How should the VESC "feel" on a board?

General topics and discussions about the VESC and its development.
arvidb
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Re: How should the VESC "feel" on a board?

Postby arvidb » 10 May 2017, 07:28

devin wrote:i've had trouble locating documentation on the topic of predictable motor amp limit effects on acceleration characteristics.
This is really very simple: motor current is directly proportional to motor torque, which is in turn directly proportional to acceleration (given a constant load). So the amp limit, in combination with your load, directly determines acceleration. Nothing more to it.

devin wrote:i'd like to be able to write an equation to predict which amp limit settings will give a predictable wattage as close as possible to 0rpm full throttle.

if the vesc detects 0.0415 ohm winding and expected pack volts with sag is 46v, is there any way to adjust the settings for full throttle = 1000w electrical as close as possible to 0rpm?

At 0 rpm your mechanical power is also zero, so all your electrical power is turned into heat loss. If you really want to waste exactly 1000 W at 0 rpm, then limit battery current to 1000 W/46 V = 21.7 A, or for 1000 W loss in the motor, limit motor current to [P = I²R -> I = (P/R)^0.5] (1000 W/0.0415 Ω)^0.5 = 155 A.

benjamin
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Re: How should the VESC "feel" on a board?

Postby benjamin » 10 May 2017, 07:33

rew wrote:Anyway, that spark might be energetic enough to blow up the FET, even when the majority of the energy is dissipated in the airgap.

The formula for the energy contained in an inductor is: E = 1/2 L I^2 . So with 50A current and 50uH inductance the energy is (someone check my math!) 62mJ. For a one-time event, this is within spec for the mosfets. They can handle about 500mJ. But if this happens multiple times per second, then things quickly get out of hand.


I think your math is correct, but in practise the power won't be dissipated in the FETs. Each half bridge is always connected to either the positive or negative rail, and the spark energy would be directed directly into the capacitors. Even if all FETs are off and a spark would be go into the phase connections, the body diodes in the FETs act as a three phase rectifier and direct the energy right into the capacitors. Therefore it is very unlikely to damage the FETs with a spark caused by a disconnected cable.

However, if you only have one FET and the polarity of the spark is such that it forces the FET into avalanche conduction (most FETs are designed to handle that) then you can compare the energy in the spark to the joule number in the datasheet. This number is temperature dependent, so if the FET is kept cool it can handle lots of avalanche energy dumps repeatedly just fine. Here is an example of that which I built many years ago where the FETs operate in avalanche every PWM cycle and generate lots of heat:



I later upgraded the power stage to different bridge configurations to handle almost 1kw driving multiple transformers, but in the video it just uses two parallel FETs that operate in avalanche.

devin
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Re: How should the VESC "feel" on a board?

Postby devin » 10 May 2017, 11:26

arvidb wrote:. If you really want to waste exactly 1000 W at 0 rpm, then limit battery current to 1000 W/46 V = 21.7 A, or for 1000 W loss in the motor, limit motor current to [P = I²R -> I = (P/R)^0.5] (1000 W/0.0415 Ω)^0.5 = 155 A.


Thank you!

"Wasting" exactly 1000w electrical full throttle as close as possible to 0rpm, and then limiting power to exactly 1000w electrical at all other physically possible rpms (or any other predictable number) is exactly what we hope to achieve because we've determined that what "feels like" even acceleration is:

full throttle = xxxx watts electrical at all physically possible rpms

^while this does not result in "actual" even acceleration due to wind/rolling resistance, grade, etc -- it feels like even acceleration because it results in even access to electrical source power -- even access to wattage at all physically possible rpms.

devin
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Re: How should the VESC "feel" on a board?

Postby devin » 10 May 2017, 14:22

In the future I would love to see a watt control mode implemented as follows:

1- user selects electrical wattage, for example "500w," equivalent to 500w electrical full throttle at all physically possible rpms

2- based on chosen wattage, battery amp limit and motor amp limit are chosen automatically based on detected winding resistance and pack voltage. the battery limit would have to change dynamically with fluctuating pack voltage to achieve this while the motor amp limit setting would remain constant.

3. user experiences full throttle = 500w electrical equivalence at all physically possible rpms, even during voltage sag

benjamin
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Re: How should the VESC "feel" on a board?

