Max amps and max voltage

[sumguy75]

Simple question: Do volts matter on an esc if the motor/esc are a proper match? I know amps do, but trying to figure out if its just amps that matter on an esc.

I'm putting electronics in a scratch build (2wd extended Bandit). I've got the new Hobbywing Max10 G2 which is 140a. I ordered a Hobbystar 2400kv 4068 motor (the 4s limit on Hobbywing's published specs for the G2). The motor also list a 110A limit and a voltage limit of 20v. I assume that means 4s capable with some headroom, not quiet 6s capable? I'm planning to run 4s almost 100% of the time.

Thing that made me curious is the 2650kv version of the motor is 122A, which would be too much for my old Max10 sct, but voltage would still be fine.

First scratch build and just trying to double check my own thinking, lol.

 

[CrashBoom]

The voltage running through your ESC must be equal to, or lower than, it's maximum rating.

The motor is 20V capable, but the most it'll get is 16.8V on 4S.

 

[PrerunninF150]

According to the specs that combo should work just fine.

 

[RCbuzz]

Yes volts absolutely matter. Volts x Amps = Watts and watts is total power.

The Max10 G2 is only a 4s capable esc, on 6s it would likely not even move. Or

The motor voltage (max) should be paired up with the voltage you are running with your ESC. In other words a 3s motor on 4s would likely work but not for long.

It's okay if the motor current is lower than the ESC, the ESC should not push more current than the motor can handle.

The voltage you are running will directly effect the top speed of the setup. That is why the higher the "S" the lower typical kV.

Motor kV x Volts = Unloaded RPM

 

[SirCrashALot]

It's okay if the motor current is lower than the ESC, the ESC should not push more current than the motor can handle.

Correction: The motor should not pull more than the ESC can handle.

 

[Hector_Fisher]

Couple of things i've learned over time: there is no such thing as "max amps". Amps only affect temperature. As long as you can cool it, or if temps aren't an issue, then there is no problem with more amps.

The only limiting factors with RC electronics are voltage on the ESC, RPM on the motor, and heat via amp usage.

ESC's usually have circuitry to prevent overvoltage -- meaning, they usually just don't work if you plug in a higher voltage battery. RPMs are limited by how much the rotor inside the motor can take from centripetal force. RPMs are affected by what voltage the motor sees through "KV", though it's a bit more complicated than that. For example, you can run 6s on a 3s motor. As long as you don't exceed the RPM limit, then you could run 6s as much as you want. Heat limits on the motor and ESC are pretty obvious. Too much heat and you'll let out the magic smoke.

 

[sumguy75]

Thanks for the replies, lots of good info.

 

[RCbuzz]

Couple of things i've learned over time: there is no such thing as "max amps". Amps only affect temperature. As long as you can cool it, or if temps aren't an issue, then there is no problem with more amps.

The only limiting factors with RC electronics are voltage on the ESC, RPM on the motor, and heat via amp usage.

ESC's usually have circuitry to prevent overvoltage -- meaning, they usually just don't work if you plug in a higher voltage battery. RPMs are limited by how much the rotor inside the motor can take from centripetal force. RPMs are affected by what voltage the motor sees through "KV", though it's a bit more complicated than that. For example, you can run 6s on a 3s motor. As long as you don't exceed the RPM limit, then you could run 6s as much as you want. Heat limits on the motor and ESC are pretty obvious. Too much heat and you'll let out the magic smoke.

Max amps are max amps. The components/traces inside the ESC are rated to a certain continuous/burst amperage, exceed rating and it will likely heat up and burn. Overcurrent does effect temperature, that is why given a poor connection when current passes and resistance rises it will heat up and burn. There is such thing as current limit in electronics which I can only hope our ESC's have and do well. Power electronics with intelligent logic are capable of much better protection.

The amp rating of the motor should be based on the max voltage specified for that motor. If you increase the voltage past the rating you will raise the current (past the max amps rating) and likely cause issues. The motor is made of up copper windings of certain diameter, length, and loops, all of this is factored into the overall design and function of the motor. These factors will give the motor the desired run voltage, current, RPM, Torque, etc. under operation.

Here is a basic example showing the correlation between voltage, amps, and resistance.

*Ohms Law
V(voltage) = I(current) x R(resistance)

11.1 (3S) = I x .14ohms(generic value chosen) --> I = 79amps
22.2 (6S) = I x .14ohms(generic value chosen) --> I = 156amps

If the motor wasn't rated to handle that voltage/current it could melt the windings, flash over and damage winding insulation, overspeed and explode, and/or draw too much from the ESC and burn it up (assuming the ESC doesn't protect itself).

Just going from memory, feel free to correct any inconsistencies.

Edit: I should probably add that the calculations assume full load. If you were to take a 3S car and speed run it at 2S vs 3S, you should see more current running 3S and more heat. ESC's use PWM modulation to reduce speed (good visual example here: https://www.rcgroups.com/forums/showpost.php?p=34847150&postcount=5). When running at lower speeds the esc will pulse the wave as needed to reduce effective voltage, reducing RPM and Current as well. Regardless, the Engineers should design these units to perform under their set limitations (some are overbuilt, I get that). An effective way to measure current is with a CT, heat sensors really should be limited to just that and aid to load reduction or shutoff.. Heat can be caused by overcurrent but also component and connection failure.

