Motor Current Limiting: More Power and Less Heat!!!

[eP]

I can agree - but that is Power_out so far.
I'm wating for Power_in values :wink:

Here's some "raw data" for a 10% slope in first gear for the "Motor Current Limited" version.

You should notice that I'm getting 1.87 hp out of a little 750 Watt motor. Normally you get about 1/4 hp less than that... so you get more power this way...

So tell us what is the acceleration at 9% slope at this case ?

 

[safe]

So tell us what is the acceleration at 9% slope at this case ?

Well, it looks like the maximum slope is 11%, so at 9% the accelleration would be next to nothing. You would be able to maintain your speed and maybe go slightly faster, but most all of the motor power would be used up fighting gravity. (which is a form of accelleration anyway)

Above 10% and the ability to climb starts to fall away... it's possible to go above 10%, but then you would be "below peak" in the rpms. The 11% slope is the ability to climb while still at "peak power". (so it's basically the steepest long hill you would want to safely climb) 15% is very rare and you would only expect to do such a hill for a few hundred feet and not much more.

Above 15% and the bike simply would fail to climb and stall... and the motor would probably heat up and melt... If you had plans to climb really steep hills you would need to lower the gearing...

 

[eP]

So tell us what is the acceleration at 9% slope at this case ?

Well, it looks like the maximum slope is 11%, so at 9% the accelleration would be next to nothing. You would be able to maintain your speed and maybe go slightly faster, but most all of the motor power would be used up fighting gravity. (which is a form of accelleration anyway)

Above 10% and the ability to climb starts to fall away... it's possible to go above 10%, but then you would be "below peak" in the rpms. The 11% slope is the ability to climb while still at "peak power". (so it's basically the steepest long hill you would want to safely climb) 15% is very rare and you would only expect to do such a hill for a few hundred feet and not much more.

So what hapened if you have to stop for a moment at 9% slope ? (whatever the reason)
How long time we need to achive the efficient speed in this case ?
What will be average efficiency for that period of time ? (from stal point to speed at max eff. )

 

[safe]

So what hapened if you have to stop for a moment at 9% slope ? (whatever the reason)
How long time we need to achive the efficient speed in this case ?
What will be average efficiency for that period of time ? (from stal point to speed at max eff. )

That's going to be difficult.

If you're big worry is "extreme hills" then you would want to gear the bike lower so that it could deal with those extreme conditions. I'm mostly concerned with normal situations and getting the most out of the motor for that. Most hills are 5% because they try to make it easier for traffic to move on them. In some rare places they have built roads that are very steep at 15% (like the passes at the top of 10,000 ft tall mountains like in the Tour De France) but most hills are within 10%.

Another option would be to get a Rohloff:

http://www.rohloff.de/en/technical/index.html

...which has a gear range of over 500%. (so you could cover every possible scenario)

 

[eP]

So what hapened if you have to stop for a moment at 9% slope ? (whatever the reason)
How long time we need to achive the efficient speed in this case ?
What will be average efficiency for that period of time ? (from stal point to speed at max eff. )

That's going to be difficult.

Why difficult ???

It is an easy task even for high school boy.
You have a constant torque, so the force is also constant.
Most of them working against gravity and the rest for acceleration (for simplicity).
So the acceleration is constant as the rest is constant (we can drop the rolling drag and the air drag or assume there are constant also).

So you have motion at constant a.
So v=at and so on...

I'm sure it is very easy task for You now.

If you're big worry is "extreme hills" then you would want to gear the bike lower so that it could deal with those extreme conditions. I'm mostly concerned with normal situations and getting the most out of the motor for that.

I dont worry about extreme hills. 9% slope is not extreme. The similar story is for 8% slope or 7% slope.
The acceleration is very important i suppose.
Try calculate average efficiency at one case and we will see how acceleration and efficiency are related if you are started from the stall point.

High acceleration margin could allow you for fast return from ineficiency region.

 

[Mathurin]

eP: Safe's motorcycle is 63.5Kg, not 50.

Safe: The only "more power" part clearly comes from a higher amp limit on the motor current limited controller compared to a conventional one. Otherwise, with equal amperage it's gutless on the whole line except when the motor revs out, where it's at par. Obviously, a bike equipped with this type of controller would be quite significantly slower from a stop.

