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?

    1. Hey Ron. BLDC motors have 3 phases but must not be confused with 3-phase AC induction motors. BLDC motors are controlled with trapezoidal signals modulated via PWM. The three phases are used to create an EMF at a fixed orientation. This EMF will “pull” the permanent magnets of the rotor, which will try to align with said EMF. However, in order to keep the motor spinning and not stall it into alignment, the three phases commute their voltage in order to change the EMF into a different angle.
      This means that it is crucial to know the speed and position of a BLDC in order to control it. It is also important to note that a higher voltage between phases will generate a stronger EMF, thus generating more torque and accelerating the response of the motor (it will spin faster).
      So, this makes the system a feedback loop. You increase the voltage -> you need to increase the frequency of commutation.
      I hope this helped a little bit. For a good reference video, you can watch https://www.youtube.com/watch?v=gNpoTPzEkco

      Regards,
      Jaime

    2. the frequency is only in ac power and the drones are powered with dc that means there is no frequency ist a direct current

    3. Ron,
      Not only that the frequency has an influence. It is the major speed factor. If you freeze the three phase-voltages (yes, they are digitally simulated/approximated sinusoidal waves) BLDC motors stop. No change in voltage levels will move it if the three (phase) voltages remain static. I guess that people miss to notice that these controllers generate three phase outputs (whatever the signals on them are like, they are in 120 degrees correlation with each other. If you switch any two of these three wires (phases), they (BLDC motors) simply spin the other way. So…there is no way you can increase the supply DC voltage and get a higher speed. Other words, you cannot get more RPM if you just replace a 3S battery with a 4S one. And the last thing is to understand the signal that an RC receiver generates to control the BLDC controller/driver. All this can be understood best if you simply use an oscilloscope.

  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.

    1. To figure this out you would need to calculate a ton of stuff including drag, weight, and the pitch of the prop put onto as well as the c rating of the battery used, it’s an extremely long process to figure that out, but essential, low Kv and big props for heavy rigs, like a camera drone, and high Kv, small props for fast, lightweight builds like racing drones, and typically higher Kv is used with lower voltage batteries, so a 2400 Kv motor for a 4s racing rig and 1700 Kv for a 6s racing rig.

  5. The motor I am using and prop per manufacture recommendation is only pulling 1/3 the amperage the manufacture states is the max the motor can handle.
    Does this mean that I can increase the dia/pitch of the prop to obtain more thrust as long as I do not exceed the max amps over a given time, in this case 60-sec? Sailplane usage, lunch only.

    Or is there a relation of thrust/amps that I need to measure. Meaning if I increase the prop dia/pitch I may draw more amps but I may not gain any additional thrust? I may actuality acquire less thrust?
    Any feed back would be greatly appreciated.
    Thanks
    Ray D

  6. If its not a 3 phase motor why does it have 3 wires and how does voltage determine the rpm?

    1. These motors are controlled by pulse width modulation, hence the three wires. Half throttle means that the motor is getting full voltage half the time. So for instance let’s say in three seconds, the flight controller sends 300 pulses to the motor. Since we’re at half throttle, 150 of those pulses will be full voltage, and 150 of those pulses will be 0 voltage. The pulses happen so fast that the motor takes an average voltage. So full voltage ~ 11 plus no voltage 0 divided by two equals 5.5 volts. So an 1800kv motor at 5.5 volts will spin at 1800*5.5 which equals 9900 rpm no load. Because as stated, the kv is rpm per volt.

      1. So continue where Noah left off 9900rpm with a prop pitch of 4.0 = 39,600 inches of travel or 37.5 miles per hour of speed.

      2. So what amp ESC what I use for an 1806 Kv?

  7. Three Phase Motor generally refers to an AC “squirrel cage” induction motor, which does not use permanent magnets. The nominal speed of the motor is determined by the frequency of the AC supply and how many pairs of coils or “poles” the motor has. There are plenty of web sites that explain it well. (For the tech buffs out there – Yes, voltage does play a part but is not the primary factor in determining AC motor speed.)

