Open Loop BLDC motor control, is it possible?

[DaveElectronics6XT]

I have a BLDC motor to run at full speed for a brief time. The motor is equipped with three hall sensors. My question is, is it possible to realise the max speed control by setting the PWM duty cicle at 100% for controlling the MOSFETs, though the hall sensor commutation logic?
Will the motor spin correctly? What problem may arise by this type of control?

 

[Diver300]

If there is the commutation logic in the motor, it may work at 100% PWM.

 

[DaveElectronics6XT]

If there is the commutation logic in the motor, it may work at 100% PWM.

Why not a 75%?

 

[Tony Stewart]

3 pole BLDCs work at any % > 0% and always start without dithering. The logic is between the Hall sensors and the drivers.

2 pole BLDCs used in muffin fans stop at the top dead centre (TDC) midpoint cog position of permanent magnet torque. The Hall sensors must lag this TDC otherwise by a small amount to always have a positive direction for torque. They allow the minimum lag for manufacturing tolerances. This is why 2 pole fans sometimes fail to start unless given a small nudge. But it has nothing to do with 100% PWM but does illustrate the difference between 2 and 3-pole BLDC motor's torque profile.

The commutation logic works externally with the logic levels of the Hall sensors. In 2 different companies where I used high-volume BLDC muffin fans, I experienced the same fan failure in a small % of the population. I designed a 30-second pulse start-stop tester for 10 fans in parallel for incoming inspection to allow the fan to stop at all 4 stop positions. This was necessary because 99 % of these fans worked from every resting position but only 1% failed usually only in 1 of 4 Hall positions which would cause it to fail to start or just dither back and forth fast. I disqualified the vendor Nidec (large expert fan co.) who accepted my test design, took over the task and immediately corrected their sensor location margin problem on future shipments.

I hope you remember this if you ever see a 2 pole BLDC fan need a nudge to start but a 3 ph fan will never behave like this even with 100% PWM

 

[DaveElectronics6XT]

in muffin fans stop at the top dead centre (TDC) midpoint cog position of permanent magnet torque. The Hall sensors must lag this TDC otherwise by a small amount to always have a positive direction for torque. They allow the minimum lag for manufacturing tolerances. This is why 2 pole fans sometimes fail to start unless given a small n

3 pole BLDCs work at any % > 0% and always start without dithering. The logic is between the Hall sensors and the drivers.

2 pole BLDCs used in muffin fans stop at the top dead centre (TDC) midpoint cog position of permanent magnet torque. The Hall sensors must lag this TDC otherwise by a small amount to always have a positive direction for torque. They allow the minimum lag for manufacturing tolerances. This is why 2 pole fans sometimes fail to start unless given a small nudge. But it has nothing to do with 100% PWM but does illustrate the difference between 2 and 3-pole BLDC motor's torque profile.

The commutation logic works externally with the logic levels of the Hall sensors. In 2 different companies where I used high-volume BLDC muffin fans, I experienced the same fan failure in a small % of the population. I designed a 30-second pulse start-stop tester for 10 fans in parallel for incoming inspection to allow the fan to stop at all 4 stop positions. This was necessary because 99 % of these fans worked from every resting position but only 1% failed usually only in 1 of 4 Hall positions which would cause it to fail to start or just dither back and forth fast. I disqualified the vendor Nidec (large expert fan co.) who accepted my test design, took over the task and immediately corrected their sensor location margin problem on future shipments.

I hope you remember this if you ever see a 2 pole BLDC fan need a nudge to start but a 3 ph fan will never behave like this even with 100% PWM

Thanks very much, so If I have a 10 poles motor, it won't never have this problem at the starting point

 

[Diver300]

Why not a 75%?

A brushless DC motor that has it's own commutation will probably work with a wide range of PWM duty cycles on its control signal.

 

[aliarifat794]

Setting the PWM duty cycle to 100% effectively means that the MOSFETs will be fully on during their respective phases, providing continuous power to the motor phases. This is equivalent to a DC supply directly to the motor windings without modulation.

 

[MaxHeadRoom78]

BLDC and 3ph PM rotor motors have the same construction and either can be made to run in both methods.
It is not recommended or, in some case, not possible to run with 100% PWM,. due to driver design.
But the normal/common method is use a higher Voltage than the motor rated value, and simply use the highest PWM % that results in the motor rated RPM.

 

[Tony Stewart]

PWM does not apply to a 2 wire PC muffin fan with Caps and internal Hall sensor & FET commutation.