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DRV8308: I sense working and stall current

Vibesh v
Prodigy 10 points
Part Number: DRV8308

Hello all ,

      We are trying to bring up a motor [micronel] through DRV8308 , we are confused as to what a stall current is?. but when we checked with the manufacturer , he says that the motor would take up a maximum current of 3.5A rms . so we decided the R_sense value to be 0.05R , and as per the calculations this would limit current of (0.25/0.05 = 5A)  which we thought would be sufficient . But turns out that the motor seems to not respond to any speed changes and when we measured with the scope we came across this voltage (as shown below) at R_sense . i.e it exceeds the 0.25V limit and probably the current limiting circuit comes into picture limiting the output to the motor. 

that said we then experimented by changing the R_sense resistor value to 0.07R (current limit of 3.57A) and noted a similar voltage reading at R-sense .

but as follows are the duty cycle and its corresponding rpm values

100% - 8200(RPM) 

75.6% - 8051 (RPM)

52.3% - 7922 (RPM)

26.1% -6389 (RPM)

we are to design the motor to spin at 40000 RPM . and we believe this current limiting is the reason where the problem is .

also we get confused as to what a stall current is . and most importantly would i spoil my motor if I try to use a  smaller value resistance say 0.03 instead.

Thanks 

Vibeesh

  • 0
    TI__Mastermind 18900 points

    Hello Vibeesh,

    Admittedly, I'm struggling to follow the exact request so I'll try my best to answer.

    Motors and stall current:

    The manufacturer might have listed 3.5A(RMS) as the max current but the situation is a bit more specific than that. RMS current means steady state or running at a desired speed. Usually, the manufacturer offers this current as no load (usually just the spinning shaft of rotor) and loaded (which could mean anything, usually something attached to the shaft that resists motor motion). Usually, these RMS currents are more expectations and might be the optimal efficiency point, or typical speed given a load, and not necessarily indicative of passing this threshold will result in damage. In addition, I expect RMS current to be >100ms of exposure (more on this later).

    Stall current is the level current of expected when the rotor is physically stopped by some force (as BEMF collapses and there's nothing to resist the current or magnetic field). As you could imagine, exposing the motor to that level of current constantly will cause damage, but we expect the motor drive to stop driving (because a stopped rotor is worthless in context of motor control). So we expect the time frame to be in a manner <1ms. 

    In the case of the DRV8308, the cycle by cycle current limit (the 0.25V spec) is usually for RMS type current and the VDS_OCP or SEN_OCP is usually in the context of stall current. Cycle by cycle doesn't stop operation (as its not supposed to) and the other OCPs are supposed to stop the motor. So when I look at your waveform and see it in the context of microseconds before decaying back to 0V instead of hoovering around 0.25V, I'm not sure what to make of it.

    Your problems so far.

    So far, I can't follow if you've proven the cycle by cycle current limit is limiting the speed. Given the device, you absolutely have to verify that you're in the one of the PWM modes, BASIC=0b0, MOD set to trapezoidal and not sinusoidal, and the PWM duty cycles <26% scale linearly (where they would flatten out as you went above 26% as alluded to in your table). 

    I'm not sure how you're monitoring speed, but probing a motor phase voltage, duty cycle, hall sensor, and ISEN should give you all the information you need to check if the cycle by cycle current limit is being hit. Specifically, the motor phase voltage duty cycle will fluctuate to a lower constant value (you can zoom very far in) even though the duty cycle is a large percentage and speed won't fluctuate that much.

    The RMS current "maximum" isn't clear enough for me to say that exposing the motor to higher would cause damage as a stall specification wasn't given. Seeing if the motor gets hot as you expose more current is a really easy way to see if you're stressing the motor. I'm surprised you weren't given a datasheet, these are pretty typical values.

    Best,

    -Cole