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.

Thanks in advance for any help you can offer.

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

The motor will have either three or four leads. If there are four, the winding configuration is almost surely a "Y" or "wye" configuration and one of the leads is the center-point.

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If you have the four-lead variety, identify the center-point by measuring the resistance between the leads. The center-point will have half the resistance to each of the other leads. Once you have identified the center-point, make note of which it is, then ignore it.

Having possibly ignored the center-point, pick any two leads. It doesn't matter which two. You can then measure the inductance and resistance between those.

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.

  • \$\begingroup\$ it is a three-lead bldc. the empirical approach is useful. i need a better multimeter to measure the inductance, but the resistance (for two phases) is ~75 Ohm. \$\endgroup\$ – tarabyte Jan 8 '13 at 20:38
  • \$\begingroup\$ it does not matter whether it's delta or star, both are interchangeable. Delta network is magnetically and electrically completely equivalent to a star network. \$\endgroup\$ – Standard Sandun Jan 9 '13 at 3:49
  • \$\begingroup\$ @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\$. \$\endgroup\$ – Phil Frost Jan 9 '13 at 3:56
  • \$\begingroup\$ I mean a delta to star conversion. See wiki page. \$\endgroup\$ – Standard Sandun Jan 9 '13 at 8:09

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