Motor Constants
The three key motor constants for predicting BLDC motor performance are: Kv (velocity constant), Io (no-load current), and Rm (winding resistance). Using these constants with simulation software, motor performance can be predicted accurately.
Kv Calculators
Kv Calculator 1 - Drill Press Method
Spin the motor at constant speed and measure AC voltage between any two motor leads.
Kv Calculator 2 - Running Motor
Run motor at full speed with no load, measure V, A, and RPM.
Motor Efficiency Calculator
Calculate motor efficiency, losses, and derived constants. Enter motor constants and operating measurements to see comprehensive performance analysis.
Motor Constants
Operating Conditions
Battery Cell Resistance (per cell)
Power
Losses
Derived Constants
Understanding Motor Constants
Kv (Velocity Constant)
Kv is the RPM per volt with no load. For brushless motors, Kv is measured in RPM/V. A higher Kv means the motor spins faster for a given voltage but produces less torque per amp.
Drill Press Method: Most accurate. Spin motor at known RPM, measure AC voltage between two leads. Formula accounts for back-EMF waveform shape.
Io (No-Load Current)
Io is the current drawn by the motor when spinning freely with no load. This represents the iron losses and friction in the motor. Lower Io means a more efficient motor.
Measure by running the motor at intended operating RPM with ESC at full throttle and no propeller attached.
Rm (Winding Resistance)
Rm is the phase-to-phase resistance of the motor windings. This determines copper losses (I²R losses). Lower resistance means less heat generation and higher efficiency at high currents.
Use the 4-wire (Kelvin) method for accurate measurement: apply known current through motor leads and measure voltage drop directly across them.