safe said:
No...It...Isn't
Only current that flows thru a resistance produces heat.
Current that passes thru an inductive reactance does not.
PWM current multiplication effect
- Thread starter fechter
- Start date
No...It...Isn't
Only current that flows thru a resistance produces heat.
Current that passes thru an inductive reactance does not.
That didn't take long.
I was fully expecting someone to point that out.
Hysteresis, eddy currents, & other core losses don't add up to a lot. If you have a better core material, the loss is less, therefore heat is less at the same current level.
Think of a magnetic ballast in a flourescent lamp. There is some heating from core losses, but it's miniscule compared to if you replaced the coil with a resistor ballast. The lamp would work the same i.e. same 'impedance' to limit the current to the same level the lamp requires, but the resistor would give off a lot more heat
I was fully expecting someone to point that out.
Hysteresis, eddy currents, & other core losses don't add up to a lot. If you have a better core material, the loss is less, therefore heat is less at the same current level.
Think of a magnetic ballast in a flourescent lamp. There is some heating from core losses, but it's miniscule compared to if you replaced the coil with a resistor ballast. The lamp would work the same i.e. same 'impedance' to limit the current to the same level the lamp requires, but the resistor would give off a lot more heat
xyster
10 MW
To my simple mind, these statements sound mutually exclusive, the second statement also not logically self-consistent.
Statement one equates to:
All Losses = Heat.
Statement two equates to:
Resistor Losses = Impedance = Heat,
but
Magnetic ballast = Impedance != (does not equal) Heat.
Clarify, please.
The difference is in that with the resistor, the impedance is coming from resistance, thus you pretty much all loss. loss does equal heat.
however with the magnetic ballast you do have impedance but a lot less actual resistance. An impedance can be looked at as an apparant resistance, for example an audio filter blocks high frequencies, at a certain frequency you can state the impedance in ohms with it going up as the frequency goes up. This doesnt imply resistance the high frequencys just arent going through. An inductor in series with a speaker is a good example it might have say 10kohms of impedance at 15khz and 1kohms at 5khz but only 0.5 ohms of actual resistance. The losses would be I^2R for the resistance plus core losses,etc I really hope that got across im not a good teacher.
Joe
however with the magnetic ballast you do have impedance but a lot less actual resistance. An impedance can be looked at as an apparant resistance, for example an audio filter blocks high frequencies, at a certain frequency you can state the impedance in ohms with it going up as the frequency goes up. This doesnt imply resistance the high frequencys just arent going through. An inductor in series with a speaker is a good example it might have say 10kohms of impedance at 15khz and 1kohms at 5khz but only 0.5 ohms of actual resistance. The losses would be I^2R for the resistance plus core losses,etc I really hope that got across im not a good teacher.
Joe
TylerDurden
100 GW
This might help:
http://en.wikipedia.org/wiki/Reactance
inductors can limit non-constant current without resistors' heat losses
(That's prolly why Turbo used quotes on 'impedance'.)
:?
http://en.wikipedia.org/wiki/Reactance
inductors can limit non-constant current without resistors' heat losses
(That's prolly why Turbo used quotes on 'impedance'.)
:?
xyster
10 MW
OK, got it. I think... 8)
tanks a lot!
tanks a lot!
safe
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So it seems that what is going on is that when the duty cycle is low enough and the discontinuous current condition exists that the pulses are unable to completely fill the "inductance pathway" in time such that any actual resistance builds up you get a special condition. The normal "peak" current and "peak" resistance might follow the:
Heat = Current Squared Times the Duty Cycle
...rule, but since a good part of that current takes place BELOW that actual resistance level in the discontinuous state of full current then the AVERAGE resistance is lower. You are in effect creating a sort of "superconductivity" because the natural resistance is never attained? (the early arriving electrons get to "run free" until the place gets crowded then things begin to "resist" the crowding)
Is this "conceptually" what is going on?
And a followup question that's been bothering me:
In practical terms how much actual power can be gotten MATHEMATICALLY SPEAKING from this discontinous condition? In the chart the current is ONLY about 1/10 of the full current possible. Does this mean that the PWM "effect" does not happen with a bike running at it's current limit? I've assumed that the "effect" was primarily rpm related (which it is) but you also seem to have to cut OVERALL power output as well. If one needs "X" amount if power to climb a hill at a certain high rpm and you allow the rpms to lower into the discontinuous level but are limited to "1/10 X" (multiplied by 10?) for power how does that improve anything? You get some extra power compared to none at all, but it's still dwarfed by "peak power" at the other end of the rpms.
