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How much torque has an EUC to accelerate/balance?


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A person who uses the possessive "its" properly instead of "it's" is worthy of the greatest respect.

Can you estimate wattage for a 100kg rider going up slopes at between 8-13 mph? I get in my opinion ubsurdly low rates like between 400-600 watts, and often much less.

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100.png.447a52d555c3ef5ce6ca4f276d20491c.png

shows mechanical power needed for slopes up to 15% for 120kg wheel+rider, where 8-13mph is pretty much 13-21km/h. Now, I am not sure how to estimate efficiency under these operating conditions. Using 1 - torque/torque_max as efficiency coefficient (EDIT: which is likely to be very inaccurate), we get for a 1200W motor (best case)

1201.png.1965847c4af718a15f34c370376e2b7b.png

At 5% slope, we see 400-700W for the above mentioned speed range, but this becomes quickly much more with increasing slope.

Edited by Mono
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Is there anything to back but theory? On your graphs, the maximum power of V8 is taken for 800 watts, look at this article https://airwheel.ru/test-monokoles-na-dinostende/, but it is in Russian, you can use google for translate https://translate.google.ru/translate?sl=ru&tl=en&js=y&prev=_t&hl=ru&ie=UTF-8&u=https%3A%2F%2Fairwheel.ru%2Ftest-monokoles-na-dinostende%2F&edit-text=. Inmotion V8 on the dyno shows 2,2 kW of mechanical power on the default battery, and 2.7 kW of mechanical power on a high-performance battery.
In addition, in half of the articles you refer to the characteristics of brushed DC motor, but we have a sensored brushless motor. It's completely different motors with completely different characteristics.

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20 hours ago, palachzzz said:

Is there anything to back but theory?

As Feynman once famously said, "If it [the law] disagrees with experiment, it's wrong. And that simple statement is the key to science. It doesn't make a difference how beautiful your guess is. It doesn't make a difference how smart you are, who made the guess, or what his name is; if it disagrees with experiment: wrong. That's all there is to it." 

The only "theory" specific to electric motors that is used is the linearly decreasing dependency between torque and speed. The maximally available torque is maximal at zero speed and goes linearly down to zero at maximal speed (that's the reason why this is the maximal speed, there is zero torque available to increase speed). This graphs is shown in the second figure.

Everything else is derived from this torque-vs-speed relation and high-school physics unrelated to EUCs. If you know a better model for the torque vs speed dependency of electric motors bring it on, I am always happy to learn.

20 hours ago, palachzzz said:

Inmotion V8 on the dyno shows 2,2 kW of mechanical power on the default battery, and 2.7 kW of mechanical power on a high-performance battery.

Interesting point that an 800W motor can deliver 2.7kW of mechanical power. Maybe it can for a short period of time, while 800W is the power it can sustain permanently. This may indeed render the above estimates for torque reserves for balancing, bumps or potholes too pessimistic.

20 hours ago, palachzzz said:

In addition, in half of the articles you refer to the characteristics of brushed DC motor, but we have a sensored brushless motor. It's completely different motors with completely different characteristics

How does the basic physics of an electric motor depend on whether it uses brushes?

 

Edited by Mono
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12 minutes ago, Slaughthammer said:

Can you please explain how this formula is derived? 

It is a guess. 

For maximal load and up to 90% speed, the efficiency coefficient 1 - torque/torque_max represents the data presented in http://www.me.mtu.edu/~wjendres/ProductRealization1Course/DC_Motor_Calculations.pdf and elsewhere very well. As I wrote before I gave the formula, I am not sure how to estimate efficiency under other operating conditions. The formula is obviously wrong at zero speed, where efficiency is always zero. 

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17 hours ago, Slaughthammer said:

Thanks for the link! The difference is, AFAICS, that they can choose the speed, so it seems reasonable to assume they chose the speed with optimal efficiency (for high loads about 80-90% of max speed). For EUCs we want to know the efficiency for any given speed. Still helpful!

