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Another brushless 'myth', lower KV = more torque

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Another brushless 'myth', lower KV = more torque

Old 02-25-2008, 08:34 PM
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Default Another brushless 'myth', lower KV = more torque

I copied and rephrased this out of another thread. I see this mis-stated so much on these boards that I thought it would be good to post it here and clear this up.

This is actually a half-truth. A lower KV motor will have more starting torque, ie. torque from a dead stop. If, for instance, you are pushing it into a wall, or towing something, the starting force will be greater per unit current.

But this is just starting torque. Once the motor starts spinning, at the lowest PWM duty cycle of the ESC, your torque is going to be (theoretically) equal no matter what the KV of the motor is. Anyone that thinks different is forgetting how an ESC works and what KV means.

Remember how the ESC works, using PWM duty cycle to create a limited voltage on the motor. For instance, a 20% duty cycle means that the voltage is 1/5th but the current will be 5x. 10% duty cycle means 1/10th the voltage and 10x the current. Etc. By 10%, I mean if the ESC provides 100A, 1V at the motor (100W), a 10V battery would see a drain of 10A from the ESC (100W) -- if the ESC were 100% efficient.

As for KV, say your battery provides 10V for the sake of simplicity. Likewise the battery can provide a fixed amount of current and no more. If the motor were spinning at 10000rpm, a 5000KV motor would require 2V (0.2 duty cycle) and the 10000KV motor would require 1V (0.1 duty cycle). Of course the 5000KV motor has twice the turns of wire so it generates 2x the torque per amp as the 10000KV motor. In theory, everything is equal in the end. Once you bring PWM into the picture, you are dealing with power and not current, you are dealing with power.

The only way a lower KV motor would have greater torque is if an ESC was just a simple linear voltage drop-down circuit whose efficiency was like a linear regulator (ie. 10V -> 5V = 50% efficiency, 10V -> 2.5V = 25% efficiency).

Now here's the 'gotcha'. Most of the time we are dealing with a 'family' of motors like Castle's CM36 series. The motor is basically the same only the turns of wire is different. That's when the above holds true.

But, ultimately the 'max torque' of a motor is determined by its design, the coil core meteriel, the rotor size, the can size, etc. As the RPMs increase, this torque is flat to a point but then starts to drop off. By the time it reaches max RPM, the torque is very close to zero.

If you take a motor like the VXL motor (3500 KV), that is a family of it's own and it's only desgined to work with 3s LiPo. This means a lower RPM and hence they can design it for a greater max torque -- for instance a fatter rotor. A rotor that might break apart at 60,000rpm, but it doesn't need to go 60,000rpm, so that's okay. If you are going from one family of motor to another, the max torque isn't going to be the same, it's going to depend on the overall quality of the design and the parameters that design was made to meet.
Old 02-25-2008, 08:42 PM
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Default RE: Another brushless 'myth', lower KV = more torque

yup
Old 02-25-2008, 08:46 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Whoops, I just realised, it should've been posted to the electric forum, oh well.
Old 02-25-2008, 11:23 PM
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Default RE: Another brushless 'myth', lower KV = more torque

This forum is fine for such a great write up. I believe it will receive plenty of views here.

Nice job [sm=thumbs_up.gif]
Old 02-26-2008, 07:58 AM
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Default RE: Another brushless 'myth', lower KV = more torque

why has no one come out with a wheel dyno yet, then we could actually put numbers to a motor, a guy that thinks he is putting out 1300 watts could only be acheiving 972 watts but never messed with the esc settings because thats still more than enough to make his buggy go pretty darn good.
Old 02-26-2008, 03:50 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Great thread Access!! A very accurate and succinct summation.

I would just like to point out two things out to the kids who might have glossed over the point in reading!

The post makes a very clear distinction between Max Torque and Torque per Amp (Kt). You need to understand the difference! And you need to understand "power". Power as in the Ohms law derived Watts=VxI.

