Burtie wrote:Excellent work Luke!
That certainly helps me understand the principals a bit better
burtie
If the average phase voltage drops, how does the average phase current increase?"anytime PWM is used to regulate battery currrent, the average phase voltage drops, and the average phase current increases to balance the power equation"
John in CR wrote:As one of those with the 2 turn per winding motors, I understand the issue. The controllers, however, have program limits on the phase current as well as the battery current limit. I accept that I probably have give up low end torque compared to a higher turn count motor by limiting the phase current, but I get higher top speed in exchange. I wonder if the width of the pulse changes to adhere to the phase current limits. If not then which current limit dominates, or does the battery current get limited when either the the battery or phase limit is reached?
John
donob08 wrote:It's not clear to me how a controller would affect a limit based on Phase current. If it found Phase Current too high and cut PWM duty cycle further it would increase Phase Current based on current multiplication. I think all it could do is shut down.
After thinking about your statement, I think I know what you're referring to. You're referring to a "full throttle" simulation graph or some such graph. If your speed decreases at "full throttle", then yes, your phase current increases but it only increases because the physical load increases i.e., when you go from cruising on flatland to climbing a hill. It's not increasing simply because your motor voltage is decreasing. If you reduce throttle, you get a "partial throttle" graph in which case your phase currents decrease.swbluto wrote:I understand your general message, but this part seems misleading or misworded.
If the average phase voltage drops, how does the average phase current increase?"anytime PWM is used to regulate battery currrent, the average phase voltage drops, and the average phase current increases to balance the power equation"
Ignoring BEMF calculations (It doesn't affect the general principle), it's simple Ohm's law. If you reduce voltage to a resistor (The phase resistance), you decrease current. What would make a motor the exception?
Another simple rule is that anytime the motor slows down in the same environment/situation, the lower the torque the motor outputs (It takes lower torque to sustain a lower speed). Since there's lower motor torque, you can deduce the average phase current is decreased.
I think everyone needs a meter that shows them phase current stats. Perhaps direct observation would be a better teacher than simple electrical laws.
BTW, on a separate tangent, I think that the "average voltage" is really just a model for understanding it. With an actual controller, I believe that the motor sees the full voltage of the battery during the ON part of the mosfet, but the motor's voltage is a fraction of that depending on the duty cycle due to inductance. Ok, now saying that, I guess that it's more than a model.
Evan,EVan wrote:donob08 wrote:It's not clear to me how a controller would affect a limit based on Phase current. If it found Phase Current too high and cut PWM duty cycle further it would increase Phase Current based on current multiplication. I think all it could do is shut down.
Controllers I'm familiar with simply switch the FETs off if the current is exceeded, which is sensed very quickly on a cycle-by-cycle basis. The phase current will start to fall (it is maintained for a while by the motor inductance causing current to flow through the commutation diodes).
Over a few cycles, this has the effect of shortening the ON portion of the waveform, so reducing the duty cycle as you said. This reduces the average output voltage, so by Ohm's law the phase current falls. Not the other way around.
His original statement:John in CR wrote:Evan,EVan wrote:
Controllers I'm familiar with simply switch the FETs off if the current is exceeded, which is sensed very quickly on a cycle-by-cycle basis. The phase current will start to fall (it is maintained for a while by the motor inductance causing current to flow through the commutation diodes).
Over a few cycles, this has the effect of shortening the ON portion of the waveform, so reducing the duty cycle as you said. This reduces the average output voltage, so by Ohm's law the phase current falls. Not the other way around.
He and LFP are both saying the power in stays the same, so the reduced average voltage out means current out must go up accordingly. I don't believe this is what necessarily happens in actual use. If it hasn't reached the phase current limit yet, then yes, but once it has reached the phase current limit then either incoming current has to get limited, or the duty cycle increased to increase voltage out. Phase current limits is a programming option in these controllers for a reason.
John
Yes, if power going in is the same and the duty cycle decreases, phase current would be higher. But, if nothing else changes, why would the power going in be the same?If it found Phase Current too high and cut PWM duty cycle further it would increase Phase Current based on current multiplication. I think all it could do is shut down.
The fact that it's not a simple resistive load........?swbluto wrote: If the average phase voltage drops, how does the average phase current increase?
Ignoring BEMF calculations (It doesn't affect the general principle), it's simple Ohm's law. If you reduce voltage to a resistor (The phase resistance), you decrease current. What would make a motor the exception?
