Why do EUCs run at 55.5-67 volts?

[Hunka Hunka Burning Love]

I'm just curious - is 55.5 - 67 volts the optimal voltage to run EUC electric motors at?  Do they have an upper ceiling or is that voltage a calculated one based on ideal energy to motion conversion calculations or something?  Why not use 100 v?  Or is it more a limitation of how many 18650 cells you can practically link together to fit the form factor or control board maximum voltage handling capability?  Would there be any benefits of going to a higher voltage (high speed/torque?)?

 

[OliverH]

Option 1) It's like putting gasoline with high octane in the car and if it's using detecting an other thresh old for uncontrolled burning it results in more performance.

Option 2) makes it more difficult to source components than in the 48 V area. 

Option 3) it lowers the current. But the depebenciy voltage to current isn't fixed.

Sorry couldn't help

/edit 100V is out of band with electric regulations. You need a lot of step down converters to get components to use this voltage.

[Hunka Hunka Burning Love]

I wonder what would happen if you over-volted the electric motor what effect that would have assuming that the control board doesn't get fried.  Kind of like how people over-volt CPUs, but of course the voltage are in decimal amounts...

Do you think this chosen voltage range has to do with the gauge of the wiring and windings?  Say someone like @TwixFix were to make a super large custom pack that put out 120 v.  Also say you had a control board and BMS that can handle that voltage.  Would it fry the electric motor just due to excessive heat dissipation byproduct or burn out the wiring?  What characteristics of the wheel would be different assuming you could program the control board to make the wheel go faster?  Is there an upper ceiling for voltage where going higher results in no advantage for these particular motors?

[Keith]

Fitting cells isn't a problem, the 32 cells in typical 16S2P pack could, just as easily (from a physical point of view) be a 32S1P pack. Giving you nearly 120 volts.

In practice, generating power by high voltage/Low current is always desirable as it is the current that results in losses as heat (the very reason that cross country power lines have such stonking great voltages). However the higher the voltage the more esoteric (and, beyond a certain point, expensive) some of the components get. Additionally, the windings for the motor set some of their own limitations. The higher the voltage the lower the kV (RPM per volt) of the motor needs to be I.e. 100V with 50kV motor gives you a maximum RPM of 5000, so does 50V with a 100kV motor. To get a lower kV requires more turns of wire on the motor, for a fixed physical size motor that means also thinner wire. The current handling goes down significantly as the wire gets thinner so there is a practical limit to how thin the wire can be so, therefore, many turns you can use and handle the power you need.

in short it is a balance of voltage against current against component costs and practicality.

[Hunka Hunka Burning Love]

From my limited understanding of how PWM electric motor systems work, and watching this video over and over again:

Is maximum speed a limit of the pulse frequency sequence?  When you do a lift test and get 50 kph, is that a hard limit or can it theoretically go faster if the pulse frequency to the coils is increased?    What would it take to get these wheels to spin at 100 kph?  If you doubled the inner motor up and had pairs of magnets and pairs of coils, would that increase the torque?  Or say if one side was wound for torque while the other side for speed, could you have a wheel with a broader band of torque vs speed characteristics?  I have a feeling that I should have gone into electrical engineering as I have like a ton of theoretical questions. 

[Chriull]

8 hours ago, HunkaHunkaBurningLove said:

I'm just curious - is 55.5 - 67 volts the optimal voltage to run EUC electric motors at?  Do they have an upper ceiling or is that voltage a calculated one based on ideal energy to motion conversion calculations or something?  Why not use 100 v?  Or is it more a limitation of how many 18650 cells you can practically link together to fit the form factor or control board maximum voltage handling capability?  Would there be any benefits of going to a higher voltage (high speed/torque?)?

Teslas on the other side use something around 400V and gets currents up to 1500A (was discussed in some other thread) - so the motor power needed, maximum speed, cost and availability of the different components will give one a practical range in which the designers can choose one solution.

Or as @Keithsaid "in short it is a balance of voltage against current against component costs and practicality."

6 hours ago, HunkaHunkaBurningLove said:

From my limited understanding of how PWM electric motor systems work, and watching this video over and over again:

That's a nice introductional video, but does not explain the details of how the motor/commutation works. For this details you'd have to look at something like:

or

(dont' forget to continue to the second part)

6 hours ago, HunkaHunkaBurningLove said:

Is maximum speed a limit of the pulse frequency sequence?

The speed of the BLDC is defined by how fast one changes the the commutation patterns. This change of commutation patterns make the magnetic field rotate and the magnets (the wheel) try to follow it. So as long as you are within the limits of the motor specification, the speed of the commutation pattern change defines the motor speed.

