Understanding KV in a motor - RCCanada - Canada Radio Controlled Hobby Forum
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Old 02-10-2014, 07:53 AM   #1
Meafordguy
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Understanding KV in a motor


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Which is something I still struggle with when choosing a motor for my RC.
From what I have learned, KV is "revolutions per volt" meaning how many times the motor turns for every volt that is put to it.

To me this means the motor spins faster with a higher KV, resulting in a higher top speed, but lower turning power (torque). So it can spin the wheels really fast, but with not a lot of power.

So, as I see it, a motor with a really high KV like 6000-8000 would be good in an on-road street racer, or a high speed monster or stadium truck like the E-Revo. You take it out onto the pavement and do top-speed drag races.

A low KV motor like a 2000 would give you good starting power, high torque, but would not top out at a very high speed. So I would use it in a crawler.

IF this is all true, why then do I look in RC magazines and why do they advertise low KV motors in the racing RCs? Particularly short course or buggys which reach high top speeds on a large dirt track, or on-road cars which also whip around at high speed, all have low KV motors?

If you don't believe me I'll have to post some scans of these ads I keep seeing.

I don't get it.
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Old 02-10-2014, 08:24 AM   #2
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Re: Understanding KV in a motor

You have to remember in a vehicle that gear ratio plays a part as well as voltage. Is the buggy setup for a 4s or 5s seup? If so the KV will be lower due to the higher voltage setup . As you have figured out a 2500kv motor turns 2500 rpm per volt so on a 2s pack going off nominal volts of 3.7v per cell it would turn 18500 rpm but that same motor on a 5s pack would turn 46250 rpm. Next you would have to calculate gear ratios which have another large factor to consider into the equation . Hope this helps some.
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Old 02-10-2014, 08:34 AM   #3
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Re: Understanding KV in a motor

It all changes with gearing too. A hpi savage flux runs a 2100 kv lots of torque, but put through the gears and on 6s or 8s you have a 70mph monster,
It can be confusing at times, take 2100 x 7.4v =15,540 rpm,

I converted a tmaxx to brushless. 2300kv motor, 19 t pinion 72 spur, tranny converted. On 7.2v it was drivable but not fast on 14.4v it turned wild.
so on 7.2v 16,560 rpm and on 14.4v its 33,120 rpm with my gearing and conversion its like a small savage flux. Also 33000 rpm is more equivalent to the rpm,s of the original nitro motor.
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Old 02-10-2014, 10:05 AM   #4
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Re: Understanding KV in a motor

Scale plays into it too... A 1/5 uses a way smaller Kv rating motor while my 1/10 SCT uses a higher Kv and my buggy and even higher. I suppose what the scale and vehicle translate into is weight.
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Old 02-10-2014, 12:34 PM   #5
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Re: Understanding KV in a motor

Sounds like I need to learn more about gearing than just judging the motor itself. What makes it the most confusing is that there are so many different kinds of motors available it's hard to know what to buy.
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Old 02-10-2014, 12:41 PM   #6
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Re: Understanding KV in a motor

For what it's worth, I run a sensored 9.5T motor in my buggy. I don't connect the sensor. I use a higher turn motor (or smaller Kv) than the guys at the track. I also know that many of them dial the speed back either through their ESC or their Tx to maintain control.

I think they would argue that, although they don't use the full speed available, it's there if they need/want it. I went with the view of having only as much speed as I can handle. I may be short sighted but it's working for the moment - and I still break parts with the speed I have.

I think you understand more than you give yourself credit for...
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Old 02-10-2014, 12:43 PM   #7
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Re: Understanding KV in a motor

I don't think any asked - What are you looking to buy a motor for anyway? 4x4, what scale, racing or bashing and all that good stuff.

You've got me thinking about resistance, ie. wheel size, transmission and the like, how much speed is required?
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Old 02-10-2014, 01:00 PM   #8
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Re: Understanding KV in a motor

Right now I'm looking to get a brushed motor for my SCX10 crawler.
I bought a $200 Novak Timbuk 2 system and it lasted about 20 feet across the parking lot and died. The warranty system they have is brutal and it will be weeks till I hear if they are going to stand behind their product or just take my money and say "Too bad, so sad".

In the meantime I just want to get a cheaper brushed system so that my new truck actually runs. I was thinking a medium turn motor like maybe a 17t? The novak was a 13.5t. Any suggestions, preferably a link, to just online order a nice waterproof system for my SCX10?
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Old 02-11-2014, 08:31 AM   #9
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Re: Understanding KV in a motor

Richard, you need to check with other crawlers. I'm a buggy and short course truck guy. I run 10-17.5 turn in SCT and even less in my buggies. Here's the thing though, even with gearing it doesn't make sense to me that you'd use a 13 or 17.5t motor in a crawler. It just seems too fast.

I'm just not the expert for crawlers, however, when I look skim through the RTR crawlers on Great Hobbies they all seem to have 20t - 27t motors... Even that sounds low to me but again, I'm not a crawler guy.
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Old 02-11-2014, 08:42 AM   #10
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Re: Understanding KV in a motor

Quote:
Originally Posted by underserviced View Post
Scale plays into it too... A 1/5 uses a way smaller Kv rating motor while my 1/10 SCT uses a higher Kv and my buggy and even higher. I suppose what the scale and vehicle translate into is weight.
More than that it is back to voltage , most large scale stuff is using 6s and up for power and most 1/10sct are using 2s for power so a much higher kv on the low voltage system and lower kv with the high voltage system.
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