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Motor Current Oscillation
3

Motor Current Oscillation

Motor Current Oscillation

(OP)
Hi to you all.

I am interesting does anyone have expirience in commissioning big induction motors supplied via VSD that workink in scalar control mode (V/f).
I had commissioning 2.85 MW motor, 660 V, 50 Hz. Drive is ABB ACS800.
Uncoupled motor was worked very bad in scalar with some high oscillation of current and in DTC everything was OK.
You can see in attach graph of uncoupled motor in scalar.
Does anyone know what could be reason of oscillations.

Best regards,

Milovan Milosevic

RE: Motor Current Oscillation

The oscillation is around 6 Hz - if I understand correctly. That is a frequency that could be the result from rotor flux, rotor inertia and stator flux forming a resonant system.

The possibility has been discussed before in this forum and e-pete has done some simulations.

Does the frequency change if you reduce excitation (lower stator voltage)? If it is the rotor swinging in stator flux, then the frequency shall decrease if excitation is reduced.

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Motor Current Oscillation

(OP)
Thanks Skogsgurra for this respond.

I didn't try with reducing stator voltage so I dont know and I am not any more in this factory so I can not try.

But this method with reducing stator voltage is only usefull to see if the problem is with this resonance, and what to do if this is true and you must run motor in scalar.
How can you decrease this oscillations.

For example some my colegue had commisionned HV motor with LV drive with step up transformer. So all recomandation is to run drive in this situation in scalar. They also had some problems with oscillations. Also they had problems with very very high current on very small frequencies but this was probably because transformer is almost short circuit for drive on small frequencies.

Do someone have experience with drives with step up transformers.

Milovan Milosevic

RE: Motor Current Oscillation

Yes. We used a 600 kW Siemens Masterdrive with a 500 V to 6 kV step-up transformer to start a large (think 6 or 8 MW) synchronous motor once.

We got bad oscillations and eventually had to switch to torque control and limit torque to positive (no braking torque allowed) to be able to control the motor.

It was a rather tiresome experience - especially after we found (somewhere on page xxx) in the manual a sentence saying that synchronous motors could not be run with that VFD.

But, we had to do it - so we did it. After lots of hours...

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Motor Current Oscillation

(OP)
Skoksgurra,
Drive was probably thought that he run iduction motor ?
So you run motor in torque control, not vector speed control.
On which frequency was oscillations.
My idea for this step up application is to have current limit that is linear function of output frequency with offset on zero frequency (motor is running pump).

Milovan Milosevic

RE: Motor Current Oscillation

I doubt it will help, but the type of resonant oscillation that skogsgurra referred to in his first post is well described in the figures here:
http://lipo.ece.wisc.edu/1970s%20pubs/T07.pdf
It completely ignores any supply/control system dynamics, just assumes a constant-frequency sinusoidal supply and which frequencies would cause resonance for given motor parameters.    

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
Skogsgurra,

Sorry I written your name wrong. I was typing on mobile phone.

Milovan Milosevic

RE: Motor Current Oscillation

LOL!

That is really one of my least problems right now!

BTW, it doesn't mean anything offensive in our language (Swedish) and I hope it doesn't in yours either...

Gunnar Englund
www.gke.org
--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

RE: Motor Current Oscillation

(OP)
electricpete,

Thanks for this paper, but it has to much mathematics and process control theory for me, but I read it.

 

Milovan Milosevic

RE: Motor Current Oscillation

A summary of the article is that a motor system fed from a fixed-frequency supply has eigenvalues that are a function of the supply frequency and of the motor parameters.  (the frequency of the supply would be fixed for a given simulation or analysis, but varying between simulations/analyses).    The unstable regions are where the eigenvalues have have a zero real part or a positive real part...i.e. the mode is not damped.    If you were experiencing this type of problem, it should go away when you vary the supply frequency (for example the motor in the article was only unstable when the supply frequency was around 0.3+/-0.1 times the base frequency).   Note that in general the frequency of this unstable oscillation will be different than the supply frequency (it is not like a resonance where the system vibrates at the excitation frequency).    

If you are interested, I can calculate the eigenvalue for your uncoupled motor.  What I would need is:
number of poles
full load: amps, power factor and efficiency
half load: amps, power factor and efficiency if available
no-load: amps, power factor  if available
motor rotating inertia
frequency of the supply at the time you experienced the oscillations.  
(I don't need locked rotor torque or locked rotor current).

