ACM now uses 12 MOSFETs instead of 6

[Mistagear]

If one individual mosfet fails, does the wheel fail ?

If so, then adding more can, both decreases and increases the risk of failure.

Whether you have a nett gain would then depend on the reason for mosfet failure ?

If the only reason they fail is from overheating then you would have a gain. If however they fail from other causes other than heat, then you may have increased the risk of failure of the wheel 

In Motorsport, there is a saying, "If it's not there, it cant fail"  

So when designing and building racecars, you always try to achieve the exact same thing with the least number of parts

[OliverH]

3 hours ago, esaj said:

As usual, this is pretty much guesswork and combining the puzzle pieces I've picked up when reading about this stuff, so take it with bagful of salt:

 Don't know about the wheels exactly, but based on what I've read and still remember, there are multiple ways to destroy a mosfet (these are probably not even all of them):

 

Someone mentioned that the "easiest" way to destroy a mosfet is applying too much voltage (difference) at the gate (going beyond the absolute maximum Vgs). The gate is what is used to control the mosfet (not conducting / "partially conducting" ie. linear region / fully conducting ie. active region). As the above picture shows, there's a layer of metal oxide between the gate and the channel that that the actual current flows through the mosfet from drain to source (for N-channel mosfet, or source to drain for P-channel). A high enough voltage difference between the source and the gate can cause a strike-through from the gate to the channel (or vice versa), shattering the microscopically thin insulator layer (that picture's not to scale). Yet for the mosfet to conduct, there must be enough voltage difference between the gate and the source...

I don't know how the actual gate drive chips in the wheels work, or if they will under any circumstance allow the gate voltage to go "out of bounds". Another possibility (although I think unlikely) could be careless handling during assembly, an electro-static discharge (ESD) from a finger could damage the insulator. Short-lived static discharges are "evil" in the sense that in case of many (active) components, they can cause internal damage that's not obvious immediately. The device can keep operating normally for a long time before finally failing:

 

Also exceeding the maximum voltage between the drain and the source can damage the mosfet, although I'm not familiar with the actual effect it has (maybe something to do with the channel structure?)

However, my guess would still be that the mosfets simply overheat under high stress situations. "Too much current for long enough time" (which, depending on current could be from very small fractions of a second to minutes) will overheat the mosfet. The are also (at least) two types of losses that occur in the mosfet: conduction losses (the "normal" voltage drop that occurs when the mosfet is fully conducting) and the "switching losses", that are due to the mosfet passing through the "linear/ohmic" region, where it actually acts more like a resistor, more "ohmic" resistance = more voltage drop over the mosfet = more power dissipation.

If the gate voltage hasn't reached high enough value (or for some reason doesn't at all), it won't be in full conduction (active region / saturation), and as long as it isn't in the active region, it's actually dropping more voltage, thus causing more power dissipation in the mosfet itself and it heats up. When the mosfet gate is being pulsed with the PWM-signal, it actually opens (starts to conduct) and closes (stops conducting) as the signal goes up and down. Each time it does that, it takes some time for the voltage to raise and drop (due to gate charge, kind of like a capacitor, it needs to be "charged and discharged" for the voltage to change), and the mosfet "travels" through the linear region:

On the latter picture in the upper graph it can be seen that while the Vds (voltage difference between drain and source) is going down (the mosfet is beginning to conduct), the Ids (current flowing from drain to source) is already starting to raise. The lower graph shows the power dissipation (losses) occurring in the mosfet, and they spike up during the transition period from off-to-on and vice versa. My guess is that that's the reason why especially high-power wheels tend to make that high-pitched noise, probably they needed to drop the PWM frequency, so the transitions aren't done as often, and the frequency is now in the audible frequency range (on the wheels you can't hear it, it's probably still there, but so high frequency that you can't hear it?).

