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

[TylerDurden]

I've got a lot of experience in how gasoline motors behave and what I'm doing is bringing the benefits of that type of powerband to the electric bike.

Let's say I never rode a peeky motorbike...

If I just let the thing lug in 3rd gear, it will just slow down.

So I downshift... the motor goes faster, but the bike doesn't.

So I upshift... the bike starts to slow down even more.

So I downshift... the motor goes even faster, but the bike doesn't.

etc. etc. etc.

 

[eP]

So please explain ...how a rider will use this "new" concept in operation... how will they know when to shift down, when to shift up?

When the rider experiences no power he knows that it's time to shift down to a lower gear.

It is time to add the power.

It is to late to shif down gears at heavy load occur.
If you reduce the gear you simply limiting your power much more as your rpms are limited by voltage.

So after shift down rider experiences immediate speed drop - this way your limit will force the driver to drive extremly slow at low gear at cases when temporary small increase of current will be enough to keep the speed at right level.

The "feel" of accelleration is based on final power output, not just torque alone. The standard controller allows a very wide powerband so it "feels" the same across a large segment of the powerband because with "current multiplication" torque actually increases in absolute terms as you go lower in the rpms which compensates for the lack of overall power that lower rpms normally mean. Power is a concept that is difficult to understand, it has two components: torque and rpm. The fact that torque can be made to increase as rpms decrease is sort of "weird" compared to gasoline motors which are based on a direct relationship to number of "explosions" in the cylinders and actual power output. Anyway, when you deprive the "imperfect rider" of his low rpm torque then he "feels" the lack of "get up and go" and realizes that he has to downshift or the bike will simply stop accellerating.

The existing controller behavior is "ambiguous". You can't know by "feel" if you are in the high or low part of the powerband because it all more or less feels the same. (only at the extremes can you "feel" power dropping away) With the change I'm suggesting the powerband will look exactly like a motorcycle powerband with a very pronounced "peak" at the top and a very linear "feel" where the power builds and builds until a peak and then falls off.

Immediate after downshift the motor will run at the idlle state until the speed drop down. (It is short period of time).
Acceleration after speed drop after downshift is more energy waste as your speed is lower (you waste the same power at much longer period of time).

It is better waste a little more power at much higer speed than wasting limited power at much lower speed (after gear down shift)

 

[safe]

Let's say I never rode a peeky motorbike...

If I just let the thing lug in 3rd gear, it will just slow down.

So I downshift... the motor goes faster, but the bike doesn't.

Yeah, you obviously have never ridden a "peaky" motorcycle. :lol:

No, when you downshift it moves the rpms up higher into the powerband where there becomes enough power to accellerate the bike. This increases the speed of the bike all the way up until you hit the "redline"... uh... a little before the "no load" limit. At this point you could try to go for the next gear, but you probably won't be able to pull it so you are best to just stay where you are.

Sometimes you might be in a gear that is so ridiculously tall that you have to downshift twice in order to get power and then you could come back up one.

Gears Need To Overlap Each Other And More Closely Spaced Gears Allow A Peakier Powerband

Your not having motorcycle experience explains the confusion... most motorcycle riders and sports car buffs know what I'm talking about...

You ought to find someone who owns a "peaky" motorcycle and try riding it!

Just be prepared to hang on for dear life!

The first chart is of my old Honda Civic Si 2000 car (sold now) which became popular in the movie "The Fast and the Furious":

 

[TylerDurden]

To accellerate will require torque,

But the torque is restricted by the current limiter.

 

[eP]

Yeah, you obviously have never ridden a "peaky" motorcycle. :lol:

No, when you downshift it moves the rpms up higher into the powerband where there becomes enough power to accellerate the bike. This increases the speed of the bike all the way up until you hit the "redline"...

You said at the begining the motor operate at top rpms so you dont have the room for higher rpms.

The negative effect of such blind current limit will be faster reduced power as the limiter cut-of the power to force rider to gear downshift.
After the downshift you will not be able to accelerate for the sake of current limiter.

For ICE - more rpms = more power, but for electric motor yours top rpms are limited by battery voltage. So you cannot increase power by limiting the current at limited voltage.
So after the downshift the power must fall 'coz it is not the ICE !!

If you are at top rpms level the only way to increase the power is increase the current.

The controlers logic should be very fast and very smart. It should calculate how fast current rising and choose the right strategy: give the signal to reduce gear or increasing the current to keep the speed at high level.

But your cotrollers is totally blind as it have no actual rpm and rpm tendency info.

 

[safe]

To accellerate will require torque,

But the torque is restricted by the current limiter.

To accellerate you need POWER.

That seems to be where you are getting confused. Torque is just one half of what makes up power. Power is the combination of torque and gearing. Raise the gearing and you lower the effective power. Lower the gearing and you raise the effective power.