This might for once and for all settle the "X vs Y, who is more powerfull?" threads? I know I for one am getting pretty fed up with them!
Old 03-07-2008, 10:47 AM
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Default RE: Another brushless 'myth', lower KV = more torque

I appreciate the myth busting.
Old 02-12-2009, 10:33 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Interesting read, and very good info to say the least.
Old 02-13-2009, 01:20 AM
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Default RE: Another brushless 'myth', lower KV = more torque

Yes good read.......Its good to learn.
Old 02-13-2009, 01:56 AM
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Default RE: Another brushless 'myth', lower KV = more torque

Well...... Thats assuming you have a battery that can put out the amps the higher KV motor needs... take a 2200kv motor and a 6000kv motor, and with sure with an awsome battery the 2200 wont have a big advantage. But run them both on a 6 cell stick pack with the same gearing and you will see a difference.... Thats the big advanage I see of a lower KV motor, less amp draw.....
Old 02-13-2009, 06:02 AM
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Default RE: Another brushless 'myth', lower KV = more torque

With PWM the the ESC is basically functioning as a 'voltage divider' and 'current multiplier'. Power stays the same (energy as a whole must be conserved). Mind you this assumes you have a high enough frequency and/or a good enough capacitor on the front end of the ESC.

Again take two motors, say we want to spin them each @ 10,000RPM, a 5000KV motor and a 10,000KV motor
Since it has twice the number of coils, the 5000KV motor produces 2 units of torque per amp, the 10,000KV motor only produces 1 unit of torque per amp. The battery voltage is 10.

1) The 5000KV motor
requires 2V, or 20% PWM duty cycle. This means the 10V the battery delivers is 0.2x (to the motor), and whatever current battery delivers is 5x (to the motor). The overall unit torque per amp is 2 * 5 = 10.

2) The 10,000KV motor
requires 1V, or 10% PWM duty cycle. This means the 10V the battery delivers is 0.1x (to the motor), and whatever current the battery delivers is 10x (to the motor). The overall unit torque per amp is 1 * 10 = 10.

So your torque per unit amp and all that other stuff is basically going to end up equal.

The oversimplification here is that we neglect efficiency. Every system has physical resistance, the wiring, the ESC, the motor, etc. Higher voltage almost always means better efficiency. So, given the choices, we should always choose an equivelent higher voltage, lower KV setup over a lower voltage, higher KV setup. We're not getting (significantly) more torque by doing this, we're just getting better efficiency, which is real important for keeping things cool. Naturally the torque might increase slightly (a few percentage points if that) b'cos of the better efficiency, but it's not going to be that significant.

For a full explanation of PWM:
20% PWM means the ESC's is 'on', or the switch is closed, 20% of the time. The ESC is 'off', or the switch is open, 80% of the time. This happens thousands of times a second, or more. A quality ESC has enough capacitance on the input end, and enough capacitance and impedience (from the motor) on the output end to level out the 'ripple' caused by the on-off. In a simple ESC, the PWM duty cycle might simply be linked to the throttle, 50% throttle = 50% PWM and so on.

Say the ESC is drawing 100A.
Current: The ESC draws 100A when the switch is closed (20% of the time). It draws 0A when the switch is open (80%). The leveling effect means in reality only 20A is being drawn from the battery.
Voltage: The ESC draws 10V when the switch is closed (20% of the time). You have 0V otherwise. The leveling effect means the motor effectively sees 2V.
Power stays the same, and energy is conserved no matter how you look at it (timebase, or Volts * Amps).

Remember this ONLY holds true if the motor is spinning. If I'm pushing the truck into a solid object (exerting a force on the solid object), the motor with more coils is going to create more torque per unit amp.