My reasoning was that at low speed and high load, the efficiency is very low. Now, what about low load? At low constant speed (ie. low load) we tend to see the largest ranges per Wh, hence rather high than low efficiency. 

Edited by Mono
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4 hours ago, Mono said:

Interesting point that an 800W motor can deliver 2.7kW of mechanical power. Maybe it can for a short period of time, while 800W is the power it can sustain permanently. This may indeed render the above estimates for torque reserves for balancing, bumps or potholes too pessimistic.

Why do you think that in V8 800 watts? Simply because it is described as the nominal power? But why don't think that this nominal power means the possibilities of a battery? It can not provide high currents for long time - otherwise it will be overheating, I think the power 800 watts (~80 v at 10 amps) is taken as the nominal for the whole EUC, but the weak point of it is battery, not motor.

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14 hours ago, palachzzz said:

Why do you think that in V8 800 watts? Simply because it is described as the nominal power? But why don't think that this nominal power means the possibilities of a battery?

Because 800W is given as motor specs, not as battery specs. I do understand that battery may also be a limiting factor, of course. 

Quote

It can not provide high currents for long time - otherwise it will be overheating, I think the power 800 watts (~80 v at 10 amps) is taken as the nominal for the whole EUC, but the weak point of it is battery, not motor.

The V8 has a 460Wh battery. Discharging at 2C^1, which is to my understanding entirely within the standard range of operation for Li-ion batteries, yields 920W. 

It would be quite funny if overheat would determine the power specs of EUCs, because that would mean their power specs were determined by the size of their heat sinks. I don't think so. 

^1 http://batteryuniversity.com/learn/article/discharge_characteristics_li

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13 minutes ago, Mono said:

Because it is given as motor specs,

Please, show me the place where it is written that this is the power of the motor.

 

13 minutes ago, Mono said:

The V8 has a 460Wh battery. Discharging at 2C^1, which is to my understanding entirely within the standard range of operation for Li-ion batteries, yields 920W. 

And again references to mythical and theoretical figures.

There is the test of LG MH1 that used in V8: 

https://www.e-cigarette-forum.com/forum/threads/lg-mh1-10a-3200mah-18650-bench-test-results-safe-at-10a-but-suffers-damage.686757/

 

Edited by palachzzz
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9 hours ago, palachzzz said:
10 hours ago, Mono said:

The V8 has a 460Wh battery. Discharging at 2C^1, which is to my understanding entirely within the standard range of operation for Li-ion batteries, yields 920W. 

And again references to mythical and theoretical figures.

Sure, maybe you also better tell that Jason, he computes 1440W:

59915e9fc624c_ScreenShot2017-08-14at10_24_47.thumb.png.f80588022196798befb34731334391ee.png5990d787a0ecc_ScreenShot2017-08-14at00_47_22.png.861110eaa7f57e9ea85c4ad2345fef5e.png

sources: 
https://www.ewheels.com/electric-unicycle-ultimate_ewheel_comparison_ips_airwheel_ninebot_king_song_gotway/
https://www.ewheels.com/choosing-an-electric-unicycle-with-the-right-battery-pack-for-you/

It may be that 80V * 10A = 800W, but the V8 has two packs in parallel, hence we need to compute 80V * 10A * 2 = 1600W, or (as recommended in the linked article) 80V * 7A * 2 = 1120W. Or more realistically as Jason 72V * 10A * 2 = 1440W or 72V * 7A * 2 = 1008W. Of course these are all only theoretical figures and they are all very mystical ;) and mysteriously all larger than the 920W I computed to be within the standard range of operation.

Edited by Mono
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14 hours ago, palachzzz said:

Is there anything to back but theory? On your graphs, the maximum power of V8 is taken for 800 watts, look at this article https://airwheel.ru/test-monokoles-na-dinostende/, but it is in Russian, you can use google for translate https://translate.google.ru/translate?sl=ru&tl=en&js=y&prev=_t&hl=ru&ie=UTF-8&u=https%3A%2F%2Fairwheel.ru%2Ftest-monokoles-na-dinostende%2F&edit-text=. Inmotion V8 on the dyno shows 2,2 kW of mechanical power on the default battery, and 2.7 kW of mechanical power on a high-performance battery.