When a bike is "bogged down" it's gearing is too tall to carry a load based on the level of torque available. When you lower the gearing you suddenly have more power available because the torque has more time to do it's job. The more you "spin" the motor the closer you get to it's efficiency zone and so not only do you get more power, less heat, but it's more efficient.

Note: Keep in mind that lowering the gearing actually increases the rpms... so you need to realize that a lowered gear ratio translates into higher rpms...


POWER = TORQUE * GEARING (rpms)

 

[safe]

You said at the begining the motor operate at top rpms so you dont have the room for higher rpms.

The "redline" ("no load") is fixed.

I've been talking about low rpms... when the bike is "bogged down" in low rpms if you can downshift it allows the motor to run faster closer to it's "ideal" rpm.

 

[eP]

You said at the begining the motor operate at top rpms so you dont have the room for higher rpms.

The "redline" ("no load") is fixed.

I've been talking about low rpms... when the bike is "bogged down" in low rpms if you can downshift it allows the motor to run faster closer to it's "ideal" rpm.

So tell us in which way motor goes to low rpms if you have magical current limiter ?
How current limiter know at how rpms is motor at the moment ?
It is very very important as at low rpms even low current is inefficient. At high rpm even big current is efficient. But i see your limiter is blind and applying the always the same motor current limit.

 

[safe]

Hub Motors

The "Hub Motor" is a "special case" where gearing is always equal to one:

POWER = TORQUE * GEARING

For the "Hub Motor":

POWER = TORQUE * 1

So in the end for the "Hub Motor":

POWER = TORQUE

 

[safe]

So tell us in which way motor goes to low rpms if you have magical current limiter ?

It's called "bogging down". When the load on the bike or car is too high then the speed will go down and the power will go down.

Here's a test:

Go out to your car and start it. Now place the car into 5th gear (assuming you own a manual transmission) and try to get going. You probably won't even get going because the engine will stall. The same ideas apply to an electric motor.

Place the car into 1st gear and you have plenty of power to get going...

 

[eP]

Hub Motors

The "Hub Motor" is a "special case" where gearing is always equal to one:

POWER = TORQUE * GEARING

For the "Hub Motor":

POWER = TORQUE * 1

So in the end for the "Hub Motor":

POWER = TORQUE

No No No

TORQUE = CURRENT * GEARING

So for hub TORQUE = CURRENT

But POWER always = TORQUE * RPM (at the motor side or at the wheel side - without gears both side are the same )

 

[safe]

The Process to Understand

This is the process that is required to comprehend the idea of this thread:

1. "Current Multiplication" - You need to be firmly grounded in how electric motor controllers that measure their current limits based on battery side current end up allowing more current to the motor on the motor side. This is being called "current multiplication". If you don't understand this then everything else will make no sense.

2. "Gearing" - You need to understand that Power is a combined value of BOTH the Torque of a motor at a given rpm and the Gearing that is has to the rear wheel. By increasing the gearing you lower the effective power at the rear wheel and by lowering the gearing you raise the effective power at the rear wheel. Torque is an "instantanious" value, so people often say "gearing raises torque" but it's probably better to talk in terms of power. You need to have this stuff clear in your mind.

3. "Motor Current Limiting" - The motor side of a PWM Controller receives a current level that increases as the Duty Cycle decreases. So the idea here is to actually compensate for this current increase by lowering the Duty Cycle to such a degree that the current actually declines based on the motors limitiations on drawing current. You have to have your mind pretty "deep" into motor theory to understand this stuff.

 

[eP]

Place the car into 1st gear and you have plenty of power to get going...
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No plenty of power but plenty of FORCE

It is a big difference !!

 

[safe]

POWER always = TORQUE * RPM (at the motor side or at the wheel side - without gears both side are the same)

POWER = TORQUE * GEARING(rpms)

Let's break these down:

Power is a combined value that is a measure of "work" over time. It can be measured in units of "watts".

Torque is a straight line measurement that is lacking a time dimension. So it's like a lever that is moving a certain amount of weight... there's no mention of how fast that weight is moving. The units are usually something like the Newton Meter. (Nm)

Gearing is the difference between the motor rpm and the rear wheel rpm. Rpms are measured in Radians per Second and represents the rate at which something is spinning. There is a "special case" of gearing where the motor rpms equals the rear wheel rpms and that's the "hub motor".

 

[safe]

No plenty of power but plenty of FORCE

Did you ever complete a college physics class?

You're talking with a guy whose college degree was related to Physics...

P.S: I'm 46 years old... are you an adult? Just checking...

 

[eP]

The Process to Understand

This is the process that is required to comprehend the idea of this thread:

1. "Current Multiplication" - You need to be firmly grounded in how electric motor controllers that measure their current limits based on battery side current end up allowing more current to the motor on the motor side. This is being called "current multiplication". If you don't understand this then everything else will make no sense.