But once there is too much load on the wheel, the magnets cannot follow the magnetic field anymore and the controller has to reduce the magnetic field rotation speed or the magnetic field and the magnets will get out of sync (something one does not want driving on an EUC)

So one has a relation of the maximum load one can put on a specific motor for a certain speed. This is show in the torque/speed diagramm:

 

 

6 hours ago, HunkaHunkaBurningLove said:

  When you do a lift test and get 50 kph, is that a hard limit or can it theoretically go faster if the pulse frequency to the coils is increased?  

If this 50kph is not some firmware implemented limit it is the maximum speed (no-load speed in the diagramm). At this no-load speed the BLDC just has 0 torque and so can no longer accelerate to higher speeds.

6 hours ago, HunkaHunkaBurningLove said:

  What would it take to get these wheels to spin at 100 kph?

You need a motor designed for a no-load speed of 100 kph  Which would be a stronger (and bigger and heavier) motor so that he can produce enough torque for low speeds and still can reach the higher speeds.

6 hours ago, HunkaHunkaBurningLove said:

  If you doubled the inner motor up and had pairs of magnets and pairs of coils, would that increase the torque?  Or say if one side was wound for torque while the other side for speed, could you have a wheel with a broader band of torque vs speed characteristics?

I hope the above videos give you an idea about the inner details of the motor. Combining high torque windings and high speed windings makes imho no real sense - if that's at all feasable to build. You would have two motors in one - almost double weight and double costs...

The solution for a "a broader band of torque vs speed characteristics" is just to take a stronger BLDC motor with the windings designed for higher speed. Which will need more batteries and stronger electronics to drive it and so makes the whole wheel bigger and heavier.

So imho the maximum speed is finally limited by the size and weight of the wheel. (and the cost - putting real money in could reduce the size and weight to some extend)

 

 

 

[Hunka Hunka Burning Love]

Thanks for taking the time to explain that.  I'll have to watch the videos a bit more in depth tomorrow.  Regarding the "dual" motor concept I was thinking that combining two inner motors, one wound for torque and the other for speed, at lower speeds, climbing hills, or to counter overleans the controller could power the torque side more than the high speed side and gradually shift power to the high speed side for cruising speed.

For example, with the Airwheel Q series dual tire motor chassis there appears to be enough room inside for one side to house a high speed motor while the other side could house a motor designed for torque.  By swapping out the two wheels with one large wheel you could theoretically have a EUC with both high torque and high speed if the controller can dynamically shift the power as needed between them.  Or perhaps having one wheel with alternating (torque and high speed) windings might work to get the benefits of both?  I'll have to do some more reading about how these motors work...

[Chriull]

An Addition to the torque-speed diagramm:

The torque is in direct proportion to the strength of the magnetic field produced by the coils. The strength of this magnetic field is again direct proportional to the current flowing through the coils.

The other thing that happens in BLDC is, that once a coil is moving in a magnetic field (the static magnetic field of the permanent magnets) a current is induced which leads to a voltage generated at this coil. This generated voltage (back emv) is of oposite polarity to the voltage put on the coil to drive/commute the motor.

The faster the wheel spins the higher this generated voltage (back emv) gets and at max speed (no load speed) it fully cancels the voltage used to commute the coil. So no current is flowing anymore -> the coils produce no magnetic field anymore -> no torque is available anymore to accelerate the wheel.

So again to @HunkaHunkaBurningLove's question for a higher max speed, there are now two possibilities:

Either use coils with less windings (less wire length -> less induced current -> less generated voltage -> higher rpms possible until the back emv cancels the commutation voltage) or use a higher voltage to drive/commute the coils.

Using coils with less windings to reach higher speeds: The generated magnetic field is not only direct proportional to the flowing current but also to the wire lenght (number of windings of the coil). So to get the same magnetic field (and finally the same torque) you have to increase the current. This means thicker wires have to be used, more batteries in parallel could be needed to deliver this current and the motor driver (Mosfets) dissipates more power (power dissipated by the mosfets is proportional to the square of the current)-> better/bigger heatsinks for better cooling are needed.

Using higher voltage: More battery cells in series are needed to get this higher voltage. But since still you need the same current as before so one still needs as many cells in parallel as before, -> more battery cells are needed -> the cell packs have to get bigger.

Additionally the controller has to limit the current to the motor at lower speeds, since with the higher voltage now the electronics and the motor could get easily overpowered (mosfets and/or motor coils burned). An other solution would be to increase the capability of the driver (more mosfets, better cooling) and als increase the capabilities of the motor (thicker wires, more "metal" to cool the motor). With this second solution the wheel gets heavier and bigger again, but could use more power (better acceleration)

[Hunka Hunka Burning Love]

 I watched the first video which is pretty clear except when they tied the windings together.  I get the push pull magnetic forces on the outrunner BLDC motor.  I'll have to rewatch it again when it's not so late at night... sleepy....

