Timing for 1s
 

So... Something that has always not sat right with me was the timing we use on motors for 1s applications. Generally speaking we want higher timing the more rpms a motor makes so it is more efficient at the top end. With a 1s we are delivering a lot less volts to the motor and because of that, the rpm will be way down compared to a 2s setup. Why would we want timing in the high 40s to low 50s when the rpm is way down?
Over the weekend I setup my motor analyzers and got to work. First I wanted to chart out a couple of my TC motors so I started on 2s settings. Then got my 1/12 motor out and started on it. Got it set, then realized I needed to drop the voltage down to 4.2v instead of 8.4v. What I noticed right off the bat, the amp draw was higher at the lowest rpm setting on 4.2v then it was on the full open setting on 8.4v. To go by the Youtube Nick guy, which I don't, 6 amps should be what you are looking for. Generally my TC motors are in the 3 amp range at the highest rpm setting. When I switched over to 4.2v instead of 8.4, the amp draw doubled. I had to drop the timing on my motor to 30 to get it to 6 amps, which is still probably too much. Will require more time with the motor. I always wondered why my 1/12 motors got so hot, they are running very inefficiently.

Anyway, it never made sense to me that we would run basically the same timing with 1s and 2s, given how timing works.

The short answer is, because it works.

The long answer has more to do with the power band. Since the rpm is so low, and in spec 12th, the cars speed is fairly consistent compared to other classes. The motor needs to make power in an rpm range that isn't going to cause spikes in amp draw and build too much heat.

its a great question. you are looking at one of the fundamental principals of an electric motor that is often misunderstood.

your 1/12 motor makes the same power (mechanical) at 4 volts as it does 8. the difference is it uses 3 amps at 8 volts and 6 at 4 volts.
so a 100 watt motor is always a 100 watt motor.
the motor spins slower at 4 volts though, correct? so rpm and voltage are related to each other.
so how does a 100 watt motor stay 100 watts if the rpm is half.
the torque doubles just like the current.
torque and rpm is power.
so when is 100 watts not 100? when you cannot provide enough current.
4 volts=5000 rpm=100 watts...........8 volts at 10000 rpm =100 watts because current and torque double
why?
when your motor is consuming voltage its also producing voltage (generates volts in between pulses or power consumption).
when it generates as much e-power as much as it consumes it wont go faster. so at 5000 rpm it uses and makes 4 volts. it wont go faster. same thing at 8 volts.
if this was all you needed to know then the timing would always be set to only 1 mark depending on voltage. 40ish degrees for 4 volts and 50 ish degrees for 8 (just an example).

the value of over timing-
4 volts-peak power of 100 watts at 5000 rpm & 40 degrees
but you need 7000 rpm
if you set the timing 50 degrees the motor doesn’t consume the e-power very well. that means it doesn’t generate e-power very well as well.
its slight misfiring on the power pulse and its slightly misfiring on the regeneration stroke. its called field weakening.
so you set the timing so the motor misfires and can now go to 7000 rpm because that KV has been bumped up.
KV is used to determine what voltage the forward and reverse (generated) power meets and it will go no faster.
you no longer have a 100 watt motor when you over time or field weaken the motor. its 80 or if you are lucky 90 watt.

so why does it work?it only works if you cannot gear the car to use 100 watts at 5000 rpm.

here is another method. ND magnets lose strength when they get hot. a little more temperature and they permanently lose strength.

if you do something to mess up the current/torque/magnet strength/pulse power it field weakens the motor. the motor will spin faster because it doesn’t generate back emf well but it will never be 100 watts if those things limit the motor. Just like purposely over-timing a motor.

so why does kv and amps on an analyzer work?
gubbs gave you the answer. because it works. not universally but its a starting point.


when back and forward voltage meet you are making heat and very little mech power.
add to that a current and you are limiting the amount of heat to a specific current (torque).
so even though kv has nothing to do with power when you add a current number to it its actually a rough estimate of the inverse of power. HEAT
it wont make peak power or peak efficiency but it applies fairly well to the surface area that allows for cooling of the motor.
the cooling properties of all motors in a given class are close enough to use kv and current as a number that tells you how many watts per second can be dissipated by the motor over a 5 or 6 minute period.

Bry195
You seem to be one of the most knowledgeable people on here for brushless motors. Can you point me in the direction of some good reading sources? Also, equipment used for testing these things, recommended dyno and software? I currently have a Trinity Motolyser v1 and a SkyRC Brushless motor analyzer. I also have the R1 Wurks digital 3 esc which records rpm, voltage, and amp draw which I plan to test with later tonight. Obviously that is probably the best since it is on the track data, but it doesn't go for 1s applications. Also hard to test that in the comfort of my hobby room, lol. Anyway, your posts are very informative, but sometimes having all the tools and going over it with someone else is priceless. I wish I could sit down in a lab with you and go over it all, but I don't think that is feasible. Although, I will be in Florida for a few days around Christmas.

Thanks for contributions, and hopefully one day I'll understand it better.

in tampa we race at ss raceway hobby on Saturdays, come by. tell me you are coming and Ill bring a mini pro dyno.