In all cases above I'm talking about setups which have roughly the same amount of full-throttle, under-load RPM (RPM-equivelent setups). If you look at a motor as a convertor from electric power to mechanical power, with some base efficiency. With equivelent battery setups (capable of providing the same amount of power) a motor that is only spinning 20,000RPM at full throttle and under load will naturally have more torque than one spinning 40,000RPM at full throttle and under load b'cos mechanical power is some function of RPM and torque. Of course if you're trying to make the max speeds the same, any gain or loss in this respect is equalized / negated by gearing. Still, sticking to a reasonable RPM for the speed you want to go, or the power you need, makes sense. B'cos of the above, increasing RPM while keeping the same battery isn't going to increase the overall torque to the wheels, the only thing increasing RPM does is increase the possible power output of the motor. Motors have a saturation point in terms of torque which they cannot exceed, and as you approach this point, they become increasing inefficient. When you start getting into things like this, though, it becomes a much more complicated topic.
Old 02-13-2009, 06:18 AM
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Default RE: Another brushless 'myth', lower KV = more torque

Wow access! I didn't know you were that smart LOL. But seriously very good read. You really do learn something new every day.
Old 02-13-2009, 07:57 AM
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Default RE: Another brushless 'myth', lower KV = more torque

Y'all are making me like my nitro
Old 02-13-2009, 08:59 AM
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Default RE: Another brushless 'myth', lower KV = more torque

Just what does KV stand for with these electric motors? I doubt it's kilo-volts, as a 5000 kilo-volt motor would be an interesting device indeed.

Never mind...I just looked it up....

"Brushless motors are given a Kv rating, which is RPM per volt"

I don't doubt your conclusions, but there's a few things that arn't clear to me. When you say:

"Remember how the ESC works, using PWM duty cycle to create a limited voltage on the motor. For instance, a 20% duty cycle means that the voltage is 1/5th but the current will be 5x. 10% duty cycle means 1/10th the voltage and 10x the current. Etc. By 10%, I mean if the ESC provides 100A, 1V at the motor (100W), a 10V battery would see a drain of 10A from the ESC (100W) if the ESC were 100% efficient."

You get the right answer, but ...you are comparing apples and oranges. For a 20% duty cycle, your voltage of 1/5th is an "average voltage" over time, however, your 5x current is that achieved during the pulse. You really can't work with those numbers. What you really have is full battery votage and current during a pulse (which is power) multiplied by the duty cycle....so naturally you get 1/5th the power.

That 10Amp draw from the battery seems misleading. It's an average of 10 Amps. The battery should be delivering pulses of 50 amps

Also, this kV rating system is for an unloaded motor. How can you really tell what's going to happen from that when the motor is under load? Although I think that's the point you are trying to make in relation to your CM36 comments.

So, does the 5000kv motor really have twice the winding turns of the 10,000KV motor? And everything else is the same? To me, that means the higher kv motor will have less winding resistance and from a start will draw more current with the same voltage applied. This means more power, and as torque is directly proportional to power, the 10000KV motor should have more "oompfh" from standing start.

Anyway...I really don't know. I have no experience with brushless motors. I'm just trying to follow your reasoning and math and find I can't.
Old 02-13-2009, 11:23 AM
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
That 10Amp draw from the battery seems misleading. It's an average of 10 Amps. The battery should be delivering pulses of 50 amps
I don't know how else to say it. On a good ESC, you have the levelling effect of:
1) The capacitor on the input end.
2) The capacitor on the output end.
3) The motor inductance.
4) A high enough PWM frequency.

http://www.youtube.com/watch?v=vqapTwk1Rpw

You can see the levelling effect in this video. This is a mamba max ESC, the ripple generally stays around 600mV (or less), and somewhat less than that when additional capacitors are added to the front end.

At very low RPMs (3) is minimized, that is why the ripple starts out pretty sharp and becomes less sharp as the RPMs go up to a reasonable level.

ORIGINAL: Argess
Also, this kV rating system is for an unloaded motor. How can you really tell what's going to happen from that when the motor is under load? Although I think that's the point you are trying to make in relation to your CM36 comments.