The 2.2/2.7 kW numbers are the absolute maximal output powers the V8 can produce with full batteries and cold motor. Also the 2.7kW are already a bit "over the maximum" - @EcoDriftblew the fuse once measuring the modified V8.

He measured the same numbers for the KS16C - which is also announced to have a nominal power of 800W. The measured 2.2kW max output power seem quite feasable with full batteries and cold motor: http://forum.electricunicycle.org/topic/7549-current-demand-versus-battery-voltage/?do=findComment&comment=106424 - just the on some sites advertised 3kW peak power are way out of reach...

 

On the other side, in a real world situation (somewhat used up batteries, warm motor - http://forum.electricunicycle.org/topic/7855-anatomy-of-an-overlean/) the limit shifted down to somewhere around ~1.5kW (also i don't know how valid and accurate the numbers reported from the wheel are...)

 

So @Mono's 800W can be reached anytime with an V8, maybe also still something around 1kW - with not too empty batteries something around this 1.5kW and with full batteries one could reach the 2.2kW...

 

14 hours ago, palachzzz said:


In addition, in half of the articles you refer to the characteristics of brushed DC motor, but we have a sensored brushless motor. It's completely different motors with completely different characteristics.

Brushed DC and BLDC motors function on the same principles - for our discussions here this simplification is totaly valid. Brushes or not, number of coils, etc has no influence on this principles.

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On 11.8.2017 at 1:28 AM, Mono said:

...

At 1/2 of the max speed, the electric input power is 1600W, the output power is 800W, the efficiency is 50%. The necessary electric input power to produce maximal torque grows to 4 times the nominal motor power at zero speed, here 3200W. The motor efficiency (all at max load) becomes the smaller the smaller the speed. Therefore, battery limitation may limit the shown electrical input and the mechanical output power to smaller values than displayed.

...

As the controller together with the motor coil works as DC/DC step down converter (for the BLDC as DC/AC step down converter) the efficiency is much higher - one does not have to burn half of the battery power to reduce the voltage to  (almost) 1/2... The main losses at the motor side are just the power "burned" at the ohmic coil resistance, magnetic losses and friction. Then there is the efficiency of the step down converter and the losses in the battery (internal resistance).

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1 hour ago, Chriull said:

As the controller together with the motor coil works as DC/DC step down converter (for the BLDC as DC/AC step down converter) the efficiency is much higher - one does not have to burn half of the battery power to reduce the voltage to  (almost) 1/2... The main losses at the motor side are just the power "burned" at the ohmic coil resistance, magnetic losses and friction. Then there is the efficiency of the step down converter and the losses in the battery (internal resistance).

So, If we consider the efficiency of the system as 80% (as confirmed by the dyno), and take the voltage / current logs in the system, do you agree that we can to know the approximately power of V8 in real world?

 

1 hour ago, Chriull said:

@EcoDriftblew the fuse once measuring the modified V8.

Yes, 2,7kW it is about 3,4 kW of electric power, it is about 45A - too much for 30A fuse in V8. But again, the motor not a weak point of the system.

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14 minutes ago, palachzzz said:

So, If we consider the efficiency of the system as 80% (as confirmed by the dyno), and take the voltage / current logs in the system, do you agree that we can to know the approximately power of V8 in real world?

Efficiency is a function of speed and load. 

But beside of this we have with the 2.2kW a "certified" (approximate) value of the maximal power the V8 can ouput with full batteries.

14 minutes ago, palachzzz said:

 

Yes, 2,7kW it is about 3,4 kW of electric power, it is about 45A - too much for 30A fuse in V8. But again, the motor not a weak point of the system.

