The Low Rpm Efficiency Myth - Busted?

[Leeps]

Ok safe i will answer your yes or no question. It is possible to have efficiency and low rpm.
http://www.agnimotors.com/home/index.ph ... &Itemid=36
There are the dyno charts to the agni motor. Notice what happens when you drop the voltage going to the motor. Granted at low voltage the peak efficiency curve gets steeper thats not the point. the point is that the efficiency is still there but the rpms are droping.
my question at this point has become irrelevent, i know what i need to know and the discussion is already launched
Joe

[xyster]

Thank you leeps, those are very interesting graphs. I would not have thought it the case, nor had this opportunity to learn something, if you hadn't taken the initiative to actually answer a yes/no question. I have questions about these graphs, but will hold off so this great debate-of-the-ages can continue

[safe]

The chart for 36 Volts is only showing the rpms from about 2000 to 1600. That's not anywhere near the percentage of rpms that we are talking about. (it's just the top 20% of the full powerband) And I don't think this is including the effects of a controller anyway... which is actually the point of the discussion.

We need a "true dyno" that shows the full spectrum of rpms and the corresponding power in and power out and then we can get the efficiency. I think in the "real world" that the PWM effect "helps" but it doesn't overcome the "overwhelming" domination of the natural charactoristics of motors.

You just can't "fix" something that is producing 50% losses so easily...

[Leeps]

well at 12 volts we have roughly 88% efficiency at 600 rpms, at 60 volts we have just over 90% at roughly 3000 rpms.
In my opinion i proved my point. If you disagree ill gladly continue.
Joe

[xyster]

In my opinion i proved my point.

Can you succinctly state that point, Leeps? I'm not sure it's the same point, or even similar, to the point Safe's been trying to make.

[safe]

We have to also keep in mind that the current limit of the controller has it's own "logic" because the lower the limit is so that the "power peak" and the "efficiency peak" get closer and closer to each other the effect is BETTER overall efficiency than if the current limit is higher.

So we have to be careful of the "apples" verses "oranges" arguments!

The PWM "effect" has a smaller influence on the overall efficiency curve than the current limit itself.

We need to stay "honest" about all the separate variables...

[safe]

well at 12 volts we have roughly 88% efficiency at 600 rpms, at 60 volts we have just over 90% at roughly 3000 rpms.
In my opinion i proved my point. If you disagree ill gladly continue.

This is for 36 Volts, but they are all like this. The rpm range is only from 2000 to 1600. You need to look more carefully...

Plus, there's no controller on these charts.. these are before the controllers (theoretical) and not dyno charts.

[Leeps]

Gladly, i really wanted him to ask for it, but ill do it for you.
The pwm effect that youve been talking about has been the voltage all along, it had nothing to do with the current. Notice in all the graphs that at a certain voltage the efficiency does go down when the rpms drop this is because the current is going up. Its not the rpms thats killing your efficiency its the current, i understand that with one graph and one voltage its easy to come to the wrong conclusion. Ive been trying to tell you otherwise but hey at least we busted the myth.
That was the purpose of my first question, it was wether or not you understood why a motor consumes the current that it does. If you understand the relationship between rpm, voltage, and current.
Joe

[safe]

....at least we busted the myth.

What?! You haven't even gotten close!

[Leeps]

88% efficiency at 600 rpm and 95% efficiency at 3000 rpm thats a spread of 5 times in the rpm range. Im sure if they gave a chart at 6 volts you would get a greater spread.
Seriously now are you going to tell me that efficiency at low rpm is a myth?!?
The pwm controller is a key to getting your efficiency at low rpm because its a really efficient way to drop the voltage going to the motor.
Are we closer?
Joe

[safe]

The "myth" is that you can take a region of a motors powerband that is "structurally" going to produce a 50% loss and through PWM recover enough of the loss so as to equal the "peak efficiency" sweet spot in the motor.

This you have not even gotten close to accomplishing...

[Leeps]

Safe do you even know where the losses are coming from? Its a yes no question if yes, do your best to explain if not ill explain it to you.
Yes i am proposing that, this is what your seeing in the graphs, and no its not especially designated to a pwm controller, its designated to any device that can change the voltage going to the motor. It just so happens that the most convenient and also acceptably efficient device happens to be a pwm controller. Your direct drive battery controller would give you the same results its just not as convenient.
Joe
edit
well it didnt equal peak efficiency at lower rpm but hey 88% it aint that bad

[xyster]

Gladly, i really wanted him to ask for it, but ill do it for you.
The pwm effect that youve been talking about has been the voltage all along, it had nothing to do with the current. Notice in all the graphs that at a certain voltage the efficiency does go down when the rpms drop this is because the current is going up. Its not the rpms thats killing your efficiency its the current, i understand that with one graph and one voltage its easy to come to the wrong conclusion. Ive been trying to tell you otherwise but hey at least we busted the myth.
That was the purpose of my first question, it was wether or not you understood why a motor consumes the current that it does. If you understand the relationship between rpm, voltage, and current.
Joe

I'm going to state my understanding from before this thread started, I think it's the same as you stated above, just worded differently.

1) Voltage In X Amps In ---> Controller --> = Voltage Out X Amps Out - losses

2) As the PWM controller lowers the average voltage (by turning off the voltage out for a longer % of time), the Amps available rises.

3)As average voltage is lowered, resistive losses increase between the controller and the motor (and inside the motor and inside the power-out section of the controller) because current is higher and power lost = I X R^2.

