Is a lower Kv motor better suited for my battery?

[hypertoric_amplituhedron]

I have a 35mm DD motor right now, with an 11.46 Kv. The battery is 24s, and will be run at 40a peaks. No long duration hill climbing, but it is 90F almost year round.

I have the option to go for the same motor build, with a 7.5 Kv. I won't ever be pushing more than 40a , and I'm running 89v nominal.

So, it seems like a lower Kv would work better for my battery. I don't care about hitting 50 mph, 40 is good enough for me. And I like acceleration more than high speed. Now, if I had a 52v 60a battery, then the 11.46 Kv would be a better match.

Thoughts?

 

[MadRhino]

There is one important factor missing, to give you good advice. You don't tell the size of the wheel.

If it is a large wheel, more than 25" tire OD, your motor is too fast. It will pull so much current, that your controller will overheat limiting it to 40A. Also, if the block time is set more than 1 second in your controller program, your battery will be asked a high Amps surge in hard acceleration. You don't tell about your battery either, you should know if it will be able to feed that kind of power demand.

Then, about efficiency. This motor winding in a 25" OD wheel, wants to do 70 Mph at 100v. Yet, your low power setting won't let it and it will lag. All the speed lost in lag, is dramatic loss of efficiency. Even more loss, if you are not going to ride fast enough to be within the motor efficiency RPM.

If you are riding a 20" wheel, the winding of your motor is OK. :wink:

 

[hypertoric_amplituhedron]

Oh, that's right. It's a 26" wheel. The battery is 38.5 Ah, which is 11p of panasonic GA cells. So, I think 40a max peaks is no problem. The pack should be capable of 27.5a continuous, and 55a peak, if using the 1/4 and 1/2 manufacturer c-rating rule. The controller is rated for 40a , but that's full throttle. I'll be cruising at 20~25mph (maybe 30 for some stretches) most of the time.

I read a discussion about the high T and low T motors making the same max torque, where the high T needed less amps to get there. It just seems like a better match for what I've got.

 

[dogman dan]

I'm just wondering why you chose 24s, if you only want to cruise 25, and have 30 mph max speed?

My gut reaction is that you should run a 14s battery.

 

[MadRhino]

Yep. Running 24s is for those riding fast, and since it does push 100v components to their extreme limit, it is also for those who are willing to compromise reliability in their quest for performance.

Then, large wheels like slow motors. Your 26" wheel can be as large as 28" OD with a 2.7 to 3.0" tire. You need to build a bike that tops no more than 20% faster than your usual riding speed, unless you don't care about efficiency. If your battery capacity does exceed your range requirement, you can keep the fast motor and reconfigure your battery aseembly to lower voltage. If you need all the range you could get, replace your motor for a slow winding AND reconfigure your battery to achieve a not so higher top speed than your usual riding speed. You like to ride 30 Mph, build no less than 35 and no more than 40, for optimal efficiency.

 

[hypertoric_amplituhedron]

I'm just wondering why you chose 24s, if you only want to cruise 25, and have 30 mph max speed?

My gut reaction is that you should run a 14s battery.

I ordered parts before doing enough research. I thought I had it figured out, lol. The bike isnt built yet, nor are the cells welded yet. So, I can modify it before going further.

I also thought going higher voltage and less amps was the way to do it, regardless of the build. But, come to find out, its more a matter of matching the battery to winding to cruising speed to top speed to unloaded speed.

Id like a top speed of 40mph, cruising around 20-30.

Yep. Running 24s is for those riding fast, and since it does push 100v components to their extreme limit, it is also for those who are willing to compromise reliability in their quest for performance.

Then, large wheels like slow motors. Your 26" wheel can be as large as 28" OD with a 2.7 to 3.0" tire. You need to build a bike that tops no more than 20% faster than your usual riding speed, unless you don't care about efficiency. If your battery capacity does exceed your range requirement, you can keep the fast motor and reconfigure your battery aseembly to lower voltage. If you need all the range you could get, replace your motor for a slow winding AND reconfigure your battery to achieve a not so higher top speed than your usual riding speed. You like to ride 30 Mph, build no less than 35 and no more than 40, for optimal efficiency.

Okay, awesome. Good info for sure. So, it seems like I can keep the battery the way it is, and get the 6T motor, with a 7.5 Kv. Looking at this in the sim matches most of what you say, minus the 20% over cruising speed, for top speed. I have more than enough range capacity, in the event that its not perfectly ideal.

 

[markz]

Direct Drive Motors
High Turn Count Motor = Low RPM/V (which is KV)
Low Turn Count Motor = High RPM/V

If you go Low Turn Motor you need more amps to do the same thing as a High Turn Motor, but the Low Turn can handle a ton more amps.

A way to get better torque is to gear it properly going from a 26" to a 20" this will reduce your top speed, but give you crazy amounts of torque because you geared right. Plus your controller wont get as hot, nor will your motor get as hot. So going Low Turn = High KV you increase the speed but since you going smaller wheel diameter the speed slows down some too. Great for climbing hills with a direct drive motor.

A Low Turn count motor has shorter strands of thicker wires, hence it can handle more amps. It also has less resistance in the windings.

A High Turn count motor has longer strands of thinner wires, hence it CANNOT handle more amps, and the resistance is higher.

If you have a choice of 3T to say a 10T, then you gotta look at what you already have. Look at your battery, whats its charged voltage, whats it discharge amps. Look at your controller, what can it handle. Look at what you want to ride, if you dont want to go fast, but you want to climb hills then a 5T or 6T may be good.

Go to http://www.ebikes.ca and look at the motor simulator and compare.

 

[d8veh]

40 mph with a 26" wheel is 536 rpm. For the 7.5kv motor, you'd need 72v to reach maximum speed. For the 11.46kv one, you need 48v. Simples.

 

[dogman dan]

Just build your battery 18s 0r 20s, unless your controller can't go that low. Or you have a ton invested in 24s charging.

Or, relace the motor you have to 20", with the needed changes to the frame for 20". THATS your best approach, 20" wheel.

 

[hypertoric_amplituhedron]

Hey, thanks for the extra replies, everyone!

I've played rather extensively with the ebike sim, especially with its new kv ratio adjuster.

All things considered and simulated, the 7.5 Kv motor in a 26" wheel, with 89v nominal, 40a max, is the best match for my build. I already have the charger and BMS, too, so I'm invested.

Loaded, the bike should top out at around 41~42 mph , which is perfect for what I want. The battery is 38.5 ah (11p of GA cells) , which I will limit to 20~25a for most riding, through the 40a controller. I don't see any major issues here.

I'll probably get an 18fet 4115 for the 100v battery, since the 4110 is rated to 95v on the safe side. Plus, I really like the idea of changing controller settings on my phone, on the fly.

 

[MadRhino]

The 4115 fets are higher resistance than the 4110, but a low Kv motor won't pull much power anyway. I believe it is a good choice for your requirements.

The 4110 are good for your battery but not all of them. I charge 105v for years, sometimes as high as 108v. I have a few 18X4110 controllers that are reliable at that voltage and feeding very high power, but I have fried many before getting there.