The motor is a main part of the "weak points" in the system. The combination Battery (voltage, internal resistance) and motor (kv - Volts/revolutions per second, coil resistance) determine the limits - one can have an overlean witht the V8 with full batteries, where it would need (much) less than this 2.2kW.

Battery current is for overleans (reaching the torque limit) at higher speed normaly not the limiting factor.

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2 hours ago, Chriull said:

As the controller together with the motor coil works as DC/DC step down converter (for the BLDC as DC/AC step down converter) the efficiency is much higher - one does not have to burn half of the battery power to reduce the voltage to  (almost) 1/2... The main losses at the motor side are just the power "burned" at the ohmic coil resistance, magnetic losses and friction. Then there is the efficiency of the step down converter and the losses in the battery (internal resistance).

I see, you are saying that Table 1 and the efficiency Graph 1 here 

http://www.me.mtu.edu/~wjendres/ProductRealization1Course/DC_Motor_Calculations.pdf

do not apply in brushless motors. Are you aware of reference figures for BLDC you can point us to? 

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9 minutes ago, Chriull said:

Battery current is for overleans (reaching the torque limit) at higher speed normaly not the limiting factor.

And yes, and not. 

When the current is increased to the limit values for the battery, the voltage in the system dramatically decreases, thus the electric power is also reduced. For example, at a current 40A on default battery the voltage drop can exceed 1V per element (see discharge graphs), i.e. down to 60 volts for a battery, this can cause even a battery shutdown by BMS, or EUC controller.

The EUC is very dynamic system. Take a look at the log for my today 17,5 km commute, peak electric power is 1852.88 watts at speed ~22 km/h, and about 600-800 watts at cruise speed (28-29 km/h) on a flat surface

2017_08_14_09_04_25_cutted.csv

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48 minutes ago, palachzzz said:

When the current is increased to the limit values for the battery, the voltage in the system dramatically decreases, thus the electric power is also reduced.

Right. Only that high speed overleans do not occur at limit currents of the battery. The higher the speed, the more resistance the battery sees from the back EMF of the motor, hence the smaller is the current we can possibly see. Generally speaking, low speed overleans are those caused by battery weakness or heat problems and high speed overleans are caused by the physical limitations of electric motors.

Edited by Mono
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I hadn't seen this

If I had, I wouldn't have started this thread in the first place :)

@Chriull was faster than me and did also much nicer graphs, IMHO. 

Edited by Mono
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17 minutes ago, Mono said:

I hadn't seen this

If I had, I wouldn't have started this thread in the first place :)

Fits good together - but still a little bit another "style" of graph.

But i still have some probs - the current need for my "acceleration" lines to not correspond to reality (http://forum.electricunicycle.org/topic/7855-anatomy-of-an-overlean/?do=findComment&comment=107766) and i did not find my fault till now... (also had not the chance till now to invest some real time into that...;( )

 

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51 minutes ago, palachzzz said:

And yes, and not. 

When the current is increased to the limit values for the battery, the voltage in the system dramatically decreases, thus the electric power is also reduced. For example, at a current 40A on default battery the voltage drop can exceed 1V per element (see discharge graphs), i.e. down to 60 volts for a battery, this can cause even a battery shutdown by BMS, or EUC controller.

That's why i don't really like 2p battery systems ;) The 4p as used with the KS16 is quite the lower limit for my "comfort level"...

... and at low battery levels one has to drive cautious anyway ...

51 minutes ago, palachzzz said:

The EUC is very dynamic system. Take a look at the log for my today 17,5 km commute, peak electric power is 1852.88 watts at speed ~22 km/h, and about 600-800 watts at cruise speed (28-29 km/h) on a flat surface

2017_08_14_09_04_25_cutted.csv

The power values are unfortionately not valid - they come from multiplying motor current times battery voltage with makes no (real) sense. For more details on this topic see http://forum.electricunicycle.org/topic/7549-current-demand-versus-battery-voltage/

 

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