DRV11873EVM: Need to make the motor run at slower RPM like 2Hz or 1.5 Hz ,but currently it is stalling below 40% PWM duty cycle

Hi Siddharth,

Thanks for posting your question in MD forum. I am not sure what you meant by setting FS pin to 125 kHz. When FS pin in DRV11873 is set to 0, the device limits maximum speed (FG) to 475 Hz. When set to 1, max speed is 950 Hz. Since you want your motor to operate at low speed, make sure FS pin is pulled low.  As you may know, DRV11873 being a sensorless device, low speed operation will be challenging due to low BEMF voltage. Also, features in DRV11873 are very limited with no options to configure/tune parameters. We have DRV10983 and DRV10987 (both sensorless) which are highly advanced and has features to tune and optimize motor performance. Again it depends on the BEMF constant of the motor and the frequency at which we drive the motor.  Can you provide the motor BEMF constant Kt, pole pairs and maximum speed of the motor? If the BEMF constant is high, it is worth exploring DRV10983 and DRV10987. 

Regards,

Vishnu.

Hi Vishnu,

Thanks for your feedback. What I meant was , I have set the FS pin high, which means the output frequency is 12Khz as per the datasheet. I will try setting it low and check if the performance improves.

The motor datasheet says the following parameters:

1. Back emf constant( Ke) = 2168 mV/min^-1

2. Torque Constant(Km) = 20.7 mNm/A

3. Speed Constant(kn) = 461 min^-1/V

I do not see anything called as Kt. Please check and let me know DRV10983 can work for this considering slow speed. If you need further information, Kindly feel free to ask.

Sorry, missed to add the pole pairs.

The motor has 4 poles which means 2 pole pairs. The maximum RPM is 5600 min-1(no load speed). I want to run it slower approximately at 2hz(120RPM).

Hi Siddharth,

Thanks for sharing the information. BEMF constant Ke is same as Kt. Ke seems to be very high for your motor. Can you recheck? Ke is the inverse of Kn. If Kn is correct, then Ke should be 2.168 mV/min^-1. What is the rated motor voltage?

Regards,

Vishnu 

Oh Yes.. Actually its a german datasheet. It is  2.168mv/min^-1. The rated motor voltage is 12V.

Hi Siddharth,

Thanks for the confirmation. Converting mV/min^-1 to mV/Hz gives 65 mV/Hz which falls within the application range of DRV10983. However, at 2 Hz the BEMF voltage will be approximately 130 mV which is quite low for the internal sensing circuits in the device to accurately measure and estimate the commutation point. Also at 2 Hz the mechanical speed is 60 RPM. PWM duty cycle at 2 Hz will be ~1% (60/5600). Minimum PWM duty cycle limit in DRV10987 is 1.5%. Below 1.5%, PWM duty cycle will clamp to either 0% or 100% depending on the register settings. Considering these challenges, I would recommend using DRV10970 which is a sensored device. Does you motor support Hall sensors? 

Regards,

Vishnu.   

Hi Vishnu,

Thanks for the clarification. When I say the speed is 2HZ, the RPM should be ~120 and if it is 1Hz, the RPM shall be ~60. Regarding DRV10970, yes, the motor has hall sensors, but the output frequency Fpwm of this only 25KHz. The recommendation is to use a driver which atleast have an output frequency in the range of 78KHz to 100Khz. Even 50 KHz may work, but need to employ series inductor with the phase lines of the motor.

Siddharth,

Sorry for the confusion. I assumed 2Hz to be electrical frequency. If 2 Hz is mechanical frequency, then at 2Hz (mech) the mechanical speed in rpm will be 120 rpm. DRV10983 and DRV10987 supports 50 kHz PWM driver switching frequency but we do not have any devices in DRV10x family that can support above 50 kHz. What is the inductance and resistance of the phase coils? I need this information to know the motor time constant. Any particular reason why you want to operate at such high PWM frequency?  

Regards,

Vishnu

Hi Vishnu,

The inductance phase to phase is 60 uH and resistance phase to phase is 0.92 Ohms. Calculated electrical time constant is 30.67KHz given by the motor vendor.

Siddharth,

I would recommend you to try DRV10987 at 50 kHz PWM switching frequency. PWM duty cycle is 2% which should be fine. Below are some useful links to tune DRV10987.

1. DRV109xxEVM Quick Start Guide

2. DRV10987 Tuning Guide

3. Integrated intelligence part 2: motor startup open loop acceleration

4. Integrated intelligence part 3: motor startup from standstill position

5. Start your BLDC journey with motor startup (Part II): Choosing your parameters

6. Start your BLDC journey with motor startup (Part III): Initial position detection (IPD)

Regards,

Vishnu

Hi Vishnu,

Thanks for the suggestion. I will try this in coming months. Right now because of tight project schedule, my plan is to go with off the shelf motor driver board from the motor vendor which I have tested.

Once we complete the design and build the device by next month, I will again explore on the driver options. If you have any other suggestions, please let me know.

Thanks for your support.

Siddharth,

Below is the recommended application range of the DRV10987 device. Your motor falls within the device operating range so it is worth trying DRV10987.  

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Regards,

Vishnu

Electrical Time constant is slightly below the lower limit but it should be fine when operating at low speeds.