# How to Find Time Constant of a BLDC Motor?

I've done this with DC motors by determining the inductance and then computing tao = L/R where L is the inductance of the coil and R is the resistance of the coil.

Is there a best-practice way to determining the electrical time constant of a BLDC motor? How should I go about measuring the inductance?

I'm interested in this because for DC motors I have been following the rule-of-thumb that your PWM frequency should be greater than or equal to one over the time constant of the motor to mitigate the torque ripple, etc.

• Are we talking about a BLDC motor with a controller included here? – Phil Frost Jan 8 '13 at 1:34
• no, just the bldc motor by itself. how might i give it a step input and quantify the rise time on an o-scope? – tarabyte Jan 8 '13 at 16:07
• is this not computed as tao = L_twoInductors / R_twoInductors? – tarabyte Jan 8 '13 at 19:40

For a more empirical approach to determining a suitable PWM frequency, pick one arbitrarily. Construct some means to measure the current flowing through the motor's windings. Measuring the voltage drop over a MOSFET in the drive circuitry is a reasonable approximation given $R_{ds\_on}$ from the datasheet. If you see the current changing significantly in a single switching period, then your PWM frequency is too low.
• @sandundhammika What? How can they be completely equivalent? They don't even look similar. To point out one pretty huge difference, if I apply a voltage across $N_1$ and $N_2$, current can flow in all three legs in a delta configuration. In a wye configuration, no current can flow in $R_3$. – Phil Frost Jan 9 '13 at 3:56