So, does the 5000kv motor really have twice the winding turns of the 10,000KV motor? And everything else is the same? To me, that means the higher kv motor will have less winding resistance and from a start will draw more current with the same voltage applied. This means more power, and as torque is directly proportional to power, the 10000KV motor should have more "oompfh" from standing start.

Anyway...I really don't know. I have no experience with brushless motors. I'm just trying to follow your reasoning and math and find I can't.
Most companies rate their motors KV under the appropriate load (for the vehicle it's designed for).

A motor is both an DC and AC device and you need to understand both sides to really understand what is going on.
If a motor was only a DC device, there would be no limit to how fast a motor could spin. A brushless motor could just charge the coils faster, and hence spin faster. What stops this is looking at the motor from an AC perspective. Every coil has inductance, impedience, making it resistant to current changes over time. In essence it takes some time for the coil to turn on, and it likewise takes some time for the coil to turn off. The more turns of wire in the coil, the longer it takes.

In a brushless motor, it is correct that a motor with fewer turns uses less wire, and hence lower DC resistance. But higher voltage helps to overcome that resistance. It's not necessarily a linear relationship, there is a 'sweet spot' for most setups. You can 'go to far', ie. a really low KV motor and 1000V for a 1/10th scale vehicle is not going to be favorable. But neither is a 10,500KV motor and 6V. Not to mention, from an efficiency standpoint, you also have to consider the whole system, not just the motor.
Old 02-13-2009, 11:43 AM
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Default RE: Another brushless 'myth', lower KV = more torque

Oh well, what do I know?....I'm a nitro guy......LOL.

Interesting video by the way. Where was the cap added? Must have been to the output side of the ESC to see the wavefrom change if it's voltage. If it were to the battery side of the ESC, the scope would have to show current I suppose, and that can be a bit tricky to set up.... Also, I couldn't see where the scope was set, but what frequency are the pulses at? Too low and you'll need a really big cap with the crazy currents you guys are running......chuckle.

Anyway, I find all this interesting, but don't know much about it. I researched the KV term and it did say "no load"...guess the website could be wrong:

http://www.rctoys.com/rc-products-ca...SS-MOTORS.html

Anyway, I would have thought:

Intial start-up = current limited by winding resistance (current goes down as motor speeds up, but high at first)
lower turns = lower resistance = high surge current
high surge current = higher starting torque

Therefore the 10000KV motor should have been better from a standing start. However as you mention, there are other variables.

Thanks for not taking this the wrong way. I really don't know much about electric and I found your initial post very interesting and informative.
Old 02-13-2009, 12:52 PM
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: Argess
Interesting video by the way. Where was the cap added? Must have been to the output side of the ESC to see the wavefrom change if it's voltage. If it were to the battery side of the ESC, the scope would have to show current I suppose, and that can be a bit tricky to set up.... Also, I couldn't see where the scope was set, but what frequency are the pulses at? Too low and you'll need a really big cap with the crazy currents you guys are running......chuckle.

Intial start-up = current limited by winding resistance (current goes down as motor speeds up, but high at first)
lower turns = lower resistance = high surge current
high surge current = higher starting torque

Therefore the 10000KV motor should have been better from a standing start. However as you mention, there are other variables.
In the video the cap was added on the battery end. The scope is showing voltage, more capacitance = less voltage ripple. It was a number of different low-ESR caps, all in parallel. One of the tradeoffs in designing an ESC is this PWM frequency, b'cos every time that switch flips, you having to move around gate charge (on the power MOSFETS), and you are having to move the mosfets through the non-saturated region of operation. Some ESCs allow you to set this PWM frequency with the advisement that higher = more heat generated by the ESC, but hopefully less at the motor.

Note that for a half-decent motor, the winding resistance isn't too significant.
A Neu 1515/1Y motor = 0.006 ohms resistance. Even if you run 100A through it, it's going to drop 0.6V, not too significant for a 2200KV motor designed to be run between 14-21V. It's only when you get to things like those cheap feigao motors or others that resistance can start to be significant. If you shorted a coil of this motor direct to a 20V battery, it would (try) to draw thousands of amps, in theory.

