processor speed

[lv_reefer]

Just wondering why an esc manufacturer would put the pwm switching rate as an adjustable option? It is the processor speed why would you ever slow it down? I realize I could make it irrelevant and just set it the the highest setting and forget about but I want to understand my hobby better? For instance the Swordfish 120 has these adjustment how would you calculate the max rpm of a 2, 4, 6, 8... pole motors? Again why slow the processor switching rate?

[siberianhusky]

Pwm is pulse width modulation, it has nothing to do with esc clock speed.
You calculate the max rpm of a motor by KV x V. so a 2000 kv motor on 14.8V = 29600 rpm.
Just leave the PWM setting at factory stock. You can damage a motor with the wrong setting.
You should do a google search for PWM so you can see what it actually is.

[NativePaul]

Its not the processor speed, its just the speed that the processor switches the FETs on and off which has nothing to do with max RPM limits.

You may or may not know that ESCs don't actually modulate voltage to control speed, lets say for arguments sake your running 4s 14.8volts but you want half throttle which would be 7.4volts, the ESC cant supply this but can approximate it by using the FETS (electronic switches) to switch the current off for half the time so you get 14.8v half the time and 0v half the time, if you do this quick enough anything with any impedance will see it as 7.4v and you get half the revs from your motor than you would at full throttle.

Generally switching rate options are 8khz and 16khz and switching in and off at 8khz (8,000 times a second) is plenty to fool your motor that it is seeing a lower voltage and is what you should be using in the vast majority of cases as there are losses involved in switching a FET and running at 16khz will increase the heat of your ESC by roughly 20% given the same load and throttle position for no benefit.

The only use I have found for increasing the PWM switching rate is to reduce cogging, cogging is something I have only ever come across in cars with the high inertia of their drivetrain making starts hard unlike planes and boats that have low inertia and can start spinning the prop very easily. Cogging is the motor rattling backwards and forwards at the start when it cant figure out which magnet to activate to spin the motor in the desired direction, and this can sometimes be reduced by increasing the switching frequency, Ive never experienced or even heard of cogging in a boat and even if I did I would probably stick to 8khz to keep the temps down as I only make 1 start per battery pack in a boat vs a dozen or so in my cars.

[lv_reefer]

Alright that makes sense. I am familiar with pwms for led lighting. I have seen some esc advertise their max rpm capability how might one find these specs on other escs i.e. the Swordfish or the Seaking 180? As to stay within its limits on say a 12-pole outrunner.

[siberianhusky]

Your main concern would be setting the timing properly for a 12 pole motor. Not PWM but actual degrees timing. Get the timing wrong and you will have issues.
The RPM specs are just there to impress people who don't know it doesn't matter.
The Castle motor in front of me at the moment has 60,000 rpm printed on it, so what it'll never turn near that fast.
You will never have a motor turning so fast in a boat that the esc can't keep up with it. You aim for about 30,000 rpm unloaded. With a prop on it you will end up in the mid 20,000 range on the water.
Amp draw is the key determining factor in your boats setup, you need to control the heat through the size of the prop and proper KV selection.

[NativePaul]

No Idea how you would find out the maximum RPM of an ESC when the manufacturer doesn't disclose that information other than by testing.

You can easily come up against the Maximum RPM limits in a boat, I have seen several ESCs that are rated at 120,000rpm on a 2 pole motor, change that 2 pole motor to an outrunner's typical 12 poles and you can only do 1/6th the speed or 20,000rpm, 50% under the rpm that you aim for. I aim for a little higher with a smaller prop and have come up against the limit myself with 6 pole Mega motors at around 40,000rpm, I cant recall what my old ESC was in my Oncilla mini hydro class cat probably some cheap noname chinese unit converted to marine use knowing me but I replaced it with a Hyperion titan 40 (marinised) and that is running well at around 50,000rpm on 6 poles (150,000 for 2poles).

[siberianhusky]

The switching speed is listed in a number of places in all programmable esc's, Guess you just really don't know what it is!

[NativePaul]

Doh! Yes I meant Max rpm not switching speed in my second post, I should have reread it before I posted, my brain doesn't know what my fingers are typing some times, thanks for the heads up I'll change it.