The situation is even "worse" when the current is flowing in reverse through the body-diodes, as those have a high(ish) voltage drop, something like 0.5-1V, and it can even go up with higher currents. Unlike when the mosfet is "normally conducting", the power dissipation won't drop at any point during which the diode is conducting. If the braking isn't done in a manner that they can open the mosfets to conduct "normally" (I believe I've read that a mosfet can allow current to flow even in reverse through the channel, as long as the gate-voltage is sufficiently different from source), the regeneration happens through the body diodes, and that could at least explain why some wheels seem more prone to blow them during power braking.

All in all, I doubt the gate voltage actually would be the problem, more likely it's simply a matter of overheating.

 

Can you explain the advantage of the IGBT over MOSFET? Why do the IGBTs in a Tesla last longer with higher current? What does Segway use?

I'm quite not sure that the piggy back MOSFET with this small heat sink (even if attached to the big one) can handle the heat transfer as the downside MOSFETs also produce heat to be transferred somewhere. My feeling: I should wait for the next revision of heat sink design Gotway should rename the ACM to DIP (development in progress) It's nice to see that they constantly improve the EUCs and specially rise the reliability of the main board. But may be it's time to split the motor driver from the core main board to a different PCB to place the MOSFET/ IGBT in a way the heat can be better transferred.

[Chriull]

36 minutes ago, OliverH said:

Can you explain the advantage of the IGBT over MOSFET?

The main differences between IGBT and MOSFET seem to be:

- The IGBT has a saturation Voltage, which means the voltage drop between Collector and Emitter ("ouput" side) has a minimal value of something about 2V (Vce-sat)

- The Mosfet has an minimal resistance between Drain and Source ("ouput" side) - Rds-on in the range of some milli Ohm (~3-10)

So the power dissipation of an IGBT is this ~2V times the current. For a MOSFET the power dissipation is Rds-on times the square of the current. So a IGBT with Vce-sat of 2V versus a Mosfet with Rds-on of 10 mOhm "meet" at 200A load current. Below the MOSFET has less power dissipation, above the IGBT.

- The IGBT has a negative temperature coefficient - under high loads the "output resistance" drops a bit. (possibility of thermal runaway!)

- The MOSFET has a positive temperature coefficient - at higher temperatures (~120°C junction temperature) most MOSFETS have 1.5 to 2 times Rds-on than with 25°C

IGBT can handle higher voltages (>1000V) than MOSFETS.

MOSFETS have better switching capabilities than IGBTs, so they are used for higher frequencies - this should not matter in any way for the couple of  kHz used in EUCs.

So as it seems MOSFETs are for EUCs definitely the better choice in because of the power dissipation. With two MOSFETs in parallel one reaches with the models used by Gotway/Kingsong (?and also now in the newest Ninebot boards) ~5mOhm even at the higher junction temperatures. At 50A one eighth of the power is burned by these MOSFETs (12,5W) than with an IGBT with 2V Vce-sat (100W).

Somewhere i have read that the theoretical lower limit of Vce-sat of a IGBT is one diode forward voltage - so about 1-1,4Vs. So even with such a model the power dissipation at 50A would be 4 times that of a MOSFET.

Additionally switching characteristics of MOSFETs should be better than the ones of an IGBT. Also in this frequency regions the difference should be marginal and more depending on the choosen specific model.

For the Tesla it should work exactly the other way round with about 1500A load current (was imho written somewhere in one of the linked articles) - here the IGBT have less power dissipation than MOSFETs. Imho also in the region of 400V (used by Tesla?) high current Power Mosfets gets rarer?

36 minutes ago, OliverH said:

Why do the IGBTs in a Tesla last longer with higher current?

???

36 minutes ago, OliverH said:

I'm quite not sure that the piggy back MOSFET with this small heat sink (even if attached to the big one) can handle the heat transfer as the downside MOSFETs also produce heat to be transferred somewhere. ... But may be it's time to split the motor driver from the core main board to a different PCB to place the MOSFET/ IGBT in a way the heat can be better transferred.

