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Part Number: DRV8837
This thread is intended to discuss, seek advice and help on analysis on maximizing the stall torque design for a particular dc motor using DRV8837 motor driver.I am not quite expert in analog domain and motor control, I have a project for door lock application using TI DRV8837. One of the design parameter that I need clarification is about maximizing the torque for driving mortise lock.We can isolate the gear ratio and resulting speed after gear reduction and focus on the origin that is the torque on motor.
Here is the graph characteristic for the dc motor:
The system works as specified:
I've done some test by measuring the current and voltage to the motor and analyze against the characteristic graph:
Here are my analysis:
Any helps or advises are much apperciated. Thank you :)
"Does this analysis is quite correct/legitimate for me to determine max stall torque that I could achieve using DRV8837 with battery as power source?"
For the motor side, the equation and calculation are correct.
The big variation is from the battery side. The V-I curve of the battery need to be studied. To me, 1.6A load for 4xAA alkaline batteries in series seems to high because the voltage dropped to 1.4V (0.35V/cell). That is much less than its peak power.
You may consider to NiMH or Li-ion battery to support such high current.
Motor Drive Solutions
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In reply to Wang5577:
Hi Wang Li,
Thank you for your confirmation. Regarding to the voltage drop:
Wang5577The big variation is from the battery side. The V-I curve of the battery need to be studied. To me, 1.6A load for 4xAA alkaline batteries in series seems to high because the voltage dropped to 1.4V (0.35V/cell). That is much less than its peak power.
This 1.4 V is the voltage across the motor. While the battery itself is more than that.
For example today, I did the same experiment but now with fresh battery and get these results:
I have some questions regarding this motor driver thing that I still don't quite understand. I have two different DC motors (one of them is the one that I mention on the first post):
The experiment that I did:
Thank you Wang Li,Pranata
In reply to Pranata:
we cannot use a lower resistance motorPranata,
1. Why for different motor, the DRV8837 seems can't give the same max current?
Stall condition means the motor doesn't have the back EMF to against the input voltage. The input voltage is applied to the winding resistor.
Motor A and B with different the voltage rating and similar stall current. That means the winding resistance is different. Higher voltage one will have a higher resistance. For example: Motor B 6V/3.8A=1.58ohm; Motor A: 3.6V/3.87A=0.93ohm
When the motor side input voltage drops to 2V with 0.93ohm winding, the winding current could be 2.15A (your measurement is about 1.6~1.7A)
When the motor side input voltage drops to 2V with 1.58ohm winding, the winding current could be 1.26A (your measurement is about 1.1A).
Bottom line: the winding current could be limited by winding resistance, not the motor driver.
2. Based on the experiment, does it means I need to drive the motor to higher voltage than its rated voltage if I want to maximize the drawn current for a given motor?
Before doing it, please check why there is high voltage drop from the battery to the motor side. DRV8837 HS + LS FET on-resistance is only 330mohm. 1.7A should only have 0.56V drop at 25C. You may be able to push the winding current a little bit higher by reducing the extra voltage drop. If we cannot reduce the driver loop resistance and we cannot use a lower resistance motor, we need a higher battery voltage..
Thank you for the explanation. That helps me to clarify the dc motor characteristic.
I think for this topic is adequate. I agree with the drop voltage on driver that seems odd. I will investiage further more and I think that would be another topic.
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