Armature Current Limiting

[safe]

Epoxy Glue

I bought some of the 2500lb strength epoxy glue and glued the Hall Effect sensor directly to the motherboard. I suspect that the board will break before this glue will let go. Compared to the weak connection that the three little sensor prongs were giving this is now solid as a rock.

Next step... soldering the main power leads to the sensor...

 

[safe]

Just waking this up for Dirty D...

 

[safe]

First Testing Completed

Well it seems to be working correctly. For more information go to my Project #002 thread in the photo's section.

So far I should be able to claim this works...

(it's a pretty simple circuit after all)

 

[johnrobholmes]

silly question here, but why not just select the proper motor or gearing in the first place? A motor that needs current limiting for a job is an improper motor or setup IMO.

 

[safe]

silly question here, but why not just select the proper motor or gearing in the first place?

Just because... :lol:

No, there's a long and complicated story to get you up to speed, but there are reasons for this. Talk to recumpence, he has some understanding of the problem. Or I'll explain later... I just logged on to recheck some numbers I posted here about the Armature Current Limiting configuration.

 

[Link]

silly question here, but why not just select the proper motor or gearing in the first place? A motor that needs current limiting for a job is an improper motor or setup IMO.

It's for the n00bs. The idea is that you keep current low when the efficiency/RPM is low, so you couldn't fry the motor. It's only of much use on a geared bike, though, and I don't know of many n00bs with a motor that drives the wheel through multiple gears. :?

Of course, the best solution is to just use a properly sized motor...

 

[johnrobholmes]

From what I gathered in the thread it is to squeeze more power out of a "legal" 750w motor? Current limiting on startup can help keep the motor cooler, but it is just as easy to have a controlled throttle hand. More modern controllers also have throttle ramping and overcurrent features that can help in the plight.

How would you get around current limiting at higher throttles? It would be totally counterproductive to current limit and overvolt at the same time. The motor will not get up to top speed with the current limited since it would take a specified torque amount to reach top speed. If the amperage to produce said torque was above the "limit" you would just have nothing but controller losses.

 

[dirty_d]

using the right motor doesn't eliminate the need for current limiting, take these rc motors for example, gear it correctly and all, and it still has very low winding resistance, 27milliohms for example, at stall at 48V thats going to be 1777A. it will linearly decrease as you get closer to no load speed, but it is going to be over the motors rated heat dissipation probably 95% of the time from 0mph to top speed. at top speed you're not going to be running into the current limit unless you're going uphill, or you set the limit too low.

 

[Link]

We're not talking about current limiting like a normal ebike controller does, we're talking about a limit that gives low power at startup and then heightens the limit once it gets up to speed, effectively removing the primary performance advantage of a motor over an engine.

Unless I'm mixing this up with another one of Safe's...er...unorthodox concepts...

 

[johnrobholmes]

While the winding might allow 1000+ amps through it, it would only be on the millisecond interval. The RC motors I commonly use in 1/10 scale trucks allow over 500 amps on startup (if the batteries allow) and they run just fine even though they are typically rated for 80a continous. If you are over the motors ratings 95% of the time it is a motor ill suited for the job- gear down or get a different motor.


You guys are thinking way too hard about this. If this was really a huge issue, they why haven't all of my motors and ESCs already blown up from over amping? My current ebike outrunner is 20mOhm resistance. I push it hard on startup and it is only rated for 84 amps. At 24v input this is 1200a at stall. Why don't I see this? Either my data logger isn't quick enough, or i am never truly stalling the motor. At either rate my temperatures do not exceed maximum so I don't care. I have bursts much higher than rated on the 1/10 second interval, but they drop off very quickly as soon as the motor starts rotating and everything is phased properly. Since the motor doesn't overheat, my average amp draw is not exceeding factory rated specs. This means my motor is properly suited for my weight and gearing of vehicle, and I need not go any further into thought.


I guess I just see this as silly because of my background in RC cars and airplanes. We would never dream of current limiting. We want everything as low of resistance as possible so the most amperage can flow through at the highest voltage while retaining a system that works for the intended application reliably.

