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About DaveThomasPilot

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  • Birthday 12/14/1954

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    RC Helicopters, PCB design, Flyball, Private pilot, scuba diving, skiing

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  1. I'm about to give up Carlos. One last time, since there's not much point in going into more subtle effects like battery internal resistance and winding resistance until the fundamentals are grasped. Where did that come from? No where is v/r referenced as a way to calculate current. The point is you can use output power and input power calculations to easily show time average battery current is higher when the battery voltage is lower. When a current (or voltage) is PWM, you do a time average over the cycle time. Take a 60% duty cycle example. Input power = (Ub * I1) * d + Ub *I2 (1-d) where d is the duty cycle The average of the two different power consumptions seen in each cycle. If I2 is close enough to zero ,then it's just d*Ub * I1 If there more phases involved that have significantly different currents, then you just weight them by the period of time each applies: Ub * t1 + Ub *t2 + Ub * t3 + ... / (t1+t2+t3+ ...) Whether the motor winding currents are higher or lower than the battery current depends on the ratio of the effective motor voltage set by the ESC and the battery voltage. But, for the battery voltages and battery windings used in EUC, yes, the winding current will be higher the battery current. Motor winding current (to a first order approximation), is independent of battery voltage (under constant output power). But, battery current is inversely proportional to battery voltage (again, to a first order approximation). I think you were pointing out that the resitive losses in the motor windings, interconnect wires, and battery internal resistance can limit the current to less than what would be required for a given torque. So, higher battery voltages will provide the ability for more current before these resistances limit winding current. Yes, that sounds plausible and we can get into that in more detail. But, that analysis starts with understanding of what current flows where so associated IR drops can be calculated. I'm not trying to say a lower battery voltage caused Marty's issue or other Gotway issues--it's pretty clear it didn't. But, the voltage drop due to battery internal resistance and wiring from the battery to the control board is probably not negligible either.
  2. So, at this point, is there any disagreement? At a fixed motor power, lower battery voltage means high battery current (easy to prove by conservation of energy). The "effective motor current" does NOT change, since power output and speed is constant. The ESC adjusts the duty cycle for the speed required. That duty cycle affects the battery input current--reduces (bucks) it when the winding voltage (sort of Um, but there isn't just one winding) / Kv of the wheel motor is lower than the battery voltage and boosts it when the the Um / Kv is higher than the battery voltage. In Marty's case, someone said the melted insulation was on a wire from the ESC to the motor. The average current in those wires doen't vary (much) with battery voltage change. But, the wires from the battery to the control board will have current inversely proportional to the battery voltage (at constant battery power).
  3. The torque the motor produces is (to a first order approximation) " is the parameter that determines how much torque is produced. " Yes, absolutely! But the average battery current required to drive the motor is higher when the battery is lower. Made a simplistic model will help. This isn't really what's going on, but its valid in an approximate way. Think of ESC as a buck/boost inverter that adjusts it's output voltage to drive the wheel at the speed you want. The motor speed, as I think you agreed, is set by the applied voltage. So, the ESC does PWM to effectively adjust the average voltage (Vm). For low speed, it's acting like a buck convertor. So, the duty cycle reduces the average current the battery sources (Ui) by the duty cycle. At lower battery voltages, the duty cycle is higher, which results in higher (cycle average) battery current. So, I think the point you're missing is that average battery current does not equal the time average current in the windings. You need to apply the duty cycle, which varies inversely with the battery voltage.
  4. Great way to clarify things! I've been referring to battery voltage (Ub) and battery current (Ib). It's important to differentiate from Ud and Id. It's harder to talk about Id, since there are multiple conductors at different phases. So, there's not just one current to consider, but three (or more)? That's why I was trying to stick with just input current (or Ib) and battery voltage (Ub). I think that the average current in each winding will be (first order approximation) proportional to the torque, or at fixed speed, proportional to the mechanical output power. I think this is what many are thinking about when they assert the input voltage doesn't matter for motor current. But, as I've tried to point out, battery current (Ib) MUST be inversely proportional to battery voltage (Ub) at a fixed motor speed and load. That principal is good to use for sanity checking a detailed spreadsheet model. If it doesn't hold up, something is probably wrong somewhere.
  5. Can this spreadsheet be used to get battery current versus battery voltage at given motor output power?
  6. Ah, I see now from your text in the post, thehorizontal axis is speed. Sorry I was able to download a spreadsheet with no problems. That's all I tried to do. I could check something else, if you want.
  7. Could you explain this plot? What are the axes? Seems motor and current are on the same axis? so, its volts or amps depending on the curve? What is the horizontal axis?
  8. Wow, hadn't thought about skin effect---but, most of the current isn't switching due to the high winding inductance. So, (I think) the skin effect would only impact power dissipation from the ripple current. So, I guess you could calculate the increased power due to skin effect by using ripple current, not total winding current.
  9. So, how can use the spreadsheet do compare two different battery voltages with the same motor load? I assume I change the UBattery average cell, until cell C20 is closed to zero (like the text says). When I do that, PMotor is higher. So, something else needs to be tweaked to get that. Or did you do that already and I missed an associated conclusion about whether battery current is (roughly) inversely proportional to battery voltage if motor load is held constant?
  10. Usually motor switching frequency is chosen above the audible range, to eliminate unwanted noise. That's why I assumed "10's of Khz". But, I don't have anything specific info for EUC wheels. I didn't pick up which wires had melted insulation on Marty's wheel? Were they the wires from the battery to the control board, or wires from it to the motor? I would think parasitic inductance of any wires driving the gate of the FETS would be important. They gate drive net is probably entirely contained on the pcb? But I'd think for any wire in series with a motor winding, it shouldn't matter--the windings themself are much more inductive than any wire parasitic inductance. If there's a snubber network of some type on the motor end of the wire, the wire would NOT be strictly in series. The current in the wire would NOT always equal current in the associated windings. But, if the current path in the wire is 100% common (in series) with a motor winding, it shouldn't matter.
  11. Also, need to clarify what current we're talking about. I'm referring to input or battery current. The motor windings that create the magnetic fields that puil the stator around will have pretty much the same current for a given power output, regardless of the battery voltage. However, the voltage duty cycle of the windings is set to get the speed you want. A longer duty cycle is required when the battery voltage is lower than when it's higher. So, a higher average battery current is required.
  12. yes. But, there may be another weak link that pops up. Needs to be tested for a rated continous power!
  13. Here's a thread I started after being challenged by my assertion that at a given output power (like going up a constant slope at a constant speed), more current will result than at a higher voltage:
  14. Wires have a rated ampacity for a given temperature rise. Here's an example: https://www.powerstream.com/Wire_Size.htm There are additional tables for bundles of wires together. Also, different wire insulations have can widely varying allowable wire temperatures--if you know the specific wire used, you can determine how much current the wire can handle without compromising the insulation. The insulation will likely melt or be destroyed before the wire acts like a fuse. The copper in the wire is a really good conductoer of heat (far better than the thermal path through the insulation), so much of the heat will be conducted to whatever the wire terminates on. Typically a heat sink on the control board. The motor windings might get a lot of airflow--I have no clue about that. But, if they do, that convection would be excellent for dissipating much of the heat. I'm not looked at the motor in EUC, so I don't know any real details, just general principles.