So that's got me confused too... and still hungover... :?
Look below at what they call "Normalized Current". How do we relate "Normalized Current" to our actual current? This formula seems to lead the way... somehow...
Question:
If you had a 40 amp current limit and wanted to get into the discontinuous condition would you need to have a duty cycle below a certain level or a current below a certain level or both?
Seems to me that current must be (in this case) BELOW 4 amps and the lower the duty cycle the more multiplication comes from the effect.
But how much power comes from 4 amps @ 10% duty cycle? Does one actually get 4 amps times ten... a full 40 amps "virtual current"?
Heat = Current Squared Times the Duty Cycle
...rule, but since a good part of that current takes place BELOW that actual resistance level in the discontinuous state of full current then the AVERAGE resistance is lower. You are in effect creating a sort of "superconductivity" because the natural resistance is never attained? (the early arriving electrons get to "run free" until the place gets crowded then things begin to "resist" the crowding)
Is this "conceptually" what is going on?And a followup question that's been bothering me:
In practical terms how much actual power can be gotten MATHEMATICALLY SPEAKING from this discontinous condition? In the chart the current is ONLY about 1/10 of the full current possible. Does this mean that the PWM "effect" does not happen with a bike running at it's current limit? I've assumed that the "effect" was primarily rpm related (which it is) but you also seem to have to cut OVERALL power output as well. If one needs "X" amount if power to climb a hill at a certain high rpm and you allow the rpms to lower into the discontinuous level but are limited to "1/10 X" (multiplied by 10?) for power how does that improve anything? You get some extra power compared to none at all, but it's still dwarfed by "peak power" at the other end of the rpms.
So that's got me confused too... and still hungover... :?
Look below at what they call "Normalized Current". How do we relate "Normalized Current" to our actual current? This formula seems to lead the way... somehow...
Question:
If you had a 40 amp current limit and wanted to get into the discontinuous condition would you need to have a duty cycle below a certain level or a current below a certain level or both?
Seems to me that current must be (in this case) BELOW 4 amps and the lower the duty cycle the more multiplication comes from the effect.
But how much power comes from 4 amps @ 10% duty cycle? Does one actually get 4 amps times ten... a full 40 amps "virtual current"?
Eh, ampmeter anyone?
Another post by nlc, (out of context):
Actually the calculations I did were to measure current in the phases on startup.
In order to limit battery current to 20a, the controller hacks the battery's current, so in the end we don't know how much current goes to the motor. What we do know is the power, wich is essentially the same before and after controller, or about 840w, and 0.4ohms.
But this is sufficiant :
P = U x I
U = R x I
Therefore P = R x I x I
Therefore I x I = P / R
Therefore I = root( P / R )
Wich gives us 45A
Another post by nlc, (out of context):
Actually the calculations I did were to measure current in the phases on startup.
In order to limit battery current to 20a, the controller hacks the battery's current, so in the end we don't know how much current goes to the motor. What we do know is the power, wich is essentially the same before and after controller, or about 840w, and 0.4ohms.
But this is sufficiant :
P = U x I
U = R x I
Therefore P = R x I x I
Therefore I x I = P / R
Therefore I = root( P / R )
Wich gives us 45A
safe
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Continuous or Discontinuous?
The foundational "concept" of the actual PWM "effect" is that the current crosses an imaginary "boundry line" where the current is considered "continuous" on one side and "discontinuous" on the other. I'm not sure if we are all on the "same page" about exactly WHEN this boundry transition takes place.
Does the current switch from continuous to discontinuous at 2 amps, or 5 amps, or 10 amps or all the way up at 40 amps? And at what duty cycle?
What are the parameters that determine when the boundry is crossed?
