DRV8871-Q1: Some motors work some don't . Current regulation setting and inrush limit

Hi Jeremy,

Can you send me some waveforms showing input (IN1, IN2) and output signals (OUT1 and OUT2) for both batches of motors. One of each should be okay.

Are the motors of the two batches the same?

Jeremy Novoa said:

What is the maximum inrush the H-Bridge can handle and for how long?

The absolute maximum inrush current the device can handle before overcurrent protection is triggered is  4.5A (typical) for 2µs.

Jeremy Novoa said:

Should I change the limiting resistor to not use current regulation?

If the initial inrush current is not sufficient to overcome the mechanical inertia of the motor, then you should probably disable the current limit. Although, if the inrush current is higher than 3.7A for longer than 2µs, OCP will be triggered.

Jeremy Novoa said:

If the stall current is 4A max should I not be using current regulation at 3.5A ?

If the stall current is higher than 3.7A, then the current limit may not work since the device will go trigger an OCP fault disabling the outputs. Current limiting is only effective when it is below the minimum OCP threshold which is 3.7-A.

Hi Pablo,

Thanks for the information. 

I am a little confuse. The data sheet states a typical OCP of 4.5A  and a peak current drive of 3.6A. The steady state of the motor is around 500mA for only 400ms.

If my inrush is les than 4.5 amps, why is the OCP been triggered (Are you using the min OCP value?)

I will send you a plot latter today, but we are driving this in forward mode while IN1 is held HIGH and we PWM IN2, so active wen PWM is LOW. 

On the bench, and just testing the motors on its own, I do see some motors where the inrush is higher that the 4.5A. This motors will trigger the OCP at 400hz but not at 150kHz. I concluded that by switching at a higher frequency (faster), we are not letting the inductance of the motor charge enough to draw more than ~2A on inrush. Can this approach be valid?  

Thank you!

-Jeremy

Pablo, one more thing. Lets say VM is set to 12V and I PWM to 75% duty cycle to get an average voltage of 9V. What voltage does the H-Bridge sees the peak (12V) or the average (9V)?

Thanks.

-Jeremy 

Jeremy,

Jeremy Novoa said:

If my inrush is les than 4.5 amps, why is the OCP been triggered (Are you using the min OCP value?)

Yes, The min OCP value for this device is 3.7-A. Meaning OCP may be triggered at 3.7 under certain conditions. If you a current probe or another instrument to measure the current through the motor, we can determine if the device is going into OCP.

Jeremy Novoa said:

On the bench, and just testing the motors on its own, I do see some motors where the inrush is higher that the 4.5A. This motors will trigger the OCP at 400hz but not at 150kHz. I concluded that by switching at a higher frequency (faster), we are not letting the inductance of the motor charge enough to draw more than ~2A on inrush. Can this approach be valid?  

What is the PWM frequency at 150kHz? is it 75% as you mention in the reply above? AT higher frequencies, the driving phase is much shorter which effectively minimizes the inrush current through the motor.

Jeremy Novoa said:

Pablo, one more thing. Lets say VM is set to 12V and I PWM to 75% duty cycle to get an average voltage of 9V. What voltage does the H-Bridge sees the peak (12V) or the average (9V)?

The voltage seem by the H-bridge will be the peak 12-V. The duty cycle will affect the average current through the motor.

Please share the waveforms when you obtain them.

Hi Pablo,

Yes we are running this motor/H-bridge form a supply voltage that has a range of 9V to 16V , we monitor the voltage via Microcontroller and adjust the duty cycle to obtain a ~9V average. Do you see any potential problems with the H-Bridge if we use higher frequency to mitigate the inrush at higher voltages?  

If the H-bridge sees the peak voltages, that means that the inrush [I=(V-BEMF/Rmotor)] that the H-bridge sees increases as the supply voltage goes up. Correct? 

Thank you!!

-Jeremy 

Hi Jeremy,

Jeremy Novoa said:

Do you see any potential problems with the H-Bridge if we use higher frequency to mitigate the inrush at higher voltages?  

An effect of driving a motor with higher current is lower motor torque. Since the current through the motor last for shorter time, the motor torque will be lower. This may be an issue once the motor is loaded. You will have to run tests with your applications to determine if there are any problems due to the lower torque.

Jeremy Novoa said:

If the H-bridge sees the peak voltages, that means that the inrush [I=(V-BEMF/Rmotor)] that the H-bridge sees increases as the supply voltage goes up. Correct?

The inrush current will depend mainly on the parameters of the motor. I suggest looking at the datasheet of the motor to get the inrush current information.

Hi Pablo,

I have attached plots to describe what I am seeing. 

Trace1: VM

Trace2: IN2 (IN1 is held High)

Trace4:Current (measure at the high side of the motor

On the first plot the motor runs as expected, on the second plot I am not sure what is going on but i was able to duplicate by using a resistive load instead of the motor. the Itrip is set to 3.5A but the plot does not show OCP. Can you weigh on this?

Thank you,

-JeremyTI.pdf

Hi Jeremy,

Thank you for providing the information. Please give me one business day to review your data. Expect a reply from me by 8/9.

Pablo, 

Correction: Trace 1 is actually the voltage across the motor (not VM)

Thanks.

-Jeremy

Hi Jeremy,

Thank you for the clarification

Hi Pablo,

I am trying to get an answer to my client sooner than later. Do you need any other information? Referring to the plots I sent, any idea why the waveforms look different when the motor does not spin? What is the state of the h-bridge here?

Thank you,

-Jeremy 

Hi Jeremy,

One thing I noticed in the waveforms is that when the motor is not spinning, the voltage across the motor does not follow IN2. The output voltage is switching at a much faster rate than the IN2 frequency of 400Hz. Can the customer retake the measurements without a load for frequency of 400Hz at 50% and 75%? I want to know if this issue is being caused by the driver or somehow by the load?

Hi Pablo,

I am sorry I did not see your last reply. We think we have figured out the difference in the motors, the one that spins does not have a filter cap across the motor contacts and the one that does not spin does have a capacitor (1uF for EMI). We think that when we drive the motor, drive -break, every time the capacitor charges the inrush is tripping the OCP, how ever the waveform indicates some sort of regulation. We have switched the PWM freq. to 150kHz and now the motor spins. The only questions I have are below ( I may know the answer to some but want reassurance)

If the H-bridge is set to regulate at 3.05A (Itrip), what happens if the inrush goes over 3.7A? Does it regulates or goes into over current? I am under the assumption that if regulation is set it should not go over that limit, but may be wrong.

What happens if the motor steady-stay is less than the regulation setpoint? Does it do any regulation or just regulates if it reaches the (Itrip)?

Last, by switching at 150kHz does the regulation even works? Since we are switching at a faster rate (6.7us w/duty of 50% to 85%) than the 25us it take to regulate. So when the device enforces the current decay we are switching about 4 times.  

We are trying to determine if we need to set the regulation or not. The motor steady-state draws about 400mA but the inrush can exceed the 3A mark although we are mitigating this with the higher freq.

Thank you so much for your help!

-Jeremy