KV58 to control BLDC with hall sensors

[a8Chcx]

Hi,

I am using KV58 to control BLDC with hall sensors and have the following issue:

1) I am trying to use the ramp control to add the power slowly and causes the motor start-up problem. Sometime, I can't start motor even with full power applied on it. When it is the low power, motor keeps bouncing back and force...

2) If I applied full power and no ramp control, it works fine...

Does anybody have experienced the similar issue and how to fix it?

Thanks,

Christie

[xiangjun_rong]

Hi, Christie,

If you apply 51% power and use bi-polar mode, there is almost no power applied on the motor, so the motor will move back and forth, this is the feature of bi-polar mode, the advantage of the mode is good motor dynamic response. If the motor moves back and forth, if the rotor happens at the boundary of hall sensor, the hall state will change back and forth. fortunately, the BLDC does not need to set up the PWM  duty cycle manually, the duty cycle is from the velocity PID.

I attach an application  note an1927, hope it can help you understand the phenomenon.

Hope it can help you

BR

XiangJun Rong

[a8Chcx]

Hi Xiangjun,

If I apply 50% of power, the motor does not move. I can understand there is no enough power to move.

But, if I apply 51% of PWM, there is enough power to make it move? If so, it should follow the commutation table, right?

What is the exact reason to cause it moving back and forth by using Bi-polar mode? Is it related to dead-time, or something else? Could you explain more on this?

Thanks,

Christie

[xiangjun_rong]

Hi, Christie,

If you use 51% duty cycle of PWM signals in bi-polar mode, because the impressed voltage on the motor is very small, I suppose the deadtime of PWM signals leads to the back/forth move.

BR

XiangJun Rong.

[a8Chcx]

HI Xiangjun,

Thanks.

Can you tell me which mode(Bi-Polar or Uni-polar) is better to drive BLDC/DC motor?

Regards,

Christie

[xiangjun_rong]

Hi, Christie,

Generally, we use bi-polar mode instead of uni-polar mode to control DC or BLDC.

BR

XiangJun rong

[xiangjun_rong]

Hi, Christie,

what is so-called "add  power slowly"? do you connect a adjustable power supply to the  DC BUS of motor, in the start-up, you control the output voltage of adjustable power supply manually so that the voltage ramps slowly? If it is the case, the example code of BLDC does not support the scenario. When you start the motor, we assume that the DC BUS voltage of Motor has reached it's rating value.

BR

XiangJun Rong

[a8Chcx]

Hi Xiangjun,

What I mean is that changes PWM duty from 50% to 90% slowly by adding 1%, then I applied these PWM on three phases based on commutation table. I am using the hall sensors as position feedback...

If I started with 70% PWM duty, it works fine. If I started with 50% PWM duty, at the beginning, the motor start moving back and forth in CW and CCW direction, after reaching 70% PWM duty, sometime, it works fine, sometime stop moving even with 90% PWM duty applied...

I really need help to figure out why? The BLDC will move back and forth with low power even with hall sensors?

Thanks,

Christie

[xiangjun_rong]

Hi, Christie,

Regarding your question, as you know in bipolar mode, the PWM signal pair is in complementary mode, in other words, the PWM2n and PWM2n+1 signals are invert for each other, this is the table for the voltage impressed on the motor branch:

for example, the PWM0/1 are connected to PhaseA, PWM2/3 are connected to PhaseB

PWM0/1 are set as 0% duty cycle, the PWM2/3 are set to 0% duty cucle, and PWM2/3  pair is inverted, in the case, PHaseB is +VBus, PhaseA is -Vbus, the motor gets maximum power.

PWM0/1 are set as 50% duty cycle, the PWM2/3 are set to 50% duty cycle, and PWM2/3  pair is inverted, in the case, PHaseB is +0V, PhaseA is -0V, the motor gets no power.

PWM0/1 are set as 100% duty cycle, the PWM2/3 are set to 100% duty cycle, and PWM2/3  pair is inverted, in the case, PHaseB is -Vbus, PhaseA is +Vbus, the motor gets maximum power.

I do not suggest you set the PWM duty cycle manually, we have the BLDC code, in the code, the velocity loop set up the PWM cycle with PID control.

Hope it can help you

BR

Xiangjun Rong

[a8Chcx]

Hi Xiangjun,

I am using the signed center mode to generate PWM duty. The adjustable range is -16383~~16383, then add 16383 to convert to 0~~32767 in order to use your function(% based) to generate 0~~100% duty. This works fine...

The only problem is if I apply small percentage PWM duty, like 51%, I got BLDC moving back and forth(in CW and CCW), hall sensor status is changing from 2-->3-->2-->3....Is it normal for BLDC if the power is not enough?

If I add more than 70% PWM duty, BLDC is moving fine and all hall status is right...

My understanding is that motor should stay in 3, right because I didn't change power? Is it possible to be related to MASK or SWAP function of KV58?

Thanks,

Christie

[xiangjun_rong]

Hi, Christie,

Regarding your question that the BLDC moving back and forth when you apply 51% duty cycle, it is normal, because the motor does not get voltage. For the explanation of the issue, assume that you control a DC motor with a H bridge, when you use bi-polar mode, in the mode, the PWM0/1 and PWM2/3 pair signal are inverter each other, it is nothing to do with center-alignment or edge-alignment, assume that you use 50% duty cycle, for HALF PWM cycle, the motor will get positive bus voltage, the motor will spin in clockwise, in half PWM cycle,the motor will get negative BUS voltage, the motor will spin in anti-clockwise. For the one PWM cycle, the motor will get zero voltage, so the motor will not spin.

Hope it can help you

BR

XiangJun Rong

[a8Chcx]

Hi Xiangjun,

If I applied 51% of power, the motor keep moving back and forth, hall sensor status is changing back and forth as well, not stay in one status. It is normal when there is no enough power? Should stay in one status?

Thanks,

Christie