 

[KnowAir]

I can't answer your question because I'm just a meathead, but your build has definitely gotten my attention. I'm dying to build a 2wd 1/8 buggy. If I had the finances, I'd be elbows deep in it for sure! What I can say is, I have an Xray buggy that I converted from nitro to brushless. I run a 4068 2560kv motor on 4s through a rtr 80a 2-4s Redcat branded (Hobbywing)esc that's 10 years old. This esc has been used on 4s since day one in a Redcat Avalanche XTE 4wd monster truck. The truck came with some undersized 36xx motor that didn't last a month on 4s. So I swapped in a Thunder Tiger Ripper 4074 2000kv. That combo outlasted the POS truck. I ended up smoking the motor in my mojo exb, but that esc runs my Xray buggy on 4s day in, day out..
Please do share more of your build when you can! I know the majority of folks have no interest in an 1/8 2wd buggy as evidenced by the fact that none exist, but I'm sure interested!!
Good luck!!

 

[KnowAir]

I think your setup sounds very reasonable. The bandit weighs next to nothing. Even with a 4s battery and 4068 hanging off the back.
Are you going to use the 272R? My brother bought a xl5 brushed bandit a couple years back, never used it after getting a vxl rustler,lol.
I have been badgering him for the bandit, it's like new. Gonna throw a brushless setup and 272R in, along with some paddle tires for winter fun..

 

[RCbuzz]

I think your setup sounds very reasonable. The bandit weighs next to nothing. Even with a 4s battery and 4068 hanging off the back.
Are you going to use the 272R? My brother bought a xl5 brushed bandit a couple years back, never used it after getting a vxl rustler,lol.
I have been badgering him for the bandit, it's like new. Gonna throw a brushless setup and 272R in, along with some paddle tires for winter fun..

Its going to rip for sure. I am running a stock VXL system on my Bandit... So much fun, ripping around a corner power sliding..

 

[KnowAir]

Its going to rip for sure. I am running a stock VXL system on my Bandit... So much fun, ripping around a corner power sliding..

What I'm thinkin!
My bro just upgraded his rustler vxl 4x4 with a max10 sct combo..so the vxl combo is up for grabs.. I honestly can't imagine a bandit on 4s.. hopefully the OP has some beefcake built into that bandit!!
I want a Raider XL that can legitimately handle 4s (even though undrivable) AND get parts for.. can't blame a fella for dreaming. LOL

 

[Hector_Fisher]

Max amps are max amps. The components/traces inside the ESC are rated to a certain continuous/burst amperage, exceed rating and it will likely heat up and burn. Overcurrent does effect temperature, that is why given a poor connection when current passes and resistance rises it will heat up and burn. There is such thing as current limit in electronics which I can only hope our ESC's have and do well. Power electronics with intelligent logic are capable of much better protection.

The amp rating of the motor should be based on the max voltage specified for that motor. If you increase the voltage past the rating you will raise the current (past the max amps rating) and likely cause issues. The motor is made of up copper windings of certain diameter, length, and loops, all of this is factored into the overall design and function of the motor. These factors will give the motor the desired run voltage, current, RPM, Torque, etc. under operation.

Here is a basic example showing the correlation between voltage, amps, and resistance.

*Ohms Law
V(voltage) = I(current) x R(resistance)

11.1 (3S) = I x .14ohms(generic value chosen) --> I = 79amps
22.2 (6S) = I x .14ohms(generic value chosen) --> I = 156amps

If the motor wasn't rated to handle that voltage/current it could melt the windings, flash over and damage winding insulation, overspeed and explode, and/or draw too much from the ESC and burn it up (assuming the ESC doesn't protect itself).

Just going from memory, feel free to correct any inconsistencies.

Keep in mind, amperage is a product, not a variable. You cannot control amperage -- you can control resistance and voltage and then get the amperage as a byproduct. Since you cannot "control" amperage directly, you can only mitigate it's side affects, which are heat. Over current protection in PC power supplies is just "thermal" protection with a different name.

Using Ohms law gives a relationship for finding amperage in a simplified circuit, but something doesn't add up here. If I increase voltage on my RC (run 4s instead of 2s or 3s), it runs cooler. Since amperage causes heat (not voltage), that insinuates amperage went down. Since this isn't a simplified circuit, how are we sure resistance hasn't changed somehow?

Initial thoughts to consider: using the same ESC/motor, can you get more power out of the truck? Yes. If you gear up and use a bigger/top end battery of the same voltage, the truck will accelerate faster, thereby more power is being used. Since you are at the same voltage level, where did the power come from? More amperage. What would likely happen if you did that to your truck? You'd run into heat issues. The answer would then be putting a better fan on the ESC and a heatsink/fan on the motor to mitigate heat.