It's up to the ESC to limit the inrush current when power is first applied to the motor. High inrush currents are a problem you see often in the real world.

The strength of an electromagnet (the coil of a motor) is proportional to the current and the number of turns in the coil. Twice the number of turns means twice the strength per unit current.

But if you read the link below, it describes how coming up with an optimum electromagnet can be a more difficult problem.
http://en.wikipedia.org/wiki/Electro...d_by_a_current

With quality motors, it's not so much an issue, see above. The effect of the resistance is quite limited.
Old 02-13-2009, 02:59 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Yes, I forgot about the number of turns. So in your example, you may have 1/2 the current (based on winding resistance only), but at twice the number of turns, the same torque is applied in either motor. And that matches up with, as you say, the winding resistance not coming too much into play. So something else must be going on there.

So I assume the reactance has more effect at turn on, or locked-rotor, than I thought. So if the reactance is lower for the 10000kv motor at full speed, it is probably still lower than a 5000kv motor at low speed, and therefore the current is higher and as P=Current squared x Impedance, it still seems like the higher KV motor will honk from a dead start. Oh well.

Battery end.....I take it that if a battery of infinite capacity was used, a capacitor added to that side whould have no effect? So I take it that it's an effective way to get more from a real world "limited" battery.

Switching issues....I'll bet. Nasty when you get two turning on at the same time effectively grounding the battery.

And the ESC has built in current limiting? I always wondered about that. Does that mean you can safely use a high power motor with an ESC designed for a low-power motor (to some extent)?

Oh....excuse me.....this thread's getting derailed (by me) and I'm asking all kinds of electric newbie questions......maybe I'll come up with some questions and start a new thread about "how to pick motors and match ESCs". Thanks for all the info to date.
Old 02-13-2009, 03:46 PM
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Default RE: Another brushless 'myth', lower KV = more torque

The ESC has some kind of power-limiting, basically it's not going to just start at 100% duty cycle on a motor that isn't turning. How this works is really up to the firmware.

I really can't go too much deeper into the theory without things getting really complicated, at some point you need to consider back-EMF, varied loads, and such. A lot of it comes down to practice.

The general key to choosing brushless motors is:
- High voltage, as high as the ESC can handle, but keep in mind charging your LiPos, etc., how many cells LiPo can your charger handle.

- RPM that makes sense for the setup, you don't need 60,000rpm with a crawler, and a 'speed' setup can't make do with 10,000rpm.
RPM ultimately determines your max power, once your motor is saturated and provided maximum torque, the only way to increase power is to increase rpm.

- Knowing the speed and the voltage, the appropriate KV isn't too hard to find.

- Then, gear based on top speed, check temperature, if temperature is too high, you're overgeared.
Old 02-13-2009, 04:32 PM
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Default RE: Another brushless 'myth', lower KV = more torque

You have completely neglected the physics of the torque of a current carrying coil, and load in your analysis of torque, thus your conclusion "your torque is going to be (theoretically) equal no matter what the KV of the motor is" is completely false and violates the laws of physics.

Old 02-13-2009, 05:55 PM
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Default RE: Another brushless 'myth', lower KV = more torque

These things were addressed in the very first post. The torque of a "current carrying coil" is proportional to the number of turns and the current, the load of the wire itself has been shown to be relatively insignificant if a motor of reasonable quality is used.

How does this violate the laws of physics? I can show that, with two equal-RPM systems, this principle practically has to be true to NOT violate conservation of energy.

When I say 'equal RPM systems', I mean for instance compare:

1) 10,000KV motor run @ 6V (60,000RPM at full throttle)
2) 6000KV motor run @ 10V (60,000RPM at full throttle)

Same battery same ESC. Compare the torque at any given RPM, that RPM being the same for both. It's almost equal, save for efficiency.