    DCBM which has three wires will have three pairs of coils or three windings whose magnetic fields interact directly with the magnets (as opposed to the AC motor where a voltage is induced into the rotor which then interacts with the AC supply fields). There are also DCBM’s that have two windings, four windings and so on, so any number of wires could be present depending on the motor and its application.

    Three wires are also present on a number of single phase AC motors. So, the presence of three wires does not necessarily mean three phase. You can argue that the DCBM controller for a three winding motor provides a three phase output, but its definitely not the same as your entity supplied 3 phase AC.

    Regarding voltage:
    In a given DCBM, the physical properties cannot be changed during operation. You can’t just add or subtract magnets or windings very easily to adjust speed, so it is done by adjusting voltage (Faraday’s Law). The resistance of the windings is fixed (although it will change a little with temperature), so to get more current flowing through the windings, the voltage must be increased. If you are not familiar with Ohm’s law, check it out and hopefully it makes sense to you. So, more voltage = more current = more magnetic flux from coil = more speed for a fixed number of turns and fixed magnetic properties.

    Hope it helps, as I said there are plenty of good explanations available online if you wish to know more.

  8. These so-called three phase direct current brushless motors are really just three pole stepper motors and the esc’s are stepper drives while the flight controller/radio transmitter combination is a pulse generator/wireless pendant.

    httpsCOLON//wwwDOTosmtecDOTcom/stepper_motor_speed_and_torque_relationship.htm

    “In summary, the current and the number of coil turns in the windings determine a motor’s maximum torque output, while the voltage applied to the motor and the inductance of its windings will affect the speed at which a given amount of torque can be generated.”

  9. If I have 2 drone motors of the same stator diameter and height and one has 20 coils and the other 12 coils but the same thickness of copper windings, which would have the higher torque and KV or is it possible to know just from this information?

  10. Sorry I meant 24 coils and 12 coils.

  11. Replacing 1404 2750KV motors with 1404 3750KV on a 4″long range. Will there be a significant flight characteristic??

  12. KV is a limit not a power. It is the MAXIMUM SPEED you can get at a particular supply voltage. But that is with no load. If you put load on the motor it will slow down. So that is not what we want. To get some power out of the shaft at that speed you will have to increase the Volts to increase the Amps to produce more Watts. Forget ‘torque’, this has two meanings and seems to be used to misunderstood sometimes. It is power that makes your machine go faster or with a bigger load, providing of course that the drive train matches the available power. That is the tricky bit. You can’t put a bigger propeller or one with more pitch unless you have the power. Unfortunately the motor manufacturers dont’ always make that clear. It seems to rely on try it or ‘that works’. These are not brushed motors which go faster offload with more Volts. The electronic control fixes the speed, the kV just stops the motor from going too fast. Adding Volts only maintains the desired set speed when a load comes on. It is a 3 phase motor just like industrial motors but is low voltage, powered by a created 3 phase supply from a battery not the mains.

  13. I apologize for “reading too much into it” as some of the commenters say here. However, BLDC motor is as much DC as it is a laptop that works on a battery (battery is DC power source driving it…everyone knows that without “reading too much into the theory”). BLDC motor is driven with three-phase alternating currents, which frequency is proportional to the rpm. The only other thing that determines the rpm is the number of motor poles (specific BLDC design). It seems that everyone here ignored the BLDC motor controller that generates these three-phase voltages… the same as ignoring a motherboard in a computer that is powered by a battery, but does all these other “miraculous” computer things including a youtube video and/or playing music. Another analogy to explain our ignorance is a hydro-power station that feeds a city with electricity. Should we say that all the electrical things in a city are working on water?
    One last thing is to warn everyone not to understand the mnemonic Kv as a kilo-volt. It actually means Konstant Velocity. It gives you an idea how fast a BLDC motor is design to run.

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