Hi everyone, my first post. I hope we have some VFD experts here. I am just having difficult time understanding some VERY BASICS of VFD drive algorithms and it bugs me a lot .
Actually, we have started doing an EV conversion at an electronics club at a local university of technology here. As we have deep level knowledge on electronics etc., we will create all the electronics by ourselves. For example, we are prototyping a simple active BMS based on bi-directional flyback transformer design on every cell module. Currently we are rewinding an industrial 13 kW(cont) 2-pole induction motor for lower voltage and 4-pole configuration; maybe we can get 20-25 kW(cont) out of it.
Let me also state that I'm a very practical or reality-oriented guy; I want to see the waveforms instead of mathematical formulas, I want to see reasons for technical decisions instead of the typical "this is just how it is done". I believe that the implementation details come quite naturally after you have nailed down what exactly to do and why; and what happens if done otherwise.
Anyway, we know the very basics of VFD's, and we already built a very simple working prototype using an FPGA for control and IGBTs for power stage. Just stored a sine wave table in FPGA's embedded memory and created the PWM on the fly. Just a dozen lines of VHDL. Then 6 optoisolated IGBT gate driver IC's and the IGBT's. Very simple. But now we need to create the "real thing" that's suitable for a car, thus, controls torque, not speed.
But, to be fair, information available on VFD's is driving me mad. I have spent days and days reading on VFD's but can't find any real information from any practical viewpoint, just mathematical jibberish; obfuscating what REALLY happens and WHY it needs to happen behind fancy names, terms, transformations and formulas.
What I mean, getting the motor spinning with variable speed was just a work of two hours or so. And it nicely did regenerative braking when I change the driver frequency to lower than the actual motor speed. This is very basic stuff.
Also, the curve of slip vs. torque is very basic stuff. Everyone has seen it; at zero slip, there is zero torque. Then the torque goes up with slip, until it hits a point of maximum torque. Then, if the slip is further increased, the torque starts reducing and this operating region ("stalling") is to be avoided. The curve can be mirrored for "negative slip" for regenerative braking. (So the VFDs start the motors at full torque by ramping up the frequency from zero to avoid the "stalling" region. At the same time, "lower voltage", or to be more exact, lower duty cycle, is used to limit the value the current in the windings will rise to due to their inductance. Simple electronics. Am I right?)
So, it appears to me that if we want to control TORQUE as is the case in EV's, we want to control SLIP. And slip is super-duper-hyper-cyber easy to control. Just add a speed sensor (encoder) to the motor shaft and adjust the drive frequency accordingly with a PI loop. Hence, the gas pedal could actually be a "slip pedal". When pressed fully down, we adjust the output frequency to the measured rpm plus "slip that gives maximum torque". When the pedal is fully released, we create output frequency that matches the real measured rpm to create zero torque; or even a bit lower frequency to apply a bit of regenerative braking.
This is called a "closed-loop (= feedback) V/f control", and it is almost unheard of.
But, apparently this cannot work well, because everyone and every reading material tells us that we need to use things that involve something like vectors from outer space and transformations named after some dead people who liked math . There is enough material to get by "how", but no one tells us "why".
Closed-loop V/f control is practically undiscussed. More sophisticated controls such as FOC and DTC are always compared against open-loop V/f which, naturally, cannot be very usable in EV.
Now, I admit that most probably everyone is right and we need to use FOC or DTC (and my experience in FPGA's allows us to do practically anything, no matter how computationally intensive, but that's not the point), but it would be nice to hear even just one argument for this, because: why do something complex if you don't understand why you do it? I have tried to catch the BASIC IDEA behind FOC and DTC, and now I'm coming to the very basic question here;
I get the idea that compared to closed-loop V/f, FOC and DTC allow controlling of different components of phase current, namely torque-generating current and flux-creating current, separately. Firstly, did I get it right? Secondly, why is this needed? If I just control slip in a closed-loop V/f control by creating sine-wave approximation by using PWM with constant phase offset of 120 and 240 degrees, and control frequency and voltage (or more correctly, a multiplier for PWM on-cycle length), what do I miss? Does the system waste energy in wrong type of two currents?
We all know that ACIMs run very well off 50 or 60 Hz 3-phase supply, except for the starting where the motor is running some time on the unwanted region below the optimum slip point. But is it true that they run well only with the rated torque reading giving the rated power? Does their efficiency fall behind if there is higher load and higher slip, even if it is still on the better side of the curve?
I would highly appreciate any pointers to practical reading material, or any open VFD designs. And, of course, publishing our results, schematics, code etc. goes without saying.
