BLDC motor speed calculation

[krunal_299]

motor speed calculation

Hi folks!

How can BLDC motor speed can be calculated from Hall sensor output?

Regards!

 

[sunderwood]

bldc motor speed calculation

measure the time between two transitions: ttr
it's the time it takes to go 60 electrical degrees (or 1/6 electrical period)
electrical frequency = (1/6) / ttr
multiply by 2xpi to get rd/s
multiply by number of pole pairs (P) to get mechanical speed
multiply electrical frequency by 60xP to get RPM

 

[krunal_299]

what determines the speed of a bldc motor

Hi again!

please correct me for folllwing example if it is wrong:
For 4 pole motor,
If i got one of 3 hall sensor output with frequency of 125 Hz, then speed will be:
2 pulse = 1 mechanical revolution.
So, for 125 Hz (125 pulse per second), speed = (125/2) revolution per second.
Which will give (62.5 * 60) = 3750 RPM

Am i right?

Regards!

 

[sunderwood]

bldc motor calculations

If you have 125Hz on one hall sensor and a 4 pole motor, your result seems correct.

 

[elmahboule]

calculation of motor speed

OK.....If you have 125Hz on one hall sensor and a 4 pole motor, your result seems correct.

 

[krunal_299]

bldc motor poles

Thanks a lot!
If the system has to be closed loop then , does above method to decide the motor present speed is accurate?
Or what would be any other more accurate approach?

Regards!

 

[sunderwood]

design calculation for bldc motor

Yes, you could have closed loop speed control with such a feedback. Just make sure your controller bandwidth isn't too much in conflict with you minimum operating speed (your speed estimation isn't "refreshed" as often at low speeds).

 

[eziggurat]

4-pole bldc

You should try looking at this application notes.

https://ww1.microchip.com/downloads/en/AppNotes/00899a.pdf

I am using the equation for my speed calculation and it seems to be accurate.