Trying to understand this basic power calculation

[ferez21]

hi,

I'm new here so sorry if this question sounds trivial, but i want to understand this basic calculation about maximum power in motors.

i see many brushless outrunner motors rated at a couple of thousand Watts @ 6S (e.g 22.2v), and i understand the basic concept of V x A = P, assuming we will use a 6S 7000mAh battery to drive this motor, how on earth and when will a battery of 22.2v and 7Ah be able to supply this amount of power?
Is the rated power of the motor even relevant?
What is the real continous power of the motor?

Any help with understanding these concepts are welcomed, thanks guys

 

[whitepony]

hey there, first of all you are mixing up capacity and discharge rate! if you have a 22.2v / 7Ah battery, then it means you have 22V x 7Ah = 154Wh energy stored in your battery. discharge rate is often given in units of its capacity, so a 1C battery will in your case deliver the current of 7A ... and actually do that for 1 hour if you are lucky!

now, your actual question is: what discharge rating should your batter have in order to power up these crazy outrunners?

for an average consumption, you could check out this link https://endless-sphere.com/forums/viewtopic.php?f=35&t=58661&start=50&hilit=battery+range#p957283
it shows a most likely quadratic fit of how much power users of DIY skateboards used on average as function of velocity. you can assume that this curve is fairly universal, since the drive system friction and air resistance is probably fairly similar among all boards - rest is up to the users trigger finger. this curve tells you: average over various users and fitted to account for quadratic air resistance term, that you are probably using up 100ish watt at 20kph, 300watt at 30kph, 650watt and 40kph and so and and so forth.

now, lets assume you ride just 20kph average, this means that your system is probably pulling somewhere around 100watt, which in return means: with a 22V battery, your battery pack needs to keep up a continuous current of 4.5A, so about 0.65C (spoken in discharge ratings of batteries). this will also tell you, that with a continuous discharge of 4.5A, your battery will be drained in about 1.5hours (since 1.5 x 4.5A = 7Ah = your battery) and youll get a range of about 30km with it (driving 20kph for 1.5hours = 30km).

this is your average consumption. of course these engines are labeled 2200Watt or similiar ridiculous values - this is pretty much just some peak consumption ... which you cannot neglect of course - I just showed you a way of getting an idea what your system will consume on average. for peak performance, youll look at much higher numbers, possibly 1000-2000 watt for double outrunner systems according to experienced users. lets assume a really worst case: you dont kick start your board, but just jerk the throttle from stand still - this situation is pretty much a short circuit for the engines - so 2x 2200 might flow through the engines if your battery can deliver (which it probably cant). if you just calculate backwards: 4400Watt/22V = 200A. its pretty unlikely that your battery can deliver that without massively increased inner resistance and a resulting strong voltage sag which will probably lead to a shut down by your ESC or BMS. to account for these currents, some batteries have a burst current rating and a continuous current rating but I havent really found that one much.

anyway, just walk your way through the numbers and if you have trouble understanding something dont hesitate to poke me. 8)

 

[Nobuo]

The power output depends exclusively of the controller. Motor power rating is only a guide of how much sustained power you can input it and won't overheat/burn

The continuous power to the motor will be managed by the controller and depends also of battery/controller voltage level in the moment of power it. When you are close to the motor max load rpm at the voltage you are pushing it, the continuous amperage will stabilize as long the power needed is the same (same terrain slope), if you go at full throttle and then you uphill, the controller will feed more amps to make again the motor run close to the max rpm, if the controller or the battery is not enough powerful, they will be just running the max power they can deliver.

if the slope is downhill, at full throttle the controller will deliver less power while the motor is stabilized at the max rpm it can deliver at that voltage (a little increasing in max rpm is always happening downhill)

 

[ferez21]

lets assume a really worst case: you dont kick start your board

I do it all the time This is actually the first time i have ever owned any kind of board, and i have it only for 3 days, hope i haven'r done any damage.
I did notice my motor (although brushed) gets incredibly hot, i will kick start from now own.

So this all Max Power thing reminds me the way they used to advertise stereo sound systems in the 90's, by noting the momentary peak power instead of the RMS....i wish they were giving some more info about the power consumption of these motors.

 

[whitepony]

@nobuo: you make it sound as if ESCs are the bottle neck, but shouldnt sufficiently sized ESCs be able to pretty much fully power up these outrunners? i ordered 2x 120A ESCs now, 150A ESCs arent very uncommon either. now Im running 2 of those in parallel and you get a current that CAN actually be high enough to hit the engine limit? and this current is only the max continuous drain again, the burst drain can be much higher!

 

[whitepony]

lets assume a really worst case: you dont kick start your board

I do it all the time This is actually the first time i have ever owned any kind of board, and i have it only for 3 days, hope i haven'r done any damage.
I did notice my motor (although brushed) gets incredibly hot, i will kick start from now own.