Thank you for your time and for the nice forum full of information. I hope I can contribute something back in the future.
Actually, we have started doing an EV conversion at an electronics club at a local university of technology here. As we have deep level knowledge on electronics etc., we will create all the electronics by ourselves. For example, we are prototyping a simple active BMS based on bi-directional flyback transformer design on every cell module. Currently we are rewinding an industrial 13 kW(cont) 2-pole induction motor for lower voltage and 4-pole configuration; maybe we can get 20-25 kW(cont) out of it.
Let me also state that I'm a very practical or reality-oriented guy; I want to see the waveforms instead of mathematical formulas, I want to see reasons for technical decisions instead of the typical "this is just how it is done". I believe that the implementation details come quite naturally after you have nailed down what exactly to do and why; and what happens if done otherwise.
Anyway, we know the very basics of VFD's, and we already built a very simple working prototype using an FPGA for control and IGBTs for power stage. Just stored a sine wave table in FPGA's embedded memory and created the PWM on the fly. Just a dozen lines of VHDL. Then 6 optoisolated IGBT gate driver IC's and the IGBT's. Very simple. But now we need to create the "real thing" that's suitable for a car, thus, controls torque, not speed.
But, to be fair, information available on VFD's is driving me mad. I have spent days and days reading on VFD's but can't find any real information from any practical viewpoint, just mathematical jibberish; obfuscating what REALLY happens and WHY it needs to happen behind fancy names, terms, transformations and formulas.
What I mean, getting the motor spinning with variable speed was just a work of two hours or so. And it nicely did regenerative braking when I change the driver frequency to lower than the actual motor speed. This is very basic stuff.
Also, the curve of slip vs. torque is very basic stuff. Everyone has seen it; at zero slip, there is zero torque. Then the torque goes up with slip, until it hits a point of maximum torque. Then, if the slip is further increased, the torque starts reducing and this operating region ("stalling") is to be avoided. The curve can be mirrored for "negative slip" for regenerative braking. (So the VFDs start the motors at full torque by ramping up the frequency from zero to avoid the "stalling" region. At the same time, "lower voltage", or to be more exact, lower duty cycle, is used to limit the value the current in the windings will rise to due to their inductance. Simple electronics. Am I right?)
So, it appears to me that if we want to control TORQUE as is the case in EV's, we want to control SLIP. And slip is super-duper-hyper-cyber easy to control. Just add a speed sensor (encoder) to the motor shaft and adjust the drive frequency accordingly with a PI loop. Hence, the gas pedal could actually be a "slip pedal". When pressed fully down, we adjust the output frequency to the measured rpm plus "slip that gives maximum torque". When the pedal is fully released, we create output frequency that matches the real measured rpm to create zero torque; or even a bit lower frequency to apply a bit of regenerative braking.
This is called a "closed-loop (= feedback) V/f control", and it is almost unheard of.
But, apparently this cannot work well, because everyone and every reading material tells us that we need to use things that involve something like vectors from outer space and transformations named after some dead people who liked math . There is enough material to get by "how", but no one tells us "why".
Closed-loop V/f control is practically undiscussed. More sophisticated controls such as FOC and DTC are always compared against open-loop V/f which, naturally, cannot be very usable in EV.
Now, I admit that most probably everyone is right and we need to use FOC or DTC (and my experience in FPGA's allows us to do practically anything, no matter how computationally intensive, but that's not the point), but it would be nice to hear even just one argument for this, because: why do something complex if you don't understand why you do it? I have tried to catch the BASIC IDEA behind FOC and DTC, and now I'm coming to the very basic question here;
I get the idea that compared to closed-loop V/f, FOC and DTC allow controlling of different components of phase current, namely torque-generating current and flux-creating current, separately. Firstly, did I get it right? Secondly, why is this needed? If I just control slip in a closed-loop V/f control by creating sine-wave approximation by using PWM with constant phase offset of 120 and 240 degrees, and control frequency and voltage (or more correctly, a multiplier for PWM on-cycle length), what do I miss? Does the system waste energy in wrong type of two currents?
We all know that ACIMs run very well off 50 or 60 Hz 3-phase supply, except for the starting where the motor is running some time on the unwanted region below the optimum slip point. But is it true that they run well only with the rated torque reading giving the rated power? Does their efficiency fall behind if there is higher load and higher slip, even if it is still on the better side of the curve?
I would highly appreciate any pointers to practical reading material, or any open VFD designs. And, of course, publishing our results, schematics, code etc. goes without saying.
Thank you for your time and for the nice forum full of information. I hope I can contribute something back in the future.