If both these systems draw the same amount of power, therefore they ought to produce the same amount of mechanical power out, barring any big differences in efficiency.

http://en.wikipedia.org/wiki/Conservation_of_energy

If you are talking about non-equal RPM systems, that is a whole different issue and I touch on that in some of the later posts in this thread.
Old 02-13-2009, 06:01 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Everything you are talking about is unloaded first of all. Next, you must consider the physics of torque of current carrying coil. You already admit that the torque is proportional to the number of turns, this does not change. Also, the duty cycle can be 100% and the rpm of the motor does not have to be at its max RPM due to that very loading that you haven't taken into consideration.
Old 02-13-2009, 06:09 PM
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Default RE: Another brushless 'myth', lower KV = more torque

Dont forget to account for the MRI bias!
Old 02-13-2009, 06:19 PM
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Default RE: Another brushless 'myth', lower KV = more torque

I see that you are going to make me pull out my physics text book, so here I go..... I will first go on to say that I have a bachelors degree in electronics engineering and a minor in math and I was *very* good in physics, such that my papers were used by my professor to teach people in elementary school based on the way my work was completed. This is actually a very simple physics problem.

Torque = (magnitude of the force) x (lever arm)

In the instance of an RC vehicle where the gearing and mass of the vehicle does not change when comparing two different motors of different windings, this will mean the lever arm will be equal. Another way to explain 'lever arm' in this scenario is that the force required to turn the spur gear is the same whether we use a 13.5t motor or a 4.5t motor, therefore, the lever arm does not change when comparing motors.

Therefore, the comparison of torques comes down solely to comparing the current passing through the coils which in turns pushes the rotor..... And, you have already admitted that torque is proportional to the number of turns.

So now it boils down to how the ESC will energize the coils in the motor. And for simplicity, I only have to describe wide open throttle, in essence, 100% duty cycle which is the total battery voltage.

So now we have it simple to where the total voltage of the battery gets used up on a coil. The 13.5t motor has more winds per coil than a 4.5t motor, winds per coil is proportional to torque, therefore a 13.5t motor has more torque at wide open throttle (available battery voltage).
Old 02-13-2009, 06:55 PM
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Default RE: Another brushless 'myth', lower KV = more torque

ORIGINAL: mattnin
So now it boils down to how the ESC will energize the coils in the motor. And for simplicity, I only have to describe wide open throttle, in essence, 100% duty cycle.

So now we have it simple to where the total voltage of the battery gets used up on a coil. The 13.5t motor has more winds per coil than a 4.5t motor, winds per coil is proportional to torque, therefore a 13.5t motor has more torque per volt at wide open throttle.
There are two seperate cases here. Since you are talking 100% full throttle,
Are we talking 1) same voltage to each motor, or 2) different voltages to each motor such that both motors spin at near the same RPM at full throttle?

Also, a brushless motor controller (ESC) cannot operate at 100% duty cycle all the time. It can only survive doing this at near its full-throttle RPM. If it does this while the motor is in the lower RPM range, it will burn up. Some people are under the misconception that, say, if your brushless motor is turning at 60rpm, the ESC is simply cycling the coils 100% duty cycle at 60rpm. But the in reality if this was happening, the system would be drawing a hysterically high current (thousands of amps in theory) and things would burn up.

One of the keys to understand is that if you look at the electromagnet equation for a 'perfect' electromagnet (assume no resistance), voltage is irrelevant. It doesn't affect the theoretical strength of the magnetic field, only current and the number of turns does. Therefore when the rotor is only turning at 60rpm, the ESC only has to provide a very small voltage with which to overcome the impedience of turning the motor coils on and off within the timeframe that the rotor comes into position of that coil.

Same thing can happen to a brushed motor if, say, I hold the shaft in place so it absolutely cannot turn, and then hook it directly to a battery. It will bake very quickly.

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