Will a direct drive motor cap my downhill speed?

[eecsflyer]

I was playing around with the motor simulator on ebike.ca and it seems if I am going down a 10% grade my speed maxes out at 25 MPH and I feed 1KW back into the battery.

https://www.ebikes.ca/tools/simulat...k=load&batt=B36LiGox8&batt_b=B36LiGox8&hp_b=0

Does this mean if I wanted to go faster, I would get increasingly more braking from regen, making it hard to go much faster?

 

[flat tire]

Who says you need to use regen?

 

[eecsflyer]

Ok, so I could disable regen through my cycle analyst and then my downhill speed will not be capped? On a similar note, I was wondering if it matters what controller I have? Can I use a 25A Grinfineon controller, or with a direct drive motor I should opt for a controller that can handle more current?

 

[Balmorhea]

My experience with relatively low rpm-per-volt motors and simple controllers is that when the back EMF of the motor rises above battery voltage, a drag is applied to the motor and that energy flows to the battery if the battery can accept it. This was very noticeable when I lived in a hillier place, and I used a 36V direct drive hub motor bike with SLA batteries. The bike had a top speed of maybe 22 mph on flat ground, but would not exceed 29 mph no matter how steep the downhill. There was no regenerative brake function per se; but the battery would begin to charge at a speed above the unloaded speed of the motor. This happened irrespective of throttle position.

I didn’t worry about overcharging, because there was an immediate 80 feet or so of elevation gain when leaving my house, and because the high performance SLAs were rated to charge at up to 4C.

A controller with more sophisticated programming, or a battery with “same port” BMS that limits charge rate, might behave differently. I can’t really say what my more recent DD hub bike does in such circumstances, because its rpm per volt is high enough that I never reach its unloaded speed.

 

[goatman]

the simulator says youre using a 3525 with a phaserunner, if youre looking at using the 3525 youre better off with the 40amp grinfinion than a 25amp.
if you get a phaserunner it has virtual electronic freewheeling
you use regen like you use your brakes, you don't have to have it on to go down a hill. I push a button to activate my regen

 

[donn]

virtual electronic freewheeling

This causes the Phaserunner controller to inject a small amount of phase current through the motor winding even when the throttle is off. As soon as the motor stops turning (based on the stall timeout) then the current drops to zero properly so you don't have to worry about it wasting power when the bike is parked.

So the application is clear when we're talking about core loss at low speed, where I guess the "regen" per se would be miniscule. A similar effect occurs during a high speed descent (as I guess would be obvious), but ... in his simulation, he has the throttle pegged. The way it looks to me, there's a limit to how fast you can drive the wheel with a given system voltage, he's going faster than that, and up there he has no way to compensate for his "regen" effect. I sure don't know, just how it looks to me.

 

[MadRhino]

Build higher voltage. Twist the grip. Ride as fast as you wish.

 

[goatman]

its a confusing post, im going to have to go see what a, hot phaserunner is? and I don't know how you put in 1000watts of regen power back into a motor doing 25mph down a hill
I read 39volts, using ligo packs. I guess I need to go figure out
I was just out on my bike, its a 3540 at 17s with a phaserunner and virtual electronic freewheeling at 1.5amps and I was coasting down a hill at 55km/h but battery was about 65v, if anything it was the wind holding me back

 

[eecsflyer]

Yeah, with a 66V battery seems I can go 40 MPH, https://www.ebikes.ca/tools/simulat...ck=load&batt=B662A123&batt_b=B36LiGox8&hp_b=0

So I guess I need to buy a higher voltage battery. Would I take an efficiency hit with a bigger battery? Seems unless I would bike faster on average, at a higher voltage I would be on a less efficient part of the power curve. Do you guys have any experience with battery efficiency in real world use cases between a 36V setup and 52V setup?

 

[NCC1941]

So I guess I need to buy a higher voltage battery.

Your other alternative, assuming you don't already have a motor in-hand, is to simply pick a higher kv motor.
Example: https://www.ebikes.ca/tools/simulator.html?motor=M3525&mass=120&frame=full&grade=-10&axis=mph&throt=100&motor_b=M3540&frame_b=full&mass_b=120&throt_b=100&grade_b=-10&cont=PR_H&cont_b=PR_H&autothrot=false&autothrot_b=false&hp=0&black=load&batt=B36LiGox8&batt_b=B36LiGox8&hp_b=0&bopen=true

Or, if you're aiming to use LiGo's as in your sim setup, you can wire them in series for 72v just as easily as wiring them in parallel for 36v.
Example: https://www.ebikes.ca/tools/simulator.html?motor=M3525&mass=120&frame=full&grade=-10&axis=mph&throt=100&motor_b=M3525&frame_b=full&mass_b=120&throt_b=100&grade_b=-10&cont=PR_H&cont_b=PR_H&autothrot=false&autothrot_b=false&hp=0&black=load&batt=B36LiGox8&batt_b=B7208_DT&hp_b=0&bopen=true

 

[donn]

It doesn't seem to me like there are enough 10% downhill grades out there, to make it worth going to 72V just for that.

 

[goatman]

I don't know, I just learned how a motor turns into a generator when going faster than the no load top speed of the motor. but I have a question. you have 4 ligo batteries?
if so and you are using 4 ligo batteries, cant you just serial connect to 72v? and I cant remember if the grinfinion 40 amp can operate from 36v to 72v but I think it can. heres a link
https://www.ebikes.ca/shop/electric-bicycle-parts/controllers/c7240-gr.html
if that is the case you should be able to run at 36v and get 4 more ligo batteries and serial connect to 72v?

