Landed on my a**, stopping on a dime

[GoinPostal]

On 9/29/2017 at 8:37 AM, JimB said:

I use ChargeDoctor, and only charge to about 90%, for exactly the reason you describe.

Which wheel make/model was this Jim?

[Mono]

On 24/09/2017 at 12:34 AM, Keith said:

it is even easier to overlean a wheel braking than it is accelerating and power demands can be even higher.

It really depends on the wheel. Only my IPS has terrible braking power as well. It's the only wheel I ever ended up in the same situation as @JimB, without even braking extremely hard. Otherwise, I can't say I have the feeling my wheels are easier to outbrake than to outlean. 

I don't understand why braking would need more power than accelerating, whereas I could understand that it needs less, for a bunch of reasons. As a force that needs to be applied against the direction of the movement, I guess that you could end up with any kind of energy demand, and, as we know, even with energy recuperation under braking.

[WARPed1701D]

5 hours ago, GoinPostal said:

Which wheel make/model was this Jim?

IPS Lhotz. OP mentions it in the 3rd post.

[JimB]

7 minutes ago, Mono said:

I don't understand why braking would need more power than accelerating, whereas I could understand that it needs less, for a bunch of reasons. As a force that needs to be applied against the direction of the movement, I guess that you could end up with any kind of energy demand, and, as we know, even with energy recuperation under braking.

Preface - I'm a software guy, not hardware.  But..

Don't electric motors have the most torque (per power) at 0 RPM?  When accelerating, you're applying high torque at low RPM.  When decelerating, you're trying to apply high torque at high RPM.  So, if I understand correctly, wouldn't it take more power to generate the same torque at the higher RPM?

Also, take into account that when accelerating, we're very aware of the amount of acceleration.  When emergency/panic breaking, it's more instinctual, and quite possible that we're breaking much harder than we would generally accelerate.

Finally, the power is working differently when accelerating and breaking.  For acceleration, we're applying power to the wheel.  When breaking, the wheel is acting like a generator, and we're applying resistive load to the wheel, right?  Is it possible that the slowing the wheel by increasing the resistive load is less efficient than accelerating by directly applying power?

[WARPed1701D]

14 minutes ago, JimB said:

Finally, the power is working differently when accelerating and breaking.  For acceleration, we're applying power to the wheel.  When breaking, the wheel is acting like a generator, and we're applying resistive load to the wheel, right?  Is it possible that the slowing the wheel by increasing the resistive load is less efficient than accelerating by directly applying power?

From my understanding this is only valid to a point...and then power must be applied to produce very hard braking.

[Mono]

1 hour ago, JimB said:

So, if I understand correctly, wouldn't it take more power to generate the same torque at the higher RPM?

We extract kinetic energy from the system. That doesn't take energy, but delivers energy. That is, we need in principle zero energy to produce as much torque as we want against the direction of rotation. The energetic engineering problem we face it rather not at all comparable to accelerating.

[mrelwood]

6 hours ago, JimB said:

Preface - I'm a software guy, not hardware.  But..

Don't electric motors have the most torque (per power) at 0 RPM?

For the type of motors used in EUCs, no. The torque curve is vaguely reminicent of a parabole, so the most torque per W is at medium speeds. There are a few measurement graphs somewhere on this forum.

At 0 RPM the electric current demand can easily be the largest you will ever generate on an EUC. That's why some Gotways got fried trying to start at a miniscule obstacle such as a small stick. And that's why I always kick a little speed before lifting my other foot aboard.

Quote

Finally, the power is working differently when accelerating and breaking.  For acceleration, we're applying power to the wheel.  When breaking, the wheel is acting like a generator, and we're applying resistive load to the wheel, right?

If you mean in electrical sense, no, the load is not resistive. It's an electric magnet. To my understanding the regenerative braking doesn't change the behavior when braking, it only applies the generated current to charging the batteries instead of only heat. For the motor there is no difference.

5 hours ago, Mono said:

We extract kinetic energy from the system. That doesn't take energy, but delivers energy. That is, we need in principle zero energy to produce as much torque as we want against the direction of rotation. The energetic engineering problem we face it rather not at all comparable to accelerating.

In princible, yes. In practice, no. Extracting kinetic energy to other energy formats by only using an electric motor is very far from optimal. It is not designed to transfer kinetic energy to heat and electricity, but exactly the opposite.

Like a microphone vs speaker. They do the exact same thing, only to the opposite direction. A speaker can be used as a microphone (and even a mic as a speaker), but the end result is lousy since the system was not optimized for that.

The max available braking power is determined by the motor alone, since it is the only part of an EUC that can manipulate the rotational speed of the wheel. The motor doesn't have separate braking coils or magnets, so essentially what happens in the motor of an EUC during braking is exactly the same as when accelerating.

[Keith]

10 hours ago, Mono said:

I don't understand why braking would need more power than accelerating,

Read what I said again, I did not say that. So there is little point in turning this, yet again into a how braking works discussion!

