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DRV8312: Assistance getting started with Sensorless BLDC control

Adam Preston
Prodigy 125 points
Part Number: DRV8312
Other Parts Discussed in Thread: DRV8320, CSD88539ND, DRV10987EVM, BOOSTXL-DRV8320H, MSP-EXP430F5529LP, DRV10983, DRV10987

Hi,

We are looking to change to BLDC motors in some of our products (which currently use brushed DC). I'm an engineer but have no previous experience working with BLDC, so am looking for a nudge in the right direction :)

In our application the motor is driving a pump (used in drink dispensing equipment). It's a fairly low load application and we just need to drive the BLDC motor FWD/REV (at top speed), brake if needed, and also run at lower speeds at times. One key requirement a fast start / ramp up time so the product responds quickly when activated. 

I'm hoping to use sensorless control, and we already have the motor we are trying to drive - it's basic specs are below:

  • Phases = 3
  • Poles = 8 (4 x pole pairs)
  • Rated voltage = 24VDC
  • Resistance (phase to phase) = 0.2 Ohm
  • Inductance = 0.45 mH
  • Rated Current = 5.5A
  • Rated Speed = 6500RPM 

Note: the rated current if 5.5A (max is approx. 8A), but in our application the motor is only pulling 2.5-3.0A and speed is approx. 7500RPM. 

I have been looking at the DRV8312 - does this seem like a good starting point?

If so, is there a way to calculate what ramp up time would be achievable from 0 to 7500RPM - or is it best to get the dev board and jump in? and also what method does the DRV8312 use for initial position detection?)

Again - I'm new to all this so please be gentle :)

Thanks!

Adam

  • 0
    TI__Mastermind 21805 points

    Hello Adam,

    Thanks for your question, and welcome to the world of BLDC motors! Let me be the first to tell you that this is a place where many engineers don't feel very equipped or knowledgeable when starting out.

    I do have several BLDC motor trainings & resources that I want to direct you towards:

    Sensorless BLDC

    Sensorless operation will be a bit harder to implement compared to sensored (Brushless-DC Made Simple – Sensored Motor Control), but given you do not have a minimum speed requirement it may be OK - operating at slow speeds is where sensorless algorithms fall apart.

    You need to start the motor sensorlessly and there are a few typical methods to implement this (Align or IPD).

    Some devices will have sensorled or sensorless capability integrated (i.e. DRV10987 or DRV8306), others will be more like just the power stage to be used in conjunction with an MCU doing the actual algorithms (i.e. DRV8312).

    On our BLDC products table, you can select "Sensor type" as Sensored, Sensorless, or Universal - "Universal" applies to devices where control is not integrated and an external MCU is used.

    BLDC motor commutation

    Whether you are sensored or sensorless, you need to apply a specific sequence to the motor to get it to spin. This is called electrical commutation (Brushed-DC motors use mechanical commutation which is why they are so easy to spin).

    Trapezoidal commutation is the easiest to implement (i.e. DRV8306), sinusoidal commutation applies a smoother waveform for quieter and more efficient operation (i.e. DRV10987).

    An example of an MCU implementing sensored trapezoidal commutation can be found in this app note: Generate your own commutation table

    Some devices will have the commutation control integrated (i.e. DRV10987 or DRV8306), and again others will be more like just the power stage to be used in conjunction with an MCU doing the actual algorithms (i.e. DRV8312).

    On our BLDC products table, you can select "Control method" as External Control, Sinusoidal Control, or Trapezoidal Control - "External Control" applies to devices where control is not integrated and an external MCU is used.

     

    Driving The Motor - Gate Driver versus integrated MOSFET

    Depending on the motor power, either a solution with integrated MOSFETs (i.e. DRV8312) or a gate driver (i.e. DRV8320) may be used. At 24V nominal supply I would recommend using devices with abs max ratings >40V.

    DRV8312 is an older device with quite a few external components needed (i.e. a secondary 12V supply for GVDD). The solution size may even be smaller with a gate driver like DRV8320 and external MOSFETs CSD88539ND.

    On our BLDC products table, you can select "Architecture" as Gate Driver or Integrated FET.

    Reference design examples:

    My Recommendations:

    • DRV10987EVM - this device will integrate the control & commutation into the driver, but the current rating may not be high enough to support your motor when it is accelerating.
    • BOOSTXL-DRV8320H & MSP-EXP430F5529LP: Power stage + external MCU

    Motor ramp up time will depend on the mechanical properties and load of the motor, as well as the current limit you allow when accelertating the motor to speed. Faster acceleration results in higher current draw. Since there are quite a few factors we normally direct customers to evaluate this by spinning the motor.

    Other content which describes a lot of what is discussed above: Brushless-DC Motor Driver Considerations and Selection Guide

    Thanks,
    Matt

  • 0 in reply to Matt Hein
    Prodigy 125 points

    Hi Matt,

    Thank you so much for the detailed response. Two quick questions before I jump in deeper:

    1. All things being equal (and configured correctly) - should I be able to achieve similar ramp up times with sensored vs. sensorless operation? 

    [Note: I found an 'IPD tuning guide' doc for DRV10983 and based on the equations provided, I don't think my motor is suitable for IPD. If so I will need to use the Align method, just noting this in case this is a factor....though I think this is still quite a fast process]

    2. I have looked at the suggested DRV10987 previously and was hoping it was an option. However as you mentioned my concern was the current rating. The data sheet mentions a '2A Continuous Winding Current'. If my motor under load pulls approx. 2.5A, does that rule it out straight away or am I misunderstanding this spec?

    Thanks again Matt! 

  • 0 in reply to Adam Preston
    TI__Mastermind 21805 points

    Hi Adam,

    1. All things being equal (and configured correctly) - should I be able to achieve similar ramp up times with sensored vs. sensorless operation? 

    --> Sensorless operation will have some additional time for the Align or IPD to execute compared to sensored operation.

    2. I have looked at the suggested DRV10987 previously and was hoping it was an option. However as you mentioned my concern was the current rating. The data sheet mentions a '2A Continuous Winding Current'. If my motor under load pulls approx. 2.5A, does that rule it out straight away or am I misunderstanding this spec?

    --> If your motor is 2.5A RMS/continuous rated, the DRV10987 will run too hot and could enter thermal shutdown.

    Thanks,

    Matt