limiting motor speed by rpm rather than power?

[jedadkins@hotmail.com]

any controllers out there support limiting speed via rpm? i am building a 5kw bike and to keep it legal on the road i wanna limit the speed to 25mph (the 5kw is for taking it off road) but my area is very steep. so steep that my last 1kw bike had trouble on some of the hills,and if i am not completely wrong most controllers limit speed by limiting the power draw of but this would also limit maximum torque correct?

 

[DogDipstick]

Chose the right Kv... and you will have what you want friend.

Choose the wrong Kv, and you will have a boggin at speed, or takeoff, or too slow bike for your likes. Important to not run the controller at 100% pwm forever, right?

This is one thing that is important in a brushless.. never reach your throttle limt too ( 100%pwm) so you will run cool.

I do it. The bursts are insane, the top speed is low, the bike is fun. This is kinda the concept behig running a small wheel and stuffing ( lots ) voltage into it. The wheel is never bogged down and usually the controller wont sit loaded for long (cause you reach top speed fast) .. but thats a lil diffent manipulating the wheel size.

It is a balet of current, load, and power. At your chosen voltage.. so not to run in the lower area of PWM OR the upper limit of PWM.. for happy controller incorporation.

For M/Sec/sec, you want amps, undeniably.

Voltage and Kv is one of the most contributing factors when choosing a controller and pairing it with a motor.... Voltage (+/-) to the speed you want, ( kv unloaded) and amps ( +/-) for the umph to get there.

Not many DIY controllers do anythig in particular to "limit " speed, sometimes with switches or dial or slider in the software, but voltage is really the thing that will depend on .

I do know of some turnkey setups that probably limit on RPM input instead of straight voltage denoting the rpm max.... Im pretty sure the Bafang BBS02 does, but dont quote me on that. ( electrical speed limit based on rpm)

 

[john61ct]

Limit speed by limiting speed if that's what you want, adding a CAv3 gives flexibility.

Battery amps can be done by any setup.

Can limit by more than one factor, heat inside the motor is a critical one with hills around.

PAS can be tweaked very flexibly.

Phase amps i.e. FOC type is the smoothest, but specialized, need a motor with hall sensors on each phase.

 

[DogDipstick]

if i am not completely wrong most controllers limit speed by limiting the power draw of but this would also limit maximum torque correct?

No. CONTROLLERS limit speed based on the wind (Kv, motor constant) and the voltage fed into that (Kv)wind. This is for general cheap non fancy controllers, that use PWM in the commutation.. ( most). Fact is, more voltage on the same controller interprets to more speed.. inherent to the motor Kv....

THROTTLE controls either ( speed based like Infineons) speed or amps ( like some others, Kelly, Tq control ) or both (a blend). The PWM duty cycle I talk about is a pulsing current and power of the processor algorithm dependent on the throttling input..... of the transistors to power the motor in sequence... (simplified explanation). Most motors dont like running at very low, or maxxed out PWM. Finding a happy medium is up to how the controller handles the throttle input and outputs its current for teh desired speed/load... and not running hot ( waste heat in motor)..... (efficient setup)..

For instance I run at 35mph at 2400w, or about ~80% of my controllers max, and that is at the maxxed out Kv for my hub. Wont go any faster at top speed, load is low, so current drops, and I cruis in a 70+% efficiency range at that speed for my hub and this wind. If my hub was 12Kv ( mph unloaded) instead of 8.5 ( 50mph unloaded), I would go faster but stress the controller. If it was lower ( 4Kv) (25mph unloaded) I would have NO top speed and probably wouldn't load up the controller much at all or use many amps cruising.. (you will just run out or KV rpm at like 20mph) and the low duty cycle.

If I used a bigger wheels, the max speed would be higher and teh load greater, and the pwm higher, and the possibility of the pwm maxxed out at 100% more time than I would like, and controller stressed.. ( controller killin zone?) If you choose to slow the motor by throttle manipulation, ( like a CA3) the motor may run in an even more inefficient zone and ( 25-40%?) and heat will be created ( in motor)( based on the nature of the wind) and the controller amps are just how fast you get there ( to the speed limit) where they will drop when the acceleration load is tapered.

My bike takes 100A for the burst peak speed but hits a happy 25mph where teh tq ( 1/2 my unloaded rpm) falls and I slow acceleration to max loaded speeds. ( 35-42mph) ( voltage maxxed out Kv ( 8.5rpm/volt for me) wind here) cruising, not stressing the controller (2400w/3300w outpt) and not stressing the hub ( 70-85% eff. range) I cruise at much less, 20-40A, the duty cycle on the controller is lower, and my controller has active freewheeling so it doesn't mind low speed /duty cycle commutation, but that is a whole other area of controller theory ( active/passive freewheeling)(low duty cycles not being hard on controller) ( cheap controllers dont like low duty cycles, like RC controllers, that blow right away at low duty cycles without freewheeling when not run at 80-90%pwm where they like to run) .

The Ca3 will absolutely give you speed limit functionality if you want, too. I hope someone corrects me if I am wrong.

 

[zeccato]

I changed hub and I'm adjusting the speed (even pas to 60%), and at 12Ah and 30 phase I still see too much power and torque, and I have to lower the continuous amps again.
I think infineon 12 fet (mine is em3ev) would work for you, or the nucular (6f?).

 

[amberwolf]

http://ebikes.ca/cycle-analyst.html or http://ebikes.ca/cycle-analyst-3.html will both do what you want.

the ca is designed as an interpreter for your throttle (and pas, if any), based on whatever settings you choose. (you should read the *entire* pages linked, including the manuals, to see all the possibilities, and all the things you'll need to setup for your particular application).

your throttle (and pas, if any) connect to the ca's inputs, and then the ca's throttle out connector sends it's own modified throttle signal to your controller, limited or augmented in whatever way you set it up to.


if you're up for designing circuits (or programming arduino or similar mcu), you can make something that takes a speedometer signal and your throttle as inputs, and limits throttle as you approach the speed limit you've set it up for.


keep in mind that limiting speed will limit throttle. throttle controls the amount of power / torque your system produces. so if your limiter is trying to limit speed, while you are trying to climb the hill, it's going to limit the power / torque, too; there's no way to avoid that.

if it is not limiting speed during the hill climb, then it won't limit the other either.

 

[dogman dan]

Yep, if you get a CA installed on your controller, you can limit amps, limit speed, etc. Then with a few easy pushed on the buttons, restore full power for the off road riding. With speed limiting enabled on the CA, you still can get your max amps, but as soon as it hits max speed, it will just stop putting in power. then you back off throttle enough to stop hitting the limiter, and ride steady power and speed. Helps a lot with the problem of throttle creep, when you have a lot of power available.

I assume you already have the motor. But next time, to still have a strong bike, yet lower top rpm, this is the only reason to choose the low rpm wind motor.

Most just build for speed, and just back off throttle for slower riding. But if you really want a slower bike, yet still have full power, a low rpm motor makes it work out to lower top speeds even when running full power.

Its NOT a more torque motor, though too many call them that. But its going to be slower at max rpm, which sounds like what you would like.

Smaller wheel will get you more torque, but surprisingly not that much less speed. You'd think a 20" wheel would be a lot slower, but it will still wind out to a very fast rpm, helped by the better torque of the smaller wheel. So just a tiny bit slower than same motor in 26"