 

[Link]

I guess I just see this as silly

Judging from some of the responses, you're not the only one...

 

[dirty_d]

john, don't the ESCs have a current limiter in them also? the current wont be high for just a few milliseconds, it will be high for a long time. as an example, take that 27ohm motor, at stall we have 1777A, and at no load maybe 2 or 3A, call it zero. if you apply full throttle at stall all the way up to full speed, then at 50% of full speed you're going to have about 888A, at 75% of full speed you will have 444A, and 90% of full speed you have 177A. it takes a lot longer than a few milliseconds to go from 0mph to full speed, and during that time the average current is 888A. the battery wont be able to supply this current, so something is going to break if no limiting is done. i don't see how limiting the current to the motor is a bad thing, if you just limit battery current then at low duty cycle the motor can have much higher current going through it, and at a certain point more current does not equal more torque because of saturation, its all just wasted power.

Link, you don't make the limit low at low speed, its a fixed current limit all the time, this gives you constant torque until the backemf of the motor limits the current at high speed.

 

[Link]

john, don't the ESCs have a current limiter in them also? the current wont be high for just a few milliseconds, it will be high for a long time. as an example, take that 27ohm motor, at stall we have 1777A, and at no load maybe 2 or 3A, call it zero. if you apply full throttle at stall all the way up to full speed, then at 50% of full speed you're going to have about 888A, at 75% of full speed you will have 444A, and 90% of full speed you have 177A. it takes a lot longer than a few milliseconds to go from 0mph to full speed, and during that time the average current is 888A.

The BEMF from the motor starts to limit current long before it gets close to no load speed.

Though some ESCs do, and these are the king you'd want to be using on ebikes.

 

[dirty_d]

Link, the backemf is linear, at 48V if your no-load speed is 3000rpm, then at 1500rpm the backemf is 24V. you will have half of the stall current at 1500rpm. that is why with such a low resistance motor, even at 90% of no load speed there was 177A.

 

[Link]

Yah, for some dumb reason, I was thinking you were advancing the throttle. D'oh. Disregard the last post. :?

 

[TylerDurden]

For the folks who don't have time to read safe's epic tomes, we will gladly provide the cliff-notes version:

Safe burned up his first motor on a hill, over a year ago. Since then, he has constantly postulated on the value of gears, armature current limiting (aka MCL), and ice-cooling theories ad-nauseum. As a wannabe motorcycle sport-rider, he has used every inappropriate ICE-based metaphor and incomprehensible charts to describe the value of his brain-damaged, bold, enlarged and colorized diarrheah-of-the-keyboard. He has enlisted the assistance of this group to create a circuit to add onto an existing controller, which senses armature current and allows the user to vary the mount of current limiting through throttle pull-down.

The sweet irony of this story is that limiting current in the low efficiency range of motor rpm also limits the torque where it is needed most. Hence "performance" (in moto-lingo) will suffer, as the motor lingers in the poor efficiency zone, generating the same or more heat over a longer period, particularly on slopes where current is not a function of the rider's control of the throttle. In the end, safe has found a way to create thousands of posts, charts, halfbaked theories and idiotic terms... in the service of limiting motor heat by simply slowing down.

:lol:

 

[johnrobholmes]

john, don't the ESCs have a current limiter in them also? the current wont be high for just a few milliseconds, it will be high for a long time. as an example, take that 27ohm motor, at stall we have 1777A, and at no load maybe 2 or 3A, call it zero. if you apply full throttle at stall all the way up to full speed

The current limiting is generally done with battery resistance, wire resistance, and fet resistance which you are ignoring in your example. Otherwise throttle ramping is used to prevent motors from producing too much torque (to protect batteries or drivetrain generally), although some industrial controllers have basic current limiting used for drivetrain and battery protection in case of accident or stalled motor (think wedged wheel). I think our opinions differ in that I believe a motor should never be ran at stall torque.