Regarding "max amps": it's a made up number in the same way C-rating is a made up number. It's a measured/calculated number based on the testing variables and specified criteria that you impose. Max amps are usually determined by testing to find out what amps can be continuously drawn through a motor for a given length of time, at a given voltage, while meeting a temperature criteria. Example: Motor A can maintain it's temperature below it's companies 180f operating limit for the companies testing time of 10 minutes if the amperage is 120amps or less. So the company would publish 120amp as the "max amps". Change any number of the criteria in that test (max allowable temp, time length needed to satisfy, running voltage, etc) and the results could change. For example, if I change the time limit to 20 minutes, the amperage may drop to 100amps (or something similar) because the motor would need to see fewer amps in order to last during the test for a longer time. If you change the time to 1 minute, you may find that you can continuously draw 300 amps and keep the motor below 180f. This is why max amps means next to nothing since you as the user will not be exactly replicating the testing conditions while you are driving. And this is why you could potentially see bursts of several hundred amps on a "100a" ESC or motor with no negative consequences.

The limiting factor here is undoubtedly heat/temperature. And apologies for the thought dump.

 

[sumguy75]

What I'm thinkin!
My bro just upgraded his rustler vxl 4x4 with a max10 sct combo..so the vxl combo is up for grabs.. I honestly can't imagine a bandit on 4s.. hopefully the OP has some beefcake built into that bandit!!
I want a Raider XL that can legitimately handle 4s (even though undrivable) AND get parts for.. can't blame a fella for dreaming. LOL

Oh yeah, its going to be VERY beefcake. Planning to do a build thread on it, will probably start it this weekend with everything I've done so far. Doing a lot of what I did to my 2wd Stampede and it has been running 4s like a champ for several months now.

I'm ordering the aluminum chassis as soon as I decide for sure on the final wheelbase length. Fast Lane has the stock chassis (11.25") and extended (12.25") and there are couple of companies that make extension kits. The extension kit is about 1.7 inches and added to the 11.25" chassis would make it the same wheelbase as the Typhon.

 

[KnowAir]

Oh yeah, its going to be VERY beefcake. Planning to do a build thread on it, will probably start it this weekend with everything I've done so far. Doing a lot of what I did to my 2wd Stampede and it has been running 4s like a champ for several months now.

I'm ordering the aluminum chassis as soon as I decide for sure on the final wheelbase length. Fast Lane has the stock chassis (11.25") and extended (12.25") and there are couple of companies that make extension kits. The extension kit added to the 11.25" chassis would make it the same wheelbase as the Typhon.

I wasn't aware of the upgraded chassis..good to know!! I won't look at them cuz I won't like the price,LOL. I've actually made a chassis from 7075 t6 for my Thunder Tiger mt4 back when. Came out quite well, bending the chassis for the kick up at the front was an exercise for sure!! LOL. Unfortunately, parts are just simply gone for that truck. It is in good running condition but I don't use a whole lot since I can't feasibly repair it..

 

[sumguy75]

I wasn't aware of the upgraded chassis..good to know!! I won't look at them cuz I won't like the price,LOL. I've actually made a chassis from 7075 t6 for my Thunder Tiger mt4 back when. Came out quite well, bending the chassis for the kick up at the front was an exercise for sure!! LOL. Unfortunately, parts are just simply gone for that truck. It is in good running condition but I don't use a whole lot since I can't feasibly repair it..

I was starting to look at options for making my own extended Stampede chassis when I ran across Fast Lane.
Bending 7075, yikes that sounds like a serious challenge!

 

[KnowAir]

I was starting to look at options for making my own extended Stampede chassis when I ran across Fast Lane.
Bending 7075, yikes that sounds like a serious challenge!

It was! Took 3 men and a baby!! LOL it definitely didn't come out perfect, but functioned quite well. Never bent it. It is in the neighborhood of 5.x mm thickness, can't remember. Over 5, under 6, definitely heavy!!

 

[KnowAir]

I took a look at the FastLane chassis, they're definitely beefcake, and beautiful!! OOS though? Are they still making them? The bandit transmission looks killer too..

 

[sumguy75]

Here's a couple of quick shots of my Stampede build with a Fast Lane 1 inch extended chassis, transmission and shock towers. Rpm front bumper has one ounce add on weight bolted on to keep the front end planted better. Still wheelies but is manageable.

Here's the transmission...

 

[KnowAir]

Here's a couple of quick shots of my Stampede build with a Fast Lane 1 inch extended chassis, transmission and shock towers. Rpm front bumper has one ounce add on weight bolted on to keep the front end planted better. Still wheelies but is manageable.

View attachment 261035

View attachment 261036

Looks pretty serious!! FastLane's pricing is actually pretty reasonable. The extended bandit chassis shows out of stock and one review from 2009.. between the pricing and old reviews, I was concerned they may not be making these anymore? I was actually on their site fairly recently as I had seen a motor mount on ebay for the xmaxx, seller said he thought it was from FastLane.. was also pretty beefcake looking but I didn't find it on their site.