Motor KV and torque ratings

[trialspower2]

Hello,

I have built an electric trials bike and custom speed controller with an 120100 outrunner motor. The bike is performing well, however I am looking to further improve it by upgrading it to a 15470 motor. The bike is geared to do around 40mph on 84V, the 120100 is a 50kv motor making around 7A no load current.

With the 15470 outrunner I have two options. I can either get a 50kv motor and keep the current gear ratio, or get a 27kv motor and reduce the reduction ratio. Now I am sure in an ideal world without any losses, at the same voltage and current this would give exactly the same result. But in this application, I am looking for as much torque as possible. Considering the losses inside the motor at low RPM, would it be better to keep with 50kv motor and high gear reduction? As the way I see it in my head, both motors have the same magnets, so this option will likely make more torque?

Or will it follow the theory where both motors have the same rating, so by changing the gearing it will effectively give the same result?

I am also interested in no load current, now in my head I am thinking the lower KV motor will offer a lower no load current, so better efficiency at top speed, but less torque......

 

[squeegee]

Hi, Did you go ahead with the new motor?

 

[thepronghorn]

At the same voltage and current geared for the same top speed, every winding configuration will be equal. If you want more torque, you need more voltage, more current, and/or lower top speed (aka larger gear ratio and/or lower Kv motor).

 

[John in CR]

None of the above. The lower Kv winding needs about twice the voltage and half the current for identical results. Then they would go the same rpm, require the same gearing, have the same efficiency (assuming same current fill), and make the same power and torque. Torque potential is determined by the magnets and steel. The copper (ie the winding) just determines the voltage and current to achieve the same result.

If the 50kv version turns the rpm you need at the voltage you use, then that's the way to go. The Kv tells you the torque constant of the motor. 9.549 divided by the Kv in rpm/volt is the torque constant Kt in Nm/A. If 2 motors have the same Kv, then they will make the same torque with the same current, so without more current you'll see no more performance.

Geared for a top speed of only 40mph a quality 120100 should be capable of more torque than you can handle unless you're talking about something more the weight of a motorcycle than an ebike, so what you need is more controller, not a change in motor.

 

[thepronghorn]

None of the above....

Read what I wrote again. I said the same thing as you. The OP needs more voltage or current aka more controller or to gear for a lower speed. Lower Kv plus same current equals more (peak) torque although it will heat up more as well. For a trials application, I assume it will be ok.

 

[John in CR]

At the same voltage and current geared for the same top speed, every winding configuration will be equal....

I read this again and it didn't get any better.

The bottom line is that different windings don't change the capabilities of a given motor. Furthermore, motors with the same Kv make the same torque with the same current as long as neither is pushed past it's current handling ability into saturation.

A quick search for both motors the OP talks about show either can handle over 20kw input. That kind of power level geared for a top speed of only 40mph should have gobs of torque and give flip you on your ass performance capable of pushing even a 200kg all up load (bike + rider). If he's coming up short then I'd say try more controller first unless the motor is being abused and already has heat issues. If the motor isn't already near it's rpm limit, then increasing the voltage and giving it more gear reduction is an option too.

 

[thepronghorn]

At the same voltage and current geared for the same top speed, every winding configuration will be equal....

I read this again and it didn't get any better.

The bottom line is that different windings don't change the capabilities of a given motor. Furthermore, motors with the same Kv make the same torque with the same current as long as neither is pushed past it's current handling ability into saturation.

A quick search for both motors the OP talks about show either can handle over 20kw input. That kind of power level geared for a top speed of only 40mph should have gobs of torque and give flip you on your ass performance capable of pushing even a 200kg all up load (bike + rider). If he's coming up short then I'd say try more controller first unless the motor is being abused and already has heat issues. If the motor isn't already near it's rpm limit, then increasing the voltage and giving it more gear reduction is an option too.

I agree that different windings don't change the capabilities of a given motor. That's not what I said.
Your statement about different motors with the same Kv is also true, but it isn't relevant to my statement because I am talking about winding configurations of a single motor.

The key part of my statement is "same top speed."

For a controller limit of 100V and 100A:
A 100Kv motor with 10:1 gear reduction will give you up to 1,000 rpm and 95.5Nm of torque at the wheel.
A 50Kv motor with 5:1 gear reduction will also give you up to 1,000 rpm and 95.5Nm of torque at the wheel.

If power is the same (controller limitation) and speed is the same, then torque will be the same. That's all my statement says.

 

[John in CR]

For a controller limit of 100V and 100A:
A 100Kv motor with 10:1 gear reduction will give you up to 1,000 rpm and 95.5Nm of torque at the wheel.
A 50Kv motor with 5:1 gear reduction will also give you up to 1,000 rpm and 95.5Nm of torque at the wheel.

If power is the same (controller limitation) and speed is the same, then torque will be the same. That's all my statement says.

...and it's still just as incorrect as the first time you said it. The only thing that will be the same other than input power potential under those conditions will be the same unloaded speed of the wheel. When you power 2 different windings of the same motor with the same voltage and current controller you get very different results that you can't make up for with a change in gearing. Assuming both models can handle that voltage and current, in actual use the higher Kv motor with greater gear reduction will significantly outperform the 50Kv rig. You only get the same results from 2 different winds of the same motor by using the correct combination of voltage and current to spin the motor to the same motor rpm.

Here's a simulation of your conditions using X5302 (2 turn higher Kv) and X5304 motors in 2 different wheel sizes for the gearing reduction. Yes at no-load they spin the same speed, but the higher Kv motor at the lower gearing will slaughter the other setup in every way. That's why guys who try to run their slow wind hubmotors in big diameter wheels burn them up on hills when they try to get any semblance of performance, while fat me climbs mountain roads with faster wind hubmotors in smaller wheels with greater all around performance without ever burning a motor up.

 

[John in CR]

That's just running the same input power. Once you add in the fact that the higher Kv motor can handle double the current due to its windings being twice as thick copper that is half as long, then the difference in performance can become drastically higher. As long as you're staying within the rpm limits of the motor it will almost always be better to spin the motor faster and gear it down more for better performance. The only exception to that is when using a very aerodynamic form and with steady state cruising on flat road in the upper ranges of speed. That's when motors with high no-load current can come back to haunt you, because it takes significant power just to spin the motor at high rpm. For operating in those conditions you'll need to do some testing to determine the best gearing to reach your planned cruising speed at greatest efficiency.