Ninebot uses quite a similar desing - that's also something a never understood. It would be quite easy to distribute the MOSFETs at least a bit more evenly around the heatsink.

But at least in the ACM the two heatsinks are screwed together and not glued with thermal paste like with the ninebot...

[lizardmech]

I'm not sure having 12 makes any sense, this is a 180A fet. 12 will produce more heat but spread over more fets. If the limitations in cooling are a result of heat being unable to escape the shell 12 fets will be worse.

[Chriull]

17 minutes ago, lizardmech said:

I'm not sure having 12 makes any sense, this is a 180A fet. 12 will produce more heat but spread over more fets. If the limitations in cooling are a result of heat being unable to escape the shell 12 fets will be worse.

If one doubles the MOSFETS the on resistance is halved and so the dissipated power.

[lizardmech]

9 minutes ago, Chriull said:

If one doubles the MOSFETS the on resistance is halved and so the dissipated power.

Resistive losses are halved per fet because current is halved. All it does is reduce the load on individual mosfets, the overall device needs more cooling with more mosfets. At best you might see less resistance due to the silicon being cooler. As soon as the heatsink gets hot you will have the same resistive losses with 2x the switching losses.

[Chriull]

1 minute ago, lizardmech said:

Resistive losses are halved per fet because current is halved. All it does is reduce the load on individual mosfets, the overall device needs more cooling with more mosfets.

Sorry, thats not right. If one takes for example 50A load and 6 mOhm on resistance one Mosfet would dissipate 15 w (6e-3 * 50²). With two Mosfets in parallel each Mosfet has a current of 25A flowing, so 3,75W power dissipation (6e-3 * 25²). So 2*3,75=7,5W in total dissipated by both mosfets.

So with 2 Mosfets in parallel just half the power is dissipated than with one.

1 minute ago, lizardmech said:

At best you might see less resistance due to the silicon being cooler. As soon as the heatsink gets hot you will have the same resistive losses

?

1 minute ago, lizardmech said:

with 2x the switching losses.

Switching losses are halved too if the circuit is well designed - in the worst case the switching losses are the same as just with one Mosfet.

[esaj]

9 hours ago, Linnea Lin Gotway said:

Everytime when I saw esaj's post I will be reminded that I am just a 3 years old child. Too professional

Haha, don't be fooled, I'm not a professional and have been wrong in the past about these things, so it's possible now also  

 

8 hours ago, John Eucist said:

Photo from @Jane Mo

Mosfets seem to be IRFB3207's, datasheets for anyone interested: http://www.irf.com/product-info/datasheets/data/irfs3207.pdf

 

8 hours ago, Mistagear said:

If one individual mosfet fails, does the wheel fail ?

If so, then adding more can, both decreases and increases the risk of failure.

Whether you have a nett gain would then depend on the reason for mosfet failure ?

If the only reason they fail is from overheating then you would have a gain. If however they fail from other causes other than heat, then you may have increased the risk of failure of the wheel 

Yes, my guess would be that it depends on how it fails, mostly (at least when overheating), it seems that the mosfets fail in a dead short, in which case the another paralleled mosfet won't help (it's bypassed through the shorted one).

 

6 hours ago, OliverH said:

Can you explain the advantage of the IGBT over MOSFET? Why do the IGBTs in a Tesla last longer with higher current? What does Segway use?

Unfortunately no, IGBTs are still a mystery to me   Haven't even gotten my head around JFETs or uni-junction transistors yet...

 

Quote

I'm quite not sure that the piggy back MOSFET with this small heat sink (even if attached to the big one) can handle the heat transfer as the downside MOSFETs also produce heat to be transferred somewhere. My feeling: I should wait for the next revision of heat sink design Gotway should rename the ACM to DIP (development in progress) It's nice to see that they constantly improve the EUCs and specially rise the reliability of the main board. But may be it's time to split the motor driver from the core main board to a different PCB to place the MOSFET/ IGBT in a way the heat can be better transferred.

