DaveThomasPilot Posted June 21, 2017 Share Posted June 21, 2017 So there is fortionately no feedback loop with the battery voltage and battery power. Just max speed is limited by lower battery voltage (due to high currents So that being said, if I am going at fixed speed, fixed wind resistance and fixed grade the amount of current through the Motor will be the same no matter if the battery volts is 80 or down to 50. We can get deep in the brushless motor theory in this thread if we want, but it really isn't necessary to understand how current must vary as a function of input voltage. Power out = Power In * efficiency. That's all you really need to know. The motors in our wheels are not resistors. They look inductive, and there are also capacitors in the power train. So, over very short periods of time, the principle that Power out = Power In * efficiency doesn't really hold. But, over longer periods of time (consistent with thermal time constants), that relationship must hold. It's a very basic conservation of energy theory. It really doesn't matter what system you are considering. A DC/DC convertor, a motor, a house, that relationship will always hold true. So, if a EUC is under a fixed load, like steadily going up a hill at a constant speed, its output power is constant. It will be the Input Power * efficiency. How much power it's putting out will be based on what speed is set by the ESC. The ESC will by turning currents on and off very quickly and the instantaneous currents that result in FETs, wires, and connectors is important. But, the average currents in these elements are what's important from a thermal analysis. Assuming the efficiency (Power Out - Power In) is constant regardless of the input voltage. The input power must be constant. Input power is Input Voltage * Input Current. So, if the input voltage is lower, the average input current must be higher. The efficiency is NOT totally constant as a function of input voltage. However, the efficiency change as a function of input voltage is small relative to the battery voltage impact on the input current. These fundamental principles are used of power and thermal analysis of virtually all power electronic systems. Let's debate this, before things get confused with the additional details on how the ESC controls motor speed and what that means for associate FET and motor winding currents. Probably, the failure to differentiate between current when devices are on, versus their time average current is the source of much of the misunderstanding about what happens when battery voltage gets lower and it might help to dig into those details. But, unless you want to argue for a perpetual motion machine, Power Out = Power In * Efficiency. Link to comment Share on other sites More sharing options...
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