How should the VESC "feel" on a board?

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

Postby devin » 09 May 2017, 18:48

i don't think the motor behaves as an inductor circuit during the "off" portion of the duty cycle.

i think there is an amp "cliff" effectively to 0a immediately after each pulse.

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

Postby benjamin » 09 May 2017, 19:08

What a waste of time... I have no idea what you are trying to prove.

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

Postby Hummie » 09 May 2017, 19:34

Maybe the current and magnetic field don't colapse as fast as the frequency?

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

Postby benjamin » 09 May 2017, 20:10

Hummie wrote:Thanks for your help all of you but I'm still confused. I understood that when an inductor circuit is off the current and magnetic field will instantly collapse as a voltage spike. How can there be a smooth current, smooth magnetic field,or even smooth torque if the pwm is turning the circuit off and on regardless of the Frequency speed and that field is constantly collapsing?

We aren't all as knowledgable as you. I don't even understand your latest answer with the visuals. But my question remains how can you have any current or magnetic field in a conductor as its off? That's a simple question an hopefully u can give a simple answer for us nubies


The circuit does not actually switch off, the motor winding gets shorted during the off time. So the PWM goes between connecting the motor to the battery and shorting the motor, meaning that the windings are exposed to a voltage between the battery voltage and 0 V just as in the simulation. As you said, disconnecting the winding would lead to a voltage spike that would be conducted in the body diodes in the FETs. Anyway, measuring the "slowly" changing motor currents, estimating the motor state and controlling the PWM to shape the motor currents for the desired torque production with minimal losses is what motor control is about.

Sorry if I got a bit impatient. After spending so much time trying to explain things and even doing a simulation and I get "I don't believe the motor behaves like an inductor" it feels like devin is just trying to be right and not trying to understand. Also, most of his posts completely ignored the points I tried to make.

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

Postby Hummie » 09 May 2017, 20:51

I changed my post earlier, can we simplify and say the magnetic field colapses slower than the frequency as it's shorted and not disconnecting the circuit?
Facinating yet confusing and have no background. Much appreciated. The vesc on foc is a dream come true.

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

Postby benjamin » 09 May 2017, 21:27

Yes, the field collapses much slower when shorting than it would with an open circuit. Also keep in mind that there is no way to create an open circuit as the induced voltage over an inductor can rise to thousands of volts to maintain the current if it gets disconnected, and well before voltage something will start conducting on the power stage: namely the body diodes in the FETs. The body diodes have some voltage drop and generate heat when conducting, so it is better to switch on the FET with the body diode that would conduct to get less losses (and a bit different behaviour when thinking in terms of BLDC and regenerative braking, but that's a different story).

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

Postby devin » 09 May 2017, 22:43

benjamin, trying my best to understand.

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.

i've had trouble locating documentation on the topic of predictable motor amp limit effects on acceleration characteristics.

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?

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

Postby Hummie » 10 May 2017, 03:28

Vedder are you saying that if I were to disconnect or break my phase wire while the motor was given a lot of current,
there would be a huge onrush of voltage building throughout the circuit, but before there would be a high voltage last spark through the air there at the break of the circuit, the high voltage would bust through the fet so no spark? So you could break your esc with a high powered broken motor wire if it was broken at the wrong time?

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

Postby rew » 10 May 2017, 06:41

devin wrote:d 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?
The wattage is proportional to the RPM. P = C * rpm * A. (C is a constant). So to calculate the current A, you'll get A = P /C*rpm which goes to infinity with RPM near zero.

Near zero RPM, you will always hit motor- or controller current limits before you'll reach the wattage limit.

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

Postby rew » 10 May 2017, 06:52

Hummie wrote:Vedder are you saying that if I were to disconnect or break my phase wire while the motor was given a lot of current,
there would be a huge onrush of voltage building throughout the circuit, but before there would be a high voltage last spark through the air there at the break of the circuit, the high voltage would bust through the fet so no spark? So you could break your esc with a high powered broken motor wire if it was broken at the wrong time?

When you suddenly break the phase wire, the motor-side of that break will take on any voltage required to allow the current to continue to flow. So the voltage was maybe 50V when the break happens, then the voltage will go 100? Current flowing? No? Ok. 300V! current flowing? No? 1000V! and so on as necessary. Then, in the microseconds after the break at some point the airgap is going to be insufficient to maintain such a voltage difference and a spark will fly. This is more-or-less what they use for sparkplugs in cars.

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.


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