So this all Max Power thing reminds me the way they used to advertise stereo sound systems in the 90's, by noting the momentary peak power instead of the RMS....i wish they were giving some more info about the power consumption of these motors.

well, my guess is that either your ESC electronic is saving you (no clue what these things can do, probably a lot!), or your battery is simply breaking down in those moments, voltage decreases due to inner resistance, it gets hot a little and woosh, youre beyond the point of stand still and everything is going back to normal quick! OR you have a really awesome battery with crazy discharge ratings and very little inner resistance and everything is fine

 

[ferez21]

The power output depends exclusively of the controller. Motor power rating is only a guide of how much sustained power you can input it and won't overheat/burn

So, how do you know what is the "strength" (for lack of a better word) of a specific engine?
In cars it's usually the engine size (although not always) that reflect how much strength will the engine have, bigger engine usually implies more horsepower, is this the same with electric engines?
63mm will usually be stringer than 50mm? This is how you guys pick the suitable motor?

 

[whitepony]

well, yea, a larger diameter means more torque, since

torque = lever length x force
with lever length = pretty much radius of the motor
and force = Lorentz force, which is proportional to the current

at the same time, the lever length means the electrons are moving faster through the magnetic field, since with constant rpm, the velocity of the electrons are pretty much rotation speed x lever length again - which in return means the maximum rotation will be smaller (its the point where lorentz force and self induction cancel each other out.)

hope i got everything right, think i had this in 9th grade

 

[rsilvers]

6S 7000mAh battery to drive this motor, how on earth and when will a battery of 22.2v and 7Ah be able to supply this amount of power?

7000mAh and 7Ah are the same thing.

It means that the battery has enough capacity to supply 7 amps for one hour.

22.2v and 7 amps is 155 watts to the motor.

If you wanted to use a 1000 watt motor, the battery would last 9.3 minutes. That is assuming it could even provide 1000 watts, which would take two things:

1. A controller that would provide 45 amps.

2. Batteries that could discharge at 45 amps.

In reality, you would not want to provide so much current in order to get to 1000 watts. So what is the solution? Don't use a 22.2 volt battery if you want 1000 watts. Go to 48 volts. Then you only need 20.8 amps.

 

[ferez21]

If you wanted to use a 1000 watt motor, the battery would last 9.3 minutes. That is assuming it could even provide 1000 watts, which would take two things:

1. A controller that would provide 45 amps.

2. Batteries that could discharge at 45 amps.

That leads me to another question,
As @whitepony said earlier, even at crazy speeds (for a skateboard) of 40kmh, the motor will use about 650W of power, that means that the ESC will provide roughly around 30A@22V - if so, why does many people use such a high rated ESCs (120A, 150A)?

 

[spinningmagnets]

Whether you are on an E-bike or power-board...when you initially take off from a dead stop, the amps will be much more, often called the "peak" amps drawn. As the vehicle accelerates and then the speed stabilizes near its top speed, the amps will slide down to a level equal to the amount of load on it, which is much less than when accelerating.

The ESCs are not designed for E-bikes or power boards. Customers who use them for that are a very small portion of the population of RC model users in the world. In a model aircraft, the motor can draw very high amps for a split second, and then the craft has lots of time to run at a much lower amperage in order to cool down. A power board will take more time to accelerate up to a stable top speed, and an E-bike will take even longer to accelerate.

An ESC that is labeled as a 160A ESC can only provide a true 160A for a very short amount of time. Therefore, if you are drawing 60A peak when your powerboard is accelerating, you will likely fry a 60A-rated ESC, because the factory catalog rating is for an RC model. This phenomenon is one of the most important reasons for a forum; to exchange information. You must "de-rate" components when you are using them in a way that is different than their original designed user-profile.

Best advice is to just copy a build that has shown that it works well when put under heavy abuse, even if that means upgrading some of the components to units that don't seem to make sense, due to the labeling.

 

[rsilvers]

I am not sure but maybe the hobby ESCs (made for RC cars) are not very conservative in their rating so that they feel like they need it to be long-term reliable. And they are not very expensive so it is easier to just pay $50 than waste time on burning out controllers. But if you were making lots of them to sell you would be more careful about spending more on a higher rating than you needed so you would do testing to be sure that you didn't spend more than you needed to on each part.

An ESC that is labeled as a 160A ESC can only provide a true 160A for a very short amount of time

http://www.hobbyking.com/hobbyking/store/__33984__Turnigy_Trackstar_150A_GenII_1_8th_Scale_Sensored_Brushless_Car_ESC_PC_Programmable.html

150 continuous, 950 burst.

http://www.hobbyking.com/hobbyking/store/__10370__HobbyKing_174_8482_150A_High_performance_Brushless_Car_ESC_.html

150 continuous 1080 burst.

