PDA

View Full Version : Limiting throttle with 6s



mariner
03-15-2014, 06:18 AM
Hello,

Just bought Impulse 31 V2, I will start first with 4s and stock setup then try with 6s.
I don't want to ruin the boat (ESC etc.) so I'm planning to use DX8 radio and limit maximum throttle.
The idea is to reach maximum speed 40 knots (45 mph), can anyone say if this can be achieved and where I should set the throttle limit ? E.g. 80 % ?

Fluid
03-15-2014, 09:42 AM
A very bad idea. Running a brushless boat only at part throttle will overheat the ESC and lead to early failure. If you want to run slower then either reduce the number of cells or run a smaller prop. Full throttle is easier on the electronics, given a decent setup.




.

sillycon
03-22-2014, 12:25 AM
Not to fly in the face of "common knowledge" but how do motors and ESCs work harder/wear out faster with less current flowing through them at a given voltage?

I'm genuinely curious to know the how/why of the oft-repeated "part throttle will kill your ESC/motor".

BHChieftain
03-22-2014, 12:42 AM
The way ESCs reduce motor RPMs is by increasing the pulse rate of current to the motor, and it is hard for the ESC to do that-- the ESC runs cooler at full throttle. THe rapid switching power on and off stresses it--- the motor wants to pull current from the batteries, and the batteries are happy to supply the current, so something has to work HARD to get in the way.

Also, I would not advise running the Impulse (or the Blackjack) at 6S. The ESC *is* rated for 22.2 volts, but it is only an 80A ESC. So you would need to use a really small prop to keep the amps under 80, and you'll end up with about the same speed as using a 4S setup with a larger prop. You will hear many people say they run their impulse/blackjack on 6S without a problem, but you will find (and I personally know) MANY people who burn out their stock 80A ESC doing this. Whereas you will hardly hear anyone having problems with this boat on 4S. Stick with 4s and use an x642 prop and you'll have a fast reliable boat. If you want to run 6S, pick up a 180A esc and you'll have plenty of headroom.

Chief

sillycon
03-22-2014, 01:58 AM
It seems odd to me that the higher the pulse rate, the slower the motor would run. I'd think that the lower the rate, the slower it would run.

Regardless, I currently run 4s and 5s setups. Thought about 6s, but "eh". The 4s/5s battery setups are less expensive (even if only slightly so), and (seemingly) less headache. Plus, I don't want to find myself going down the upgrade rabbit hole. Already done that with too many other hobbies/toys. Trying to keep it simple this time.

For the stock motor and ESC would you consider an x642 to be too much prop at 5s? I've been torn between picking up an x442 or x642, but wasn't sure of the risk of burn-down on an x642 at 5s. I'd rather buy right and buy once rather than have a bunch of extra parts kicking around collecting dust.

ETA: What about the Octura M545? It appears to have about the same total pitch as the x642, but is de-tongued.

Also, thanks for the explanation on the ESC deal.

BHChieftain
03-22-2014, 10:52 AM
Here's a link to Castle Creation's FAQ page, and they address this

http://www.castlecreations.com/support/faq/faq-general.html

///
6. Can I control how much current passes through my speed controller by limiting full throttle travel on my transmitter with endpoint adjustments/trim? In other words can I use a 25 amp speed controller with a motor that will pull 45 amps but lower my top throttle endpoint on my transmitter so my watt meter only shows 25 amps at full throttle? Will this be OK?
NO!!! A speed controller controls power to the motor by turning full throttle current on and off really fast, 11 to 13 thousand times per second (Pulse Width Modulation or PWM). The percentage of each on/off pulse that is off compared to the part that is on determines how much power the motor sees. I.E. With a pulse that is 50% off and 50% on the motor will see 50% power*. Because each on pulse is 100% of full throttle current, a system set to pull 20 amps at full throttle through a Phoenix 10 will not last if you are throttled back to the point where you only see 10 amps on a wattmeter. The ESC in this case is still switching 20 amps, which it can’t do for long. Actually it is worse than the simple example above. Because an electric motor will always to try to pull as much power as is available to get to its rpm (volts times Kv), when you are running the motor below its Kv speed by switching power on an off, each on pulse will actually be way over the full throttle amp draw. That is why ESCs work harder at partial throttle than full throttle and why we underrate our ESCs. We underrate not so they can handle more current than their rating at full throttle, but so they can handle extended partial throttle operation with no problems.

* Actually, electric power is not linear as in this example, but you get the idea.
///


Chief

sillycon
03-22-2014, 12:30 PM
That makes sense. I didn't dawn on me last night that at 100% pwm the circuit is always closed so there are no amperage spikes. Thanks for posting that!

Now, not to be a pain, but might you have any input regarding my prop question above?

Thanks again!

BHChieftain
03-22-2014, 05:18 PM
I have not tried the m545, but I have the m445 and the x642. The m445 has less total pitch but seems to be faster than the x642-- I think the 1.4 pitch (and detounge) lets the motor unwind and hit higher rpm. But the hub on the m445 is a bit big and you need to modify the drive dog spacing, whereas the x642 hub fits perfect. I also prefer less diameter on monos-- a bit less chine walking-- so overall for speed+handing I'm a big fan of the x642. I'm talking 4S... on 6S I think an x or m 440 (or maybe x442 if everything is running cool) would be the way to go.
Chief