A DC PM motor will produce max power at around half the stall torque, which just happens to be 1/2 the no load speed. This is not the most efficient place to run the motor, it is around 10%. As you load it more the efficiency drops further. A motor ran at 90% no load speed under stalling torque would not continue to run at 90% rpm, it would fall to zero rpm. If it is ran at 1/10 stalling torque it will run at 90% of the average applied voltage x KV aka no load speed. Trying to run a motor at stall is the worst possible way to extract power.


Where I am confused is here- If you are overloading the motor and use current limiting to prevent damage, you are reducing the torque of the motor. This will reduce the motor speed until the stalling condition is relieved. Basically you are trying to build an electronic device to keep an under sized motor from stalling by simply limiting power. A much cheaper and more effective means to protect the motor is to properly select it in the first place.


I shall step out of here now as I am not contributing to the discussion at hand. I will put my efforts into proper component selection and vehicle gearing.

 

[dirty_d]

actually the max power produced at 1/2 stall torque and 1/2 no load speed is at about 50% efficiency. all motors will start out stalled, it takes time to accelerate from 0rpm to whatever, if you only have one gear available you are going to need to limit the current, like if you start up a steep hill current is going to rise, if there is no current limit, then you're going to need to have an ammeter and constantly be playing with the throttle to keep the current below the max the motor can take. its definitely not a good thing to rely on battery resistance for current limiting, the 10 or so milliohms of FET resistance plus the 30 or so for the motor are pretty much a dead short for the battery.

TylerDurden, your avatar is most disturbing.

 

[safe]

In Depth Analysis of ACL Circuit

First let me post the simulation that I actually just did yesterday. The whole time at the beginning I was doing it "without a net" so to speak and it's comforting to know that with the net it still gives the results originally intended:



Okay, so here's what's happening...

My controller delivers a very accurate 5.0 volt power source, but things start to get more complicated after that. When you use the power source of 5.0 volts and run it through JUST the throttle you get:

Throttle Closed - 0.6 volts
Throttle Opened - 4.5 volts

...now if you based everything on that (which I did because that's the spec on throttles) then everything would have been perfect from the start. But what actually happens is the controller sinks a significant amount of the throttle voltage and so in the "real world" the controller ends up with a voltage more like:

Throttle Closed - 0.6 volts
Throttle Opened - 3.4 volts

...which means that about 1.1 volts is getting consumed by the controller in the act of reading the throttle voltage and doing whatever it needs to do. When I attach the ACL circuit it gets worse and now we get for the final throttle voltage measured:

Throttle Closed - 0.6 volts
Throttle Opened - 2.6 volts

...so on top of the 1.1 volt "sag" we now have another 0.8 volt "sag" to deal with. So we are down almost two full volts from what the throttle gave us. :?

The Solution?

One solution (not the best, but it works) is to insert a couple of small cell batteries in between the 5.0 volt power source and the ACL circuit and that makes it so the final voltages are:

Throttle Closed - 0.6 volts
Throttle Opened - 3.8 volts

...which means that the full normal throttle range is restored and while I have not been able to ride today (since it's pouring rain outside) the motor does seem to have the "pep" that it should in the bike stand. More testing to be done later and a possible redesign might be in order if there was any way to do this without resorting to the supplimental batteries to overcome the voltage "sag".

Otherwise, it does seem to work and the effect is rather fun because the power comes on later than normal. Usually you get that "lurch" and then the power "falls" off from that. With ACL the power slowly builds until it's natural peak of horsepower, but the torque is a constant (by definition) so it's a more predictable powerband and does lend itself to "knowing" where the good power is. (it will allow the rider to keep things in the peak of the powerband easier) It also sort of makes me realize (by comparison to the stock controller) how difficult it was to get the perfect accelleration using the "throttle fidding" approach. This technique GUARANTEES the optimal throttle setting at full throttle all the time... that's "by definition" what it does. If you align your power peak and your efficiency peak to be the same spot (something you can calculate in advance) this makes for the most efficient possible motor. And even when you set the "boost" to a higher level you still rise in a linear way, it's just a faster rise. (the heat is still less than stock)

But it does eliminate much of that "lurch" feeling... and despite the negatives that produces as far as efficiency and heat people do like that a lot.

