Hi Mike:
While not the last word on KV here's a short article I wrote. I have tried to keep it as simple as possible so that I do not confuse people with a whole lot of technical stuff. I hope this helps answer some of your questions concerning kv.
Regards
Eric Rule
WHAT ABOUT KV?
Many customers ask me “what is the importance of the kv number on a motor?”
The simplest answer is that the kv number means the maximum rpm per volt of battery power that the motor can develop. For example you have a motor with a kv of 1000 and you are using a 3 cell battery. 1000 (kv) X 11.1 (volts) = 11,100 rpm. Simple, Right? Well not so fast….. 11,100 represents the maximum rpm with no prop. Since we are going to have to use a prop on the motor we should only assume 80% as what we could refer to as “effective maximum rpm”. That means we really only have about 8,880 rpm available with that motor. For most of us a maximum rpm of 8,880 is going to be too low. So we are left with the choice of using a motor with a higher kv rating or using a battery with more cells.
If we use an example of a glow powered model that we would normally put an OS.40 FP on as our basis for calculation we would expect to use a 10-6 prop and 9000 rpm and the model would probably weigh in the 42 oz range. OK, what do we need in an electric motor?
Just like a glow engine an electric motor needs to have some load on it in order to operate efficiently. If you put an 8: prop on your FP.40 the model would fly but would not perform well enough for you to do the pattern. Same thing would happen if you put an 11 or 12” prop on the FP.40. You need a prop that allows the motor to develop horsepower in its “happy range”. The electric motor is exactly the same as the glow engine in that you have to match the motor with a prop that develops the power you need without under or overloading it. A good “rule of thumb” is to use a prop that is 1” larger in diameter on an electric motor than what you would use in a glow engine. One point to remember is that we use governors on electric stunt so therefore the rpm is more or less constant and does not get any faster after take-off like with a glow engine that can easily gain 1000 to 1500 rpm in the air.
That weight of model will require an electric motor capable of developing 400 watts of power on a 3 cell battery running an 11X5.5 prop (electric motors develop more torque than a glow engine so we use a larger diameter prop) at 8,800-9,000 rpm. Here’s the calculation to determine the kv you will need. Rpm of 9,000 X 1.2 (remember the 20% reduction due to needing to use a prop) =10,800 /11.1 volts = 972 kv minimum. Since you will want to have a bit of “head room” at the top of the rpm range I would suggest choosing a motor that has 1050 kv, or as close to that as is available.OK, so why don’t we just pick up a motor with a high kv like 1400 or 1800? Seems that more kv would be better, right? Well….. not really.
In an electric motor the higher the kv rate the lower the torque. Lower torque means that you will need to use more power to turn the large prop at the desired rpm. In electric control line power usually equates to heat. More power will create more heat in the motor, Esc and battery. This will degrade the efficiency of the motor. That will lead to more energy being pulled out of the battery which will inevitably lead to using a higher Amp ESC and a larger capacity battery. It is easy to see that this process is similar to a glow engine where the venture hole is too large. We don’t get any more power. We just use more fuel.
What we need to do is to select the motor with sufficient kv rating to provide the rpm range we require for efficient power generation. Like Goldilocks porridge kv it is either too small to large or just right.
Topic: Electric motor prop charts (Read 2260 times)