ESC Sizing per motor

[mschaffer66]

Is there a scientific way to figure out what size esc you need for a given motor? I know a lot of things affect the current draw for a motor, voltage, prop size, drive gears, etc... but there is there a way to calculate a worst case scenario or something similar?

One would think the EE would already know this....but oh well :)

[m4a1usr]

Is there a scientific way to figure out what size esc you need for a given motor? I know a lot of things affect the current draw for a motor, voltage, prop size, drive gears, etc... but there is there a way to calculate a worst case scenario or something similar?

One would think the EE would already know this....but oh well :)

One of the most common methods to size the ESC you desire is to use amps x volts. This equals watts. Or volts divided by watts. This would be amps. So if you have a motor that is rated at 1500 watts continous, and plan on using say a 4S pack for power AND you picked the motor in the correct RPM range you need. Hydros 30 to 38k. Cats 27 to 33k and monos 24 to 30k. (those are my own rpm limitations with the exception of SAW setups.

So for a 4S pack the voltage would be 14.8 volts(nominaly), the motor is rated at 1500 watts continous. That means 1500 watts divided by 14.8 volts equals 101 amps. I like to use the 60 to 80 percent rule when sizing an ESC. So if the numbers say 100 amps then plan on adding 20 to 40 percent additional capacity. 100 amps calculated then use at minimum a 120 amp speedo but a 160 would be better. Now that doesnt mean you are going to burn 1500 watts or use 100 amps in your boat. It just means those are the initial numbers. You might be higher or lower depending on how well the hull is running, the prop you choose, the hardware selection, heck even the wiring configuration. Lots of variables. But at least its a good place to start.


John

[mschaffer66]

I get the calculation part, but when it comes to what the rated power of the motor is how is that affected by prop size? Like for your example of the 1500w motor, what size prop(hypothetically) will that be running? and how would that 1500w change if you went up or down a size in prop?

[m4a1usr]

I get the calculation part, but when it comes to what the rated power of the motor is how is that affected by prop size? Like for your example of the 1500w motor, what size prop(hypothetically) will that be running? and how would that 1500w change if you went up or down a size in prop?

The honest answer to that question is daunting at best. I would ask around here to make sure you start with the right prop of the smallest size for any given hull. Hydros could run big props because of their smaller ride surface area, but because they tend to run high rpm's they need to have smaller props then the other hulls. So a clear answer is not so obvious. The rule for motors is run no more then the continous wattage rating. Thats a normal setup. As you go up in prop size so does the power consumption. So too big a prop and you go beyond the continous motor rating. Then its inverse going the other direction.

An example would be running a 55mm prop on a 30 inch hull of any design. Hydro, Mono, Cat. That would be an absurd prop diameter unless your running a submarine turning 500 RPM's. So the load is somewhat proportional to prop diameter. Unfortunately it becomes exponential beyond a certain point. Then its all over. Thats where you dont want to tread. There is a gas boat web site that has a good prop guidance/recomendation list for given hull sizes and motors. That might be what you are looking for. Let me search thru my saved documents and see if I kept it. That might give you some ideas.


John

[m4a1usr]

Here are some prop examples for boat classes off the Neumotor web site. Might help you with making some choices.

John

http://neumotors.com/Site/Boat_motors.html

[mschaffer66]

I'm resurrecting this post because I have some new questions.

I'm picking up one of those 34" H&M Supercats off of a buddy of mine. It has a gear drive in it. I'm planning on running a 2800ish KV motor on 4S to start and work up to 6S.

First question is what would be like a standard starting point for a prop and what kind of current would it be pulling?

I'm trying to size an esc and figure out what C rating I need on my batteries.

Thanks

[Make-a-Wake]

What size 2800ish KV motor.................. so many different diameters, lengths, brands etc..................

[ED66677]

sizing the ESC according to motor? hum... if you plan to run the motor somewhere around its max efficiency amp draw then the ESC must be at leat 20 to 30% higher capable!
but I would say always get the biggest you can afford, the biggest ESC has the less internal resistance and then allow better performance!

back to the last question, 2800KV motor on a 34" cat might be a bit too much unless you want to do SAW!, target 30000rpm (unloaded) and choose motor/battery according to this, under 4S that would be a KV around 2000rpm/v, under 6S a 1400KV is correct if you refer to this you'll find that for such a hull it is even better to run 8S with a smaller KV of course, around 1000rpm/V in any case the motor should be capable of delivering at least 1800W continuous!

[Simon.O.]

I will agree with the science that John has provided and the guidance that Emmanuel has stated with alteration that I choose a higher rpm and aim for low 30krpm loaded giving me mid-high 30krpm unloaded with a smaller prop.

Deffo get an ESC that is 125% or more of your anticipated load.
Choose batteries to suit that possible draw too !!
A BL motor can pull insane amps if it wants to and the first to blow is the ESC and the packs will follow very quickly.

I anticiapte a motor load and then add 10% before I do anything else.
Then I choose an esc at 125% of that and packs to suit the esc.

That is my odd way of doing it.

[mschaffer66]

The cat has a gear drive in it. I plan on stepping that 2800 down to something like maybe half of that? Dunno, I guess thats the nice thing about the gear drive is I can experiment with different ratios.

But as far as a 2800kv motor I was thinking either the Feigao 6XL or something with similar dimensions.

I suppose my question about current draw goes more into my batteries. I plan on getting the Turnigy 180A water cooled esc for it, which should give me plenty of overhead for the esc(i hope).

The question I have is what C rating on the batteries am I going to buy. Basically I can get any amp rating from 100A continuous to 200A continuous. And there is nearly a 2:1 price difference between the two so I really don't want to spend the extra coin if it will only run at like 80A continuous(just throwing a number out there).

Am I making sense or am I off base?

[ED66677]

Gearboxes were usefull in the past when KV choice was limited, with nowday's BL system, it is easier to be right on the spot, don't bother with gears, this is just an extra source of faillure and efficiency loss.
You have to decide what voltage you want to run first, I would go 6S, there are more 6S ESC than 8S and they are cheaper, also 6S means less amp draw than 4S at same power, P=UI! target 30000rpm (more or less as Simon stated) I prefer a bit less than a bit more, more means smaller prop, less means bigger prop, bigger prop means better prop efficiency with less slippage, if you're on a budget a Feigao 540XL might be a bit small, go for a 580 10L (1326KV) and the Turnigy 180A ESC is the very minimum you need. 80A continuous is not a stupid number, under 6S (22.2V) that's 1776watt witch is exactly what the 580 can safely deliver continuously!

[Make-a-Wake]

Consider this 36 x 88mm XXL Ammo by Electrifly. They are known to be quaulity motors and at 88mm it should have some good mojo on 6s at 1280kv...........28.5k rpms at nominal. I have been wanting one but dont have anything to put it in right now................

http://www.offshoreelectrics.com/pro...od=dh-gpmg5325

[ED66677]

this motor is given for 55A constant, that's pretty much the same as a Feigao 12 or 13XL, same diameter so torque must be in the same range, just $30 extra!

[Make-a-Wake]

this motor is given for 55A constant, that's pretty much the same as a Feigao 12 or 13XL, same diameter so torque must be in the same range, just $30 extra!

This is only a 1280kv..............with the same amperage constant as a 1445kv Feigao XL, so it is apparently a more powerful motor..........its 14mm longer. Its better quality as well.

Plus...... "Powered by "rare earth" Neodymium magnets for high torque and unmatched heat resistance."

[ED66677]

Feigao must be using Neodymium magnets as well to reach this level of performance!
for an extra $20 he can have a monster... a 580L with much more torque!