What I said is that in an emergency the natural reaction is to back away so, instinctively, if suddenly exposed to danger ( like a car turning across your path etc)  you will lean back hard and before you have time to think and control your actions, whereas you are likely to accelerate with some care. So if your wheel has limited torque (and all wheels have some limit) then it is easier to overlean when emergency braking than accelerating.

10 hours ago, JimB said:

When accelerating, you're applying high torque at low RPM.  When decelerating, you're trying to apply high torque at high RPM

I had not even thought about that but, yes, good point that is also a factor.

There is also the factor that accelerating is an everyday activity, emergency braking, unless deliberately practiced, is not, so users are likely to have poorer control when an emergency occurs. Emergency stops should be practiced IMHO.

[Mono]

5 hours ago, mrelwood said:

The max available braking power is determined by the motor alone

How do you know it is not determined by the battery?

[Mono]

5 hours ago, mrelwood said:

The motor doesn't have separate braking coils or magnets, so essentially what happens in the motor of an EUC during braking is exactly the same as when accelerating.

When accelerating we always need to put energy in the system, while for braking we don't. I am not sure that this should qualify as happening exactly the same. For example, in case of braking we could well be limited by the heat capacity of the wires storing the kinetic energy back as heat, which is not the case when we accelerate (also not the reverse, because we cannot use the heat to re-accelerate). We are bound to exactly the same physical laws indeed, and in either case electromagnetism plays a decisive role, but after that what happens inside the motor is not quite exactly the same.

[Mono]

18 hours ago, JimB said:

Don't electric motors have the most torque (per power) at 0 RPM?  When accelerating, you're applying high torque at low RPM.  When decelerating, you're trying to apply high torque at high RPM.  So, if I understand correctly, wouldn't it take more power to generate the same torque at the higher RPM?

It helps to understand why electric motors have their maximum torque at 0 RPM. The reason is the backward electromagnetic force, back EMF. It is a force that works against the direction of spinning. So if you want to accelerate the wheel it works against you. If you want to decelerate the wheel it works in your favor. This even suggests that you may get, in principle, the more deceleration the higher the speed (I don't know if this is so). At least it strongly suggests that the reason why torque decreases with increasing speed only holds for acceleration but not for deceleration.

[Chriull]

Whats true for electric motors _all_ the time:

- the torque is direct proportional to the (signed) current. (Ignoring losses, magnetic saturation, etc)

- the motor generates a voltage direct proportional to the rotational speed (one has to regard the voltage drop at the coils by the flowing motor current)

The difference between (regenerative) braking and accelerating is the way the controller for the three half bridges work.

While generating "positive" torque (accelerating, "normal" driving/cruising) the controller lets a needed "positive" current flow to achieve the according proportional positive torque. This is achieved by changing the pwm duty cycle. (Beside the controller also commutes the coils in the right sequence - this has to correspond to the actual speed, but is of no importance for this discussion. The principle is the same for 3 phase bdlc as for normal (1 phase) dc motors).

So the motor generates a voltage proportional to the current speed, the battery voltage is stepped down by the pwm duty cyle and by the difference of these two voltages and the resistance in the path (mainly coil and internal battery resistance) the desired current is flowing generating the desired torque. ( Edit: as far as i understand like this also "plugging" brsking can be achieved. One just needs an other commutation sequence...)

What happens while (regenerative) braking:

The low side mosfets are driven to switch on/off with a specific frequency. This together with the motor coils step up the generated motor voltage. The frequency is choosen by the controller so that the motor voltage is stepped up high enough (above the battery voltage) so that a (negative) current is flowing generating the desired braking torque.

[Chriull]

By this principles while generating positive torque this torque is limited by the max torque over speed graph ( max torque a 0 speed to 0 torque at max speed).

While generating negative torque (regenerative braking) the limit is

–the bms cutting off at any cell reaching ~4.2V. Here the cell charge determines the cell (internal) voltage. Additionally the internal battery resistance by the flowing current generates a voltage drop raising this cell voltage. And imho the cell chemistry raises the internal resistance of a burdened cell. So the braking torque is limited by the maximum current that can flow through a cell before it reaches ~4.2V.

this can only be prevented/delayed by switching a power resistor in parallel to the battery. Here once a promising try was posted using halogen bulbs. Unfortionately there was no follow up – or i missed it?

so braking is not (1) limited by/depending on speed but and (1) by battery charge/„chemistry“ state and the internal battery resistance.

only at lowest speeds the step up capabilities could maybe not be sufficient to achieve a high enough voltage to perform this regenerative braking? But then the firmware could still switch to plugging braking. Maybe?

edit: (1) the no speed dependence is not true. By the voltage up conversion the current is lowered by the same factor. So at the same motor current (torque) the battery current is the lower the lower the speed is!

Edit: This is all nicely described and explained (unfortionately just in german) here: https://ces.karlsruhe.de/~BUB/Umwelttechnik/Elektromobilitaet_TGJ14_2012.pdf

It's also focused on e-bikes but most points are the same or quite similar for EUC's. All the principles are explained with 1-phase DC motors, which are applicable to the 3-phase BLDC motors...