 

[eP]

eP: Safe's motorcycle is 63.5Kg, not 50.

Safe: The only "more power" part clearly comes from a higher amp limit on the motor current limited controller compared to a conventional one. Otherwise, with equal amperage it's gutless on the whole line except when the motor revs out, where it's at par. Obviously, a bike equipped with this type of controller would be quite significantly slower from a stop.

Hi.

We don't talking here Safe's motorcyccle weight.
50kg was simply an example. The real weight could be lower or higher it is not very important.

We taking here about Safe's idea. I've similar opinion: possible efficiency gains would be insufficient to sacrifice acceleration.

 

[safe]

The only "more power" part clearly comes from a higher amp limit on the motor current limited controller compared to a conventional one. Otherwise, with equal amperage it's gutless on the whole line except when the motor revs out, where it's at par. Obviously, a bike equipped with this type of controller would be quite significantly slower from a stop.

Slower from an absolute stop... until you get up to speed and then you would be faster than a standard motor because on the standard one you COULDN'T run the higher amp limit without burning up the motor. (so we're talking about the first 20 feet off the start)

That's the whole idea... :wink:

If you tried to run the higher amp limit on the motor it's technically possible to ride it "perfectly" and never produce excessive heat, but that demands a "perfect" rider that is always in the right gear which is hard to make a reality (even when you are trying to be "perfect"). The idea is that you SACRIFICE your lowest rpms so that you can run an amp limit that you normally wouldn't be able to run without heat related issues. Certainly this idea would be TERRIBLE for a hub motor. It would be the "worst of all possible worlds" because you would lose all your low end torque and your hub motor would only work on flat land. But for gears this is the "best of all possible worlds" and actually when you really think about it anyone that is running gears and is using a "standard" controller is not taking advantage of what gears have to offer which is flexibility in torque production. The geared bike can decide how much torque gets to the rear wheel... you are in effect "unbound" by the motors torque limitations.

This logic is really a "repeat" of racing technology in gasoline engines. An Indy 500 race car revs to really high rpms and has very little power down low. Every once and a while they even stall their cars in the pits because those engines produce 1000 hp on the top end, but they don't have that much down low. So the logic is "familiar"... at the race track you tend to see people sacrifice the low end so that the top end is optimized. It's a tradeoff.

Most people wouldn't like driving an Indy Car to work...

 

[eP]

The only "more power" part clearly comes from a higher amp limit on the motor current limited controller compared to a conventional one. Otherwise, with equal amperage it's gutless on the whole line except when the motor revs out, where it's at par. Obviously, a bike equipped with this type of controller would be quite significantly slower from a stop.

Slower from an absolute stop... until you get up to speed and then you would be faster than a standard motor because on the standard one you COULDN'T run the higher amp limit without burning up the motor. (so we're talking about the first 20 feet off the start)

That's the whole idea... :wink:

If you are SURE we COULDN'T so lets calculate average efficiency for 40 Amps battery limited at 400m (1/4 mile) %9slope and the same for the motor limited in your way.
So we will see what real efficiency is better and at which case you have more chance to burn your motor.

If you tried to run the higher amp limit on the motor it's technically possible to ride it "perfectly" and never produce excessive heat, but that demands a "perfect" rider that is always in the right gear which is hard to make a reality (even when you are trying to be "perfect").

But this is what you tried to do. You want to force rider to drive faster at higer Amps limit (50A instead 40A at motor side).
If you will go to inefficiency region (afetr slow down or stop) it is much harder to you live this region very fast. So you are much more prone to burn your motor at low speed and low rpm.

The idea is that you SACRIFICE your lowest rpms so that you can run an amp limit that you normally wouldn't be able to run without heat related issues.

You are TOTALLY wrong. You don't sacrifice low rpms.
You sacrifice your TORQUE margin. So you are much more prone to go to low rpm region and what is more worse you are not able live this dangerous region very fast even at moderate slopes (7% - 9%).

So you are limited to driving mostly at flat or low slopes.

 

[Lowell]

I don't see why said bike couldn't climb slopes. Gear it right and it will go as fast as the power level allows. You will only be at low motor RPMs on starts, and with a low first gear there will be enough wheel torque to get moving smoothly.

