How to improve commutation with BLDC motor with Hall effect sensors

Thread Starter

auzzy_t

Joined Jan 30, 2024
1
I'm working on a power tool project and am observing a rough feeling motor (noise and vibration) and excess heat. Any methods to determine if the motor is commutating ideally?

Thanks!
 

MisterBill2

Joined Jan 23, 2018
18,928
LQC is probably correct, because noise and overheating are symptoms of motor overload. BUT with a BLDC motor it may also be that it is not being commutated correctly. That adjustment will require following a procedure that I am not familiar with.
 

MaxHeadRoom

Joined Jul 18, 2013
28,759
BUT with a BLDC motor it may also be that it is not being commutated correctly. That adjustment will require following a procedure that I am not familiar with.
Generally for a BLDC motor, the three sensors, Hall effect , etc, are fixed at the time of manufacture.
One other method with BLDC motors fitted with a position encoder, there is a a track for commutation purposes and these are easily aligned using a double beam 'scope, and back feeding, rotating the motor externally in order to generate.
Types with the three fixed sensors can also be confirmed.
Some motors, such as now used in HVAC etc, have no sensors, per-se, but exercise the motor at power up and locate the position of the rotor magnets, register this position, and keep track after that.
 
With a BLDC motor the Hall sensor transitions should immediately cause changes in power bridge inverter state (commutation).

Are you controlling power applied to motor through PWM duty cycle control of power bridge?

Do you have sufficient dead time around power bridge transitions?

Do you have an outer velocity control loop and/or an inner torque (current) control loop?

Are velocity and/or torque being sensed correctly?

We know nothing about how you're controlling your motor, so it isn't possible to answer your question other than to guess. A mistuned PI or PID control loop can produce the misbehavior you describe. If you're using an off the shelf controller there is likely a fair amount of configuration state to double check and sanity check.
 
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MisterBill2

Joined Jan 23, 2018
18,928
I realize now that we did not get any hint as to the type of motor from the thread starter, except for the title of the thread. And also no responses to comments or questions.
 

MaxHeadRoom

Joined Jul 18, 2013
28,759
The other thing to consider and have to accept, is that a BLDC motor runs rough at lower to med RPM's, due to the very coarse commutation, the exception is when some kind of feed back, PID etc, is used.

P.S. Another OP that ask the question and then is not seen again, apparently took off immediately after posting??
 
Right! We don't know if this is a constant or variable velocity or torque application, if he is using trapezoidal commutation with PWM or sinusoidal commutation with SVPWM, or what velocity and/or torque feedback mechanism(s) and related control loop(s) are used, if any. A number of things could produce the behavior described, we can only guess.

I've seen BLDC motors with hall sensors work very smoothly at low RPM with no load. Things get dicey when you try to apply a lot of torque at low RPM, particularly when the load situation is dynamic. It's mpossible to say what's going on here, we don't even have a number for "low" RPM in this application. Dead horse...
 

MaxHeadRoom

Joined Jul 18, 2013
28,759
I've seen BLDC motors with hall sensors work very smoothly at low RPM with no load. Things get dicey when you try to apply a lot of torque at low RPM, particularly when the load situation is dynamic
I have used them in CNC applications with PID controllers down to zero rpm with no problem, ;)
 

MisterBill2

Joined Jan 23, 2018
18,928
Certainly there must be a wide variety of BLDC motors, including both high speed motors and multi-pole motors smooth enough for audio tape players. It may even be that the TS wants to replace the rough running motor with a BLDC motor. ONE SIZE does not fit all.
And I am unwilling to make blind guesses.
 
I have used them in CNC applications with PID controllers down to zero rpm with no problem, ;)
Yes, with a high resolution position encoder (vs. Hall sensors) I would guess! I've never worked with CNC controllers, but my understanding is they have a position feedback loop controlling inner torque and/or velocity feedback loops. These controllers can really make a motor dance.
 

MaxHeadRoom

Joined Jul 18, 2013
28,759
Yes, with a high resolution position encoder (vs. Hall sensors) I would guess! I've never worked with CNC controllers, but my understanding is they have a position feedback loop controlling inner torque and/or velocity feedback loops. These controllers can really make a motor dance.
The encoder has hall effect equivalent tracks on it as well as the hi-res encoder, they operate separately.
For DIY projects I use one of the Galil motion cards that fit in a PC slot, using PID control and they make any BLDC motor as smooth as you would ever wish for ! ;)
 

MaxHeadRoom

Joined Jul 18, 2013
28,759
Certainly there must be a wide variety of BLDC motors, including both high speed motors and multi-pole motors smooth enough for audio tape players. It may even be that the TS wants to replace the rough running motor with a BLDC motor. ONE SIZE does not fit all.
And I am unwilling to make blind guesses.
BLDC motors generally go to 8pole-10pole, still very coarse operation at low RPM, without suitable feedback etc.
 
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