The output of the analysis would be:
frequency of oscillation
damping associated with oscillation... (indication of whether stable or not and how much margin).

There can also be some what-if sensitivity analysis...how does the eigenvalue change when you change the inertia or add a load torque or change the supply frequency or voltage.

I don't' know much about vfd's, but as far as I know scalar control is a simple open-loop control with constant supply voltage magnitude and frequency (as long as the setpoint isn't changing).  In that case,  Krause's analysis would seem appropriate for scalar control .

By he way, that is the biggest low voltage motor I have ever heard of! (2.85MW, 660V).
Are you sure you have the rating right (2.85MW, not 285KW).
Is it a squirrel cage induction motor?
 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
electricpete,

Motor ratings are OK so 2.85 MW, 660 V, 3000 A, 50 Hz, 992 rpm, power factor 0.86. This is all parameters I know right know.
You dont need to struggle whit those equations to find critical frequency.
This oscillations was appeared only on some frequencies (somewhere between 10 and 15 Hz) and only during acceleration or deceleration (motor was uncoupled).
Also on faster ramps oscillations was smaller (probably because of faster passing through this critical area).
I can attach some pictures that illustrate this if you are interesting.
 

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
electricpete,

I was reading some your old post in which you said that oscillation of torque during direct on line start of induction motor is because of DC component in stator current. Are you sure in this.

Milovan Milosevic

RE: Motor Current Oscillation

.

Quote:

I can attach some pictures that illustrate this if you are interesting.
Yes, I am interested.
 

Quote:

You dont need to struggle whit those equations to find critical frequency.
This oscillations was appeared only on some frequencies (somewhere between 10 and 15 Hz) and only during acceleration or deceleration (motor was uncoupled).

The the oscillation only occurred at certain frequencies is consistent with the Krause paper (there are certain zones of instability, primarily 0.3* base frequency for his motor).

The objective of the study would be to determine the damping factor of those modes to check whether they are in fact predicted to be unstable.   Whether the oscillating frequency matches observed oscillating frequency (for a given supply frequency) would be a factor that could be used to corroborate or validate the model..  

I already have a program ready to go and have done it before for another motor discussed in this other thread:
thread237-249262: Low power factor, recip pump

I was not able to post the full results of the eigenvalue sensitivity analysis in that thread, but I have attached them to this thread.  It starts with a numerical sensitivity analysis of the eigenvalue.  From the numerical sensitivity analysis we can see that the eigenvalue frequency is proportional to voltage, inversely proportional to square root of inertia, inversely proportional to the sqrt of sum of leakage reactances.    This suggests a simpler model which matches the behavior of the eigenvalue as shown on slides 3-5 (not as good as full eigenvalue analysis, but a little more intuitive).

But there is not really enough data to get a good model.  Do you have motor inertia and full load efficiency?
 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

Quote:

I was reading some your old post in which you said that oscillation of torque during direct on line start of induction motor is because of DC component in stator current. Are you sure in this.
I am sure they are related somehow.

Attached is simulation of start of a 2250hp motor.
On slide 1 you can see the 60hz torque oscillations which die out at approx t=0.8  seconds.
On slides 5, 6, 7, you see the stator phase currents.  

Let's analyse the shape of the phase currents:

If you follow the simpler approximation discussed in the other thread by Rockman, we can say we have a sinusoidal component plus a decaying dc component that is gone by around 0.2 seconds.

But if we take a closer look, that dc component is not simply decaying, but instead it goes up and down while decaying for the period from t=0 to t=0.8 seconds.  I am not sure what the exact origin of that is (I am inclined to think it may be related to rotor frequency because the "frequency" of that decaying component starts slow and then increases, which matches what we expect from rotor current frequency).

At any rate, the time period of the torque oscillation in slide 1 matches the time period of the superimposed decaying oscillating dc in the phase currents in slides 5,6,7, so they are somehow related.

In my mind it is expected because we expect steady torque only during balanced conditions and we expect oscillating torque when unbalanced.  Up until 0.8 seconds the phase currents are never balanced so we have oscillation.   Maybe there is a better / alternate explanation (?)
 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

Just in case I wasn't clear, the key thing I was pointing out was that the period of the torque oscillations (0-0.8 seconds) is the same period as which the currents have an added oscillating/decaying dc component.