 

[Nobuo]

@nobuo: you make it sound as if ESCs are the bottle neck, but shouldnt sufficiently sized ESCs be able to pretty much fully power up these outrunners? i ordered 2x 120A ESCs now, 150A ESCs arent very uncommon either. now Im running 2 of those in parallel and you get a current that CAN actually be high enough to hit the engine limit? and this current is only the max continuous drain again, the burst drain can be much higher!

indeed ESC's controllers (or also batteries/BMS's) are always the bottle neck that avoid a motor to burn :lol: so, not the bottle neck in the bad way if you have the power you want. Just take into account the motor won't ever limit the power in any way, you can virtually have the same torque with almost any motor if you put the enough power to them and they work within the same overall conversion power efficiency, some's can take the power and dissipate the heat, some will burn, some will have wiring over-current...

spinningmagnets explained it crystal clear, about what bursts and continuous drains you must be concern about. I will add you have two basic limits in a motor:

- One is the heat produced inside the motor, always depending of power output. "watts" = amps * volts. Both amps or volts can exclusively burn a motor. If the motor can't dissipate the heat enough will smoke/burn wires, or you will damage magnets making them weak.

- Second is heat produced on the wiring, windings included. Exclusively depending on Amps, independently from volts. Phase wires and windings can burn / melt. (motor should have added always a max sustained current rating, this would avoid too much phase wiring melted in uphills, you can take phases gauge size to guess more or less the amperage limit)

You can never know exactly how much sustained power a motor can resist, which wire or portion of it is the most weaken in the motor, that will burn the first. So this is why manufactures rate the motor for a sustained power watts.

Because all the harming is because of heat, Generally speaking in Canada you can run a motor with more power than in California, in winter you can push more power than in summer.

The power output depends exclusively of the controller. Motor power rating is only a guide of how much sustained power you can input it and won't overheat/burn

So, how do you know what is the "strength" (for lack of a better word) of a specific engine?
In cars it's usually the engine size (although not always) that reflect how much strength will the engine have, bigger engine usually implies more horsepower, is this the same with electric engines?
63mm will usually be stringer than 50mm? This is how you guys pick the suitable motor?

This is a good question. The size says too much about how big can be the magnets and how much power can manage, but because there is not a standard "fuel" as you use in a combustion engine, you can't rate directly the torque or max speed. You can push a motor with different voltages and amperages, so the strength is not a constant.

Motors has a Kv rating to know how much rpm they can deliver with no loads per volt. And has an efficiency conversion rating (close to 90% at high rpms and stabilized speed) so you can calculate the strength taking into account motor power output.

If the Kv rating is not specified, max rpm at a voltage given always are, so you can calculate again with the power output rating

 

[torqueboards]

Great tips for beginners. This should be added to the sticky.

We shouldn't need much more then a 6S 5ah 10C battery for most cases. I've only used 6S-12S 5ah 10C/20C and all function the same at the 10C rate. Of course >6S will result in more power and range due to pack watt hours which is worth it. A higher discharge pack will can give you most power but the price IMO doesn't justify. I would much more rather use 18650 packs in parallel for higher amperage but doesn't seem like it's needed.

You can get even more power from your motors by wiring more packs in parallel to increase your discharge rating. Although, this would increase in space.

6S 5ah will usually result in about 6-7 miles with regular riding. 12S 5ah will result in about 10-14 miles of regular riding.

Adding in a dual motor does not increase speed but will increase hill climbing power by utilizing two motors to do the job. This is recommend for trying to climb 20-30% hills. 10-15% hills should be fine on a single motor if your gearing is correct for your motor.

The average amps used on start-up can range from 60A to 80A peak at max and regular average speed (once up to speed) seems to drop to about 20-30amps cont.

You can always attach a watts up meter to check your peak amps. You can also use the eLogger v4 for ease of use as it will log the amperage during your run.

We use higher rated ESCs just as a pre-caution to not burn out a lower rated esc.

Your motors should never get hot during operation even when riding for 30 minutes to 2 hours. If your motors are real hot to touch for more than 3 seconds, you are most likely running incorrect gearing and/or are over-stressing your setup. Steps should be taken to fix this issue otherwise, you have a chance of burning out the components.

The heat comes from a setup being overworked and/or pushing more amps to accomplish an easier goal fixed by changing simply the gearing ratio.

From my own experience, if you are running a single motor setup a 63mm is definitely a better choice because it can dissipate heat much better and since it's bigger it provides more torque and it does not get hot as fast.
For a dual motor using two 63mm motors, the board can weigh much heavier. It's ideal to switch to two 50mm since dual 63mm would usually be overkill with the added weight.

