Speed Calculator

[BrianG]

Ture, lipos peak at 4.2v/cell, but the voltage does not stay there very long at all when running - just take a look at any discharge graph. People generally use 3.7v/cell because that is the voltage the cell is at for the majority of the time. As a matter of fact, depending on setup, the voltage may actually fall to 3.5v/cell or lower during higher current bursts.

Also, it depends somewhat on the user.

- If you are looking for max torque specs, use a lower voltage/cell rating. High torque is generated by high current. High current causes the cell voltage to drop.

- If you are looking for max speed spec, there will be less current draw when the motor is spun out, but the cell still won't be at 4.2v because the current drawn will still pull the voltage down.

It depends on gearing and weight too. If you are geared high and/or have a heavy vehicle, expect more v drop on take-off acceleration due to the current needed. If geared low and/or have a lighter vehicle, v drop will be less.

It also depends on the battery quality and capacity/C rating. A higher quality/capacity/C rating battery will maintain a more constant voltage under load.

[macr0w]

Originally Posted by BrianG

Ture, lipos peak at 4.2v/cell, but the voltage does not stay there very long at all when running - just take a look at any discharge graph. People generally use 3.7v/cell because that is the voltage the cell is at for the majority of the time. As a matter of fact, depending on setup, the voltage may actually fall to 3.5v/cell or lower during higher current bursts.

Also, it depends somewhat on the user.

- If you are looking for max torque specs, use a lower voltage/cell rating. High torque is generated by high current. High current causes the cell voltage to drop.

- If you are looking for max speed spec, there will be less current draw when the motor is spun out, but the cell still won't be at 4.2v because the current drawn will still pull the voltage down.

It depends on gearing and weight too. If you are geared high and/or have a heavy vehicle, expect more v drop on take-off acceleration due to the current needed. If geared low and/or have a lighter vehicle, v drop will be less.

It also depends on the battery quality and capacity/C rating. A higher quality/capacity/C rating battery will maintain a more constant voltage under load.

I did some speed runs back and forth in front of my house on and off for 30 minutes or so and still had 15.4 volts on my battery this afternoon.

It was SCREEEEAAAMING!!!! The speed and acceleration is mind blowing! I now know what "Stupid Fast" really means.

[Patrick]

Originally Posted by macr0w

I did some speed runs back and forth in front of my house on and off for 30 minutes or so and still had 15.4 volts on my battery this afternoon.

That would be unloaded voltage though. Voltage under load is what's most important, and is one of the main differences between a good battery and a not so good battery. 15.4v does mean that the pack probably had about half is capacity left though.

[macr0w]

Yeah, it's a 25c 5000 mah 4s Polyquest/Enerland pack.

So far I'm really happy with it.

[brushlessboy16]

Brian could you add some specs for Outrunners?

[BrianG]

I generally limit the motor choices to what Mike carries. As you can see, the motor list is getting quite long already. As a mattery of fact, I'm thinking of removing the LMT motors altogether...

[lutach]

Originally Posted by BrianG

I generally limit the motor choices to what Mike carries. As you can see, the motor list is getting quite long already. As a mattery of fact, I'm thinking of removing the LMT motors altogether...

Please add the Megas Acn22 series.

[BrianG]

I will. I was waiting for the Muggy diff ratio from BlackedOutRevo...

[BrianG]

Griffin: So, it's not just a pure kv factor, it matters how many rotor/stator poles a given motor has as well. Hmm. I was hoping for something like "min rpm for proper ESC operation is motor kv/10", or something along those lines. I really don't feel like adding a couple more specs to each of the motors in my list.

Most of the time, manufacturers don't supply the stator and rotor pole count. Is there a way to get this information manually simply by turning the motor shaft and counting the magnetic "detents" during one full rotation? For example; a Feigao/Wanderer/Nemesis is a two-pole and has two magentic detents per shaft rotation. A Neu is a 4 pole motor, but has 12 magnetic detents per rotation.

macrow: ballooning depends a lot on the tire and the foams used inside. For a tire to balloon out, the sidewalls have to contract and good/harder foams will help limit this. 1" ballooning on a 4.25" tire is a 23.5% increase in tire size! That would make the tires look like pizza cutters and would seriously hamper handling. The calc only show theoretical speed, ot theoretical controllable speed.

[GriffinRU]

Originally Posted by BrianG

Griffin: So, it's not just a pure kv factor, it matters how many rotor/stator poles a given motor has as well. Hmm. I was hoping for something like "min rpm for proper ESC operation is motor kv/10", or something along those lines. I really don't feel like adding a couple more specs to each of the motors in my list.

You should be able to do that by turning the motor shaft by hand and measuring the back EMF. (Eagletree is sufficient to do this...)
And you need to check only one phase.
Count the number of zero crossings per one rotation and than the inverse of that will give you the minimum rpm. (1/# of zero crossings)

[macr0w]

Originally Posted by BrianG

macrow: ballooning depends a lot on the tire and the foams used inside. For a tire to balloon out, the sidewalls have to contract and good/harder foams will help limit this. 1" ballooning on a 4.25" tire is a 23.5% increase in tire size! That would make the tires look like pizza cutters and would seriously hamper handling. The calc only show theoretical speed, ot theoretical controllable speed.

I'm running these in the road. And they do look like pizza cutters at full throttle. And handling goes out the window. From the looks of it 1 inch ballooning was a conservative guess. That would only be a half inch expansion all around.

[BrianG]

If I don't have the eagletree, could I still rotate by hand and simply count the magentic detents?

[GriffinRU]

Originally Posted by BrianG

If I don't have the eagletree, could I still rotate by hand and simply count the magentic detents?

Not with every motor...
For Feigaos and Novaks it might work.
You can use 2LED's backtoback and count flashes :)

[BrianG]

lol, that works. I'm sure I have LEDs hanging around in my parts bin.

Ok, so I count the flashes for the various motors and invert that number to get the minimum rpms? Example: If I count 4 flashes, that means the motor has to spin no lower than 0.25rpm for proper ESC operation? Seems kinda low...

Thanks for your assistance!

[GriffinRU]

Originally Posted by BrianG

lol, that works. I'm sure I have LEDs hanging around in my parts bin.

Ok, so I count the flashes for the various motors and invert that number to get the minimum rpms? Example: If I count 4 flashes, that means the motor has to spin no lower than 0.25rpm for proper ESC operation? Seems kinda low...

Thanks for your assistance!

You welcome:)
You need to count only flashes from one LED, we are searching for zero-crossing. Otherwise you need to divide total number of flashes by 2 to get zero-crossings not peaks. Sorry for confusion, I do prefer scopes :)
And it should be sufficient for ESC based on zero-crossing. There are ways to do the same with interpolation and curve-fitting which requires less rotation...
And just to make sure, it is not 0.25rpm it is quarter rotation. You can convert it to RPM but to do that you need to know the minimum voltage range for ESC analog to digital convertor. BackEMF value based on RPM, but zero crossing is dependant of that.

Bottom line if we assume that 0.25V of backEMF is sufficient for ESC than quarter of rotation would be enough for ESC logic to get going. The RPM for given motor to get 0.25V of backEMF would be driven by motor Kv. You can put this all together.