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

Quote (electricpete):

Do you have motor inertia and full load efficiency?
Whoops.. sorry - we've got voltage, full-load current, power, and p.f....so efficiency is computable. How about inertia?
   

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
electricpete,

I dont have data for inertia but I was calculate it according to graphics that I recorded.
So I get this values:  105, 107, 100, 100, 110, 98, 111, 99 (all in kg*m^2).
This is values from differents accelerations and decelerations that I record.
So you can use mean value for inertia and that is 104 kg*m^2.

 

Milovan Milosevic

RE: Motor Current Oscillation

Thanks.  I am pretty confused as to what I'm looking at.

Just to check for understanding.
The magenta (purple) curve is rms current.
The dark blue curve is voltage.

I assume that both types are rms-averaged current, rather than an instantaneous current (waveform), correct?

And why is voltage swinging like that... is there a current limit active during this period?

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
electricpete,

Voltage and current is rms values.
Current limit is not active. This blue curve that look like current is torque. Did you meant on this curve. Voltage curve is almost same as speed.

Milovan Milosevic

RE: Motor Current Oscillation

Quote:

This blue curve that look like current is torque.
OK, that explains it.  I was looking at Scalar_2.JPG and the dark blue color seemed to match the voltage label color. Torque makes a lot more sense.

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

Last question – is the value of no-load current at full speed (line frequency) available?   It seems like it may be retrievable from within this test data.

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
Yes, you can use value 764 A.

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
electricpete,

And one queston regarding torque oscillation during DOL start of induction motor.
I simulated induction motor supplied with 3 current sources
with zero speed all time (in Matlab). So no any DC component and you have again torque oscillation. How you explain this.
Here is results.
First motor speed and torque.

Milovan Milosevic

RE: Motor Current Oscillation

What I see is 3 stator currents that all simultaneously jump from 0 to sinusoidal without any transient.    It is not the expected behavior if the machine is powered from a voltage source (there will be a dc offset).   Maybe you could get close with a current-controlled supply  on the stator, .... but your rotor currents show the same pattern of simultaneously jumping from 0 to sinusoidal without any transient.   Imo it cannot be accomplished unless we had doubly-fed wound rotor motor with current controlled power supply on both the stator and the rotor.

From a visual look at the current waveforms, my guess would have been that there is no torque oscillation occurring because the magnitudes are balanced and as far as we can tell the currents are sinusoidal with unchanging magnitude and phase are 120 degrees apart.    But, we certainly cannot easily visually judge small changes in phase relationships using a/b/c phase currents (these sometimes show up much better in synchronous reference frame d / q representation of the currents).   

So... I give up.  Do you have in mind an explanation or a lesson from these waveforms?  
 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

First step to do simulations and analyses would be to determine equivalent circuit parameters.  I think any of the situations we will look at involve low slip frequencies, so we only "fit" the running data... no locked rotor data considered, no deep bar correction required.  That makes it a lot simpler and the results more reliable. Results as follows (curves attached):
                    

Quote (Model):


Induction Motor Equivalent Circuit Fit Results            

===============Model Parameters Solution================            
Name    Value    Units    Description
R_NL    1000000000    ohms    Resistor simulate portion of No-Load losses - connected direct in parallel with the source
R_1    0.0028121    ohms    Stator Resistance
X_1    0.019762324    ohms    Stator Reactance
R_2    0.000978489    ohms    Rotor Resistance refd to stat
X_2    0.019760511    ohms    Rotor reactance refd to stator
X_M    0.478990949    ohms    magnetizing reactance
FullLoadSlip    0.007999851    none    Full Load Slip
BarDepthR    0.001    meter    Equivalent Depth of rectangular rotor bar - Used for deep bar correction of R2
BarDepthL    0.001    meter    Equivalent Depth of rectangular rotor bar - Used for deep bar correction of X2
============Selected Inputs ====================================            
VLL    660    volts    Line To Line Voltage
SyncSpeedRPM    1000    RPM    Synch Speed in RPM (like 1200, 1800, 3600 etc)
BarMat    Copper        Select Aluminum or Copper for use in deep bar correction                
factor_SPT    1        Slip at Peak Torque calculated from R2 and L2 but cannot apply deep correction since S_PT not known before calculated => Apply iterative correction based on slip that produces peak torque in Model Output                
factor_SHL    1        Slip at Half Load initially assumed half of full-load slip.  But slip is actually somewhat non-linear with power -> correct iteratively based on slip that produces half power in model output.                 
                            