 

[eP]

I don't see why said bike couldn't climb slopes. Gear it right and it will go as fast as the power level allows. You will only be at low motor RPMs on starts, and with a low first gear there will be enough wheel torque to get moving smoothly.

But if you have limiter motor current you will have enough torque to accelerate smoothly even at moderate slope.

So as a consequence you will stay for a long time in low rpms inefficiency region.

 

[Lowell]

I don't see why said bike couldn't climb slopes. Gear it right and it will go as fast as the power level allows. You will only be at low motor RPMs on starts, and with a low first gear there will be enough wheel torque to get moving smoothly.

But if you have limiter motor current you will have enough torque to accelerate smoothly even at moderate slope.

So as a consequence you will stay for a long time in low rpms inefficiency region.

Isn't that what I said? There will be enough wheel torque to pull off smoothly if the gear ratio is low enough. Obviously you have to be sensible with the motor wattage vs. bike weight. A 300lb rig isn't going to pull off smoothly on a 10% hill with a 100 watt toy car motor no matter what the gearing is.

 

[eP]

I don't see why said bike couldn't climb slopes. Gear it right and it will go as fast as the power level allows. You will only be at low motor RPMs on starts, and with a low first gear there will be enough wheel torque to get moving smoothly.

But if you have limiter motor current you will have enough torque to accelerate smoothly even at moderate slope.

So as a consequence you will stay for a long time in low rpms inefficiency region.

Isn't that what I said? There will be enough wheel torque to pull off smoothly if the gear ratio is low enough. Obviously you have to be sensible with the motor wattage vs. bike weight. A 300lb rig isn't going to pull off smoothly on a 10% hill with a 100 watt toy car motor no matter what the gearing is.

The real problem is: 300lb rig isn't going to pull off smoothly on a 10% hill with a 2 000 watt huge monster too, as a Safe calculated.

So the Safe's idea simply failed.

 

[Lowell]

300lbs @ 10% grade takes a mere 65 watts to get rolling at 1mph. 330W @ 5mph, and 685W @ 10mph. It would not be difficult to gear a bike for that powerband.

 

[eP]

300lbs @ 10% grade takes a mere 65 watts to get rolling at 1mph. 330W @ 5mph, and 685W @ 10mph. It would not be difficult to gear a bike for that powerband.

Yes it it true. But what will be the max. speed (at flat) at those cases ?
Pretty low i'm afraid.

And what about with acceleration at motor current limited ?

 

[Lowell]

Max speed will depend on gear ratio spread. With a 500% internal hub 12mph in 1st and 60mph in top gear would be possible. Even an 11-36 freewheel would give over 40mph top speed and keep a granny gear for those hill starts.

 

[eP]

Max speed will depend on gear ratio spread. With a 500% internal hub 12mph in 1st and 60mph in top gear would be possible. Even an 11-36 freewheel would give over 40mph top speed and keep a granny gear for those hill starts.

So what will be the acceleration at 7% slope ( at 12mph max 1st gear) ?

 

[Lowell]

Max speed will depend on gear ratio spread. With a 500% internal hub 12mph in 1st and 60mph in top gear would be possible. Even an 11-36 freewheel would give over 40mph top speed and keep a granny gear for those hill starts.

So what will be the acceleration at 7% slope ( at 12mph max 1st gear) ?

Safe will have to punch that up on his spreadsheet for exact results.

 

[eP]

Max speed will depend on gear ratio spread. With a 500% internal hub 12mph in 1st and 60mph in top gear would be possible. Even an 11-36 freewheel would give over 40mph top speed and keep a granny gear for those hill starts.

So what will be the acceleration at 7% slope ( at 12mph max 1st gear) ?

Safe will have to punch that up on his spreadsheet for exact results.

Safe calculated the acceleration is next to nothig, so it is pretty bad if the motor current is limited.

 

[safe]

Track Testing

Let me summarize all these issues...

If you were testing your bike on a certain track you would ideally use an onboard computer that would record all the telemetry data so that you can see what gear and what rpm and how much power you were creating at all points. This is standard activity at race tracks now. In the process of pre-race testing you would arrive at your ideal gearing so that you could be certain that you would be able to have the right gear for all situations.