Postby benjamin » 10 May 2017, 17:03

devin wrote:^while this does not result in "actual" even acceleration due to wind/rolling resistance, grade, etc -- it feels like even acceleration because it results in even access to electrical source power -- even access to wattage at all physically possible rpms.


You got this completely wrong. The acceleration you feel is the torque, and the torque is the current. Wattage is not proportional to the acceleration, the torque (or acceleration) from a given wattage is completely speed dependent. With wattage control the throttle will give very strong acceleration at low speeds and make you fall with the slightest throttle input, and at high speed wattage control will barely provide any acceleration at all and you will slow down when going up hills until you are going slowly enough for the wattage to give sufficient torque at that speed.

If you want the throttle to control the acceleration you feel at any speed without battery voltage dependence then current control is what you are looking for - it provides exactly that. Wattage control does not make any sense in most practical situations because it won't feel natural at all.

devin
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Re: How should the VESC "feel" on a board?

Postby devin » 10 May 2017, 17:25

Let's imagine a motor controller that is programmed to the following:

full throttle = 10 battery amps at all physically possible rpms

(current control)

(50v pack voltage assumed - 500w is desired)

full throttle = 10 battery amps x 50v = 500w at all physically possible rpms (with no sag)

if sag brings pack v to 46v under load:

full throttle = 10 battery amps x 46v = 460w at all physicslly possible rpms (40w loss electrical from 500w desired)

if battery amp limit of the controller was programmed to fluctuate dynamically to maintain 500w electrical

when v sag to 46v occurs:

full throttle = 10.86 battery amps at all physically possible rpms (10.86 battery amps x 46v pack sag = 500w electrical desired)

devin
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Re: How should the VESC "feel" on a board?

Postby devin » 10 May 2017, 17:31

if the winding resistance detection was 0.0415ohm x 2 = 0.0830ohm, and the motor amp limit was set to 77.61 motor amps, then 500w and 10 battery amps would be "allowed" to occur at full throttle, even close to 0rpm

in this theoretical battery current control esc, if the motor amp limit is set below 77.61 motor amps, then 10 battery amps would be impossible at rpms close to but still above 0rpm because 500w and therefore 10 battery amps would be "forbidden" by the lowered motor amp limit.

devin
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Re: How should the VESC "feel" on a board?

Postby devin » 10 May 2017, 17:44

for example if motor amp limit were set to "40" with the 0.0415ohm x 2 = 0.0830ohm winding

40 motor amps limit only "allows" 132.8watts close to 0rpm full throttle (40 motor amps x 3.32v pwm effective volts = 132.8w)

132.8watts / 50v battery volts = 2.65 battery amps

therefore with a battery current control esc, desired 10 battery amps all possible rpms full throttle, desired 500w (50v), with 10 battery amp limit, and 40 motor amp limit settings, the most battery amps possible accelerating full throttle at rpms close to 0rpm is 2.65 battery amps or 132.8 watts electrical which is nowhere near the 10 battery amps & 500w desired or expected at all rpms full throttle.

77.61 motor amps limit "allows" 500watts close to 0rpm full throttle (6.44v effective pwm volts) and therefore 10 battery amps for current control = 10 battery amps full throttle at all possible rpms

77.61 motor amps limit x 6.44v effective volts = 500w
10 battery amps limit x 50v pack volts no sag = 500w
40 motor amps limit x 3.32v effective volts = 132.8w
2.65 battery amps x 50v pack volts no sag = 132.8w

benjamin
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Re: How should the VESC "feel" on a board?

Postby benjamin » 10 May 2017, 18:54

What exactly are you trying to achieve? Was it not acceleration that feels even? I get the feeling that you are completely ignoring what I'm writing...

hexakopter
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Re: How should the VESC "feel" on a board?

Postby hexakopter » 10 May 2017, 18:58

Please stop it devin. You are continuing the nonsense threads/discussions you already started on the electric-skateboard forum.
You are just ignoring what the people tell you and steel their time with talking about "XXXw at all physically possible rpms", also they told you x times before that this isn't useful. Benjamin has make an effort to explain it to you, but you are just ignoring it with your hardheaded opinion.
I think it isn't unfounded that you got banned on the electric-builders forum. Please dont do it here again. :|
Because you already created your own forum especially for this topic, maybe it would be a good idea to discuss it there.


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