=========== Model Performance Against Targets==============                            
Perf Variable    Calculated Value    Units    Target Value    FractionalError    Weight Factor    Weighted Squared Fractional Error    Comment
FullLoadAmps    2999.93955    Amps    3000    -2.01499E-05    1    4.06019E-10    INPUT
FullLoadEff    0.966460276    none    0.966319161    0.000146034    0    0    Redundant - not used
FullLoadPF    0.859909912    none    0.86    -0.000104753    1    1.09732E-08    INPUT
FullLoadPower    2850060.177    watts    2850000    2.11148E-05    1    4.45836E-10    INPUT
FullLoadTorque    27435.55025    N*m    27434.97507    2.09651E-05    0    0    Redundant - not used
HLEfficiency    0.980834557    none    0.928    0.05693379    0    0    No target
HLPowerFactor    0.822013709    none    0.84    -0.021412251    0    0    No target
LRC    9790.801207    Amps    15000    -0.34727992    0    0    No target
LRT    2478.419793    N-m    49108.60537    -0.949531864    0    0    No target
NoLoadCurrent    763.9952299    Amps    764    -6.24362E-06    1    3.89828E-11    INPUT
BD_Tq    46295.05236    N-m    83951.02371    -0.448546899    0    0    No target
X2overX1    0.999908267    none    1    -9.17332E-05    0.001    8.41498E-12    Thumbrule - split evenly
R2overR1    0.347956608    none    1    -0.652043392    0    0    No target
X1overXm    0.041254456    none    0.05    -0.174910878    0    0    No target imposed
Full Load Slip    0.007999851    none    0.008    -1.85694E-05    1    3.44824E-10    No target imposed, but constrain to within 2.5 rpm from nameplate
BarDepthR    0.001    m    0.005    -0.8    0    0    REMOVED FROM OPTIMIZATION (LEAVE AT 0.001)

                    SWSFE    1.22173E-08    


 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
I dont know answer for torque oscillations during DOL start.

Just this explanation that is because of DC component in stator current is not enough convincing for me.  

Milovan Milosevic

RE: Motor Current Oscillation

Good point. I'm not sure exactly what you're comment was, but I certainly don't claim to fully understand those twice line frequency oscillations - but I note that their time corresponds to the period in which there is an added slowly oscillating dc current.

One thing to mention: if we look only at an individual phases, the current goes to zero twice per cycle and the associated torque goes to zero twice per cycle, so we do in fact expect twice line frequency oscillation for the torque associated with a single phase current.   And it is only when the current end up balanced in both phase (120 degrees apart) and magnitude that the sum of the three phases will not have any twice line frequency variation.  So from that standpoint it makes sense that until the transient settles down we have some torque oscillation.  But there's a flaw in this simple view... the individual phase torque oscillation would be twice line frequency, but the oscillation seen in the simulation of start of a three phase motor is in fact one times line frequency (!?!).

Another view is not exactly a physcial explanation, but a look at where these oscillations come from within the Krause's "transient equivalent cirucit" which I have attached.  Let us select the synchronously rotating ref frame, which means we set the variable w in that diagram equal to 2*pi*Line Frequency.  The source is a dc circuit.  At first glance, we wonder where the heck is twice line frequency variation going to come from in a dc circuit.  The answer from my view point is that the twice line frequency is sort of a "resonant frequency" of the stator portion of the circuit, and that resonant frequency is excited when we hit the circuit with a step in the Vq dc source from 0 to constant.  Gotta run...will post just alittle bit more on this later.

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
Frequency of oscillations is not twice line frequency. Frequencies are same.

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
I like to simulate motor with locked rotor and in this case you dont have this slowly oscillating dc current.
Only clasical DC component.
 

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
So,

What have no sense to me is if you try to simulate motor with locked rotor, there are no this slowly oscillating dc current only fastly decaying dc component.
Theoreticaly this DC component will never fall to zero,
but if you compare this DC component in let say 50th ms from simulation start and in 500th ms difference is huge, and difference in amplitude of torque oscillation is not so big (I am speeking in bigger motors 100 - 200 kW).

