Unstable current waveform from Bldc controller - Page 1
Forum section: electronics / projects

author: Kenny_047
author_rank: Newbie
timestamp: September 24, 2021, 05:23:52 pm
content: I am currently working on a BLDC motor drive project where i have connected the inverter to an evalkuation board to control a BLDC machine, The schematic of the connectionfor the evaluation board,for the inverter board, and the physical system is [ Specified attachment is not available ]. The system runs but the system stops working at speeds above 270 rpm i have yet to determine what causes it and i also noticed when i compared the Pwm siganls at the gate of the mosfets to the current waveform of the motor at speeds above 200 rpm it looks, and at speeds below 200 rpm it looksthis matches what the waveform should be according to the theory. I am open to all suggestions to fix the waveform and make the motor run at 1000 rpm and above.

author: Benta
author_rank: Super Contributor
timestamp: September 24, 2021, 10:49:18 pm
content: Schematic is unreadable. Perhaps if I buy a 200" screen with 8000 x 12800 resolution, but I'm not going to do that.

author: Kenny_047
author_rank: Newbie
timestamp: September 25, 2021, 07:45:14 am
content: it has been modified

author: Benta
author_rank: Super Contributor
timestamp: September 25, 2021, 07:47:31 pm
content: I see no Hall feedback, so this is either:A sensorless BLDC drive.A PMSM drive.Either way, both are difficult to start (none of them will start under load, you need Hall sensors for that) and have completely different dynamic characteristic. Let us know.

author: Kenny_047
author_rank: Newbie
timestamp: September 26, 2021, 08:05:26 am
content: The motor is hall sensor based and its controlled using six step commutation sequence i have commented in the image above to show the hall signals

author: Terry Bites
author_rank: Super Contributor
timestamp: September 26, 2021, 03:24:43 pm
content: A rectangular pulse waveform driving an inductive load will create a trapeziodal current waveform. The edges of the trapezoid are not straight as a result of series resistance. So its what you expect to see. You can force the motor into a known alignment by driving only one phase before start up.Is your curent feedback compensation correct, U2 aqnd U3c sections? look at their waveforms. Read AN5397 from STImages look fine on my 25 inch @1440?

author: jmelson
author_rank: Super Contributor
timestamp: September 26, 2021, 03:32:39 pm
content: Well, one possibility is the microcontroller is not fast enough, or the program is not efficient enough, to keep up with the motor at higher speeds. I'm just taking a guess. But, you do need to respond pretty quickly to changes in the commutation signals. I use a CPLD to decode the Hall signals, and the logic produces the dead time for the half-bridges, and responds in 400 ns after that.Jon

author: Benta
author_rank: Super Contributor
timestamp: September 26, 2021, 08:00:32 pm
content: Thanks for clarifying the Hall sensor issue.I have problems understanding your 'scope recordings.Do they depict one phase line during commutation plus PWM on top? What exactly are you measuring?

author: Benta
author_rank: Super Contributor
timestamp: September 26, 2021, 08:02:06 pm
content: Quote from: Terry Bites on September 26, 2021, 03:24:43 pmThe images have been changed by OP. Please read complete thread.

author: Kenny_047
author_rank: Newbie
timestamp: September 27, 2021, 07:18:31 am
content: yes it shows the commutation at one high side mosfet and the corresposing maotor phase current waveform, i am measuring the current ripple in the motor waveform

author: ace1903
author_rank: Regular Contributor
timestamp: September 27, 2021, 11:15:11 am
content: Power supply wires between boards look thin compared to size of the motor. Wondering if it shifts the ground between boards at higher loads.

author: Kenny_047
author_rank: Newbie
timestamp: October 01, 2021, 08:51:00 am
content: Hello now using the same cable to power the board and the motor, the system still does not run above 300 rpm, i think it might have something to do with the current sense network and the amplifier to fix the challenge