Brushless Motor Kv and Torque Constant Calculator (BLDC, Drone Motors)
The torque constant, no-load speed, speed and torque under load, shaft power and efficiency of a brushless motor from its Kv rating, voltage, current, no-load current and winding resistance.
Results
The dashed line marks the value you entered.
What this tool does
A motor's Kv is its no-load speed in rpm for each volt, and it is the same quantity as the back-EMF constant seen the other way round: the torque it makes per amp is K_t = 60 ÷ (2π K_v), about 10.4 mN·m/A for 920 Kv. A high Kv means a fast, low-torque motor for a small propeller; a low Kv, a slow, high-torque one for a big one. Under load the winding resistance and the no-load current take their share: speed falls to K_v (V − I R) and the torque is K_t (I − I₀), so the efficiency of a real motor is a function of its current. The model treats the motor as an ideal DC motor; heating and iron losses are ignored.
Formula
K_t = 60 ÷ (2π K_v) [N·m/A] · no-load rpm = K_v × V · rpm = K_v (V − I R) · T = K_t (I − I₀) · P = T × 2π × rpm ÷ 60
Variables
| Symbol | Meaning | Unit |
|---|---|---|
kv | Motor Kv rating | rpm/V |
v | Supply voltage | V |
i | Current drawn | A |
i0 | No-load current | A |
r | Winding resistance | Ω |
KT | Torque constant Kt | mN·m/A |
NL | No-load speed | rpm |
NS | Speed under that load | rpm |
TQ | Shaft torque | N·m |
PO | Mechanical power at the shaft | W |
EF | Efficiency | % |
Worked example
- Motor Kv rating920 rpm/V
- Supply voltage14.8 V
- Current drawn20 A
- No-load current1 A
- Winding resistance0.05 Ω
- Torque constant Kt10.38 mN·m/A
- No-load speed13,616 rpm
- Speed under that load12,696 rpm
- Shaft torque0.197 N·m
- Mechanical power at the shaft262 W
- Efficiency88.6 %
Limitations
- The formula assumes ideal conditions: no friction losses, no air resistance and no efficiency losses unless you enter them.
- Coverage and consumption figures vary between products. Take the values from the manufacturer's datasheet for the product you are actually using.