Continue to Site

Welcome to our site!

Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

  • Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

How does coil current build up quickly in a 3 phase BLDC?

Status
Not open for further replies.

Flyback

Well-Known Member
Hello,
In a three phase BLDC motor, (driven by a standard voltage source inverter) the three coils are obviously switched in one following the other. When the motor is spinning at nominal speed, and the motor is also full loaded, how does the current build up in a coil just after the instant that it is switched in?
The point is that if the motor is spinning, then there will be a back EMF in the coil at the instant that it is switched in. Thus there will actually be very little actual driving voltage across the coil inductance. Therefore, by Lenz’s law, (di/dt = V/L), the di/dt in the coil will be very low. So in this case, how does the current in the coil manage to quickly build up to the nominal current level in the coil?
 
The back EMF in the coil is such that the delta voltage (input voltage minus EMF) is sufficient to generate a current through the winding inductance as needed to power the motor and load.
If not, then the motor slows down, reducing the back EMF to the point where the current is sufficient for the load.
It's all self regulating, rather like negative feedback.
 
Thanks, so in a BLDC running at nominal speed, and fully loaded, just after the instant that a coil is switched in, the current in that coil has immediately risen up to the full current level in that coil?

That is, I am wondering what is the di/dt of the coil current when each coil is switched in? Certainly with stepper motors, there is a di/dt whereby the higher the DC driving voltage, the quicker the current builds up in the bipolar stepper motor coil.
 
This BLDC coil drive sim (extracted from a sim for a different thread) may help to explain it :
BLDC-coil-drive.gif
 

Attachments

  • BLDC-coil-drive.asc
    3.1 KB · Views: 104
thanks, though I must say that that sim contains some symbols etc not in LTspice library and it wont run,
 
Which symbols? The only non-LTS one is a CD4093 Schmitt gate (used to square-up the scon signal and allow adjustment of the mark/space ratio by editing its Vdd value). The Modulate device is in the 'Special Products' folder.
 
Status
Not open for further replies.

Latest threads

New Articles From Microcontroller Tips

Back
Top