Testing WeakResponse 6 

If you set WeakResponse to 7:none, field weakening will be off. The dashboard will still show you when back EMF is limiting your speed and it will display “Weak”, but it won’t apply field weakening. The motor rpm will stop increasing. If you use WeakResponse 0-6 once you see “weak”, the motor speed will continue to increase as field weakening current is added.

@1:12 with field weakening off (WeakResponse 7), you see I give full throttle and amps goes to 90a, which causes voltage sag to about 60v (nominal). The bike speeds up then back EMF is now 60v already so “weak” appears in the dashboard. This forces current to decrease which allows voltage to increase some, which does give me some top speed but even it settles out at 40a DC for 70kph.

@4:45 I have field weakening on. (WeakResponse 6, but anything 0-6 is on). Now i give full throttle and as it speeds up and you see “weak” appear, DC amps stays at about 80a. Back EMF cant push it down to 40a because field weakening has reduced back EMF. At 80 kph it begins death wobble so I let off throttle. As long as battery voltage at the Fardriver stays at or above nominal, (60v in my case and 72v for you), field weakening will make it faster. But if there is severe sag, either due to battery cells, series buss, battery cable from battery to controller, field weakening will go into a death spiral because battery voltage drops and the controller sees it needs to add more field weakening current, which causes more voltage sag, and eventually you are burning the motor up going very slow. This is where ratios in speeds can help prevent death spiral by limiting phase current as the motor gets faster. This in turn limits DC current which in turn limits voltage sag, preventing the death spiral situation.

Mainly here on RU-vid. I myself am still learning about the field weakening settings and some other settings not well explained in the Fardriver documentation. I try to devise tests that I can do using different values for WeakCharacter and WeakResponse to see what they do but it hard to see it without going faster than my ebike is stable at. Lol There are so many interrelated factors, and no single set of settings is ideal for all ebikes, or else they would just lock the setting. I also try to compare other controller brand FW settings to see if there is any correlation. We know the KO MOTO uses basically the same software as fardriver, especially the PC app, and they don’t explain settings well either, they just say they recommend using WeakCharacter RSP 0 and use the current limiting function of Ratios in Speed to rein it in. The Sabvoton seems to let you specify how many amps of field weakening to allow, which seems much easier to set, as each amp limit would result in a new higher max RPM as long as the battery was not sagging more than the FW current was decreasing back EMF voltage. So in my tests im trying to see exactly what WeakResponse changes. Is it an amp limit? Is it a percentage of total amps allowed as FW current? Or is it just using a different algorithm to get to the same objective? In the test in this video WeakResponse 6, which is supposed to be the lowest level of field weakening, seemed to be self restraining before reach any RPM or current limits. Settling in at 80a (of the 100a dc setting) and 90a (of the 200a phase limit). Whereas if set on WeakResponse 0, its going to keep the DC amps at max (100a in this case) and anytime back emf pushes it below 100a, it would divert that to FW current, until everything comes into balance. The reason WeakResponse 0 needs governing by ratios in speed current limit is because no motor can survive max amps of the correct fardriver for sustained periods without frying. All the fardrivers are capable of giving about 4x the rated current of the recommended motor, where rated power is the maximum continuous power without going over 70c. If you experiment with WeakResponse7 (no FW), at max acceleration you will feel (and see in the app) when back EMF has decreased the voltage differential between the battery and motor to the point it can no longer push max amps, so amps will slowly decrease until the motor reaches a balance. But if you set a limit on rpm and set WeakResponse 0-6, it will hold current higher by adding field weakening during acceleration, and then at the top FW current should go back to near 0 because the speed limit or current limit is back down to the same current as without FW. In my testing, because my noFW top speed is 72kph, it’s not worth 100% more amps to get 10% faster, because my motor will heat up twice as fast. I also try to watch other RU-vid videos where they show the settings and the dashboard screen recording, but these are very rare. The dashboard will show if FW is on but it won’t show you the weakresponse setting or ratios in speed table.

If you set WeakResponse to 7:none, field weakening will not come on, but you will still see “Weak” appear on the app when the motor is running at rated rpm, meaning its generating the back EMF equivalent to your battery voltage at that time. If you can make a screenrecording, showing your fardriver settings, then a acceleration and top speed run, and post it, you can show what your controller is doing and I can look and see why it’s doing it.

If you want to do a simple test to see when field weakening is actually working, and when its not, do this very simple test: Set WeakResponse to 7:none, which disables field weakening. Make sure you have set correct pole pairs so you get accurate rpm reading. Set MaxSpeed rpm to 2x the rated speed of your motor so it’s unrestricted. Raise the rear tire off the ground and gently increase throttle until full throttle. Note the RPM, this is rated RPM at that voltage. Next, add 20% to the rpm above and enter that in MaxSpeed field. Example, you had 1000 rpm, add 200 to get 1200 rpm. Next set weak response to 0, and gently increase throttle to 100%. You will see now the motor can run 1200 rpm instead of 1000 rpm. These are unloaded conditions which have very little back sag. On the road the same test with field weakening off will be lower rpm because the voltage will sag, and then when you turn field weakening on, if ratios in speed are set to create a taper, it will find a balance point. Once you understand the very basics, you need to understand how the battery’s power affects the stability. If you have a weak battery that sags more than 0.5v/ cell (10v for a 72v li-ion pack), it means your MaxLineCurr is set too high and the battery sag will fight with field weakening. Remember field weakening is needed when the motor creates more voltage than the battery, so as battery voltage sags, the controller will send more field weakening current and less current for making the bike go faster. If this current is causing battery to sag too low, the controller will add more field weakening current. If it sags way way too much you will get LVP cutout on your controller, or the BMS will get LVP. All this will be shown on your screenrecording of your fardriver if you start screenrecording, scroll through the settings top to bottom, then set it on Graph and record the dashboard behavior during full power acceleration and top speed.

@@imho7250 ill do that next time I ride as I should have a tune that is much better but ill let you look over it and @ you when I post it