Brushless Motors: Kv vs. Turns

When you want to buy a new brushless motor for your RC truck, it’s only natural and pretty prudent to want to compare offerings from different brands. The only problem is the manufacturers don’t make it easy. Some market their motors by Kv rating and others use the term turns.

Kv
Kv is a measurement of performance. Think of how full-size manufacturers try to impress us with big horsepower numbers. While Kv isn’t horsepower, it does allow us to compare a performance attribute of motors. Specifically, Kv is how fast (rpm) a motor spins when one volt is applied to it. So, in theory, the higher the Kv rating, the faster the motor. For example, a 9000Kv motor would be considerably faster than a 2200Kv motor.

Turns
If Kv made us think of horsepower, turns reminds us of displacement. That is to say, like displacement, turns is a physical attribute of a motor. The number of turns indicates how much wire is wrapped inside the motor. A higher turn number means more wire which means more resistance which means a slower motor. So, turns is just the opposite of Kv in the sense that with turns a lower number means faster. For example, 4.5-turn motor would be considerably faster than a 15.5-turn motor.

What it all means
The bottom line is that Kv and turns are two very different things. One, Kv, is a measurement of performance output. The other, turns, is a physical attribute. Again, it’s like comparing horsepower and displacement. Generally, fewer turns means more Kv (speed). It’s just like more displacement generally means more horsepower. You probably know that all engines of the same displacement don’t put out the same horsepower. It’s no different in RC; just because two motors have the same number of turns doesn’t mean they have the same Kv ratings.

 

Notes:
It’s unlikely you’ll find an RC company using horsepower to measure performance of its motors. This is because most RC brushless motors produce less than a single horsepower. A better measure to use is watts. You can even convert watts to horsepower.

Brushed and brushless motors are both measured in turns, but the numbers are not directly comparable. That is to say a 10-turn brushed motor does not deliver similar performance as a 10.5-turn brushless motor. For years, a 27-turn brushed motor was the standard of “stock” racing. The new standard is 17.5-turn brushless.

 

You can find more information on sensorless vs. sensored brushless motors here.

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Filed Under: FeaturedGeneral RCTech

Matt Higgins About the Author: Matt has over 25 years of experience in RC and has worked professionally in media for over a decade. Matt enjoys everything from racing to rock crawling to bashing, and he believes RC should be all about having fun. Matt is as at home covering a world championship in an exotic country as he is showing a new hobbyist how to set gear mesh. His desire to share the hobby with as many people as possible inspired him to create RC Truck Stop and RCTruckStop.com.

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  1. Charles says:

    Matt. Should i go brushless or stay brushed. Im not into speed like a lot of people just do some back yard bashing and some custom work. but everyone keeps telling me i need to go brushless.

    • Matt Higgins Matt Higgins says:

      I prefer brushless and I generally prefer sensored systems over sensorless. That said, if your brushed motors are delivering enough power for your tastes, run them into the ground. Compared to brushed motors that wear out, brushless motors pretty much require no real maintenance. I’d say brushless is better than brushed, but that better comes at a price.

      • Vaqas Afzal says:

        matt, if the watts given on motor has no relation to the thrust produced by it as u described thn ow can i find the right motor for my VTOL model….or then how can i calculate a motor that need to vertically lift a 1kg weight…..please help me out i am really struck here…..and i need to proof it with formulas….can u help me???

        reply

        • Matt Higgins Matt Higgins says:

          Can’t help you too much with a VTOL model. I’m kinda a RC truck guy. Watts is power and power makes thrust…but a few other variables also impact thrust. If you are not scratch building, I would suggest contacting the manufacturer.

  2. Austin Leonard says:

    Hello! Great post! One thing to add though. Yes lower Kv’s is not as fast, but it has more torque. More torque is better in situations like a crawler motor. Thanks! :-)

    • Matt Higgins Matt Higgins says:

      That is generally true, Austin, and a point worth mentioning. Thanks for commenting. A lower Kv motor will provide more torque than a higher Kv motor at the same current. Motor design, however, is a better way to get power and torque. For example, for a crawler who is seeking lots of power and torque, I’d suggest going to a 4-pole motor and not getting too low with the Kv rating.

  3. james says:

    ok i was wondering is a 2.5t motor with a 2s = to a 8.5t with a 3s

    • Matt Higgins Matt Higgins says:

      Depends more on the actual Kv rating of the motors. That said, say the end rpm was the same, the setups are not the same. The higher turn motor on 3S is far better as it will run cooler and last longer.

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