AC Induction motor running with sensorless FOC control

Hello Rasit,

1) these currents look pretty good, especially at no load and almost no amplitude. Load the motor and you will see them clean up. Also, if your motor is only a couple amps when you design your inverter stage you can rescale the current sense to give even better range.

1)  Rs for our algoritm is measured line to neautral, I assume your 7.4 is line to line. This is why the value is essentially half. Your Rs will change with temperature (and current usage), so 7.0 vs. 7.4 datasheet is quite normal

The Flux variables are a bit confusing. Here is what happens, MOTOR ID

-           finds Id rated at USER_RATED_FLUX setting

-          Uses a % of Id rated (default of ½) to then find Flux_VpHz

-          If all goes well you should expect USER_RATED_FLUX about 2x Flux_VpHz

(which you did!)

Hello again,

I run the same AC  induction motor with V/F scaler control. Attach file shows no load motor terminal current at 1500rpm with VF control and no load motor terminal current at 1000rpm with FOC.  No load V/F control current shape much better than FOC control current shape.  I was expecting current shape will be less harmonic and sinusoidal form with FOC control than V/F control even in no load condition.

am I right?

Best Regards,

6406.AC Induction Motor Terminal Current- No Load Scaler VF _FOC.docx 

Hello Rasit,

In your first post, you mentioned that you are using a commanded value of Id=0.  While this is normal for permanent magnet machines, it is not acceptable for AC Induction machines.  Id is used to control the flux in the machine.  Since a permanent magnet motor already has flux from the permanent magnets, you can set Id to 0.  But an AC Induction motor has no permanent magnets.  So when you set Id to 0, you are essentially trying to run the motor with no flux!  With V/Hz control, the flux level is held at its rated value by changing the voltage to be proportional to the operating frequency.  So your comparison is not "apples-to-apples" since in one case the motor has flux, but in the other case, it doesn't.

When I simulate the effect of setting Id to 0 with an AC induction motor, I see that the current amplitude is so small that the ADC resolution can no longer faithfully reproduce the current waveform accurately, which causes distortions in the current waveshape.  But since V/Hz control does not rely on the resolution of the current signals, it doesn't have this problem.

I suggest that you rerun the test; but this time, set the commanded value for Id to be the rated magnetizing current for your motor.

-Dave

I completely missed that you were manually setting Id = 0. I assumed you were running InstaSPIN-FOC as-is, where with induction motors the Id is automatically set to the magnetizing current as Dave notes should be done.

Hello Dave,

As shown in attach file in FOC block diagram Id_ref current entering to Id_PI controller equals to sum of the currents Irated (estimated) which is one of the FAST estimator output and User_IdRef value. In my code   User_IdRef value=_IQ(0.0)  I think this is default value for PMSM and AC Induction motor . I read Irated (estimated) current value from watch window as _IQ(0.15). So Id_ref current entering to Id_PI controller is not equal to zero. It is equal to _IQ(0.15).

I tried to increase User_IdRef value current value up to_IQ(0.5) using CTRL_setId_ref_pu(ctrlHandle, User_IdRef value);. function. Using watch window I increase  User_IdRef value. As you said motor current value is getting increasing but current shape is almost the same. 

Using watch window I check that Id feedback current value enters to Id PI controller is equal to reference value Id_ref.

In my code I am reading estimated Irated current value and Id feedback current value using code shown below.

idrated=CTRL_getIdRated_pu(ctrlHandle); // estimated Irated current value

idfdb=CTRL_getId_in_pu(ctrlHandle);  // get the feedback value

So I wonder that should I set User_IdRef value to zero and motor rated magnetazing current (pu) comes from Irated which is FAST estimator output 

or

should I set User_IdRef value to motor rated magnetazing current (pu).

As I said in both situation motor terminal current is not sinusoidal. I think its not related current magnitude because 350rpm waveform is better than 1000rpm waveform.

Regards,

2818.FOC Block Diagram.docx

You should NOT set the User_IdRef (gMotorVars.IdRef_A is the variable we use to interface to this in MotorWare when using field control), through the following function.

