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Original question:

MCF8329A: Optimizing motor efficiency

MCF8329A: About BASE_CURRENT

Nguyen Thao
Prodigy 40 points
Part Number: MCF8329A

Tool/software:

Hello TI Expert,

In the before topic, I received some corrections for the EVM from TI about [How to Calculate Base Current].

Please reference this link: e2e.ti.com/.../5316025

In another application with MCF8329A, I have those values: R_sense = 1 mΩ, CSA_GAIN = 40V/V.

So the base current in ampere has to be 37.5A and the BASE_CURRENT in GD_CONFIG1 = 0x400.

Additionally, I configured the following fault limits: ILIMIT = 40%, HW_LOCK_ILIMIT=80%, LOCK_ILIMIT=65%.

I start the motor and increase the load until it stops due to the HW_LOCK_LIMIT error with the phase current (peak)= 16.5A, torque = 0.16Nm.

Then I want to increase the torque limit, so I adjust the CSA_GAIN to 20V/V but keep the BASE_CURRENT at 0x400.

However, using the same fault limit setup, the motor can be driven at phase current (peak)= 33A and torque = 0.38Nm until it is stopped by the HW_LOCK_LIMIT error.

The value of BASE_CURRENT in register GD_CONFIG2 appears to have no effect on the base current, which is used as a reference and current threshold.The base current must be 75A and BASE_CURRENT = 0x800 when CSA_GAIN = 20V/V and r sense = 1 m.Despite the BASE CURRENT being set to 0x400 (meaning 37.5A), the actual results demonstrate that the referenced values, such as ILIMIT and OL_LIMIT, were referenced to 75A.

However, the motor initiation fails when I set the BASE_CURRENT=0x800 and CSA_GAIN=20V/V.

What solutions do you know for this issue?

Regards, 

Thao N

  • 0
    TI__Expert 6580 points

    Hi Thao N,

    The 1mOhm shunt assembled on the EMV is showing up additional impedance for switching frequencies and the scaling is getting affected.

    If you find smaller package of 1mOhm (we will soon replace the part number and publish) or change to higher RSENSE .

    I will provide you some part number by early next week .

    Thanks and Best Regards

    Venkatadri S

  • 0 in reply to Venkatadri Shantaram84961
    Prodigy 40 points

    Hi Venkatadri S,

    Thanks for your reply.

    I apologize for my unclear question.

    I don't want to find a smaller 1 mOhm package or go to a higher RSENSE.

    I just don't understand why the value of BASE_CURRENT in register GD_CONFIG2 appears to have no effect on the base current, which is used as a reference and current threshold?

    When I modify the value setting of CSA_GAIN from 40V/V to 20V/V while keeping the base current at 0x400. Despite maintaining the same current limit threshold (%BASE_CURRENT), the phase current limit was increased?

    Could you please explain what is causing this problem?

    Thanks and Best Regards

    Thao N

  • +1 in reply to Nguyen Thao
    TI__Expert 6580 points

    Hi Thao,

    I understand your question. 

    The base current is (VREF-VREF_SEL)/(RSENSE*CSA_GAIN) , we need to program the BASE current register appropriate to CSA GAIN and do not change the reference like you are doing. 

    I will confirm to you about possible errors if we do like this.

    I recommend to change RSHUNT or CSA gain and program the BASE_CURRENT as per these selections.

    Thanks and Best Regards

    Venkatadri S

  • 0 in reply to Venkatadri Shantaram84961
    Prodigy 40 points

    Hi Venkatadri S

    I understand my problem. Thanks for your answer.

    Thanks and Best Regards

    Thao N

  • 0 in reply to Venkatadri Shantaram84961
    Prodigy 40 points

    Hi Venkatadri S

    I'm looking forward to hearing from you soon on the potential issues if we set the GD_CONFIG2 different than base current.

    Thanks and Best Regards

    Thao N

  • 0 in reply to Nguyen Thao
    TI__Expert 6580 points

    Hi Thao N

    I will reply to you early next week on this.

    Thanks and Best Regards

    Venkatadri S