Motor Current Oscillation 3

[Masbibe]

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

 

[Masbibe]

And one more picture where you can see how is oscillation amlitude depends on ramp time.
So on this graph frequency setpint is 12 - 14 - 12 - 14 Hz, but with different ramps.


Milovan Milosevic

 

[electricpete]

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?

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(2B)+(2B)' ?

 

[Masbibe]

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

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[Masbibe]

Yes, you can use value 764 A.

Milovan Milosevic

 

[Masbibe]

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

 

[Masbibe]

And stator and rotor currents.

Milovan Milosevic

 

[electricpete]

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?


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(2B)+(2B)' ?

 

[electricpete]

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):

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

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(2B)+(2B)' ?

 

[Masbibe]

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

 

[electricpete]

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.

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(2B)+(2B)' ?

 

[Masbibe]

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

Milovan Milosevic

 

[Masbibe]

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

 

[Masbibe]

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

 

[Masbibe]

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

 

[Masbibe]

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

 

[electricpete]

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

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.

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.

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.

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.

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.


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(2B)+(2B)' ?

 

[Masbibe]

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

 

[Masbibe]

"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