At least when mosfets are being used as load (ie. deliberately dissipating power in them to limit the current), they should be placed as close together in the heatsink as possible.

From:  https://cache.nxp.com/documents/application_note/AN11599.pdf  (NXP's application note,  Using power MOSFETs in parallel):

"To get the most from the MOSFET group, the individual MOSFET should be mounted in a way that causes their mounting base temperatures to be as similar as possible and also as low as possible. To realize this goal, the thermal resistance between each MOSFET (mounting base) and the mounting bases of all the other MOSFETs in the group should be matched and minimized. They should be mounted symmetrically and as close together as possible on a thermally conductive surface. Heat flow can be considered to be analogous to electric current flow; so the thermal bonding points of the MOSFETs (usually the drain tabs) should be on a thermal ‘ring main’. The low thermal resistance path allows heat to flow easily between the MOSFETs. When heat flows easily between all the MOSFETs in the group, their mounting base temperatures track together closely. Note: This arrangement does not promote equal current sharing, but promotes better die temperature matching. The temperatures of all the MOSFETs in the group can rise more before the temperature of the hottest MOSFET reaches 175 C. Hence the power dissipation capability of the group is maximized. "

The PTC (positive temperature coefficient) -effect should take care of "balancing" the load on the mosfets, as when any of the mosfets starts to conduct more current than the others, it will heat up faster, causing the internal resistance to go up, which in turn causes it to conduct less and "share" more of the load with others. Of course then the others heat up and the one that was conducting more cools down, but after a while, a sort of an equilibrium should be reached.

"Serious" electronic loads use separate op-amp per mosfet-gate and separate (high precision) current sensing resistors to control each mosfet (I once read an article about a company that uses power mosfets for power station load-testing, and they were dissipating something like 120kW at best ). The idea is that the separate op amps controlling the gates can then individually control the amount of power dissipated per mosfet.

 

5 hours ago, Chriull said:

The main differences between IGBT and MOSFET seem to be:

- The IGBT has a saturation Voltage, which means the voltage drop between Collector and Emitter ("ouput" side) has a minimal value of something about 2V (Vce-sat)

- The Mosfet has an minimal resistance between Drain and Source ("ouput" side) - Rds-on in the range of some milli Ohm (~3-10)

So the power dissipation of an IGBT is this ~2V times the current. For a MOSFET the power dissipation is Rds-on times the square of the current. So a IGBT with Vce-sat of 2V versus a Mosfet with Rds-on of 10 mOhm "meet" at 200A load current. Below the MOSFET has less power dissipation, above the IGBT.

Sounds plausible, but shouldn't you also consider the switching losses on a mosfet over time, as they're much higher (momentarily) than the "plain" conductive loss causes by the Rds(on). I don't know enough about IGBTs to say whether they're better or worse for the wheels... the high Vce(sat) is probably an issue, but it also depends on how high the switch losses on mosfets are (still could be much better than IGBTs, don't really know).

 

3 hours ago, Chriull said:

Sorry, thats not right. If one takes for example 50A load and 6 mOhm on resistance one Mosfet would dissipate 15 w (6e-3 * 50²). With two Mosfets in parallel each Mosfet has a current of 25A flowing, so 3,75W power dissipation (6e-3 * 25²). So 2*3,75=7,5W in total dissipated by both mosfets.

So with 2 Mosfets in parallel just half the power is dissipated than with one.

?

Switching losses are halved too if the circuit is well designed - in the worst case the switching losses are the same as just with one Mosfet.

Ways to "fight" switching losses that I know of are lowering the PWM frequency (less on/off -switching over same period of time vs. higher frequency) and using more current to charge / discharge the gates faster (the voltage raises/lowers faster, so it spends less time in the linear-region). Some of the gate drive ICs I've taken a quick look at have stated something like 3A-9A current to/from the gate   Quite a lot for a small IC, but then again, the transitions probably happens over nanoseconds, so they're just small spikes.

