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                      Thanks to Those Who Take the Time

The Rc car i've been using died due to its Esc being burnt out by a 3S lipo battery, and i want to ultimately replace it with something. If i were to be creating an electronic speed controller (ESC) for a specific motor. Whereas specs such as the max amount of current under load you can put into the motor without having it meltdown, do not appear on the motor and are not listed elsewhere cause concern. this keeps me in a certain situation where i am stumped on planning the ESC. On the other hand my understanding on how the ESC works with controlling the power is missing a few things.

For my motor situation, the motor itself is rated at 4000Kv (rpm per volt), i am assuming that is the unloaded max speed it will go. i am also under the impression it will eventually get really hot, and burn up if i continue to have it operate for a prolonged period of time at that rate. I do not what that to happen, but i want to be able to have the ability to go just as fast as the stock ESC, and not have it burn out the motor or ESC while under load. To do that i do not have a clear understanding. Im thinking i need to find max amount of current it will take without burning under load. in order to be able to program the ESC and plan the hardware to fit the specific motor. Is that True, is there a different way i should be looking at this?

For the ESC Issue, its unclear to me what causes it to fry. if it cannot resist the load to the motor to have it operate at a safe temp will it fry certain parts as my old one did? where does the hardware come into play to prevent this? How do i make the ESC to be able to manage a 2S lipo that has 7.4v, and a 3S Lipo that has 11.1v battery without burning up and be able to keep the motor at a safe temp. With features of a forward, reverse, a BEC for Servos and etc, a over heat failsafe if ran too long, a low voltage cut off to save batteries, and control acceleration? That being said what do you think would be some other things i would need to consider?

To conclude do i need to find the "max amount of current under load" in order to keep my motor safe, what else am i missing? When a ESC fries what causes it and how could i prevent it? How should i be viewing its overall programming process and hardware operation of controlling current along with other features to Then optimize speed and avoid burning up itself and the motor.

Here is some info from the HPI Wesbite about the motor

"Upgrade to a zero-maintenance brushless Flux Vektor 4000Kv motor for the >ultimate in performance and ease of use! The Flux Vektor 4000Kv brushless motor >is standard equipment in the Savage XS Flux, making it the perfect partner for >any off-road vehicle for fun running. It can handle 7.4 volt (2S) or 11.1 (3S) >LiPo batteries as well as 7.2v NiMH batteries for a range of possibilities! " >Features: - High torque/high temperature neodymium magnet

  • Oversized precision sealed bearings
  • 3.2mm 1/8" hardened steel shaft
  • 4 Pole rotor for more torque
  • Triple insulated windings for long life
  • Black anodized case
  • Easy to Install
  • Compatible with any sensorless ESC

-- P.S. if you can point me in the right direction of some useful text or experience that would be also appreciated.

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closed as unclear what you're asking by Olin Lathrop, laptop2d, Daniel Grillo, Autistic, Dave Tweed Nov 29 '16 at 17:47

Please clarify your specific problem or add additional details to highlight exactly what you need. As it's currently written, it’s hard to tell exactly what you're asking. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

