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DRV8308: Need 0.5 RPM control - can't seem to get there with the DRV8308

Part Number: DRV8308

My customer is having problems running his motor down to 0.5 RPM with the DRV8308:

A couple of months ago I purchased a brushless DC motor controller eval board called the TI DRV8308EVM. I looked into the possibility of using this IC as a common controller for our pumps. It worked very well over certain ranges. However, I could not get it to work at the very low speeds that is required for our pumps, which is 0.5 RPM. If there is something that I missed as I evaluated the DRV8308, please let me know.

 

In general I considered the DRV8308 just right because of its SPI interface and closed-loop PWM control. But its inability to control at those low speeds I mentioned is causing us to reconsider using it. Again, please let me know if there is anything that I am missing.

Motor Parameters / 1/10th HP  
Number of Poles 4
Winding configuration 3-Phase WYE
Hall effect angle 120° electrical
Continuous Torque (Tc) 35 oz-in
Rated Speed @ Tc 3500 RPM
Rated Current @ Tc 2.31A
Peak Torque (Tp) 105 oz-in
Torque constant (Kt) 15.62 oz-in/A
Back EMF Constant (Ke) 11.57 Vp/kRPM
DC resistance (Rt) 1.59 Ohms
Inductance (Lt) 4.62 mH
Rotor Inertia (Jm) 0.67 oz-in2

Thanks for you help!