Understanding Motor Specifications

Haha~

Do you know different motor specifications's mean?

Today, i will tell you the motor specifications's mean.

Most electric airplanes either come with a motor or the manual will tell you exactly what size motor it needs. If you're converting from a brushed motor or glow engine to a brushless RC motor, understanding what these designations mean will be important.

1.)Voltage Constant
Kv is the "Voltage Constant". This is how many RPM's the motor turns for each volt applied. Actually, it's a tad less because even brushless motors aren't 100% efficient.
For example, if you apply 12volts to a 200Kv motor, it will turn at just under (12 x 200) 2400 RPM's.

2.)Torque Constant
Kt is the "Torque Constant". For all motors, Kt= 1355/Kv. You'll have to ask someone smarter than me where the 1355 comes from, cause I don't have a clue! Just accept it for what it is.
In a nut shell, this means the faster the motor spins for a given voltage (Kv), the lower the output torque will be (Kv).

Outrunners generally have a lower Kv, which in turn produces more torque at a slower speed for spinning those larger props. The opposite is true with inrunners.

3.)No Load Current
Io is the "No Load Current". This is the amount of current it takes to spin the motor with no prop.

For example, if your motor is pulling 25Amps, and the Io is 2Amps, then you really only have 23Amps turning the prop.

4.)Terminal Resistance
Rm is the "Terminal Resistance". This is the internal resistance of the motor measured in Ohms. The higher the Rm, the less efficient the motor is.

5.)Current and Power
The maximum current and power is what determines how large of a prop and what size plane can be used with the motor. Simply multiply the current by the battery voltage to get power.

The larger the propeller's diameter and pitch, the more current the motor will draw for a given RPM. For example, the 200Kv motor will run at 2400 RPM's regardless of whether it has a 10x6 prop or a 11x5 prop. But, the 11x5 prop will cause the motor to draw more current. Drawing too much current will destroy the motor.

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