Now in "real world" conditions you are sometimes thrown a massive hill climb that happens only every once and a while. 99% of the time you can do just fine if your bike can climb a rare 10% slope and since most slopes are 5% you usually don't have much to worry about.

If someone were to know that they were going to need a very wide range of gearing then they would simply be forced to get something like a Rohloff hub that has a range of 500%. Such a range is so wide that you could easily gear for a hill of 20% or more.

Now I've just had my morning coffee... but I'll be back in a minute after I make up a spreadsheet that shows how really wide the Rohloff hub range really is. I think you're going to laugh at the results... care to climb a brick wall? :lol:

Okay, that was easy... the results really are laughable. My spreadsheets are designed to only go up to a 25% slope and the results are above 25% so I really don't know how far up a brick wall it would go, but it's very possible that you could climb something close to a brick wall.

Top speed in first gear is 12.3 mph! (peak power is 9.9 mph) :lol:

So the "bottom line" is that there is no problem with hills if you use the right gearing... (and you can still go 50 mph in 14 gear)

From a standing start on a 10% slope you would accellerate with ease.

 

[eP]

Track Testing

Let me summarize all these issues...
...
If someone were to know that they were going to need a very wide range of gearing then they would simply be forced to get something like a Rohloff hub that has a range of 500%. Such a range is so wide that you could easily gear for a hill of 20% or more.


Top speed in first gear is 12.3 mph! (peak power is 9.9 mph) :lol:

So the "bottom line" is that there is no problem with hills if you use the right gearing... (and you can still go 50 mph in 14 gear)

From a standing start on a 10% slope you would accellerate with ease.

So give us an example: P_out, P_in, speed, torque and acceleration at 10% slope. (at 500% hub range )

 

[safe]

So give us an example: P_out, P_in, speed, torque and acceleration at 10% slope. (at 500% hub range )

My spreadsheet doesn't have accelleration built into it because that's time dependent. I'm more concerned with being able to climb a certain hill at peak rpms, not that first 20 ft off the start, but it would still accellerate fast in those first 20 ft anyway. This is a far steeper slope (20%) so you would be able to accellerate with ease. It's a constant torque accelleration for any specific gear.

Check out that "Hub Torque" figure. That's right at the limit of a Rohloff hub which lists it's maximum hub torque as 100 Nm. So any steeper hill and lower gearing and you get to the point where you start to blow up the hub from excessive torque.

101 Nm at the Hub! Yikes!

(4 Nm at the motor becomes 101 Nm at the rear wheel)

 

[safe]

This chart compares the torque that the motor produces before gearing becomes a factor. In some ways this is easier to comprehend because gearing can effectively be "whatever you want" so there are an infinite number of "what ifs" that can be generated. Given that we want a rider to be "imperfect" and not have to "magically" know when to shift or have to stare at an ammeter (the way people might know how much current is flowing now) this "Motor Current Limited" will be easy to ride because you would "feel" your shift points just like a racing motorcycle gives feedback. There are no ambiguous situations with such a motor.

Top end torque is better because of the higher allowable current limit.

Low end torque is lower, but efficiency is terrible there so no intelligent person would choose high heat and low efficiency if they had a choice. So that low end stuff is for people that don't know how to ride. (that's the "imperfect" rider behavior I was describing)

Standard Current Limit - 40 Amps

Motor Current Limit - 50 Amps

 

[safe]

In my opinion if you wanted to make an "Electric Hillclimb Bike" that was specifically designed for hill climbing that you would be better off using a standard controller. When low end torque is the primary goal of your machine then having to rev to the higher rpms to get power is not a good thing. So you need to adapt your controller technology to the usage that you have.

A "Road Racer" with gears would use "Motor Current Limiting".

A "Hill Climber" would use "Battery Current Limiting".

 

[Lowell]

Safe: "Okay, that was easy... the results really are laughable. My spreadsheets are designed to only go up to a 25% slope and the results are above 25% so I really don't know how far up a brick wall it would go, but it's very possible that you could climb something close to a brick wall."

Thanks for proving what most people knew already. As usual eP folds again. Shall we deal the next hand?