BLDC motor

Think of a 3-phase BLDC motor like above in the picture. L1, L2 and L3 are the armature windings and there is a rotor in the center. As you know when sequentially magnetize the armature windings with a certain phase difference, it causes to eddy currents in the rotor and it causes to move.

Armature windings are basically inductances when you think them standalone. But when you magnetize them they act like transformers since they induce current in rotor. So here is my question:

enter image description here

When you increase or decrease the motor load, how would it effect the armature windings inductance? For instance, would they increase in higher motor loads?

Or, the inductance remains the same but the current flow increases only?

  • 1
    \$\begingroup\$ Just think it through. When initially turning on an inductor (AKA "winding"), no current flows. But after some time, current flows as if it were a short circuit. Some where between those two moments in time, the motor will move and switch to a new inductor (or so we hope). If there is a load on the motor, the motor will move slower. A slower moving motor will leave an inductor on for longer. It is expected this will allow the inductor to conduct more and for the overall average current to increase. \$\endgroup\$
    – st2000
    Dec 24, 2016 at 17:58
  • \$\begingroup\$ Ok I got it but what about the inductance? Is it fixed all the time? But if it is fixed, how its impedance stays fixed (Or how does the current increase)? \$\endgroup\$
    – Alper91
    Dec 24, 2016 at 18:31
  • 1
    \$\begingroup\$ Oh, someone may say differently. But I think you can assume the inductance is fixed for this thought exercise. The current increases because the inductor (AKA "winding") acts like an electro magnet. When you "try" to change the current from nothing to something, a magnetic flux is generated. Which opposes the current. So the current can only slowly increase. Read more about inductors here. \$\endgroup\$
    – st2000
    Dec 24, 2016 at 18:53
  • 1
    \$\begingroup\$ See if this MIT web page answers your questions more clearly. \$\endgroup\$
    – st2000
    Dec 24, 2016 at 19:05
  • \$\begingroup\$ "L1, L2 and L3 are the armature windings" - No, they are stator windings. "it causes to eddy currents in the rotor and it causes to move" - AFAIK only induction motors use this effect. What type of motor are you actually describing? You say BLDC but it appears to be a variable reluctance or induction motor. \$\endgroup\$ Dec 26, 2016 at 16:59

1 Answer 1


This increase in current when you load a motor more is not related to inductance. It is mostly because of the mechanism of conversion of electric energy to mechanical energy, which follows Lenz's laws (among others). There is no change in inductance happening.

When you increase the load on a motor, you are in effect putting more load torque on the motor, which opposes the torque produced by the motor. This motor-produced torque is directly proportional to the current flowing in it. You applying the load reduces the speed and draws more current. It is like you lifting a 5 kg load up a hill and then someone puts another 5 kg weight on you. You would need to supply more of your muscular energy to maintain the same speed, just like the motor draws more current.

The inductance present in the coils is due to the leakage inductance, which itself is present due to the imperfect (rather incomplete) coupling of the magnetic fields of the rotor magnets and the stator coils. This remains constant and is largely fixed by motor construction.


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.