y 1100Kv)

6S lipos 22.2V X 80A = 1776W

4S lipos 14.8V X 120A = 1776W

You're getting all the right answers for all the wrong reasons. :)

The higher voltage is the reason for the lower amp draw, not the KV of the motor. If you're doing the same work, you're doing the same work. If the efficiencies of the motors are the same, then the heat level is going to be the same. If we assume all of those motors are equally efficient at 90%, then they'll radiate 177.6w of heat, regardless of current draw.

The only way to make the same power with less heat is to get a more efficient motor.

[lutach]

Originally Posted by sleebus.jones

You're getting all the right answers for all the wrong reasons. :)

The higher voltage is the reason for the lower amp draw, not the KV of the motor. If you're doing the same work, you're doing the same work. If the efficiencies of the motors are the same, then the heat level is going to be the same. If we assume all of those motors are equally efficient at 90%, then they'll radiate 177.6w of heat, regardless of current draw.

The only way to make the same power with less heat is to get a more efficient motor.

Do you have first hand experience? I do with all the set ups I mentioned. Guess which one runs cooler and longer. Try it out and then tell us how a 8-10S with the appropriate Kv motor compares with a 4S set up and again with the appropriate Kv motor. What you saying will work on paper, but in real world I've found out that it doesn't work out. The numbers I posted was for AMP spikes and the avg. I get from my 8S set up is only 5A while I've seen over 10A with 6S. I haven't had much data on 4S, but with a similar set up as the 8S I've found that my ESC, battery and motor were all much warmer. I've heard of guys avg. over 20A on a 4S set up and that's where you see losses due to heat. You can have all the watts, but electric motors also makes power from torque and that's where a lower Kv motor (Brushed or Brushless) will have the advantage.

[sleebus.jones]

Originally Posted by Arct1k

Not true losses increase with square of current W = I * I * R...

Yes, that's correct. Higher voltage will be more efficient.

Originally Posted by lutach

You can have all the watts, but electric motors also makes power from torque and that's where a lower Kv motor (Brushed or Brushless) will have the advantage.

Torque produced by a BL motor is a function of the can length. Once you saturate the magnetic field, they all make the same torque. So a 7700kv can make the same torque as a 4600kv, provided you can supply enough power to saturate the coil. Once it's saturated, all the power in the world won't change matters. If you want more torque, you need to go to a longer can.

This is further backed up by Castle's site:

A high power capable brushless motor in electrical engineering and physics terms, has unlimited torque. We live in “the real world” so technically for us that’s not totally true, but – a brushed motor has a torque level that due to its design has an upper limit, regardless of how much power is being applied to it. That limit is low enough that you can see it clearly on an average track On the other hand, a high power brushless motor’s limit to torque in an RC vehicle is not within the bounds of the motor itself so much, but rather falls on the ability of the battery to deliver current to it. We generally don’t describe these motors in terms of “one has more torque than the other”, but rather “the 7700Kv motor is faster and draws more current than a 5700Kv motor in the same vehicle”. It draws more current, because it’s making the car go faster and doing more work than the 5700 motor is. As long as the batteries used are very good at supplying current without an excess of voltage depression (low internal resistance is good) both motors will appear to have the same torque, even though one is much faster than the other.

and this:

So think of torque as a function of battery capability only.

So low kv does not equal more torque.

[lutach]

Originally Posted by sleebus.jones

Yes, that's correct. Higher voltage will be more efficient.



Torque produced by a BL motor is a function of the can length. Once you saturate the magnetic field, they all make the same torque. So a 7700kv can make the same torque as a 4600kv, provided you can supply enough power to saturate the coil. Once it's saturated, all the power in the world won't change matters. If you want more torque, you need to go to a longer can.

This is further backed up by Castle's site:



and this:



So low kv does not equal more torque.

Castle Creations (With the Chinese slotless motors) is new to the brushless motor scene compared to Aveox (Neu included since Steve used to work at Aveox), Plettenberg, Lehner, Hacker, Mega and some other companies. Yes the can length has to do with torque mainly because of the length and/or diameter of the magnet. Take a lower turn Kv Nue motor and put it against a higher turn Kv motor and see which one has a higher Kt. I know first hand that my Neu 1515/2Y has more torque than my Neu 1515/1.5Y. I thought the .5 turn difference wasn't going to be noticed, but it is not by much though. Aveox has a nice way of calculating all of this, but I only have PDF files of it. I'll probably take a screen shot and post it here.

[sleebus.jones]

Originally Posted by lutach

Aveox has a nice way of calculating all of this, but I only have PDF files of it. I'll probably take a screen shot and post it here.

Cool, I'd like to check that out.