2. "Gearing" - You need to understand that Power is a combined value of BOTH the Torque of a motor at a given rpm and the Gearing that is has to the rear wheel. By increasing the gearing you lower the effective power and the rear wheel and by lowering the gearing you raise the effective power at the rear wheel. Torque is an "instantanious" value, so people often say "gearing raises torque" but it's probably better to talk in terms of power. You need to have this stuff clear in your mind.

3. "Motor Current Limiting" - The motor side of a PWM Controller receives a current level that increases as the Duty Cycle decreases. So the idea here is to actually compensate for this current increase by lowering the Duty Cycle to such a degree that the current actually declines based on the motors limitiations on drawing current. You have to have your mind pretty "deep" into motor theory to understand this stuff.

ad1. At top rpms there are no multiplication !!!

ad2. Power and Torque are two different things !!
If you have ICE you could do this error/mistake because when ICE's rpms go high the power goes high.

But at electric you want work at TOP rpms (or very close to them) so you have no room for higher rpms ten TOP.

ad3. Dont worry about my mind

And rethink all again - plese !

 

[eP]

No plenty of power but plenty of FORCE

Did you ever complete a college physics class?

You're talking with a guy whose college degree was related to Physics...

P.S: I'm 46 years old... are you an adult? Just checking...

I'm a little bit younger
But a little bit.
So don't worry.
I complete also college ( electrician and mechanical not finished).

 

[Toorbough ULL-Zeveigh]

safe, just press onward.

The summations tie up a lot of the points rather well.

 

[eP]

POWER always = TORQUE * RPM (at the motor side or at the wheel side - without gears both side are the same)

POWER = TORQUE * GEARING(rpms)

Let's break these down:

Power is a combined value that is a measure of "work" over time. It can be measured in units of "watts".

So tell me Mr Physic where come from plenty of POWER if the actual speed is very close to ZERO (if you starting from stall point) ?

My asnwer is POWER is also close to zero !!

What is your answer MR Bachelor ?

 

[safe]

I'm a little bit younger
But a little bit.
So don't worry.
I complete also college ( electrician and mechanical not finished).

Okay, so you are sincere. I was starting to think you were a "troll", but I guess you really believe what you are saying.

So let's get back to it...

ad1. At top rpms there are no multiplication !!!

Correct.

ad2. Power and Torque are two different things !!
If you have ICE you could do this error/mistake because when ICE's rpms go high the power goes high.

Correct. ICE is easy... high rpms translates into more power. The electric motor with the standard controller has a sort of "reverse logic" where at lower rpms it's possible to have higher levels of torque (which boosts power at lower rpms as well) but doing this causes more current to flow and that makes more heat.

My idea is to make the electric motor "act like" an ICE motor and eliminate the weird effect of low rpm extra torque and power. The advantage of this is that since the rpms of the motor in the "real world" goes up and down in relation to conditions you end up with an

AVERAGE

...value for heat production. If I'm thinking in terms of "average heat" then it's possible to allow a higher overall current limit but then if you restrict the low end it makes the average heat production lower.

Look at these two charts... they spell out the difference.

The 50 Amp chart has low average heat. (Motor Limited)

The 40 Amp chart has the lowest heat at high rpms, but grossly higher heat at low rpms. (Battery limited)

 

[TylerDurden]

You're talking with a guy whose college degree was related to Physics...

Related by birth? :lol:

 

[safe]

Related by birth? :lol:

It was the old "Physical Sciences" Major. Once long ago people wanted to be "renaissance men" that knew a little about everything. The old major required that you were somewhat knowledgeable in every discipline. These days people tend to specialize and my second B.S. Degree was actually Computer Science. So I have two science degrees. Kind of wish that I added the Electrical Engineering minor along the way though... :wink:

 

[eP]

Look at these two charts... they spell out the difference.

The 50 Amp chart has low average heat. (Motor Limited)

The 40 Amp chart has the lowest heat at high rpms, but grossly higher heat at low rpms. (Battery limited)
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But you should keep in mind that motor at current limiter on its side could goes faster to inefficiency region.
You dont know efficiencies related to both charts.
To know that you need to know actual rpms, winding resistance, motor's Kv.

ps. Sorry i have to log out for a few hours.

 

[safe]

You dont know efficiencies related to both charts.

I have those values too.

Note: The Efficiency value needed to be scaled to fit the chart and uses something like a scaling factor of 20. The power output and heat are in watts and are actual values.

The Motor Current Limited Motor (with 50 Amps Controller) produces about 250 watts more at it's peak than the Battery Current Limited Motor (with 40 Amps Controller) and the average heat output in real world conditions would be about the same.

That's 250 Watts of "Extra Power" without significant heat consequences!

 

[safe]

Combining Both Charts

This is what a mess you get when you take both charts and combine them into one. Very hard to read... :?