I appreciate the compliment. I work in servos and brushless motors and have for awhile. that means I have had to overcome allot of problems and I understand the language and science. The important part of that statement is dealing with problems (not me). there are brilliant guys all over this forum that have encountered more problems than I in RC. Practice and experience in RC will beat science and language all day.


out of circumstance or ego some people find themselves in a battle over motor knowledge with me (not my choice but I’m also not perfect). there is no good way to get people to listen. you are either in that mode or you are not. everything changes eventually. If everyone felt invited to talk so that I could listen I would be happier than I am. Because this is why i come here. i listen. That’s my advice. listen long enough and you will be able to hear gems. don’t get irked by someone who isn’t listening and exchanging ideas (but it will happen). You might even twist the screw on someone that should get allot of respect for their experience but s**t happens and their is always a chance that something better comes tomorrow. maybe a really experienced friend who listens and has a new found respect. I am on both sides of that coin.


here is a great document that I scanned quickly that might help

http://ww1.microchip.com/downloads/e...tes/00885a.pdf


i’ve seen quite a few posts from others on good stuff. My suggestion is take it slow. one piece at a time. start with power, then torque, then heat, then timing, then field weakening. electric motors aren’t like a car engine but they have some common themes. then KV, then ohms law (maybe start there). you get nothing for free with an electric motor. You always trade one thing for another because almost everything is an extension of ohm law. which by law forces one thing to be exchanged for the other.

Bry, your above explanation also explains why severely under gearing an electric motor creates excessive heat. I had never understood why that happens.

20% beyond (rpm) max power is max efficiency. 10% beyond that and most current is turned to heat without field weakening.
the ratio of current to heat on the run up to max power is a little more forgiving. the efficiency is usually allot wider which sounds great.
the problem is that below a couple 1000 rpms the motor needs allot of current to accelerate so more current and lower efficiency creates allot of heat.
push the motor to max power by timing 1 lap and set the gearing to the fastest lap. let the motor cool.
push the rpms up to about 20% more and the motor will cool down allot and slow a little but now you can average your time over 5 minutes and be the fastest you can be without overheating.
set the timing to take heat more heat out at that point.
maybe bump the gearing back towards lower rpm a hair to put out more power and heat and adjust the timing to take a little more heat out.
take note of the rpm you are at.
that will be peak power and peak heat that the motor can ever dissipate over a 5 minute run.
if the next track is 10% shorter take that rpm and lower it with gearing by 10%.
do not adjust the timing again.
if you remove friction or figure out a way to cool better you will be able to gear the motor to run closer to peak power without overheating. (closer to peak power than peak efficiency).
alternatively you could also over time to field weaken and run at an rpm above peak efficiency for a long track but the car wont be as punchy.
if you drive in a way that keeps you away from very low rpms and very high rpms you will avoid both areas of the curve that are below 20% efficiency which means you will make allot less heat.

Hello Bry, that's a great explanation.


Generally speaking, what creates more heat, over gearing a motor or over-timing to compensate for lack of RPMs? Or do both those kind of fall off the efficiency curve about the same and it becomes a moot point? I suppose this would also depend largely on the motor?


Does increasing the timing lessen the peak current demand (because the motor is working less efficiently) or is it just trading more KVs for more heat and still demanding the same current? The thought process behind that question is wondering if you can compensate for a mediocre battery with extra timing?


I am a nerd and find electric motor physics fascinating. :lol:




Sean

Bryon thank you for your precise explanation of how to time and gear brushless motors for racing. This is the best explanation I have seen so far on what to do for timing and gearing.

I want to think of a good way to explain this. Here is my unprepared answer. timing efficiency is on an rpm curve. current to torque is on a separate efficiency curve. but you synchronize the curves to rpm. until you need more rpm than the motor likes.

if you have a row boat what wastes more energy? steering wrong or paddling wrong? Sorry its the best analogy I could give that might be understandable to the masses. not trying to be cute but it was the best i had. you want the coil to fire at the point it induces the most movement. when this happens you get the most back emf. when the voltage back equals the voltage forward you dont get anymore speed. unless you limit the amount of forward voltage which in turn limits the amount of return or back emf. voila, more speed by lifting the rpm cap on the motor.

What I recommend is finding the rpm for peak power and the single most efficient timing mark for it.
find peak efficiency and the single most efficient timing mark for it (based on rpm of the motor)
the timing between these two points with be a ratio of timing and rpm
closer to peak power it will be hotter closer to peak efficiency will be cooler.
until you exceed peak efficiency and then think about field weakening.

but before considering field weakening get yourself familiar with converting lap time and wheel speed into average motor rpm.
when I say exceed peak efficiency I dont mean exceed it once. I mean exceed it by the average motor rpm of the entire lap.
this will let you center a couple curves.
the torque demand for acceleration
the efficiency curve for torque
the power curve for sustained high speed
the timing mark for the average motor rpm to average the inefficiency that timing introduces to the torque curve.

once you can work within the power range of the motor you will know when you are above it because wheel speed + gearing wont get you between peak power and peak efficiency.

lets say a normal curve without over timing or minimizing peak power is 10 squares wide by 10 squares tall or 10x10
if you find that the track is longer and needs an rpm above peak efficiency you can trade squares with field weakening
you will get something that is 12 wide but 7 tall.
that is the first benefit
the second benefit that isn’t created by the field but comes from understanding motor rpm versus lap time is this:

you need more rpm.
but what you really need is a higher average rpm
so the time spent at low rpm is minimized
so the low efficiency zones in the low rpm aren’t converting as much current to heat for acceleration
but if you weaken the field too much you 7 tall curve becomes 6 tall and the little bit of time you spend at low rpm generates 20% more heat than it did if you weakened the field enough to extend you average motor rpm out far enough to get you around the track in the time you need.


that was a long way around to make a recommendation. Manual timing is on a curve. its only most efficienct at 1 timing mark and 1 rpm. if its set for 10000 rpm and you are at 100 rpm it will be off significantly but the gap shrinks as you approach 10000. You’ll have to measure it. all motors have a different resistance. but it seems like there is a knee in the curve (takes on a second rate of timing efficiency) just above the rpm that would correspond to 40 degrees. this is why boost is good to cover the fron half of the neutral plane error and turbo is good to cover the second rate.