AC VFD for Brushless DC motor control


Thread Starter

malcolm burke

Can you use a VFD to control a brushless DC motor? Is the sinusoidal wave form from a VFD compatable with the DC brushless motor? I recall that some servos are designed for trapezoidal.

Any help would be great.

Malcolm Burke
Only if the drive mfg explicitly states that a certain drive model can be set-up to run different types of motors. While both induction motors and permanent magnet DC brushless motors run off a 3-phase PWM power stage, that is basically where the similarity ends. The induction motor is based on the V/HZ or vector principle and feedback is typically none (open-loop), resolver or encoder. PMDC brushless works on electronic commutation principle - typically using Hall sensors, encoder or both (hybrid). The control schemes are completely different. Furthermore, commutation can be trapezoidal or sinusoidal depending on motor type. But, because of the common PWM power requirements, some mfg's have incorporated both control schemes into the same drive - thanks to the benefits of digital drive technology.

Trapezoidal is not the issue. For example, a brushless motor with Trap back EMF can only run smooth with Trap Hall commutation. A brushless motor with sinusoidal back EMF can only run smooth with sinusoidal commutation (Hall or encoder). A sine shape motor run on Trap commutation will have about 14% ripple.
How large of a brushless DC motor do you want to control? I've controlled as high as 24VDC brushless motors with a simple PWM (pulse width modulation) circuit.

Hakan Ozevin

Why do u need a VFD? A brushless DC motor can be controlled by a simple variable DC source, instead of a variable frequency/voltage source as in VFD.

If you really want to do it, place a rectifier with filter (LC circuit) at the output of the VSD.
The dc motor comes with its own dc control ckt.
You have to remove the electronics and run the motor from a vfd to apply 3 ph power.
All DC brushless motors are DC motors and the core will get very hot if you run from a vfd,due to the high frequency components present in the VFD output. The VFD does not produce a clean sine wave output, but composed of 2-5 kHz frequency bursts.
If yo connect the VFD through the electronics, the vfd will not allow DC current to flow through and will go to fault.
A brushless DC motor can NOT be controlled by a simple variable DC source as you say, like "brush" motors. Brushless motors require not-so-simple electronic feedback/commutation control which makes brush type armature control impossible! Also, brushless motors are typically 3-phase. Your LC circuit for VFD also makes no sense.

You can learn more about brushless motors from textbooks and industry/manufacturers literature.