motor torque

[Gary]

I am a little confused on "motor torque". Lets say we have...just and example, a 2200kv motor on 6s and we switch to a 2000kv motor on 7s. Will the 2000kv produce more "torque" . Will a 1800kv produce even more torque? All that I have researched says yes and no.

[Speed3]

The motor that trying to rev the highest will produce the most torque. Example you have 3 motors equal in size, one is a 2200 kV, another 2000 kV and the next 1800 kV. The 3 motors has the same prop. The difference is number of cells between the 3. You have one with enough cells to rev 35000 rpm, unloaded, the other setup to run 40000 unloaded and the last to rev 45000 unloaded. The one with the highest unloaded rev will produce the most torque.

[Peter A]

While not being an expert, my understanding of electric motors is that they produce maximum torque when stalled. RPM has nothing to do with it. The amount of torque a motor produces will depend more on the rated wattage and size of motor, also the number of poles.

[Speed3]

The higher reving unloaded setup will cause the motor when under load to produce greater torque to maintain higher rpm underload that the other setups. So the rpm may drop to 33000 loaded rpm, while the other setups will have less loaded rpm. The lowest unloaded rpm will have the least torque under load. And I think it requires greater torque to spin a 445 at 33000 rpm than at 30000 rpm.

[Gary]

OK just so I have this correct. Lets say one motor example is 1900kv on 7s ...that would be 55,800rpm unloaded. Next motor 2200kv on 6s 55,000rpm . Both rpm's pretty close. So both will produce the same theoretic speed. Correct?
Speed3 I get what you are saying to a degree but if the motor does not have the grunt to spin a certain prop but is capable of "out revving" another motor unloaded it will be slower when the prop is in the water.

[CraigP]

Assuming you operate the motor within the manufacturer's limits, the biggest effect on torque from an electrical input perspective is voltage. Increasing the prime mover directly moves motor torque up. So Gary, the setup mentioned above... The one with the most volts will produce higher torque.

From the load side, mostly meaning prop, it gets MUCH more complicated! So staying with the same prop for comparisons is a good play!

[Speed3]

If they have the same winding pattern, example y wind. The torque would be close. The 1900kv would win in that case because the 2200kv would pull more amps and case the battery to fall off much quicker. So the 1900 would walk away, also winding lose would create in efficient power delivery at higher amps again favouring the 1900.

[Speed3]

I actually did this scenario in a similar boat prop, the only difference was cell count. Both motors had around the same unloaded rev like I described. The boat in the two different configuration did the same speed but one had much more runtime.

[Gary]

Ok so if I had the 2200 kv on 6s and swapped over to a 2000 kv on 7s....same prop..... boat would pick up speed because of more voltage being supplied.

[NativePaul]

Torque will be similar for similar size and mass motors of similar efficiency. KT is a linear torque constant and the lower the KV is, the higher the KT will be for a given size motor. That may lead you to think that the 1900KV motor has more torque than the 2200KV, but KT is torque per amp, so while the 1900KV has more torque at any given amp draw, the 2200KV motor gets its higher KV by having less turns of thicker wire giving it less resistance, and makes it capable of handling more amps. and pushing the same prop at the same speed with one cell less will make it draw those extra amps.

There is no big winner or loser, you just pick whether you want to run high voltage or high current, and as long the current is at a good point in the motor's efficiency curve you will get a similar result.

While I normally favour higher voltage over higher current for lower wiring losses, in that example I would avoid the 7s setup due to the plethora of cheap 6s ESCs.

[CraigP]

I'm running 8s and have to run an external UBEC. I would prefer higher voltage and less current because it makes run times longer and is much easier on the entire electrical system. The problem is if you don't control the flow of current, with setup factors, then you are much more vulnerable to saturating the motor with over current and real bad things start to happen.

I'm working on a PCB now that I call a Power Controller (PC). I reads the system current using a Hall effect current sensor and a uP reads that value. You program the PC with your laptop, like hooking up for a data logger. You set the system for max amps. If the current draw exceeds that, then the throttle signal, which is also being fed into the PC, is reduced by the uP and fed out thru an output on the PC. If the current is OK, then the original throttle signal coming in gets passed directly to the output signal unfettered. This way, you can set a given power, in KW, and test many setups for efficiency at given power settings. I also have a "Turbo Mode", where if you are at full throttle and the power is being limited, you can back the throttle off slightly, go back to full throttle, and the system will use a higher current setting for a programmed period of time. It's like Push-to-Pass button in Indy cars.

[Gary]

Wow great information. All these years of running boats and my thought process was lower kv...more torque! Now I know the facts! Thanks guys!!

[Speed3]

Like one person said lower kV is easier on the electrics, less amp. But two motors equal in size has the same torque potential. And like I said I tried it out with two similar size motors with different kv's. So I see it first hand.