current measurement in control of 3 phase motor

Bui Van Linh

Hello experts,

I have a question about the current measurement in control of 3 phase motor.
In the star connection we have the equation

Ia + Ib + Ic = 0

It means that we only need to measure currents from 2 phases. We can calculate the current of the third phase from the above equation. In your publications I have seen very often that you measure the currents in all 3 phases.

(see: https://www.ti.com/lit/eb/ssiy006/ssiy006.pdf?HQS=sys-md-robotics21webinar-tr-ebook-postwebinarww-eu&ts=1632212696927&ref_url=https%253A%252F%252Ftraining.ti.com%252F

What is the purpose of measuring currents in all 3 phases? What are the advantages?

Thank you for your explanation - Bui

over 4 years ago

0 Ramesh Ramamoorthy over 4 years ago

TI__Mastermind 27240 points

1. one can verify if ia+ib+ic =0. If this is not happening based on sensed phase current values, then the feedback is faulty.

0 Matt Hein over 4 years ago

TI__Mastermind 21805 points

Hello Bui,

When using two current sense amplifiers, you need to calculate the third phase of current. This calculation introduces error when one of the two phase currents are very low or very high - your calculation of the third phase current may be inaccurate.

When using three current sense amplifiers, all three phases can be measured and used, rather than calculating. This helps remove the possible calculation inaccuracy.

Thanks,

Matt

+1 Bui Van Linh over 4 years ago in reply to Ramesh Ramamoorthy

Intellectual 660 points

Hi Ramesh,

thanks for the answer. It makes sense to me.

Regards - Bui

0 Bui Van Linh over 4 years ago in reply to Matt Hein

Intellectual 660 points

Hi Matt,

thank you for your answer.

It is not clear to me what you mean. Can you perhaps give an example with numbers to clarify how the inaccuracy can arise.

Regards - Bui

0 Matt Hein over 4 years ago in reply to Bui Van Linh

TI__Mastermind 21805 points

Hello Bui,

For example if one phase is at 10A and another phase is at -0.5A, you have 10+/-0.2A - 0.5+/-0.2A to calculate the third phase.

Bear in mind current is sensed when the phase is low and current is flowing through the low-side sense resistor. That means that if you are driving high duty cycle (close to 100%) there is barely any time to measure the current.

Thanks,

Matt

+1 Bui Van Linh over 4 years ago in reply to Matt Hein

Intellectual 660 points

Hi Matt,

it's clear now.

Thanks a lot - Bui

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current measurement in control of 3 phase motor