4)It is the increased resistive losses at voltages fractional of the available total (the battery voltage) that equates to decreased system efficiency at lower RPMs (RPMs fractional of the total available for a given battery input voltage).

[safe]

You might not know how to read a chart.

By the time you get to "low rpms" your efficiency has dropped to 40% and it only gets worse as you approach zero rpms which is "off the charts".

[Mathurin]

*headdesk*

[Matt Gruber]

i investigated low rpm power use.
i was concerned that the frequent starts/stops in a store would kill my sla's, and i might not make it home.
my psev w/250w 24v MY1016 draws~8-10 amps
to get moving in a store. This was hurting my range. Sometimes i would park it and walk around.

my mbike 500w 36v MY1020 is twice the weight and i feared it would really suck amps. Much to my surprise in stores it draws less than its' no load wheel up WOT amps!(2.5). i can easily ride around 1-3mph at 1-2 amps. This looks really efficient to me, at least comparing the 2 scoots. The 250w is too small and wastes power.
Does anybody else ride a lot at low rpms? I think not.

[safe]

Are we closer?

Either you are joking (in which case ha, ha, ha) or you SERIOUSLY don't have a clue about how to read these charts.

Low rpms gives you 40% efficiency...

[Leeps]

My god buddy, believe me i can read a chart. What did i say in my other post. The key is changing the voltage, i see no changing of voltage. What happens if you change the voltage.
Ok heres an exercise. Take the 60 volt graph look at the efficiency at 600 rpm. Its bad, well this is what happens when you dont have a controller you have full voltage, full voltage at low rpm gives you bad efficiency. Now look at 600 rpms with 12 volts, you have good efficiency. The controller lets you go from 60 volts down to 12 and back again.
Are we closer?
Joe

[xyster]

Either you are joking (in which case ha, ha, ha) or you SERIOUSLY don't have clue about how to read these charts...

Safe, if as impartial umpire I had either an effective carrot, or a fearsome stick, I'd beat you over the head with it for trying to incite the opponent to violence.

[Matt Gruber]

88% efficiency at 600 rpm and 95% efficiency at 3000 rpm thats a spread of 5 times in the rpm range. Im sure if they gave a chart at 6 volts you would get a greater spread.
Seriously now are you going to tell me that efficiency at low rpm is a myth?!?
The pwm controller is a key to getting your efficiency at low rpm because its a really efficient way to drop the voltage going to the motor.
Are we closer?
Joe

Good job Joe! Even if safe is a little slow, others will read this topic and gain a lot of understanding.

[safe]

It took me a moment to realize what you were talking about...

Yes, in the "perfect scenario" where the "efficiency peak" and the "power peak" are one in the same then you will get the kinds of high efficiencies that you have pointed to. This is one of those "apples" verses "oranges" situations were we are talking about two tihngs at once. The PWM has NOTHING to do with this... this is all about matching the voltage to the rpms so as to hit the "sweet spot" of efficiency within the "normal" physics of the motor. That's why in the past I've stressed the idea that you need to align these two things and then the controller will automatically produce a very efficient overall powerband.

Back to the "myth" however...

The PWM "effect" simply gives you a little "extra" torque in areas that are nice to have... if you use a very high current limit YOUR LOSSES ARE STILL BAD. (PWM doesn't change the overall motor charactoristics that much, just a little "extra" in certain rpms)

The point here is that it's the motor configuration and the current limit that determine MOST of how your overall system behaves, not PWM.

The myth stands as a myth...

[safe]

Good job Joe! Even if safe is a little slow, others will read this topic and gain a lot of understanding.

Sorry, you don't realize that there are two separate variables:

1. Current Limit

2. PWM "Effects"

They BOTH contribute separately to the overall motor perfomance.

The "Heroic" PWM Controller Myth is still BUSTED!

[Leeps]

The pwm effect is just changing the input voltage to the motor, thats it its just that simple. It does not in any way shape or form change the motor characteristics nor does it give you extra torque. The motor characteristics are constants and as such stay that way. Torque is proportional to current and the current that a motor will draw can only be externally effected by voltage and shaft rpm. Even current limiting works in effect by lowering the voltage going to the motor in order to reduce current draw at the motor, there is a problem in the controller measures battery current and thus the dc-dc conversion doesnt allow the current limit to be a constant value, i suppose this is the torque boost you are speaking about, the same rules still apply the controller is droping voltage to try and keep the current in check and maintain efficiency.
I see things being very apple to apple or orange to orange depending on which fruit you prefer.
Joe

[safe]

Hopefully you guys haven't walked away before realizing the conclusion on this one. The PWM "effect" is totally unrelated to the choice of current limit for the controller. For a novice that knows nothing about this stuff they don't know the difference between the "current limit" and the PWM "effect" they just think "controller". So if they read this they will get the wrong idea.

It's true that a low current limit makes for a more efficient powerband and a higher one tends to give more power, but it also means that you lose that "sweet spot" of being in the "peak efficiency" of all the various voltages as you go up and down the rpms.

[xyster]

The "Heroic" PWM Controller Myth is still BUSTED!

I don't see that the myth ever existed.

Safe's focusing on whether or not controller characteristics can overcome motor inefficiencies. The answer, I agree and have agreed, is no. But I don't see Leeps has ever argued otherwise.

Leeps has been focusing on controller efficiencies at different voltages, and is correct in his assessment too that the controller is a very efficient voltage converter, especially when compared to not using a controller and instead using some extraneous load whether physical or electrical to moderate motor speed. I've not perceived Safe arguing otherwise.

You both win. Now hug and make up...