| Electronics > Projects, Designs, and Technical Stuff |
| 100+kW Motor Controller |
| << < (6/8) > >> |
| moffy:
About regen, you need somewhere to dump the energy otherwise your DC bus will go excessively high. In diesel/electric locomotives they use massive fan cooled resistor banks, for yours, if battery fed you would need a safe means of getting the energy back into the battery. |
| marcos_:
--- Quote from: moffy on May 03, 2019, 12:14:17 am ---About regen, you need somewhere to dump the energy otherwise your DC bus will go excessively high. In diesel/electric locomotives they use massive fan cooled resistor banks, for yours, if battery fed you would need a safe means of getting the energy back into the battery. --- End quote --- Regen is only dangerous if you are operating the machine at a speed that produces a BEMF higher than the battery voltage. In a nissan leaf for example I think its ~120kmh. Below that rpm, you won't have an uncontrolled energy flow situation, since you can clearly define the maximum regen current limit and the act of regen requires a well coordinated firmware effort. Without PWM switching no current flows into the battery. (attached is the GUI to set this) If you are above said rpm, you better be prepared for some fireworks, because in a fault situation the motor will dump an uncontrolled amount of energy into the battery. If the powerstage and battery can handle that current, and the vehicle and passengers can handle the huge deceleration, no worries! At least the battery will clamp the voltages on the system to safe levels until speed decreases to BEMF=Vbatt. If not, you have to embrace the possibility of blowing the battery fuse or tripping the main contactor... now you exchange uncontrolled currents for uncontrolled voltages and the IGBTs will have to endure whatever BEMF voltage the motor generates. If your BEMF at 100kmh is 400V, then your BEMF at 200kmh could be 800V. You will *really* hope the designer chose at least a 900V IGBT. If he chose a 650V device you will have instant, regretful fire. The BEMF comes from the magnets, thats why Tesla can use 650V IGBTs, an undriven ACIM motor won't produce any BEMF, even if its spins at a bazillion rpm. And when they switched to permanent magnet motors, they now needed an extra voltage buffer: enter wide bandgap SiC mosfets. Beside havin less switching losses, they can easily achieve 900V rating, so the introduction of PM motors and SiC drives are tightly coupled. |
| marcos_:
This week we added Field Weakening and MTPA support to the vesc firmware and GUI in our github repo. Here is a writeup about that: https://hackaday.io/project/164932-axiom-100kw-motor-controller/log/162931-field-weakening-support FW has been probably the most requested feature of the vesc platform for years, as it allows to operate the machine in the constant power region. It was just simply not possible to go beyond the constant torque region, as torque would drop to zero. Not every motor has a flat constant power region, but a good automotive motor will. |
| Yansi:
--- Quote from: langwadt on April 30, 2019, 12:43:56 am --- --- Quote from: marcos_ on April 29, 2019, 11:48:28 pm --- --- Quote from: jeremy on April 29, 2019, 11:00:27 pm ---Very nice! If I may ask, is the FPGA completely necessary? It seems like the MCU is doing the PWM (and possibly the calculations ?). All I can really think of other than that is the encoders, but there are many ARM mcus that can directly interface to incremental encoders nowadays. --- End quote --- Thanks! Incremental encoders is already supported, but for high performance you want to know the rotor position from the start, otherwise you can't apply full torque at 0rpm, in a launch situation for example. That can be done with resolvers or high speed absolute encoders, a common comm protocol for those is BiSS which I'm not confident can be managed by the already loaded cpu. The other potential usage is fitting 7 delta sigma demodulators to digitize the 4 voltages and 3 currents directly at the busbars, without any analog signal path. This way everything stays digital, filtering can be tweaked from firmware, no drift, no offsets, etc. We are already using sensors that come with delta sigma variants. In this scenario all motor signals are being input directly to the FPGA and output PWM is an fpga output,so you could run the control loop at ludicrous frequencies inside the fpga and using the arm mcu as an application processor. Also the fpga is actually not much more expensive, early versions had a bunch of discrete logic gates doing the basic protection job and the costs sum up, I think overall its a $3 difference that gives you the opportunity to do interesting things. --- End quote --- and afaiu in safety related systems FPGAs are sorta seen as "hardware" and considered more reliable than software, which make sense considering the many many permutations of what happens in a cpu once you throw in a bunch of interrupts --- End quote --- Now go and tell that to those stupid formula student germany (FSG) rules makers. |
| marcos_:
--- Quote from: Yansi on May 05, 2019, 03:43:09 pm ---Now go and tell that to those stupid formula student germany (FSG) rules makers. --- End quote --- May I ask what's the problem? |
| Navigation |
| Message Index |
| Next page |
| Previous page |