 

[eecsflyer]

I have not bought this yet. So I could get a higher voltage battery. My only concern there is efficiency when going up hills slowly. It would seem the higher voltage could kill my efficiency on the uphill..

 

[goatman]

pulling 40 amps out of a 4p battery isn't wise, I do from my 25r's but that's just a fun bike I use for quick rips less than 20km round trips not made for range. im planning on buying that 3525 from grin when they are open for pick-up, just to see if it will help on the steep hills and towing a trailer. im still learning and personally im trying to stay "bicycle" but "fun bicycle" meaning a 40amp 65lb bike that doesn't need motorbike rims and 65km/h is fast enough. I went 17s because when my battery is dead at 51v its like the feeling of a freshly charged 48v bike. you already jumped from 36v to 72v Dogman Dan mentioned he doesn't run 72v in the summer or he cooks motors

maybe explain what you need from your ebike and other more experienced members here can help you.

 

[DogDipstick]

I regularally go right near to my no load speed of 48.5mph on long downhills. 45, 46mph downhill tucked? pushing full throttle but at this point the motor isnt doing much?

Pushing itself on the flat my loaded speed is 40mph..... and top no load speed is 48.5 mph.. when you pick the rear wheel off the ground and spin it... uphill is anywhere from 30-34.

No regen on the bike, controller. No drag. Just coasting really really fast.. easy over my flat full speed...

 

[john61ct]

Going to a higher kV motor

or higher voltage battery to get higher to speeds

does not lose any efficiency when climbing hills at the lower speeds.

In fact the lower amps due to higher voltage will help a lot, since heat buildup becomes the limiting factor as low voltage torque increases.

HOWEVER having higher speeds easily available on the flats WILL result in much lower average range "pushing the wind"

unless you are one of the very tiny percentage with willpower

or actually set a self-enforced speed limit, as with a CAv3.

However, not having gearing will limit the weight and slope you can do, at all, never mind efficiencies. Trick at the limits will be keeping your speed up, there may be times standing starts are too hard, need to go back down to the bottom to get a running start.

The real ideal there is a mid-drive + powerful gearing, and scale up to the separate-chain big block style if you have to for heavy weight and/or very steep hills

 

[MikeSSS]

Like Balmorhea said, back EMF limits ebike top speed for a direct drive, often to less than pedal bike top speed.

 

[JackFlorey]

Going to a higher kV motor or higher voltage battery to get higher to speeds does not lose any efficiency when climbing hills at the lower speeds.

You will lose a bit since
1) Higher voltage FETs have a higher RDSon, in general
2) Very low pulsewidths are less efficient from a controller perspective. (Higher peak currents, and losses go as I2R.)

In fact the lower amps due to higher voltage will help a lot, since heat buildup becomes the limiting factor as low voltage torque increases.

Only true on the battery -> controller side. From the controller to the motor, you have the same current you did before (for the same torque.) So if motor heating is the issue, that won't help.

The real ideal there is a mid-drive + powerful gearing, and scale up to the separate-chain big block style if you have to for heavy weight and/or very steep hills

Agreed; that's ideal for hills. (And with a mid drive there's no issue with regen above base speed of course.)

 

[dogman dan]

Backing up to the original post, the answer is yes.

Coasting down a hill, throttle at zero, your motor will slow you down. A faster RPM motor will still let you hit 35 mph easy, and on some hills that can be a good thing. Especially if your weak frame lets a bike overloaded with batteries wobble at 40 mph. I chose low rpm motors for my cargo bikes for just this reason, I liked it better limited to 25 mph down the hills. I live in the rockies, so my hills can be 10 miles long. Regen slowed me too much, to 5 mph.

To eliminate that resistance, open the throttle. its not using much watts, but it opens the motor to go at a fast rpm without any resistance. RE Mad Rhinos comment about 72v, the higher voltage the bike is, the higher its max no load rpm. So a 72v bike with the throttle on can coast down hills VERY FAST. Past max no load rpm with max throttle on, the motor will again, begin to resist.

To enjoy normal bike coasting speeds, go to a geared motor, or mid drive, which have freewheeling.

 

[DogDipstick]

Backing up to the original post, the answer is yes.

Coasting down a hill, throttle at zero, your motor will slow you down.

I beleive the experience, friend, but how does this explain my 8.5Kv motor feels 0 drag as I coast downhill at higher than my top loaded speeed? Am I special? I have a ton of datalogs showing it. I top out at 40mph but can run downhill faster than that. Is it because I give full throttle, and if I lower to 0 throttle, the drive will drag? Here in the log you see the highest top speed is 43mph but only 40mph loaded ( 24.4sec loaded full th, 24.8sec unloaded, full th) I guess I am not really coasting.

TY DogmanDan, for clarifications..

 

[donn]

Can you get data for the motor with really no load, like propped up with the rear wheel off the ground?

 

[DogDipstick]

Can you get data for the motor with really no load, like propped up with the rear wheel off the ground?

Sure here:

 

[donn]

So do I see you running over 48 mph with no load? The way I understand what we've been talking about here, you aren't ever going to notice this effect, because there's no such thing as a 10% grade where you can get up to 55 mph.

 

[DogDipstick]

So do I see you running over 48 mph with no load? The way I understand what we've been talking about here, you aren't ever going to notice this effect, because there's no such thing as a 10% grade where you can get up to 55 mph.

Not over, but right near, and higher thatn the loaded rpm on the flat. But not coasting.

Feels like 50mph is right there. If I had a bigger hill maybe, but yea never over the no load rpm....so far.