Timeline for Why is my DC motor losing torque when over voltaged
Current License: CC BY-SA 4.0
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| Oct 18, 2018 at 22:53 | comment | added | Phil G | If the controller is getting to 100% PWM, the output at the motor should be a DC voltage, reduced by the drops across the FETs and the current shunt resistor, both of which should be fairly small for this, so the performance should be near identical. Are you sure that the controller does get to full on state? You could just turn the ouput pin on, or tie the PWM input to the +3.3V. The default PWM frequency for Arduinos is only 490Hz, really too low for motor control. During the off period, the current can not only fall to zero, it can start to reverse, braking the motor. | |
| Oct 18, 2018 at 22:45 | comment | added | user201365 | How did you measure the "7V", if using a DMM, the PWM waveform will be converted into an equivalent RMS value (if the DMM is decent), but if the PWM frequency is to low then the motor may have problems starting as well. If possible check the PWM output with an oscilloscope. It's quite possible that at certain speeds the commutator may be not really utilizing the PWM waveform. | |
| Oct 18, 2018 at 22:31 | comment | added | Yrra Uy | Thank you, i will try your suggestion. On the edit what can you say about the driver dilvering lower speed and torque at the same voltage compared to connecting directly to the power supply? | |
| Oct 18, 2018 at 21:20 | comment | added | Phil G | I hadn't noticed that the Vref was brought out to a connector, that'll allow you to set it without hardware modification. If you don't have a datasheet for the motor with that maximum current stated, just start low - something just over the maximum steady current the motor draws. The current limiter will then operate during run-up, so it'll be a little slower, but there's no harm in letting it do so. I can't read the marking from the pics of the controller, but it looks like it's a ti.com/lit/ds/symlink/drv8701.pdf the datasheet has some explanation of the current limit operation. | |
| Oct 18, 2018 at 21:10 | comment | added | Yrra Uy | also ,as i mentioned in the edit. that when i directly connect the motor to the power supply at the same voltage as my driver is outputting, the motor is now more powerfull and faster than with the driver | |
| Oct 18, 2018 at 21:07 | comment | added | Yrra Uy | it just so happen that with the driver that i am using i can change the current limit. how do i know / calculate the appropriate current that i should limit the motor? | |
| Oct 18, 2018 at 21:03 | comment | added | Phil G | That's right. You need to have a current limiter that prevents the field generated by the windings from getting strong enough to demagnetize the magnets. With a 3V supply, the winding resistance alone probably prevents that from happening, but at higher supply voltages the inrush current will likely be too high. | |
| Oct 18, 2018 at 20:58 | comment | added | Yrra Uy | Thank you for your explanation. So by letting it get all the current it want, i have effectively weaken the motor? am i getting you right? | |
| Oct 18, 2018 at 20:42 | history | answered | Phil G | CC BY-SA 4.0 |