Just an example:

LM5114 - Single 7.6A Peak Current Low-Side Gate Driver   

More examples: http://www.ti.com/lsds/ti/power-management/mosfet-and-igbt-gate-drivers-products.page

[Chriull]

54 minutes ago, esaj said:

...

Sounds plausible, but shouldn't you also consider the switching losses on a mosfet over time, as they're much higher (momentarily) than the "plain" conductive loss causes by the Rds(on). I don't know enough about IGBTs to say whether they're better or worse for the wheels... the high Vce(sat) is probably an issue, but it also depends on how high the switch losses on mosfets are (still could be much better than IGBTs, don't really know).

My practical knowledge about IGBT is also not existing - that's all just from articles... And one of the recommendations for Mosfets vs. IGBT is to take Mosfets for high frequencies. Also a "bad" turn off behaviour is often mentioned for IGBT. (Reminds me of bipolar transistors in saturation, which causes them to turn off slower, too - and an IGBT should be similar to a bipolar transistor on the "output side")

So my cautious conclusion for now is, that Mosfets are at least as good regarding switching losses as IGBTs.

54 minutes ago, esaj said:

Ways to "fight" switching losses that I know of are lowering the PWM frequency (less on/off -switching over same period of time vs. higher frequency)

Imho they PWM frequency of EUCs is already more than low enough - i slowly get used to a permanent tinitus  ( Unfortionaltely i forget most of the times my eQuiet ear plugs...;( )

A nice workaround could be variable PWM frequency - a higher (not hearable) one for all the times there is no real load on the driver and the low frequency for high load situations... If it is not only switching between two frequencies but really depending on the load this could maybe also give a nice sound/feedback while driving.

[lizardmech]

Best solution is probably SiC or GaN mosfets. Higher RDs on but something like 10% of the switching losses. You're right about the lower resistance on parallel fets, I forgot it's squared instead of linear. From what I recall switching losses were almost 50% of the heat when I found a calculator on a mosfet manufacturers website a while ago.

[John Eucist]

2 hours ago, esaj said:

Mosfets seem to be IRFB3207's, datasheets for anyone interested: http://www.irf.com/product-info/datasheets/data/irfs3207.pdf

I have the original photo so I was able to "enhance!"   You are spot on.

[Jane Mo]

9 hours ago, OliverH said:

Can you explain the advantage of the IGBT over MOSFET? Why do the IGBTs in a Tesla last longer with higher current? What does Segway use?

I'm quite not sure that the piggy back MOSFET with this small heat sink (even if attached to the big one) can handle the heat transfer as the downside MOSFETs also produce heat to be transferred somewhere. My feeling: I should wait for the next revision of heat sink design Gotway should rename the ACM to DIP (development in progress) It's nice to see that they constantly improve the EUCs and specially rise the reliability of the main board. But may be it's time to split the motor driver from the core main board to a different PCB to place the MOSFET/ IGBT in a way the heat can be better transferred.

toooooooooooooo professional.

[Lawrence Law]

On 6/23/2016 at 10:15 AM, Linnea Lin Gotway said:

Everytime when I saw esaj's post I will be reminded that I am just a 3 years old child. Too professional

I am better, 10 years old who can read but don't understand a single word :-P

[Linnea Veteran]

2 minutes ago, Lawrence Law said:

I am better, 10 years old who can read but don't understand a single word :-P

You type wrong, 40 years old, not 10. 

[John Eucist]

I just know IGBT looks too much like LGBT

[TremF]

On 6/22/2016 at 5:52 PM, John Eucist said:

I'm not too sure about how long they've been using these boards for either but I suspect a couple of weeks.  

Well they haven't been using the boards for a couple of weeks. Jane confirmed with Gray that we have the original board which means whatever they did when they held onto our ACM's for the low speed frying issue wasn't the board.

