Darren's Blog

Kv, Turns, and Voltage -- definitions and relationships

What do all those numbers on a brushless motor mean? Which one should I buy? Which battery, gearbox and prop should I use with it?

These are some common and confusing questions given the all the options available today. I'll briefly go over some definitions and strategies to pick "the best" match for any given application. First, some definitions:

Kv: Revolutions per Volt. This is how fast, in RPM, a motor will spin without a load for every input volt. For instance, a 5300 Kv motor will spin 53,000 RPM with 10V input (assuming no losses, which there always are). The higher the Kv number, the "hotter" the motor will be -- meaning it will draw more current and put out more power per input volt. On Himax and other motors, the last number of a non-geared motor is approximately its Kv -- the higher the number, the hotter the motor.

Turns: Turns of wire on the Stator. Mega, Aveox, and others rate their motors in turns of wire rather than Kv. But turns are related inversely to Kv, so the fewer the turns, the higher the Kv. A Mega 4T, for example, has a Kv of 2200 while a Mega 5T is 1800. High Kv motors have fewer turns of bigger wire: lower resistance able to handle more current. Low Kv motors spin slower so they can handle higher Volts before spinning apart.

So let's start with a simple no-load relationship. Lets say we want to spin a motor at 30,000 RPM with a 3s Li-poly pack. A 3s Li-poly pack runs approximately 10V under load. So I need a motor with a Kv of 3000 (30,000RPM/10V). But say I want to switch to 2s packs that are closer to 6.5V? A higher Kv motor will spin exactly the same speed, so I pick a 4600Kv motor (30,000RPM/6.5V).

Effects of Load:

Unfortunately putting a load (prop) on the motor complicates matters a little. A 3000Kv motor won't spin 30,000 RPM at 10V with a prop on it. The actual RPM will vary with load. The bigger the prop, the more load and the slower the motor will turn -- and the more amps it will draw. Because actual prop RPM is related both to Voltage by Kv and to Amps by Load, Power capability (rather than simply Amps or Volts) is the true measure of a motor.

A more practical example:

Many of us are familiar with the Mega 16/15/x line of motors used in Unicorns. Many fly a Mega 16/15/4 (2200Kv) with a 6x5.5. On 3S lipos (10V) this set up draws about 26A and turns 17,500 RPM. Watts in (10 Volts from the battery times 26A pulled by the motor) is 260W.

However I flew the Mega 16/15/5 with 4s lipos with almost identical results. How did this work? Since the 5T has a lower Kv, it will spin slower for every volt but draw fewer amps. The set up draws 21A at 12.5V for 262W and 17,600 RPM -- Note power input and RPM match very closely.

Which is better? At first glance, a lower current means smaller ESC's and less loss in the wires. However, higher Voltage means more loss in the BEC. As far as the Motors are concerned, there is no difference. The lower input current in the 5T is offset by a higher resistance in the longer, thinner wire.

What's the bottom line? How do I use all these related variables to my advantage? Since many of us are switching to Li-poly packs with larger voltage steps (2s and 3s are the only practical options without a higher-tech charger or harnessing), selecting motors of differing Kv can offer the same results that "adding or subtracting a cell" used to do.

It is now possible, and preferable for charge safety reasons, to standardize on a single battery voltage (I'd suggest 3s for reasons I'll discuss later) then select different motors and/or gearing for the desired power results.

Gearing can fill in gaps, but is a topic all it's own.

Happy Flying!
Darren

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