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Wide open throttle for brushless motors

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Old 07-11-2018, 01:46 PM
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Default Wide open throttle for brushless motors

In full size boat engines (and probably in all full size engines but I donít know) thereís a term called wide open throttle, which is a range for the max rpm for the engine, if the engines max rpm isnít in that range it means that the engines prop is too large/small, and is damaging the motor.
When you look at brushless motors, thereís no wide open throttle or any similar term which tells in which rpm the motor should run. There is KV, and KV*V = Max rpm, but surely running the motor at that rpm is not good for the motor.
So my question is this: how do you find/calculate the recommended max rpm for a brushless motor?
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Old 07-11-2018, 03:06 PM
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to large a prop of pitch will have a lesser rpm .kv is a no load rpm a prop in water will have a load.again less rpm in the end.
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Old 07-11-2018, 03:25 PM
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Is this a brushless motor in a car or a boat?
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Old 07-12-2018, 06:59 AM
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I like to go by temps.... I try to keep my motors under 160įF when possible and gear appropriately.... for stock racing classes I will contact the manufacturer of the motor to get a recommended amp draw to set the timing on a motor analyzer and gear per their recommendation for a given class.
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Old 07-12-2018, 03:59 PM
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Originally Posted by urnotevenwrg2 View Post
Is this a brushless motor in a car or a boat?
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in a Boat
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Old 07-13-2018, 05:15 AM
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Originally Posted by Krallgren View Post

in a Boat
To answer your original question, you can only put so much voltage to a motor before it spins too fast and shells out the rotor. This is why almost all motors come with a safe voltage range listed on the box.

RPM is only part of the equation. The torque curve is the other part of the equation. You have to match the load on the motor to keep it in the sweet spot of the rpm/torque.

Boats create a large load on the motor, so it is important that the prop is correct. If you are running a popular boat you could ask around some RC boat forums to learn some common motor/prop combinations. If you are building something of your own, your best bet will be to try several props to see which one works best for whatever you are using.
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Old 07-13-2018, 08:53 AM
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I have a lot of experience propping full size boats, and I think it's slighty different with Brushless vs gas engine. The difference being a brushless motor can draw more power to make up for a FDR that is too tall. I would think of it like changing FDR on a car to keep the temp safe for the amount of time you plan to run it, except that you are using prop pitch instead of gearing to adjust your FDR. Then I would look into prop profile/surface area to get the holding / planing power you need. Good luck.
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Old 07-13-2018, 11:17 AM
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Comparing electric to IC engines is apples to oranges- there is no comparison other than they are used to propel craft. Electric motors usually do not have a stated RPM range that they can run at MAX, as there are too many factors. Theoretically, electric motors can run at an unlimited RPM in a perfect environment, like a vacuum. What you need to instead look at is the maximum voltage and current range the motor can run within, and keep in mind that as the voltage of your battery decreases, so will motor RPM's and therefore current. The only way to ensure that you are maximizing your systems power output is to put a current meter on it and run it to see what your voltage sag is and what your maximum current draw is.

To give you an idea of why stated electric motor RPM ranges are useless, I had an airplane motor was was only designed to run on 10s max at 5200rpm. We routinely ran them on 12s and spun them around 6500rpm for many years with zero measurable effect on the motor itself. You only need to pay attention to current draw at that point, being that we were within it's 100 amp range (78 amps), and the motor never exceeded 140F, we concluded that no damage would ever be done to the motor and that we were maximizing its power capability.
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Old 07-13-2018, 05:30 PM
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Originally Posted by Xpress.. View Post
Comparing electric to IC engines is apples to oranges- there is no comparison other than they are used to propel craft. Electric motors usually do not have a stated RPM range that they can run at MAX, as there are too many factors. Theoretically, electric motors can run at an unlimited RPM in a perfect environment, like a vacuum.
I don't know where you got that idea from, but rotors will generally disintegrate if they spin too fast due to the centripetal forces involved.
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Old 07-17-2018, 09:14 AM
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As stated above by Bill, temps are going to be what you need to go by in order to dial in any brushless setup. Of course the max RPM of the motor cannot be exceeded, but the max potential RPM is easy to ballpark by multiplying your voltage by the KV. Electric motors don't have a "powerband" in the way an IC engine does, so the RPM limit is really about keeping the rotor from exploding. Beyond that max RPM, temp is your only limiting factor in extracting more power from a brushless motor, we have a good quick read on the subject from our "RCJuice University" here if you are interested: https://www.rcjuice.com/power_system_setup
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Old 07-17-2018, 02:23 PM
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Originally Posted by gigaplex View Post
I don't know where you got that idea from, but rotors will generally disintegrate if they spin too fast due to the centripetal forces involved.
That's why I stated "theoretically"
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Old Yesterday, 06:00 PM
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Originally Posted by gigaplex View Post
I don't know where you got that idea from, but rotors will generally disintegrate if they spin too fast due to the centripetal forces involved.
Centrifugal, not centripetal. Centripetal force holds them together.
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Old Yesterday, 07:54 PM
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Originally Posted by ta_man View Post
Centrifugal, not centripetal. Centripetal force holds them together.
Ah, this debate again. The motor is actually applying centripetal forces to the rotor, accelerating it in the direction of the centre of the rotor as it moves in a circular motion. The centrifugal effect is just inertia as the rotor mass resists the acceleration.
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Old Yesterday, 07:55 PM
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Originally Posted by Xpress.. View Post
That's why I stated "theoretically"
Well then no motor has a theoretical wide open throttle limit to reduce damage if you ignore damage potential
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Old Today, 05:27 AM
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Originally Posted by gigaplex View Post
Ah, this debate again. The motor is actually applying centripetal forces to the rotor, accelerating it in the direction of the centre of the rotor as it moves in a circular motion. The centrifugal effect is just inertia as the rotor mass resists the acceleration.
The only possible debate is among people who don't remember (or haven't yet taken) their high school physics class and therefore are not aware of the scientific definitions.

Debate away!
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