  • \$\begingroup\$ This will come across somewhat critical and somewhat pedantic BUT the aim is to get you a better reception than what you'll otherwise get. There are people here who can help BUT you have to make the question easier for brains to follow. You probably have too much unexplained terminology for many people here AND you have quite a few typos that make some terms gobbledygook. eg "at 4000kw". If that was 4000 kW you should stand well clear. But, it's not. And sloppy 11.1 (no units) and a style approaching slang with words omitted (4000 rating) and unnecessary words added (7.4 volt naturally).. \$\endgroup\$ – Russell McMahon Nov 28 '16 at 10:36
  • \$\begingroup\$ With no motor spec the max allowed current is the current it is not damaged by. If you don't know that we can't tell you. A say 4000 kv motor will nominally run at about 30000 RPM on a 2S LIPO and about 44000 RPM on a 3S LIPO and ... . You cannot tell from that what is safe. eg at 88000 RPM on a 6S LIPO it's a race between whether it or you will die first. Wire thickness, body size for type (in/out runner) and comparison with similar with known specs will give some guide. | ESC current is limited by PWM of available voltage.Making your own implies enough knowledge or a cookbook or ...? \$\endgroup\$ – Russell McMahon Nov 28 '16 at 10:44
  • \$\begingroup\$ How do you propose to go about making your own? | ESC current capability is limited by the ratings of the power FETs used and by the cooling provided. \$\endgroup\$ – Russell McMahon Nov 28 '16 at 10:46
  • \$\begingroup\$ If the problem (killing the ESC) is max current, that can potentially apply whatever the voltage, because voltage and current draw are only loosely related in a motor. To limit max current, you need to limit torque. That means you may be equally safe at 8V (32000rpm) and 12V (48000 rpm) provided you limit the acceleration (or vehicle weight, or uphill gradient) to limit the torque. Or reduce the gearing, to get the same top speed at higher RPM for faster acceleration. \$\endgroup\$ – Brian Drummond Nov 28 '16 at 12:03
  • \$\begingroup\$ So @BrianDrummond would i then program the esc to detect what type of battery or accept some type if signal to choose the type Then have it run through whatever functions i need it to do based off that selection? So if i have a 3s lipo running (48000rpm) with a 24 tooth pinion, opposed to a 28 that would be used on a 2s setup at 32000rpm? that would be increasing the torque not decreasing it? \$\endgroup\$ – Dan JJ Ber Nov 30 '16 at 1:00
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MUCH more information is needed.

With no motor spec the max allowed current is the current it is not damaged by. If you don't know that we can't tell you.

A say 4000 kv motor will nominally run at about 30000 RPM on a 2S LIPO and about 44000 RPM on a 3S LIPO and ... .
You cannot tell from that what is safe. eg at 88000 RPM on a 6S LIPO it's a race between whether it or you will die first.

Wire thickness, body size for type (in/out runner) and comparison with similar with known specs will give some guide.

ESC current is limited by PWM of available voltage.Making your own implies enough knowledge or a cookbook or ...? So - how do you propose to go about making your own? Do you have a design or circuit or ...?

ESC current capability is limited by the ratings of the power FETs used and by the cooling provided. Thermal power loss is largely a factor of FET on resistance and maximum current.
Resistive thermal power per FET ~~= (Imax)^2 x Rdson_FET / 3.
eg for a say 60A ESC Imax = 60A which each FET handles for 1/3 of the time.
For a say 10 milliOhm Rdso FET tyermal power per FET ~~=
(Imax)^2 x Rdson_FET / 3
= (60)^2 x 0.010 /3 = 12 Watts
Some serious heatsinking is needed.
IF you can use FETs with Rdson = 2 milliohms hot at full load the power/FET drops to 2.4 Watts nd far more modest heatsinking.
For more $v you can get 1 milliOhm Rdson FETs (and lower) and may 'get away with minimal heatsinking if 60A is used only occasionally in brief bursts. Thereare other losses (switching, gate drive capacitance loss, ...) but raw resistive switching tends to dominate until you get quite capable low Rdson FETs.

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The brushless motor says it is safe for 4000kv on a 3s LiPo... Since i have no motor specs

You seem to be confused about what Kv means. It is a motor spec. Kv is the motor's 'velocity constant' which is the rpm it will do per volt applied (when unloaded). So at 7.4V it will spin at 7.4 x 4000 = 29600rpm, and at 11.1V it will do 11.1 x 4000 = 44400rpm. Whether this is safe depends on whether the rotor and bearings can handle that rpm.

would it not be operating at 4000kw?

No. I assume by 'kw' you mean 'kW' ie. kiloWatts (capitalization is important!). Watts = Volts x Amps, so to consume 4000kW it would have to draw 360000 Amps at 11.1V. Kv is just the velocity constant, which is not directly related to Volts, Amps or Watts.