Most 50mm and 63mm motors with a KV rating less than 300KV will work fine. I've tested a few higher KV motors 300/400KV and they don't seem to have enough power and to get the gearing reduction for them to be worth it still doesn't seem to work as well as a motor with 300KV or lower.

RC Motors seem to last forever (2,000+ miles) for the most part if they are not overheated and have an equal pull, not stressed. If they don't last this long, I usually suspect it can be due to a bad motor overtime, a motor somehow get's damaged in between the 2,000 miles that it should work for. Received a lemon motor and/or similar. But these motors seem to have a long life span if taken care of. Always protect the c clip and double check the set screws on the end of the motor shaft which keep the motor in tip top shape. Usually, I make sure the c-clip does not have any obstruction when spinning and it can effectively spin around the motor mount. I replace the set screws on the back of the motor with actual bolts as you can torque down much harder than with a set screw. Medium threadlock will also help from loosening the bolts but putting too much will get the bolt stuck in there forever. Just a little is needed.

I also don't think that there are many bad motors, most if not all motors are made very similar as far as 50/63mm motors simply just different designs and lower kv. Small differences don't matter too much.

I have ridden many motors for 1,000+ or more from SK3 63mm, Sk3 50mm, NTM and the ones which I sell. I'd ridden 1,000+ miles without keyways climbing 30% hills.

The new difference with the newer customized motors for our own application are simply just the added keyway slot which is a nice to have as well as the option for lower KV motors.

 

[Hummina Shadeeba]

@nobuo: you make it sound as if ESCs are the bottle neck, but shouldnt sufficiently sized ESCs be able to pretty much fully power up these outrunners? i ordered 2x 120A ESCs now, 150A ESCs arent very uncommon either. now Im running 2 of those in parallel and you get a current that CAN actually be high enough to hit the engine limit? and this current is only the max continuous drain again, the burst drain can be much higher!

indeed ESC's controllers (or also batteries/BMS's) are always the bottle neck that avoid a motor to burn :lol: so, not the bottle neck in the bad way if you have the power you want. Just take into account the motor won't ever limit the power in any way, you can virtually have the same torque with almost any motor if you put the enough power to them and they work within the same overall conversion power efficiency, some's can take the power and dissipate the heat, some will burn, some will have wiring over-current...

how can you put two esc's in parallel to run one motor? I can't imagine that.

I thought you want to chose an esc that can handle the motor and not the other way around. Does the motor pull amps and the esc have to be capable of supplying them, or does the esc feed amps and the motor has to be capable of not burning up...or is it all just load dependent and the only thing saving the motor or the esc is an amp limit set within the esc? I still don't get it. Assuming the battery can put out enough amps to blow either

 

[whitepony]

I bought 2 ESCs to run 2 motors, guess that could be done with 1 ESC too, but better be safe than sorry (over-engineering stuff)!

your 2nd thought is valid: esc feeds amps and the motor has to be capable of not burning up!

 

[Hummina Shadeeba]

I think you need an esc for each motor.

 

[Hummina Shadeeba]

I just had another converstation trying to explain/understand how volts x amps = watts yet amps are the only thing creating inductance in a motor. I don't understand. My neighbor tells me that when he uses higher voltage he wil have greater acceleration but with the same amps used. So 72 volts with ten amps vs 48 volts with 10 amps the 72 will produce more torque. How can a motor's inductance be solely decided by amps yet volts x amps = watts?

Amps and volts go hand in hand as v= I x R, yet I believe the esc has a variable resistance in the mosfets so it is possible to kick a motor with either 1 amp at 100 volts, or 100 amps at 1 volt. What's going on in these scenarios or is it still not possible to have such a situation?

Or is the esc converting the higher voltage to amps and my friend may be reading the 10 amp use on his watt meter but it's not truely what the motors are getting. I'm believing this is what's happening

 

[Trbt555]

Here is a post by Benjamin Vedder, which is very useful.

Here is another post for some background info.

 

[randyc1]

Great tips for beginners. This should be added to the sticky.



"6S 5ah will usually result in about 6-7 miles with regular riding. 12S 5ah will result in about 10-14 miles of regular riding."

? I thought it was the (Ah) that had to be increased for more range ?

 

[Hummina Shadeeba]

Watt hours is a true revealer of your stored energy and distance possible. Voltage times amp hours.

But those links above don't explain how volts relate to torque. I've been to vedder's site before and the stack exchange link, while it does have one equation which incorporates voltage it's not intuitive or practically understandable. From what I get from the stack exchange link, at least the words used describes amps as the sole source of torque.

 

[whitepony]

volts does not directly relate to torque, but volt x ampere does. V*A = Watt = power your engine currently sucks up and that power is proportional to the torque of the motor:

V*A = torque x (rpm*2*pi/60)

(simplified, ignoring losses, stall torque, ...). hope that helps