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Understanding Kv Ratings

Understanding Kv Ratings

What is the Kv rating of an electric motor?

Answer: “Kv” refers to the constant velocity of a motor (not to be confused with “kV,” the abbreviation for kilovolt). It is measured by the number of revolutions per minute (rpm) that a motor turns when 1V (one volt) is applied with no load attached to that motor. The Kv rating of a brushless motor is the ratio of the motor’s unloaded rpm to the peak voltage on the wires connected to the coils.

Knowing the Kv rating of a motor will help you determine how fast that motor will rotate when a given voltage is applied to it. For example, a 980Kv motor powered by an 11.1V battery would spin at 10,878 rpm (980 x 11.1) with no load. A change in voltage will change the rpm and will require changing the propeller to avoid overloading the motor. Kv allows you to get a handle on the torque that can be expected from a particular motor. Torque is determined by the number of winds on the armature and the strength of the magnets. A low Kv motor has more winds of thinner wire—it will carry more volts at fewer amps, produce higher torque, and swing a bigger prop. A high Kv motor has fewer winds of thicker wire that carry more amps at fewer volts and spin a smaller prop at high revolutions.

Knowing the Kv rating of a motor is helpful to determine which motor belongs in which aircraft. An FPV racing quad, for example, requires high rpm for high speed, so you would use a high Kv motor and a small-diameter prop. On the other hand, you would use a lower Kv motor in a heavy-lift multirotor because you want to turn a large prop at lower rpm and obtain high torque.

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  1. if i put a heavier camera on a drone that came with a light weight camera will the motors wear out or burn out quicker? there does not seem to be a problem with the drone lifting the heavier load.

  2. Since these brushless motors a in reality 3 phase motors, frequency has an influence on rpm at any given voltage. In determining the Kv of a motor, is there a certain frequency used to arrive at the Kv rating?

  3. Point accepted but I think your reading too into the theory.

    Lower better for bigger, Larger better for smaller.

    1. We would agree with that.

  4. It would be useful to know the relationship between the RPM at the rated maximum power and the ‘no load’ RPM indicated by the kv figure.

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