I wonder what would happen if you took two different motors like from the MCM4 and one from the MCM4 HS and fused them side by side together.  If one large tire could then be fitted and two control boards attached with dual batteries and have it all wired up to power on at the same time.  Keeping it simple and assuming the electronics are in sync what characteristics would the ride exhibit?   Both control boards would issue the same balancing signals to each motor, and since they are fused together with a single tire would it be a cooperative effort?  Or would one side screw up the timing of the other side, and there would be interfering forces against each other?  But if the control boards could sync the rotational speed of each side in perfect harmony...

 

[Chriull]

5 minutes ago, HunkaHunkaBurningLove said:

...

I wonder what would happen if you took two different motors like from the MCM4 and one from the MCM4 HS and fused them side by side together.  If one large tire could then be fitted and two control boards attached with dual batteries and have it all wired up to power on at the same time.  Keeping it simple and assuming the electronics are in sync what characteristics would the ride exhibit?   Both control boards would issue the same balancing signals to each motor, and since they are fused together thna single tire would it be a cooperative effort?

Could easily be that the two control loops trying to balance the wheel start to swing -> you get an unridable or burning wheel.

If they control loops can work together it could be that they combine their forces and one gets more torque driving up to the max-speed of the slower speed motor (?MCM4?) or they still "fight" and hinder them a bit on driving and you get a low power version just burning energy...

But once one reaches max speed of the slower speed motor (I think to remember that at max-speed the MCM4 just shuts off) one just drives on with the MCM4 HS motor and would have to restart the MCM4 again at lower speeds/standstill.

But after all you have two wheels in one - double size and double weight. Just taking a stronger motor, electronics and batteries should be way more efficient to reach higher speeds.

[Hunka Hunka Burning Love]

My brain likes to theorize about what ifs I guess.  . Thanks for humouring my strange ideas and thinking a bit through them.

One other what if.... Say you mounted some circular patterns of magnets to the both inner sides of the motor shell covers and placed additional windings to align up with them.  Could you get some additional torque from spinning the motor shells in addition to the outer rim of the motor?  It would add additional weight, but maybe the extra rotational force would make up for that?

EDIT:  Interesting videos @Chriull.  I watched both part 1 and 2.  Can the speed of EUCs be calculated by the frequency of the commutation cycles over time knowing the radius of an inflated tire?  So it looks like the speed is a function of how quickly the commutations can cycle through, but is there any "slippage" that occurs under load where the magnets are over-powered by the load so the wheel cannot keep up with the commutations?  Would more powerful magnets ensure reduction of slippage if it does occur?

[MetricUSA]

Also the lower the voltage, the higher the current, and thicker the wire, and bigger the motor, for the same power...

[Chriull]

23 hours ago, HunkaHunkaBurningLove said:

EDIT:  Interesting videos @Chriull.  I watched both part 1 and 2.  Can the speed of EUCs be calculated by the frequency of the commutation cycles over time knowing the radius of an inflated tire? 

Yes. And the number of magnets/windings.

23 hours ago, HunkaHunkaBurningLove said:

So it looks like the speed is a function of how quickly the commutations can cycle through, but is there any "slippage" that occurs under load where the magnets are over-powered by the load so the wheel cannot keep up with the commutations? 

yes. Its an asyncronous motor. The more load the more slippage. If the motor is overloaded it goes out of sync. This was already reported here some time - when the wheel makes a loud "rattling" noise it is out of snc and slips back to the next magnets.

23 hours ago, HunkaHunkaBurningLove said:

Would more powerful magnets ensure reduction of slippage if it does occur?

Yes. The more powerfull one makes the motor, the less is the slippage. Like with a normal combustion motor - the more power it has the later it stalls.

[Jurgen]

In fossile fuel engines they had a similar dilemma: low end reactivity vs. High power.

Solution= bi-turbo; one small turbo for the low end (torque) and one big one for high end (power) that starts to take over at higher revs. I see  no reason why this could  not be done in EUC (at a weight and cost penalty).

[Hunka Hunka Burning Love]

Well the dual motor in one idea was what I was trying to get at to try to combine high torque with high speed to get the benefits of both worlds.  If you take a look at this 14" motor:

If you mounted another ring of windings with say thicker or thinner wires facing the cover plate and attached a matching ring of magnets to the cover plate on both sides it would be like 3 motors in one.  The cover plates might have to be redesigned so they are wider to provide enough room, but I wonder if the extra dual "side" propulsion would add some torque/speed capability.  You already have those large cover plates - why not attach some magnets to them to use them for propulsion?

[Chriull]

@HunkaHunkaBurningLove: In some form this dual motor is already in place as https://en.m.wikipedia.org/wiki/Dahlander_pole_changing_motor. Or just a switching from delta to star connection should also change the speed/torque characteristics! (Factor squareroot of 3 ?)

[Rehab1]

@HunkaHunkaBurningLove @Chriull Many any thanks for posting the instructional videos! So informative!