Sabvoton MXUS 3k V3 3T Top Speed issues

marcchep

10 µW
Joined
Aug 19, 2019
Messages
5
Hello people,

I recently finished building a 26' electric bicycle with a Sabvoton 72150 and an MXUS 3k V3 3T motor with 20s lipos. The top speed is only around 85km/h on 100A DC Current and with 50A Flux weakening current. The bike basically accelerates really hard up to 85km/h and then just stops accelerating, it feels like it runs into some sort of limit. Has anyone an idea what that could be?

Regards Marc
 
How fast does the wheel spin off ground?

If it's exactly the same as the on-road speed, then the controller is limiting it. THat's the most likely given the description.

If it is faster, by 10-20% or so, then it may be limited by the motor's top speed at the voltage you're at, or by the power available.

What is the actual current / power being drawn from the battery? (not what the controller or display says it is, but measured with an ammeter or wattmeter)
 
How fast the wheel turns of the ground I need to find a way to test that. I just hooked an ammeter to the bike and when going top speed the motor only draws around 76A, but when accelerating it is closer to 100A.
With the 11.9kV of the 3T motor it should get to 100km/h easily on 74V.

Thank you both for your help thus far! Really appreciate it!
 
marcchep said:
How fast the wheel turns of the ground I need to find a way to test that.
How are you measuring on-road speed? If the kit has a speedometer, move it to the motor wheel (if it has one but no sensor you can move, it's probably reading the halls in the motor and would just work either way).


I just hooked an ammeter to the bike and when going top speed the motor only draws around 76A, but when accelerating it is closer to 100A.
Then it's probably not power limited.
 
I measured on-road speed with GPS.

Today I increased the dc current from 100A to 120A and the bike still only reaches around 85km/h.

Could it be a controller setting that's not right?

Regards, Marc
 
Sure, there could be a speed limit in play. That's where measuring on-ground and off-ground speeds to see the difference (if any) would be useful.

You could get a cheap bike speedometer to do it, but if the system has an LCD display, I'm surprised it has no speedometer built in.
 
It does not have an LCD, so there is no speedo. But I figured out another way to measure it, and I basically get the same speed off-ground that I get on-ground.

So with the speeds basically the same, what should I do?

Regards Marc
 
Pole pairs 23, max erpm of controller (maybe) 24000, —>max possible rpm 1043

85km/h= 23m/s=1380m/min
Circumference 26”x2” wheel: 2.05m

1380/2.05=673 rpm or 15500 erpm
Nope, it’s not erpm limit of controller..

kv 11.9*72V*0.85=728rpm=89km/h
That’s not far off from your measured speed. 0.85 factor in calculation is an assumption on loaded/unloaded rpm, it’s ( in my experience) roughly correct. Then you have the field weakening which increases effective kV at the cost of max torque so you will be bale to get more speed if the power and torque is enough.

I guess you are limited by being close to max speed combined with too low power or torque. Let’s check the needed input vs output

Power output from battery 72V*100A*( 85% efficiency )= 6120W

Power needed: 5845W (see link)
https://www.gribble.org/cycling/power_v_speed.html?units=metric&rp_wr=90&rp_wb=30&rp_a=0.6&rp_cd=1.15&rp_dtl=0&ep_crr=0.01&ep_rho=1.22601&ep_g=0&ep_headwind=0&p2v=200&v2p=85

It roughly checks out. Depends on your frontal area/position, drag coefficient and tyres etc. You could need a lot more than this in worst case. if you’re a small skinny guy, tucked riding position, road tires rider then your achieved speed seems too low but not otherwise :D

You can check noload speed at full charge and low charge and compare them, if they roughly scale vs same kV then you’re not limited by other than the performance limitations of your kit.
 
Thank you for your expansive answer, I understand what you are saying but the problem is that even unloaded the speed is the same.
And when riding at full speed I am not even at 100A. Adding to that I am a really skinny dude (55kg) and when doing the full speed test I rode tucked. Also raising the Current to 120A did not help at all.

Regards Marc
 
It sounds about right to me. I have a qs 205 50h 4t, that has very similar kv as the maxus 3t.
I have a smaller wheel though, but I measured 80km/h with a 3.00 17" and about 80V.
Then I was drawing 70-80A, the motor looses power when it is "catching up" to the voltage.

Up to about 60km/h it can take a lot of A, but then it tapers off.
I have only started to play with field weakening, but so far I have "only" reached 92km/h.
That is what the display showed, I haven't confirmed with gps yet. And it sure was picking up slow after 80km/h
 
larsb said:
Might be field weakening cannot give anymore speed or it doesn’t work in controller?
Not all motors have large FW capability either..

What current can your battery supply? Is it close to 150A?

My max battery A setting atm is 300A. I haven't tried higher yet, I have just started using this controller. I dont know if I can log sag with this controller, but when I maxed out the SFOC5 with 150-200A I had about 4V sag.
It is hard enough to control the bike with these settings (500A phase and 300A battery) with dirtbike tires in the cold, dark and wet weather we have now. (probably about the same weather you have :wink: )

I can set up to 350A with a short burst of more. Now the burst is set to 25A, I don't know how high I can set that.
I am using my nucular 24fet I got at the start of this week. The 80km/h with a 3.00 17" tire is older though.
Now I have 3.00 16", but the speed seems to be about the same.
I Just checked with gps, my readings are just a hair high. I would say my 92km/h with field weakening is 90km/h with gps.
( I only tested about 75km/h, but the readings was 1-2km/h high all the time)

It sure is faster than without field weakening, so it must be working. I have tried 30A and 60A field weakening, I didn't notice very much difference. But it was only one try each. I am not sure witch run I got 92, but they were rather similar.
 
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