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
I dont understand this:
"One thing to mention: if we look only at an individual phases, the current goes to zero twice per cycle and the associated torque goes to zero twice per cycle, so we do in fact expect twice line frequency oscillation for the torque associated with a single phase current. "  

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
Actually, oscillations of torque have frequency of rotor current.
So if speed is zero frequency is same as line and if speed if half of synchronous frequency is half of line frequency.
 

Milovan Milosevic

RE: Motor Current Oscillation

Great discussion! I'm glad to see someone else is interested in these type of questions.

Quote:

Frequency of oscillations is not twice line frequency. Frequencies are same.
My comment was – steady state torque oscillation frequency in a single phase motor  is at 2*LF... torque oscillation during start of a 3-phase motor is 1*LF.  It was a simplistic attempt to try explain the complicated 3-phase behavior by breaking it into single phase pieces.  What was missing from my previous discussion is mention that the expression for torque from a/b/c currents includes not only terms involving a single phase, but also cross terms involving multiple phases... so we don't just add up three single phase torques.

Quote:

Actually, oscillations of torque have frequency of rotor current.
So if speed is zero frequency is same as line and if speed if half of synchronous frequency is half of line frequency.
That's an interesting idea that the frequency of torque oscillations is the frequency of rotor current, but I believe it's incorrrect.  I did a simulation of normal unloaded start using your motor paramters above (even though I didn't try to model the locked rotor parameters welll... still gives us a qualitative idea).  The results are attached and slides 1 and 2.   As close as I can tell the frequency of oscillation is constant and does not slow as the rotor accelerates (see slide 2).  So I think it is line frequency, not half of rotor current frequency.

Quote:

I like to simulate motor with locked rotor and in this case you dont have this slowly oscillating dc current. Only clasical DC component.
Good point.  Attached slide 3 and 4 are simulation of DOL rotor start for my model of your motor, with the rotor locked.  It has only the decaying dc... no oscillating component.    It leads to the same conclusion I was thinking about 11 Oct 10 17:14  ("I am inclined to think it may be related to rotor frequency because the "frequency" of that decaying component starts slow and then increases, which matches what we expect from rotor current frequency").  Now it is solidified that the oscillations in the envelope of the stator dc component occur at rotor slip frequency... thanks for helping me understand that better.

 

Quote:

dont understand this:
One thing to mention: if we look only at an individual phases, the current goes to zero twice per cycle and the associated torque goes to zero twice per cycle, so we do in fact expect twice line frequency oscillation for the torque associated with a single phase current. "
It applies to single phase motor, which does have torque oscillation at twice line frequency.  It is not particularly relevant to 3-phase motor as discussed above since cross terms are not included.

Quote:

What have no sense to me is if you try to simulate motor with locked rotor, there are no this slowly oscillating dc current only fastly decaying dc component.
Theoreticaly this DC component will never fall to zero,
but if you compare this DC component in let say 50th ms from simulation start and in 500th ms difference is huge, and difference in amplitude of torque oscillation is not so big (I am speeking in bigger motors 100 - 200 kW).
Yes, I agree.  And it argues that it is not strictly the dc component causing the torque oscillations as you said. Instead there are subtle changes in phase angles going on that contribute to these torque oscillations. I believe it is better viewed and explained in the synchronous d-q reference frame, where we see that the d and q variables oscillate at 60hz because it is a resonant frequency of the stator circuit.  I still plan to post a little more on that.
 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
Thanks, I like to discuss but my english is not so good so sometimes I miss something or write something that you maybe dont understand well.

This your statment:
"My comment was – steady state torque oscillation frequency in a single phase motor  is at 2*LF... torque oscillation during start of a 3-phase motor is 1*LF.  It was a simplistic attempt to try explain the complicated 3-phase behavior by breaking it into single phase pieces.  What was missing from my previous discussion is mention that the expression for torque from a/b/c currents includes not only terms involving a single phase, but also cross terms involving multiple phases... so we don't just add up three single phase torques."

I think it is not adequate, because it is totaly different fenomena in 3 phase and 1 phase motors. In 1 phase motors puslation of torque is because direct and inverse component of current (or flux). And frequency of pulsation is ws + wr (synronous speed + rotor speed). So I think that has no any connection with 3 phase motors.