 

I have had mine drop on me twice now when setting off! The first time it seemed like it flipped on me but this time, because I have been cautious when setting off, I noticed that the ACM just dropped on me! I turned it on, pulled down the pedals, rolled it back and forth to make sure the wheel was free then got on while holding onto my fence while I got my feet comfy. As I leaned to set off - only slight lean for a slow start - the ACM just dropped!!

It hasn't dropped while riding yet but twice in a week when getting on and about to set off. Needless to say I'm not too happy because not only am I having that issue but my casing has gotten quite scratched from it hitting the ground!

[US69]

26 minutes ago, TremF said:

Well they haven't been using the boards for a couple of weeks. Jane confirmed with Gray that we have the original board which means whatever they did when they held onto our ACM's for the low speed frying issue wasn't the board.

 

I have had mine drop on me twice now when setting off! The first time it seemed like it flipped on me but this time, because I have been cautious when setting off, I noticed that the ACM just dropped on me! I turned it on, pulled down the pedals, rolled it back and forth to make sure the wheel was free then got on while holding onto my fence while I got my feet comfy. As I leaned to set off - only slight lean for a slow start - the ACM just dropped!!

It hasn't dropped while riding yet but twice in a week when getting on and about to set off. Needless to say I'm not too happy because not only am I having that issue but my casing has gotten quite scratched from it hitting the ground!

Wow! That sounds like a really heavy bug?! Means when you want to start wheeling the ACM just "falls down" like on a Motor cut off?

[TremF]

10 minutes ago, KingSong69 said:

Wow! That sounds like a really heavy bug?! Means when you want to start wheeling the ACM just "falls down" like on a Motor cut off?

Not sure exactly because I have never had this before with my Ninebot. It's a bit like getting on without turning it on and when you go to lean it falls over. 

[Keith]

5 hours ago, John Eucist said:

I just know IGBT looks too much like LGBT

Well they do convert D.C. To A.C.

Don't forget, in these discussions, putting two transistors in parallel halves their total resistance and therefore the heat loss. It doesn't simply distribute it across the two devices.

IGBT's do look to have very promising properties, I wonder if cost or lack of experience with them is why they are not yet considers for EUC's although Wikopedea does have this to say on the subject: "In general, high voltage, high current and low switching frequencies favor the IGBT while low voltage, low current and high switching frequencies are the domain of the MOSFET." 

[HEC]

12 minutes ago, TremF said:

Not sure exactly because I have never had this before with my Ninebot. It's a bit like getting on without turning it on and when you go to lean it falls over. 

Did the wheel completely shut off (including the LED lights) or was it still in "on" state but simply not balancing so you can tilt it front back with (almost) zero resistance?

[TremF]

7 minutes ago, HEC said:

Did the wheel completely shut off (including the LED lights) or was it still in "on" state simply but not balancing so you can tilt it front back with (almost) zero resistance?

I didn't notice the lights BUT when I stood it up it was as if there was no power and the wheel moved freely instead of trying to stay upright. I then turned it off then back on again as I did the first time this happened.

[Chriull]

 

15 minutes ago, Keith said:

IGBT's do look to have very promising properties, I wonder if cost or lack of experience with them is why they are not yet considers for EUC's although Wikopedea does have this to say on the subject: "In general, high voltage, high current and low switching frequencies favor the IGBT while low voltage, low current and high switching frequencies are the domain of the MOSFET." 

EUCs are quite sure low frequency applications. The faster switching capabilities of Mosfet could just help here to reduce switching losses. Also in most articles the "current-tailing" of IGBTs is mentioned (bad turn off speed )

High and low voltage/current are quite relative. From some articles i read (don't really know how valid they are) the high/low voltage "barrier" is somewhere in the range of 200-500V. Above 1000V it's definitely the area of IGBT.