However due to the way a permanent magnet DC motor works, its 'torque constant' (Kt) is the inverse of its velocity constant (Kv). Kt is the torque produced per unit of current drawn (typically measured in Newton Meters per Amp). The higher the motor's Kv, the lower its Kt and therefore the more current it must draw to produce the same torque. But this not the full story because the gearbox lowers rpm at the wheels and increases torque by the same ratio - so a high Kv motor with deep gearing could draw the same current and power as a low Kv motor with shallow gearing. Also a high Kv motor on low voltage can perform the same as a low Kv motor on high voltage, the only difference being higher current draw at the lower voltage.

If you know the motor's Kv (which is a ratio, not a maximum voltage or rpm rating) then you also know its Kt. But this doesn't tell you how much current it can draw. For that you need to know its internal resistance, Rm. This is determined by the length and diameter of the copper wire in its stator windings. The larger the motor, the more space it has for copper and so the lower its internal resistance (for the same Kv). A motor wound for higher higher Kv will have lower Rm because it has less turns of thicker wire. This also means its stall current will be higher because current = voltage / resistance.

So unloaded rpm is determined by Kv x voltage, and stall current by voltage / resistance. But what current will the motor draw in actual operation? This depends on how much load is applied. Torque, speed, current and power are all interrelated according to the graph below:-

enter image description here

Ultimately this means that motor Kv, battery voltage and gear ratios all have to be matched to the vehicle. If your car is designed to run on 7.4V then that is what you should use unless you increase the gear ratio or change the motor to one with lower Kv.

all id have to program for a "ESC max amp..or failsafe" in order to not blow my motor, would be to not exceed a RPM of (kv x volts)?

No. Kv x volts only determines the maximum unloaded speed. When a vehicle is accelerating from a standing start the motor is stalled, so current draw is very high (much higher than its continuous rating) because it is limited only by resistance. The ESC must either be rated to handle this current or limit it to a safe value. Active current limiting is desirable because as well as protecting the ESC it also limits torque applied to the mechanics (gearbox, tyres etc.), producing maximum safe acceleration without having to go easy on the throttle.

assuming I have planned my PCB and all of its components in order to make my ESC function just as good as the stock,

Designing a good high power ESC is not an easy task. Unless you are willing to do a lot of research and experimentation, it would be better to just buy a suitable ESC. The stock Savage Flux XS ESC is known to have reliability issues. I suggest browsing rc car forums to find out what other people are replacing it with.

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  • \$\begingroup\$ Thanks you for all your wisdom cleared up a lot of uncertainties. When you say, "No. Kv x volts only determines the maximum unloaded speed." how would it work while on load? would it cause more resistance on the motor and heat up because its not using the all the energy applied? \$\endgroup\$ – Dan JJ Ber Nov 29 '16 at 1:05
  • \$\begingroup\$ ive always had a thing for electronics, and in turn ive found an interest for research and experimentation of the creation of more advanced electronics. i took two years of c++ code and the build up from where i would have left off shouldn't be too bad. I was looking at designing a board using Peripheral Interface controllers and MOSFETS to control the direction of the motor. But as you can tell i am way off in terms of a plan. gathering information and doing small projects is what im doing right now. \$\endgroup\$ – Dan JJ Ber Nov 29 '16 at 1:13
  • \$\begingroup\$ "how would it work while on load?" - Examine the graph. As load is applied current increases and rpm decreases (due to voltage lost in the winding resistance) eventually reaching zero rpm at stall. Maximum power occurs at 50% of no-load rpm, and highest efficiency at ~80-90%. Best operating range is between these points. You choose a motor Kv and battery voltage to get the rpm you want at cruise (taking into account gearing, wheel size etc.), and limit current to prevent burning out the motor or ESC during heavy acceleration. \$\endgroup\$ – Bruce Abbott Nov 29 '16 at 2:41
  • \$\begingroup\$ A different way of looking at it is that Kv * V determines the back EMF, so as you add load, the motor slows, the back EMF reduces. Now the rest of the supply V is dropped across the winding resistance R giving current I, (V=IR) where I produces the torque needed. Think about that V=IR : that represents wasted power (heat in the windings) and you'll see the motor is most efficient at low load - Bruce's graph confirms that. \$\endgroup\$ – Brian Drummond Nov 30 '16 at 10:25

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