If I'm understanding this chart below correctly the "effect" that comes with discontinuous operation doesn't occur until the load drops waaaaaay off... so is this true or false? Are we confusing the "regular" continuous condition with what might actually be a more rare discontinuous condition? Or is the current ALWAYS discontinuous and this chart is misleading? (the definition of "Normalized Current" might be creating a wrong impression of the behavior since it is scaled to be around 10% - 15%)
If you look at the chart and ponder the implications of it you have to come to the conclusion that the PWM "effect" ONLY TAKES PLACE when under a light load... (so trying to "load up" at low rpms will make you fall out of the discontinuous condition) A throttle on "full" at low rpms will always allow a full current limit to pass... so you would need low rpms AND the throttle opened to just a trickle so that the current is kept really, really low. With so little current the discontinuous condition kicks in and the voltage multiplication allows for more current to be passed through even though it "appears" that very little current is being used. This then increases torque. So if you knew "when" the boundry is crossed (with a light) then you would know that you are "throttle fiddling" correctly. Knowing the amps alone doesn't allow you to know because it's a complex boundry line that is related to the duty cycle AS WELL AS the current. This chart shows how the "boundry" moves all over the place depending on the duty cycle.
The foundational "concept" of the actual PWM "effect" is that the current crosses an imaginary "boundry line" where the current is considered "continuous" on one side and "discontinuous" on the other. I'm not sure if we are all on the "same page" about exactly WHEN this boundry transition takes place.
Does the current switch from continuous to discontinuous at 2 amps, or 5 amps, or 10 amps or all the way up at 40 amps? And at what duty cycle? What are the parameters that determine when the boundry is crossed?If I'm understanding this chart below correctly the "effect" that comes with discontinuous operation doesn't occur until the load drops waaaaaay off... so is this true or false? Are we confusing the "regular" continuous condition with what might actually be a more rare discontinuous condition? Or is the current ALWAYS discontinuous and this chart is misleading? (the definition of "Normalized Current" might be creating a wrong impression of the behavior since it is scaled to be around 10% - 15%)
If you look at the chart and ponder the implications of it you have to come to the conclusion that the PWM "effect" ONLY TAKES PLACE when under a light load... (so trying to "load up" at low rpms will make you fall out of the discontinuous condition) A throttle on "full" at low rpms will always allow a full current limit to pass... so you would need low rpms AND the throttle opened to just a trickle so that the current is kept really, really low. With so little current the discontinuous condition kicks in and the voltage multiplication allows for more current to be passed through even though it "appears" that very little current is being used. This then increases torque. So if you knew "when" the boundry is crossed (with a light) then you would know that you are "throttle fiddling" correctly. Knowing the amps alone doesn't allow you to know because it's a complex boundry line that is related to the duty cycle AS WELL AS the current. This chart shows how the "boundry" moves all over the place depending on the duty cycle.
safe
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In the end I'm stuck with another set of questions:
Does torque significantly increase because of the PWM "effect" or are you simply able to achieve comparable torque with less motor effort? (less heat produced allows the motor to handle the load better)
And the followup on this is that if REAL torque gains are to be found (not just ways to avoid overheating) then why would a controller be set up to even allow full throttle (peak current behavior) at low rpms?
Why even allow the "less desireable" condition to exist?
Couldn't a controller be made "smart" so that it exploited the PWM "effect" (discontinuous operation) so the user never even knew that the optimal behavior was being performed?
What if a "Cave Man" wanted to ride an electric bike? How might he "adapt" to the PWM "effect"?
Does torque significantly increase because of the PWM "effect" or are you simply able to achieve comparable torque with less motor effort? (less heat produced allows the motor to handle the load better)
And the followup on this is that if REAL torque gains are to be found (not just ways to avoid overheating) then why would a controller be set up to even allow full throttle (peak current behavior) at low rpms?
Why even allow the "less desireable" condition to exist?Couldn't a controller be made "smart" so that it exploited the PWM "effect" (discontinuous operation) so the user never even knew that the optimal behavior was being performed?
What if a "Cave Man" wanted to ride an electric bike? How might he "adapt" to the PWM "effect"?
knightmb
100 kW
safe said:
Cost related. If you were going to use a certain "motor" for a product, you spend the R&D money to figure out it's best operating range at a given power. So if 'Motor A' doesn't make any more low RPM torque at 40 amps as it does at 20 amps (because the extra amps just produce more waste as heat), you program a controller to recognize this lower RPM and use less amps. When the speed increases, so do the amps as the motor winds up to it's most efficient RPM and torque usage. When it comes to e-bikes, it's easier and cheaper just to make a "one size fits all" power output then to go more complicated.
Couldn't a controller be made "smart" so that it exploited the PWM "effect" (discontinuous operation) so the user never even knew that the optimal behavior was being performed?
What if a "Cave Man" wanted to ride an electric bike? How might he "adapt" to the PWM "effect"?
