Low Currents at Non-stall loads

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SBitney
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5 replies posted 5 sign-ins First reply posted

Hi,

I'm having issues reaching higher currents when loading down a motor driven by a IMC101.

I've got a maximum current limit of 3.5A set, and it will achieve that at stall or near-stall conditions. However, loading the motor so it just slows down, but doesn't stall, the controller wont drive it at more than around 1.5A and the motor will continue to run at the slower speed. It won't increase the current any more than that to hit the target speed. Only by bringing the motor to a near stop will it hit the 3.5A limit.

I've tried changing the speed control PI gains, and while that affects behavior and recovery from stall, it doesn't make any difference to this lower current limit.

I'm a bit stumped as to what could be causing it.

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1 Solution
Krupashankar
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500 replies posted 50 likes received 25 likes received

Hi @SBitney,

This problem can be due to the following reasons:

  • Improper Kp Ki tuning 
  • Improper motor parameters 
  • Improper current regulator tuning
  • Improper Current measurement

Could you please try increasing the current regulator Bandwidth ( Ques 58) and observe the behavior of the motor and current waveform, share the current and motor speed trace from MCE designer.

Could you please share the details of the Motor Datasheet, Powerboard and .mc2 file in a zipped format.

This will help us to understand the issue better.

 

Thanks,

Krupashankar

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11 Replies
Krupashankar
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500 replies posted 50 likes received 25 likes received

Hi @SBitney,

This problem can be due to the following reasons:

  • Improper Kp Ki tuning 
  • Improper motor parameters 
  • Improper current regulator tuning
  • Improper Current measurement

Could you please try increasing the current regulator Bandwidth ( Ques 58) and observe the behavior of the motor and current waveform, share the current and motor speed trace from MCE designer.

Could you please share the details of the Motor Datasheet, Powerboard and .mc2 file in a zipped format.

This will help us to understand the issue better.

 

Thanks,

Krupashankar

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Hi Krupashankar,

Thanks for the reply.
I tried increasing the current regulator bandwidth from 600 rad/s to 800 and 1000, but there wasn't any noticeable change
.

I've attached a copy of the driver circuit, motor datasheet, wizard and parameter files, as well as some current and speed waveforms that show the issue.

Regards,

Scott

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Krupashankar
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Hi @SBitney,

We had a look into the motor datasheet and MCE Wizard configuration .mc2 file.

We found that in MCE Wizard configuration Ques 7 Motor Back EMF Constant ( Ke) - 485 V( ln-RMS / KRPM)

In the Motor Datasheet, we found Ke - Back EMF constant 4.44 V/Rad. S-1 if we convert 268.31  ( ln-RMS / KRPM) 

Configuration parameter is different from the motor datasheet,  Could you please check this? 

You could also refer to this document for measuring the motor parameters: How to Measure Motor Parameters

 

Thanks,

Krupashankar

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The datasheet already lists the Back EMF as Line to Neutral, so shouldn't it convert out to 465 (ln-RMS / KRPM)?
We're using 485V in the config as we found the actual Back EMF when measured to be slightly higher than what's specified in the datasheet.

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Krupashankar
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Hi @SBitney,

Yes, We wanted to confirm that value. Have you got a chance to measure the back-emf according to the document?

Could you please try increasing Ques 52 Motoring Current Limit? As this parameter will limit your motor operating current. But limit this value less than maximum current rating of motor.

 

Thanks,

Krupashankar

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We'd previously measured the back-emf using a method similar to that in the document, but using the scope calculated RMS value instead of converting from peek-peek, over a range of speeds, and got an average value of 485(ln-rms)/krpm, which is pretty close to the provided value of 465.

The Motoring Current Limit is currently set to 100%, and our Motor Rated Amps is set to 3.5A, which is just under the limit of our driving circuit.
I tried setting it to 125% just to test, but saw no increase in power when loaded. Both at 100% and 125%, the current would not exceed around 1.2A when loading 250RPM down to 150RPM.
It was however delivering more current when stalled as it would trip over-current protection, which is to be expected.

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Krupashankar
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Hi @SBitney,

Can you try running in the current loop and check if you are able to achieve a higher current?

You can refer to this document to run the motor in the current loop: AN2020-12 Driving high inertia loads with iMOTION™2.0

 

Thanks,

Krupashankar

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Same thing unfortunately.
Setting IqRef_Ext to the max current rating (3.2A), it's still only reaching that max current at near stall.

SBitney_0-1652084686825.png

 

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Krupashankar
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Hi @SBitney,

1: We could see that the DC bus operating voltage range is from 120V  to 450V. Can you tell us the application and need for such a wide operating voltage window?

2: Could you please share the trace of DC bus voltage and motor current at high motor load?

3: In the meantime, you can enable Over modulation in the configuration and check if there's any improvement in the performance. 

Thanks,

Krupashankar

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Hi Krupashankar, sorry for the late reply.

The DC bus limits were just based on the limits of the hardware, not the expected application. These can be reined in if they're causing issues.

I've attached traces of the DC bus vs current at low, medium and high loads.

I've tried enabling overmodulation, but it doesnt appear to make any difference. I've attached captures with both overmod on and off.

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Krupashankar
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Hi @SBitney,

We could see that during high load the DC bus voltage is dropping to 250V Could you please tell us if this is desirable and please confirm if there is no current limit to the source.

Can you please check if the hall angle offset 120 deg? 

 

Thanks,

Krupashankar

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