John in CR wrote: Evan,
He and LFP are both saying the power in stays the same, so the reduced average voltage out means current out must go up accordingly.
Miles, are you ever going to provide a specific counterpoint?Miles wrote:The fact that it's not a simple resistive load........swbluto wrote: If the average phase voltage drops, how does the average phase current increase?
Ignoring BEMF calculations (It doesn't affect the general principle), it's simple Ohm's law. If you reduce voltage to a resistor (The phase resistance), you decrease current. What would make a motor the exception?
swbluto wrote: Let's take that one at a time.
Inductive - this just shapes the current curve and causing it to ramp up. What's the deal here? The average current would be less than the ohmic predictions. How much less depends on the electric RPM, and the L/R ratio. From a phase current perspective, less current would be less harmful, so no problems there.
swbluto wrote:Once you understand that I already understand, you should graduate from the level of competitiveness measurements to getting a degree in truth acknowledgment.
Wow, you know what.. if it isn't obvious by now, I created the simulator. See the link below. It's not like someone had handed me the equations and I created a wrapper for it. I understand the law of "conservation of power"/energy (Power is just the time rate of energy), and I further understand the law of induction. Yes, I know that a collapsing magnetic field induces a voltage value on the coil via Lenz's Law and that PWM allows one to control the ratio of collapse/field-growth to create an average voltage of your choosing. Once you get that average voltage, you calculate average current, after subtracting the bemf from the motor voltage, via ohm's law. When you calculate the power via the motor current and motor voltage, minus the bemf (And diode voltage for more accuracy), it's *gasp* equal to the power of the input battery. I_1*V_1 = I_2*V_2. when V_2 goes down (The motor's average voltage), I_2 goes up to compensate.liveforphysics wrote:swbluto wrote:Once you understand that I already understand, you should graduate from the level of competitiveness measurements to getting a degree in truth acknowledgment.
OMG... you've gotta save these posts for LOL value to be re-read for the time when you understand how phase current multiplication works.
I guess we should start at a very basic circuit, like a bucking LED driver in a flashlight that uses a FET and an inductor. Maybe it runs a pair of lithium cells in series, so it takes in 7.4v at 300mA, it switches the FET at around 40% duty cycle through the inductor, and ends up outputting something like 3v at 750mA to the LED. Are you capable of grasping this concept so far?
John in CR wrote:Let's talk about how this issue occurs in real world use. eg Taking off from a stop once I get past about 1/3 throttle acceleration, sounds, etc. all seem the same whether it's 1/2 throttle or WOT, so isn't that all at full duty, not PWM chopping? If I use slightly less throttle, then I do hear some noise that seems to come from the motor. A cross between a click and a groan is the best I can describe it. That's the only time I hear that noise, which I take to be the audible result of PWM under load.
I don't expect to tune my HV controllers to high current, so either very easy acceleration using low power or maximum acceleration is easy, and I already avoid that middle ground that the motor warns me about.
The instance where my 100v100a Methods controller failed was while going up a hill that was gradually increasing in grade, but traffic was slowing me to less than full throttle. With PWM in play, I guess the phase current skyrocketed with the high power demand of the hill, but at less than full duty. If that is indeed where the risk lies then I can make a point to avoid it. If I'm on the right track here, I can't really think of another time I'd be at high load and less than full duty unless I tried cruising along at a 60-70 mph that was somehow partial throttle using a single motor. With the dual motor I don't think the load would be high enough, and the only way I'd get the dual motor under high load will be a top speed attempt, so WOT.
What am I missing?
John
As the physical load increased, i.e., the hill, your phase current would've sky-rocketed whether you were going full throttle or partial throttle. If you're at full throttle, the phase current will be even more than at partial throttle BUT the mosfets endure switching losses while PWMing during partial throttle, so if you're not at low enough partial throttle, the mosfets will endure more instantaneous heat than at full throttle.John in CR wrote: The instance where my 100v100a Methods controller failed was while going up a hill that was gradually increasing in grade, but traffic was slowing me to less than full throttle. With PWM in play, I guess the phase current skyrocketed with the high power demand of the hill, but at less than full duty. If that is indeed where the risk lies then I can make a point to avoid it. If I'm on the right track here, I can't really think of another time I'd be at high load and less than full duty unless I tried cruising along at a 60-70 mph that was somehow partial throttle using a single motor. With the dual motor I don't think the load would be high enough, and the only way I'd get the dual motor under high load will be a top speed attempt, so WOT.
John