For the same reason above I believe anyway. I've been very impressed with my 20 amp cyrstalyte controller because it produces good speed and range. The controller and motor never feel to get hot even on steep hills. Sure it's slow on steep hills, but I just use some pedal effort and I'm still moving along very fast. I know that when I swap in the 40 amp controller it climbs hills faster at the expense of more power, but the top speed doesn't change much (30 mph vs. 35 mph)
So I'm willing to climb hills slower (and use my own pedal power to assist) to give me more range than have rocket starts at the sacrifice of range.
safe
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In some simulations of controllers with their motors (like the 5304 that Xyster uses) they include the PWM "effect" as a boost in torque in the areas between a duty cycle of 100% and a duty cycle of 0%. (anything less than the "peak power" rpm) My question would be whether that PWM "effect" takes place if you are holding full throttle in a low rpm condition? (at full throttle does it revert back to the "linear" relationship like you expect from the continuous condition?)
Don't we need a "special set of circumstances" where the duty cycle is low AND the current (the load) is low so that the discontinuous current allows the motor to run with less difficulty?
Such a simulation program seems to portray "ideal throttle usage" to get the full effect of the discontinuous current behavior. So if you were to simply use the throttle without regard for the PWM "effect" would you get the extra torque?
Would full throttle usage (thereby operating outside the discontinuous condition) eliminate the PWM "effect" at low rpms?
Does less throttle actually give MORE torque or just less heat?
("Hub" represents the torque at the Hub in Newton Meters)
Don't we need a "special set of circumstances" where the duty cycle is low AND the current (the load) is low so that the discontinuous current allows the motor to run with less difficulty?
Such a simulation program seems to portray "ideal throttle usage" to get the full effect of the discontinuous current behavior. So if you were to simply use the throttle without regard for the PWM "effect" would you get the extra torque?
Would full throttle usage (thereby operating outside the discontinuous condition) eliminate the PWM "effect" at low rpms?Does less throttle actually give MORE torque or just less heat?
("Hub" represents the torque at the Hub in Newton Meters)
Aww jeez.... you're making my head hurt again.
If you're go up an increasingly steep hill and hold the throttle on full, the motor will behave like a normal brushed motor curve until you hit the current limit.
After that, the overall system changes because you're trying to exceed the limit. As you continue up the hypothetical hill, the duty cycle will steadily decrease, along with your speed, even though you have the throttle nailed. If you back off part way with the throttle, you won't notice any difference until you back off far enough for the duty cycle to be less than what the limiter is making it do.
If you're go up an increasingly steep hill and hold the throttle on full, the motor will behave like a normal brushed motor curve until you hit the current limit.
After that, the overall system changes because you're trying to exceed the limit. As you continue up the hypothetical hill, the duty cycle will steadily decrease, along with your speed, even though you have the throttle nailed. If you back off part way with the throttle, you won't notice any difference until you back off far enough for the duty cycle to be less than what the limiter is making it do.
safe
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fechter said:
I know I'm "pushing peoples minds" again and asking the really hard questions but I think I'm getting close to really understanding the fine points of the PWM "effect".
fechter said:
But let me get back into those "painful" questions... the "true" boundry is not between 100% duty cycle and a low percentage duty cycle, but the boundry between "continuous" current and "discontinuous" current. The controllers current limit is a pretty arbitrary thing that is specific to the motor configuration. The "boundry" point where the current is LOW ENOUGH for the transition to take place is where my central line of questioning is going. (and I think the current must be very, very low)
How does one know WHEN the "boundry" is crossed and you are ACTUALLY getting some "discontinuous" current taking place. There are no bells or whistles or even set current values that you can base your "boundry crossing" on because it's a condition that is variable...Attachments
xyster
10 MW
I think, Safe, you're closing in on the holy grail of physics, the TOE (Theory Of Everything). The problem as I understand it has to do with the inability to unite Quantum Mechanics and General Relativity. After you grasp the PWM effect, you should see what happens when you graph:
Quantum Mechanics X PWM = General Relativity
Then take a break, go for a long ride, come back to hole up in your garage working on your flux capacitor until next winter.
Oh, and Einstein: 'boundry' is spelled 'boundary'. That's about all the help I can be at this point :wink:
safe
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Okay Xyster...