 

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
"That's an interesting idea that the frequency of torque oscillations is the frequency of rotor current, but I believe it's incorrrect.  I did a simulation of normal unloaded start using your motor paramters above (even though I didn't try to model the locked rotor parameters welll... still gives us a qualitative idea).  The results are attached and slides 1 and 2.   As close as I can tell the frequency of oscillation is constant and does not slow as the rotor accelerates (see slide 2).  So I think it is line frequency, not half of rotor current frequency.
"

I am not sure when I look on this graphs what is with frequency, because oscillations ends on very small speed.
Probably you have better zoom so I only can trust you. But on my simulations frequency of torque oscillations are same as rotor currents. Can you try simulate some flystart (starting motor that is already running for example on half of synchronous speed). Then you will be sure what is with frequency.

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
"Yes, I agree.  And it argues that it is not strictly the dc component causing the torque oscillations as you said. Instead there are subtle changes in phase angles going on that contribute to these torque oscillations. I believe it is better viewed and explained in the synchronous d-q reference frame, where we see that the d and q variables oscillate at 60hz because it is a resonant frequency of the stator circuit.  I still plan to post a little more on that.
"

I notice also. Torque oscillations are because stator and rotor angle oscillate. This is main factor for such high torque oscillations.
So only question is why angle between rotor and stator current oscilate.

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
Sorry not stator and rotor angle I meant to say angle between stator and rotor current oscilate.

Milovan Milosevic

RE: Motor Current Oscillation

(OP)
sorry,
One more mistake. Frequency of pulsation in 1 phase motor is 100 hz or 120 not as I said before. You was right. I was think a little bit.

Milovan Milosevic

RE: Motor Current Oscillation

Quote:

I am not sure when I look on this graphs what is with frequency, because oscillations ends on very small speed.Probably you have better zoom so I only can trust you. But on my simulations frequency of torque oscillations are same as rotor currents. Can you try simulate some flystart (starting motor that is already running for example on half of synchronous speed). Then you will be sure what is with frequency.  
I stand corrected.  You are absolutely right, the torque oscillations occur at a frequency of LF*(1-slip).  This is demonstrated attached using lower inertia (100 kg*m^2 changed to 10). I'm definitely learning something here, thanks!

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

It's been a good discussion, I've learned a lot, I hope it continues.

I'd like to summarize some of the features of the DOL start that we've talked about, and add a little bit of an attempt at explanation/analysis.

First there are two frequencies of oscillation we'll mention:  LF*s and LF*(1-s)

1 - The oscillation at LF*s occurs in the torque and is obvious in the d and q currents. (see slides 1 and 2)

2 - The oscillation at frequency LF*(1-s) occurs in the DC offset of the currents when viewed as the a./b/c stationary variables (as we expect to measure them in the field).  This is shown in slides 3 and 4. Some notes to add about this oscillation:
2A – It starts off very closely resembling "classical" simple decaying dc component  (as discussed in thread237-283053: DC offset in induction motor inrush current  ) perhaps because initially the frequency LF*(1-s) is zero.... but as LF*(1-s) increases the oscillation becomes evident (as expected).  
2B – For locked rotor condition, LF*(1-s) remains zero throughout, and the dc offset remains a classical simple decaying dc throughout, with no oscillation in that dc offset (as expected) as shown in slide 6.  Locked rotor torque is shown in slide 7 and of course frequency LF*(1-s) remains constant at LF.  Also the oscillations seem to last much longer at locked rotor conditions.
2C – It is postulated but not confirmed that LF*(1-s) frequency oscillation in the offset is related to the LF*(1-s) frequency of the rotor current when viewed in the rotor reference frame (slide 5)

I have a vague unproven belief that if we study the synchronous-reference frame d-q transient equivalent circuit of Krause, we might gain some more understanding in a manner similar to how the steady state equivalent circuit helps us understand other things.  

I think we can roughly identify where these oscillations at frequencies of s*LF and (1-s)*LF come from by studying Krause's transient circuit (slide 8).

Marked in red on Slide 8 is a path for circulating current in the rotor circuit which a resonant frequency of s*LF... I believe this is where the LF*s oscillations come from.  There is analytical "proof" on slide 9.  Slide 10 shows that the phase relationships in the simulation are in agreement with the proof.  

Slide 12 shows a path for circulating currents in both rotor and stator circuits with a resonant frequency of (1-s)*LF.   I believe this is where the LF*(1-s) oscillations come from.  The analytical proof is not shown but it should be very easy to apply the exact same logic shown in slide 9 to this scenario.  Slide 13 shows that the phase relationships resulting from the simulation do obey the expected phase relationships for this circulating path.

