You may have noticed the acronym “KV” with a value next to on the labels affixed to many of your brushless motors and wondered what is this “KV” malarkey all about?
Kv (big K, little V not to be confused with kV which is kilovolts) specifically relates to “motor speed” or its correcty scientific term “motor velocity constant”, put simply its a way to measure the speed of a motor in relation to the voltage you supply it with.
The K represents revolutions per minute (rpm) and the V is “volts”, so its telling us how many times per minute the motor will spin when you apply 1 volt with no load (i.e. the motor is not driving or moving anything).
Its all theoretical because in the real world there will be factors such as load of simply turning the motor however it does give a reasonable representation of how the motor will behave and it allows you select the right motor for the right job and choose an appropriate matching propeller or gear.
What gives a motor its Kv value?
A motor is basically magnets and coils aligned in such a way that when power (current) is applied to the coils they are repelled away from the magnets causing the motor to turn. The Kv value of a motor is defined by the number of turns that are made on the coils, the fewer turns the faster the motor will spin.
How does Kv affect me?
Motors of differing Kv values have very different properties and are suited to different applications; generally speaking a high Kv motor is used to turn a small load very quickly and a low Kv motor would be best suited to turn a larger load more slowly.
Also the Low Kv/big prop combination can produce more torque so therefore is good for moving heavy craft and the high Kv/small prop combination is the opposite: its good for high speed as its more suited to overcoming the air-speed differences once the craft is moving.
Because of these different properties you find that low Kv motors are best suited to heavy fixed wing craft and multi-rotors (as they have no gliding ability) and high Kv motors are best suited to EDF’s, pusher models (flying wings etc.) that have smaller props and helicopters (which are generally geared down but require high head-speeds for stability).
Your normal flight battery voltage can play a big part when it comes to choosing a motor with the right Kv rating; for example a 2s setup running a 2000Kv motor will be roughly equivalent to running a 1000kv motor on a 4s setup.