 

[safe]

Otherwise throttle ramping is used to prevent motors from producing too much torque (to protect batteries or drivetrain generally), although some industrial controllers have basic current limiting used for drivetrain and battery protection in case of accident or stalled motor (think wedged wheel).

Yes, some more advanced programmable controllers already offer "Throttle Ramping" or "Armature Current Limiting" (they can mean the same thing) in their products.

My goal here was to simply apply the concept to cheapo $50 controllers for ebikes.

A much cheaper and more effective means to protect the motor is to properly select it in the first place.

In an industrial application where weight does not matter you can afford to increase the motor size without any real limit. On a "legal" ebike you are limitied to a rated load of 750 watts which translates into a heat dissipation rating. In order to get more power from a little motor you overvolt, narrow the powerband to reduce heat and then add enough gears so that you can be humming along at your peak rpm in any circumstance. Without multiple speed gearing you would not get much advantage with Armature Current Limiting.

Notice what happens when a little 350 watt rated motor gets tricked out with all this stuff:

(almost double the peak power while maintaining a lower heat profile)

 

[safe]

Notice For Newbees

Armature Current Limiting (Throttle Ramping) is an accepted technique that exists within the electric vehicle world. It is, however, NOT for everyone.

On an ebike that is a one speed (like a hub motor) using this technique will only make your performance worse!

Using this technique in combination with a multiple speed transmission (derailler or internally geared rear hub) that the motor runs it's power through will give the performance benefits that have been discussed previously.

So for the newbees that might hope that there is some "magic technique" to improve your bike this is NOT going to help you any. This is only a useful technique in the limited circumstance I've mentioned.

Hopefully this will act as a "disclaimer" so that I don't get people excited about something that is out of their technical reach to actually implement.

 

[TylerDurden]

Armature Current Limiting (Throttle Ramping) is an accepted technique that exists within the electric vehicle world.

Armature Current Limiting is not Throttle Ramping. They are separate things: throttle-ramping can be used for current limiting, motor-side or battery-side.

 

[Anonymous]

.

 

[safe]

...charts and graphs are interesting ... but so they say anything can be "proven" on paper. Show us results of implementations.

This was a "build" thread for the circuit.

I actually built this one and this is the testing phase.

The result is that it "worked", but that the circuit (and controller) are straining the power supply available, so I'm looking at ways to improve on it. One "hack" is to place a booster battery in series with the circuit to overcome the voltage sag.

This was not "idle" theory... I've actually ridden this circuit and it does exactly what it's supposed to do.

But what does it do?

What it does is effectively REDUCE the riders choices.

You say:

"Why the heck would you want to reduce the rider choices?"

...the whole idea of having multiple speed gears attached to an electric motor (what I'm doing) is to get out of the low rpms where all the excess heat is produced. Horsepower is made up of two factors: Torque and Rpm. Just like with the RC motor thread about high rpm motors delivering better power than expected the same idea applies to all motors. If you can force the rider to shift gears when the motor rpms drop by TAKING "DIRTY" POWER AWAY you force the rider to be in the optimal gear for peak horsepower all the time.

Armature Current Limiting "filters" the "dirty" power out of an electric motor in favor of the "clean" (efficient) power you find up higher in the rpms. It reduces motor heating.

But for people riding "one speeds" (hub motors) this whole idea is so alien and incomprehensible that you will not likely have the intuitive grasp of what it might feel like. I'm used to electric motors with gears... I'm in a seriously small minority today. (it might be different in the future though)

 

[TylerDurden]

This was not "idle" theory... I've actually ridden this circuit and it does exactly what it's supposed to do.

Not yet:

Using this technique in combination with a multiple speed transmission (derailler or internally geared rear hub) that the motor runs it's power through will give the performance benefits that have been discussed previously.

The circuit works. So far, it has only succeeded in slowing down the motor. I don't think that is the "performance benefit" safe has been asserting with 18 month's of charts and poppycock.