If we are looking for physcial insight, we observe that we have currents when represented as d-q variables in sync frame are  sinusoids of constant magnitude.  Oscillating between d and q axes.  This represents phase change of the sinusoidal currents which is what we had already speculated.  

Maybe (hopefully), someone can chime in if there are more or different conclusions we can draw on all of this.
 

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

I think the slide 8/9/10 explanation for the loop that creates s*LF was good and it shows up in the q and d currents in sync ref frame.

However the slide 11/12 discussion of a loop creating (1-s)*LF was incorrect. This frequency of (1-s)*LF does not show up in the sync ref frame currents.  Maybe instead it has something to do with the rotor reference frame rotor currents shown in slide 5.  

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

I had another thought about the physical interpretation of the LF*s oscillation which shows up in the torque and in the synch ref frame d-q currents.

Focus on the rotor d-q currents in synch ref frame.  They represent components of current which are physically 90 degrees apart on a ficticous rotor that is rotating at sync speed.  What we see in the simulation (slide 12) is that the oscillating component of these currents is equal magnitude and are 90 time degrees apart (90 time degrees as defined by the frequency s*LF).  What does that tell us?  

It doesn't tell us that the resultant synchronous field is just oscillating back and forth about some equilibrium.  It is in fact rotating (the sum of two sinusoidal fields displaced by 90 degrees in both time and space produces rotation).  They are rotating backwards with respect to the sync field. It is somewhat analogous to a phenomonon of pole slipping, but it occurs continuously as this component of the rotor field moves steady backwards with respect to the stator field. No wonder there are large torque oscillations predicted.  

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

(OP)
electricpete,
Thank you so much for such effort you make to explain all off this. I need few days to think a little about what you wrote and to make final conclusions.

Milovan Milosevic

RE: Motor Current Oscillation

Thanks. I still have a few errors that I've got to fix. I reversed s*LF and (1-s)*LF in some places.  Also I calculated rotor current in rotor ref frame wrong (plugged wr*t into the transformation from sync to rotor instead of wr*t-we*t), but that doesn't change the picture much. All of that is corrected in the new attachment labeled "Rev2"

One more thought I have had about these torque oscillations at s*LF is that when we transform a rotating field (like the one we see in the Iq & Id oscllation at s*LF in sync frame), it's frequency changes by the same amount as the change in ref frame speed.  So if we transformered that s*LF oscillation to the rotor ref frame, maybe we would see dc.  That would lead to a much simpler explanation: the resonant frequency in the rotor ref frame transforms to zero which is what we expect out of simple R/L circuit... this is simply a dc current in the rotor ref frame that is dieing away slowly due to rotor inductance.
At first glance, that seems logical.  If I calculate the time constant associated with that loop, I get
(LM+L2)/R2 =[(XM+X2)/<2*Pi()*50>] / R2 =[(0.4789909
+0.0197605110515279)/<2*Pi*50>] / 0.000978488734217218 = 1.62247673 seconds.   That looks like roughly the same constant that is evident in decay of torque oscillations slide 7.  So it seems like it might lead to a simple explanation that the dc in the rotor tends to remain at the same location with respect to rotor and causes torque oscillations at s*LF as the synch field passes by it at a rate of s*LF.

But unfortunately I still have one fly in the ointment. The rotor current in rotor ref frame does not seem show any dc component  (slide 5).  I have a suspicion that maybe I still have some error in my transformation of rotor currents from sync frame to rotating frame. After all we see what appears to be frequency getting faster... but it should be getting slower. Will keep working on checking that transformation.

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

On attached slides 11 I added corrected rotor currents in rotor reference frame (Now I integrated wr instead of multiplying by t as discussed in other thread).   The main frequency is now s*(1-LF) as expected (starts slow and speeds up).  There is superimposed a high frequency ripple, either LF or s*LF... I can't tell (tried a few things on slides 12-14 to try to figure it out but doesn't provide any clues).  There is no sign of the dc that I thought would be there. Back to the drawing board in terms of an explanation for this torque oscillation at s*LF.

=====================================
(2B)+(2B)'  ?

RE: Motor Current Oscillation

I suspect the rotor current plot in rotor frame is still wrong - I will try to correct that in the next few days.

=====================================
(2B)+(2B)'  ?

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