The <30A max continous current for EUCs are presumably considered as "low current". I just looked up mouser and found the the lowest Vce-sat for IGBT's are around 1V. Compared to the ~6mOhm Rds-on from currently used MOSFETs this still gives a power dissipation at 30A of 30W for the IGBT's and 5,4W for the Mosfet. Equal power dissipation would be around 160A, which are never reached with EUCs.

Additionaly by paralleling the mosfets the total power dissipation can quite easily be reduced. Due to the fixed Vce-sat power dissipation cannot be reduced with IGBTs - just distributed.

So the voltage, current and frequency used in EUC's nowadays can be handeled easily by both. Just in this operational range Mosfets perform far superior in regard of power dissipation. And cooling seems to be by now one of the bigger problems with the new EUCs.

Looking at the available models of IGBT/MOSFETs for 400V/1500A (peak?) (as used by Tesla) the picture changes. In this range the Rds-on for Mosfets gets higher and IGBTs can handle more current than Mosfets in this area... And with this currents the fixed Vce-sat of 1-2Vs gets an advantage over the 15-30mOhm of here applicable Mosfets.

1 hour ago, TremF said:

...

I have had mine drop on me twice now when setting off! The first time it seemed like it flipped on me but this time, because I have been cautious when setting off, I noticed that the ACM just dropped on me! I turned it on, pulled down the pedals, rolled it back and forth to make sure the wheel was free then got on while holding onto my fence while I got my feet comfy. As I leaned to set off - only slight lean for a slow start - the ACM just dropped!!

It hasn't dropped while riding yet but twice in a week when getting on and about to set off. Needless to say I'm not too happy because not only am I having that issue but my casing has gotten quite scratched from it hitting the ground!

I encountered quite comparable situations with my KS16 (after updating to Firmware 1.20) - i had two starts with the wheel stopping to balance once i put my second foot on and accelerating. Seems to be some overcurrent protection (strangly only sometimes effective?) - but i wanted to observe this a bit more before posting it here (in the right sub-forum;) ). Could not say by now if i stepped on/accelerated a bit to harsh or it was just some "malfunction" of the Kingsong.

But definetely a situation i never encountered with my Ninebot One E+.

[TremF]

4 minutes ago, Chriull said:

I encountered quite comparable situations with my KS16 (after updating to Firmware 1.20) - i had two starts with the wheel stopping to balance once i put my second foot on and accelerating. Seems to be some overcurrent protection (strangly only sometimes effective?) - but i wanted to observe this a bit more before posting it here (in the right sub-forum;) ). Could not say by now if i stepped on/accelerated a bit to harsh or it was just some "malfunction" of the Kingsong.

But definetely a situation i never encountered with my Ninebot One E+.

That could be the change they put in place for mine. It has been said the ACM seems to act similar to KS in some respects. 

I know for sure that, after it happening the other day, I have been super cautious since and this morning was no exception yet it dropped on me.

Similarly I never experienced it with my Ninebot One E+ either.

[HEC]

18 minutes ago, Chriull said:

I encountered quite comparable situations with my KS16 (after updating to Firmware 1.20) - i had two starts with the wheel stopping to balance once i put my second foot on and accelerating. Seems to be some overcurrent protection (strangly only sometimes effective?) - but i wanted to observe this a bit more before posting it here (in the right sub-forum;) ). Could not say by now if i stepped on/accelerated a bit to harsh or it was just some "malfunction" of the Kingsong.

Did you need to power cycle (switch off / on) the wheel to get rid of this status or did it recover itself after a brief moment while the wheel was still on? Is it 840 Wh model?

[Chriull]

2 minutes ago, HEC said:

Did you need to power cycle (switch off / on) the wheel to get rid of this status or did it recover itself after a brief moment while the wheel was still on?

I did turn it off and on - did not try or remember if i really needed to do so.

2 minutes ago, HEC said:

Is it 840 Wh model?

Yes