CTRL_setId_ref_pu(ctrlHandle, _IQmpy(gMotorVars.IdRef_A, _IQ(1.0/USER_IQ_FULL_SCALE_CURRENT_A)));

Id_ref is set by default to 0 for PM and MAGNETIZING_CURRENT for ACIM.

I'd have to test this out to see what happens if you set gMotorVars.IdRef_A to 0 while using an induction motor....I would think the Id_ref of 0 would overwrite the MAGNETIZING_CURRENT.

Hello,

I test again Id current values for AC induction motor.

Id_ref =_IQ(0.15) // reference current value for Id PI controller

id_fdb=_IQ(0.15) // feedback value for Id PI controller which is equal to motor magnetizing current value

where

USER_MOTOR_MAGNETIZING_CURRENT = 1.5A     and     USER_IQ_FULL_SCALE_CURRENT_A = 10A

 So I think measured results is OK . Dou you agree?

Note: In previous mail Id reference current value can be confusing.  It will be better to write following equation to make clear issue

Id_ref  = User_IdRef  + Irated

where

Id_ref= PI controller input

User_IdRef =_IQ (0.0) // The variable can be seen in FOC block diagram

Irated= pu motor magnetizing current which is _IQ(0.15)

In my code I didnt set  Id_ref PI controller input to _IQ(0.0).  User_IdRef variable is set to zero.

Best Regards,

Hi Rasit,

Thanks for the clarification.  I now understand how you are setting the d-axis current.

I went back to look at your waveforms in an effort to search for clues as to what might be going on.  To be honest, I don't think the FOC waveforms look that bad.  In fact, it looks like the V/Hz waveforms have pronounced zero crossing distortion with a flat top (a characteristic trait of dead-time distortion).  But I don't see that with the FOC waveforms, since current regulation removes most deadtime distortion.

Do you notice any difference in the way the motor runs, or are you just not satisfied with how sinusoidal the waveform looks?  Also, does the actual Id feedback variable track Id_ref closely?

-Dave

Hello again,

During running AC Induction motor with FOC everything seems OK except current waveform. In actually I didnt satisfy this waveform because I think it can affect motor efficiency. I didnt test motor with load but considering current waveform I think V/F efficiency will be better.

Other than current waveform dynamic performans of the motor is good as expected from FOC. As an example Lab6a works well. Speed fdb, Id fdb, Iq fdb track reference well. Id fdb track reference max %3 error. 

If possible can you send current waveform of AC Induction motor you work at 1000rpm and no load or can you compare with the my motor current.

Best Regards,

Hi Rasit,

I will try and locate a current waveform per your request.  In the meantime, please be mindful of the fact that motor efficiency is related to the i²R losses in the stator, the i²R losses in the rotor, and the core losses in both the rotor and stator.  Also, keep in mind that the current you are looking at with no load is the magnetizing current.  It can be shown that under no-load conditions with an induction motor, efficiency is affected MUCH more by the amplitude of the magnetizing current than slight deviations in its waveshape.  Unless you are using slip control, this represents a distinct advantage of FOC over V/Hz control, since you have direct control over the d-axis current level.

Even better, InstaSPIN-FOC has a special mode of operation (called PowerWarp)^TM which uses a special algorithm to calculate the optimal value for I_{d ref} in an attempt to maximize induction motor efficiency!  In independent testing in an agricultural application, PowerWarp was able to achieve 70% better efficiency compared to a V/Hz drive.

Again, I will try to find a current waveform to send you, but I just wanted you to be aware that if you are concerned about efficiency, there are much bigger issues to take into consideration than the shape of the magnetizing current waveform.

-Dave

Dear Dave,

Especially I concern about controller in my work. Whether It works as should be or not.  I have experice with PMSM motor drive before but this is my first experience running AC induction motor with FOC. That is why I am not sure about current waveforms.

Again I want  to indicate that feedback values tracks referance values max %3 error in PI regulators (spd, iq, id) and no load current waveform I sent you before.

Considering all of this can we say that FOC of the AC induction machine works as should be?

Please send the no load current waveform of your motor. I want to compare with other motor waveform to be sure.

Best Regards,