Speed Calculator

[GriffinRU]

Originally Posted by BrianG

Getting back on topic; anyone know this?
What is the "rule of thumb" regarding the minimum rpm a sensorless motor has to turn for the ESC to get the proper back-EMF signals? Something like kv * 0.1?

I want to try to include this calculation in my top speed estimator to notify the user that they are geared too high for proper ESC operation...

A simple rule is zero-crossing.
Zero crossing would be sync with every pole, so for given motor number of magnet poles and armature poles per rotation would tell you what min. RPM would be sufficient for ESC to get it going. But to get direction correct you might want to get one extra, but it can be much simpler as well. If hardware can do that, than software is your limiting factor...

[macr0w]

I figure the balloon effect is at least 1 inch?

Differential Ratio: 3.3333333333333335
Transmission Ratio: 1
Other Ratio: 1
Spur Tooth Count: 46
Pinion Tooth Count: 12
Total Voltage: 14.8
Motor KV: 3300
Tire Diameter (inches): 4.25
Tire Ballooning (inches): 1
Motor Current Draw: 125
Motor coil Ω: 0
Spur/Pinion Ratio: 3.83 : 1
Total Ratio: 12.77778 : 1
Tire Circumference (inches): 19.63 inches (498.73 mm)
Total Motor Speed: 48840 RPM
Vehicle Speed: 71.07 mph (114.16 km/h)
Effective Speed Rate: 4.8 mph/V (7.71 kmh/V)
Estimated Adjusted Speed: 64 mph (103 km/h) - 10% loss
Effective KV Value: 3300
Motor Torque: 0.27 ft-lbs
Final Torque: 3.41 ft-lbs
Final Power: 1850 watts (2.5 HP)

[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)

[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.

[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.

Sorry for the late response Brian

Drive train ratio is 14.48:1

Pinion is 11
main gear is 45

That what I found searching, so if its wrong, someone please say so, but I think its right

thanks!

[BrianG]

@Griffin: lol, yeah, I like scopes too, but I no longer have access to one. Poo.

I don't think this is gonna work for what I wanted to do. It would have been nice to be able to display a minimum vehicle speed where ESC would work effectively, but it seems like there will be too many assumptions, not to mention all the types of motors out there to test. People are just gonna have to use common sense.

Oh well.

@macrow: the only time the ballooning figure would be nice to enter is if you are just doing straight-line top speed. I always use 0" of ballooning because I want to know the max controllable speed.

BlackedOutRevo: OK, thanks. I'll add the diff ratio. I was also working on adding all the Mega motor data so there is just one update. There are a LOT of 22 series motors! I might just add the two Mike has in his store...

[GriffinRU]

Originally Posted by BrianG

@Griffin: lol, yeah, I like scopes too, but I no longer have access to one. Poo.

I don't think this is gonna work for what I wanted to do. It would have been nice to be able to display a minimum vehicle speed where ESC would work effectively, but it seems like there will be too many assumptions, not to mention all the types of motors out there to test. People are just gonna have to use common sense.

Oh well.

I will get back to you on this,

[BrianG]

- Added Medusa Motors

- Removed LMT motor (so few now)

- Added the number of poles and the max recommended power rating to the Data Tables page. If anyone knows the power ratings for Neu and Feigaos, let me know.

- Added dropdown box to select vehicle type. Used to make drag-adjusted speeds more accurate. However, the drag values are the same for all types right now because I don't have enough real-world samples to go on. At least the functionality is there...

[Arct1k]

Any chance of adding the Novak HV motors - not for me but was trying to demo the app and they used them. thx

[BrianG]

I was thinking of it but I try to limit the selection to what Mike carries. It is for general purpose R/C, but I do have his logo on there. Just my way of supporting the monster without actually buying stuff.

If you supply the following data, I will do it:

- model name
- kv
- if kv is loaded/unloaded/unknown
- max rpm
- motor length
- motor diameter, weight in grams,
- flange diameter (the raised area where the shaft exits the can)
- shaft length
- shaft diameter
- motor resistance (if available)
- recommended max power (if available)
- number of poles
- roughly equate it to an existing Feigao motor with respect to power. I use a funky rating system for the motor selector.