Can you say without a shadow of a doubt when your bike is actually crossing the boundAry between continuous and discontinuos operation? (I've been using the George Bush spelling of "boundry", but I'll change to the more standard form)
You might "think" you know, but in reality you probably have no way to actually know. Simply knowing your current level is not adequate to know... unless your current reading is coming from the motor side, then you could "guess" based on the point of current reversal... but you have to be careful there because you might be going back up the continuous side and not know it... (no way to tell)
My observation is that the slightest slip of too much throttle and you "drop out of warp" so to speak and are simply going will low throttle with nothing to show for it. If there were some way to VERIFY the actual effect was talking place then you could ride accordingly.
It's even possible that people are almost never actually using the effect because it occurs at such a low current as to be below normal usage.
I have begun playing with that formula in my spreadsheet and it does create the curvature that one wants... it's going to take a while to get the values of the Inductance right and the period T as well. And dealing with the boundary issue will be tricky. I've seen the Inductance listed as 2.6 Micro Henry's for the PMG 080 motor, so that might be a "ballpark" number to start with... we will see... the main point is that this "effect" seems to only occur at low current.
I think I'm going to have to reinvent my spreadsheet so that it focuses on the duty cycle FIRST and then let's all the other parameters fall out of that. The controller ends up dominating the way the motor behaves, so the motor formulas need to be dependant on the PWM continuous verses discontinuous behavior. It's not easy. (but for me getting this wrong might mean breaking a hub)
Can you say without a shadow of a doubt when your bike is actually crossing the boundAry between continuous and discontinuos operation? (I've been using the George Bush spelling of "boundry", but I'll change to the more standard form)You might "think" you know, but in reality you probably have no way to actually know. Simply knowing your current level is not adequate to know... unless your current reading is coming from the motor side, then you could "guess" based on the point of current reversal... but you have to be careful there because you might be going back up the continuous side and not know it... (no way to tell)
My observation is that the slightest slip of too much throttle and you "drop out of warp" so to speak and are simply going will low throttle with nothing to show for it. If there were some way to VERIFY the actual effect was talking place then you could ride accordingly.
It's even possible that people are almost never actually using the effect because it occurs at such a low current as to be below normal usage.
I have begun playing with that formula in my spreadsheet and it does create the curvature that one wants... it's going to take a while to get the values of the Inductance right and the period T as well. And dealing with the boundary issue will be tricky. I've seen the Inductance listed as 2.6 Micro Henry's for the PMG 080 motor, so that might be a "ballpark" number to start with... we will see... the main point is that this "effect" seems to only occur at low current.
I think I'm going to have to reinvent my spreadsheet so that it focuses on the duty cycle FIRST and then let's all the other parameters fall out of that. The controller ends up dominating the way the motor behaves, so the motor formulas need to be dependant on the PWM continuous verses discontinuous behavior. It's not easy. (but for me getting this wrong might mean breaking a hub)
xyster
10 MW
Yah, when I hit a curb going 30.
Seriously, you're way beyond the point I stop trying hard to comprehend something, for doing so takes too much time away from trying to half-way understand something else -- just enough to make good use of the info, and basically follow the conversation of those folks more in the know.
So you see, by pointing out the spelling error and offering the above sarcastic reply, I was teasing my epistemological approach as much, if not more, than yours'.
safe
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Hopefully Fechter will add more to this issue...
Basically it would be nice to know when the "actual" effect where the current goes discontinuous takes place. I really don't see how it could happen at high rpms (and high duty cycles) unless you somehow could lower the load. Normally when you look at a motor performance chart you are "assuming" that the current limit of the controller is the limiting factor of the motors behavior. But the PWM "effect" is sort of "vertical" rather than "horizontal". In other words you can have partial throttle at many different rpms and achieve the PWM "effect" since it's a:
RELATIONSHIP of duty cycle to current.
Load is the sort of weird "wildcard" because any time the load is high in the continuous condition there can be no discontinuous condition. The two "states" are mutually exclusive of one another.
So the "bottom line" is that half throttle at half rpms might not produce ANY PWM "effect", but 1/8 throttle at half rpms very well might. The simulation programs likely takes the "perfect throttle" scenario where you are delicately balanced in the discontinuous zone so as to get the current multiplication and the "effect".
So the simulation might be "overly optimistic" in that most people would "drop out of warp" unless they got the throttle setting absolutely right.
All the more reason to have a PWM "effect" light or something... then you know you are really getting something worthwhile and not fooling yourself with partial throttle that doesn't do any good.
