minde28383 said:
This is first time I see someone using word "buck" converter when talking about controllers. And mentioning such thing like, - "increase phase current above battery current if the voltage is less". I know just principal that battery voltage drops more the more current you draw from battery the longer the time you do current draw. Never thought in a way that controller is DC-DC converter. What did you exactly mean by, - "The controller is a buck converter, it can only increase phase current above battery current if the voltage is less"? Did you wanted to say that I won't have 72v during heavy load times?, eh this is just my guess.
Buck converters are converters that efficiently step down voltages.
You can step down voltage with a resistor - but to do that you have to dissipate the rest of the voltage as heat. Let's say you want 5 volts/1 amp out, but you have 10 volts in. You could use a resistor to drop that down (and in fact that's how linear regulators work.) But if you want 5V 1A (5W) out, that means you have to also dissipate 5 watts in a resistor or other device somewhere. (10 watts in, 5 watts out, 5 watts dissipated as waste heat.) Very inefficient - and hot.
A buck converter is more efficient. Instead of a resistor it uses a switch, a diode and an inductor. It opens and closes the switch rapidly and the inductor "smooths out" the choppy voltage that results. (Also generally uses a capacitor for filtering, but that's not essential.) Now if you want 5 volts 1 amp out you only need to supply 10 volts at half an amp - much more efficient.
Now, a controller for a 3 phase BLDC motor is not designed to be a buck converter. But it has all the same parts. It has a switch (the FET) a diode (the body diode of the opposing FET) and an inductor (the motor winding.) So if you do PWM drive of the FET - which most controllers do - you have, effectively, a buck converter. This is great, because it's more efficient. And since you get the effect of a buck converter, it's possible to get 1 amp through the winding when the motor is moving slowly (=lower voltage needed) when you are drawing half an amp from the battery. That means more torque at low speeds.