Here's how the light might work:
Any time the current is continuous then the duty cycle equals the voltage. When the current goes discontinuous then the voltage breaks free of the constrant of being equal to the duty cycle and begins to deviate. So the light could simply test to see if the voltage has strayed away from equal and when it does it goes on..
Simple! (well probably not in practice, but conceptually it's simple)
Basically it would be nice to know when the "actual" effect where the current goes discontinuous takes place. I really don't see how it could happen at high rpms (and high duty cycles) unless you somehow could lower the load. Normally when you look at a motor performance chart you are "assuming" that the current limit of the controller is the limiting factor of the motors behavior. But the PWM "effect" is sort of "vertical" rather than "horizontal". In other words you can have partial throttle at many different rpms and achieve the PWM "effect" since it's a:
RELATIONSHIP of duty cycle to current.
Load is the sort of weird "wildcard" because any time the load is high in the continuous condition there can be no discontinuous condition. The two "states" are mutually exclusive of one another.
So the "bottom line" is that half throttle at half rpms might not produce ANY PWM "effect", but 1/8 throttle at half rpms very well might. The simulation programs likely takes the "perfect throttle" scenario where you are delicately balanced in the discontinuous zone so as to get the current multiplication and the "effect".
So the simulation might be "overly optimistic" in that most people would "drop out of warp" unless they got the throttle setting absolutely right.All the more reason to have a PWM "effect" light or something... then you know you are really getting something worthwhile and not fooling yourself with partial throttle that doesn't do any good.
Here's how the light might work:
Any time the current is continuous then the duty cycle equals the voltage. When the current goes discontinuous then the voltage breaks free of the constrant of being equal to the duty cycle and begins to deviate. So the light could simply test to see if the voltage has strayed away from equal and when it does it goes on..
Simple! (well probably not in practice, but conceptually it's simple)
Yes, I suppose you could design a circuit that would tell you when you went discontinuous, but as you point out, this occurs at a current below the normal operating area. In practice I think this would only happen going downhill or with a major tailwind.
I guess I don't see why you would want to know this. An indcator of overall efficiency might be much more useful.
I guess I don't see why you would want to know this. An indcator of overall efficiency might be much more useful.
safe
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fechter said:
But this is the big "realization" that should be understood. The much admired and almost "worshipped" PWM "effect" that is supposed to cure cancer and make hub motors able to leap tall mountains really only works when the load is small. The actual "effect" demands low current and that's a problem on a hill. You can make hill climbing "better" by exploiting the "effect" and going slowly, but you can't "force" the "effect" into areas that it can't apply.
The big advantage is for a slow speed "crawl" where you keep your efficiency high by exploiting the "effect". So the PWM "effect" light might have value for those that live in flat areas.I'm just starting to wonder how much danger this really represents to the internal multi-speed hub bike because it doesn't seem like you really get as much out of it as you might imagine.
safe
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Economy Mode?
While the PWM "effect" can is certain limited cases apply to hillclimbs the place it "might" be useful is in getting into a kind of "Economy Mode". If I run too tall of a gear then my motor overheats and I don't really get all that much power even if the throttle is full. If I'm in the correct gear then if I want full throttle I can do it and it will run efficiently. But it seems like a third option is to run a gear lower and then (since the load is now less) I can drop the throttle to almost "off" and I "presume" that I've now gotten into the PWM "effect" and can carry this lower level of torque with little effort.
The question becomes:
"Other than the fact that I'm going a lot slower and therefore don't need as much power to propel the bike have I gained anything in efficiency by doing this?"
I could achieve the same speed if I dropped down two gears and then "spun" the motor to it's "efficiency peak" and then let off the throttle. It just doesn't make much sense... How do I use the PWM "effect" to an advantage while riding? What is the "real" efficiency of the PWM "effect" compared to the "efficiency peak" value? How do we even calculate the efficiency of the PWM "effect"?
The more I know the less I seem to know... :?
The two "ideal states" to be in are:
1. The Peak Efficiency RPM
2. A Low Enough Current and Duty Cycle so as to invoke the PWM "effect".
By definition the "peak efficiency" requires a duty cycle of 100%, so we can exclude the possibility of the two states intersecting anywhere. The duty cycle ranges from 100% down to 0% beginning at the peak efficiency rpm OR the peak power which can be at a lower rpm if the controller has a high current limit. Basically the entire right side of the chart after the peak power is at 100% duty cycle and the left side slowly declines until you reach 0%. Within the outlines of the powerband you have this "sub-band" which is the partial throttle, lowered duty cycle, reduced continuous current OR if the duty cycle is low enough a discontinuous current. Once you reach this "sub band" then the voltage suddenly reverses it's decline and suddenly "perks up". With the increased voltage the motor can now carry more current and so you get more power... but only as long as the load remains very low... the moment the load rises (like a hill) then the current demands increase and the "effect" switches back again. You've dropped out of warp.
So somehow you have to use as little throttle as possible because it's as though the slightest resistance to the torque you are making causes the whole thing to stop working for you.
While the PWM "effect" can is certain limited cases apply to hillclimbs the place it "might" be useful is in getting into a kind of "Economy Mode". If I run too tall of a gear then my motor overheats and I don't really get all that much power even if the throttle is full. If I'm in the correct gear then if I want full throttle I can do it and it will run efficiently. But it seems like a third option is to run a gear lower and then (since the load is now less) I can drop the throttle to almost "off" and I "presume" that I've now gotten into the PWM "effect" and can carry this lower level of torque with little effort.
The question becomes:"Other than the fact that I'm going a lot slower and therefore don't need as much power to propel the bike have I gained anything in efficiency by doing this?"
I could achieve the same speed if I dropped down two gears and then "spun" the motor to it's "efficiency peak" and then let off the throttle. It just doesn't make much sense... How do I use the PWM "effect" to an advantage while riding? What is the "real" efficiency of the PWM "effect" compared to the "efficiency peak" value? How do we even calculate the efficiency of the PWM "effect"?
The more I know the less I seem to know... :?
The two "ideal states" to be in are:
1. The Peak Efficiency RPM
2. A Low Enough Current and Duty Cycle so as to invoke the PWM "effect".
By definition the "peak efficiency" requires a duty cycle of 100%, so we can exclude the possibility of the two states intersecting anywhere. The duty cycle ranges from 100% down to 0% beginning at the peak efficiency rpm OR the peak power which can be at a lower rpm if the controller has a high current limit. Basically the entire right side of the chart after the peak power is at 100% duty cycle and the left side slowly declines until you reach 0%. Within the outlines of the powerband you have this "sub-band" which is the partial throttle, lowered duty cycle, reduced continuous current OR if the duty cycle is low enough a discontinuous current. Once you reach this "sub band" then the voltage suddenly reverses it's decline and suddenly "perks up". With the increased voltage the motor can now carry more current and so you get more power... but only as long as the load remains very low... the moment the load rises (like a hill) then the current demands increase and the "effect" switches back again. You've dropped out of warp.
So somehow you have to use as little throttle as possible because it's as though the slightest resistance to the torque you are making causes the whole thing to stop working for you.
xyster
10 MW
Ah yes, the asymptotic learning curve runs smack into a wall at some point. As I study any subject in increasing depth, my learning efficiency (info per time) declines until the point where I give up, figuring my time is better spent learning something else I don't know as well, and so can make great headway with a small brain investment. :wink:
This discussion passed that point for me about 3 pages ago.
But I'm still reading the thread anyway...go figure!
safe
1 GW
- Joined
- Dec 22, 2006
- Messages
- 5,681
xyster said:
I was watching "The Apprentice: Los Angeles" last night and they had Arnold Schwarzenegger who talked about how life is like this. At some point you always hit the "wall of pain" and most people stop there and go no further. It's the few that can fight through the pain to get to the other side that actually "get there".
I'm feeling that there is less and less of a fuss to be made of the PWM "effect". It does exist, but for the "Sport Riding" style that I'm doing with gears and all it is of no practical use. I'm less worried about the torque claims because I think people are using incorrect logic in their simulations. If you can achieve three times the torque of "what you might expect" of one third the throttle setting then you still end up pretty much where you started. I don't think that the PWM "effect" is going to be the cause of a multi-speed hub blowing up because it only exists at small fractions of full throttle and only under low load conditions. The moment the load presents itself to "break the hub" the PWM "effect" will "drop out of warp" and be unable to do any damage. The "effect" is limited to rare special conditions... namely low load, low duty cycle, low current.
Before I can be "sure" however I'm going to do a conversion of my spreadsheet so that it runs off the duty cycle rather than voltage. Once I make that change then I can simply add the discontinuous equation and see what happens. Everything needs to be redone to do that...
