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quiz - can a DOL-start unloaded induction motor "overshoot" sync speed
3

quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
If I were to perform a direct-on-line start a large motor with no connected load, would you expect the speed to overshoot synchronous speed?

(note the word quiz - that's a clue that I know the answer - just asking for fun)

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
On what basis?

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Because my understanding of the process is that the rotor is trying to get up to the sync speed by being pulled by a force moving at that speed. As that speed is approached the pull is rapidly dropping.  Yes, there may be some overshoot over the 'slip' speed but not past the sync speed.  Further, any over speed would be significantly curtailed by the motor generating, and hence being electrically 'braked'.

This is not like a generator being driving by a large engine.  The engine not having any real synchronous relationship could easily over shoot the generator.

All this is seat-of-the-pants conjecture..

Keith Cress
kcress - http://www.flaminsystems.com

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

I don't think so. As the motor accelerates the accelerating force produced by the slip also reduces. Can a canoe go faster that the paddle?
If instrumentation indicated an overshoot, I would seriously check the instrumentation and recheck the supposed overshoot with different types of instruments.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Are there any assumptions built into the torque speed curve?

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
By the way, my answer is yes it can exceed sync speed.  One reference shows a simulation of a large motor started unloaded DOL where it exceeds sync speed by 2-3% (estimating from a graph).

There are assumptions built into the torque speed curve...

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

You gave the answer too soon, Pete!

I did a recording once showing that it can, indeed, overshoot. Especially it there is an inertial load and very little friction. The reason is that acceleration is quite high and that it doesn't go to zero immediately when you reach synch speed.

That is also why an induction motor can oscillate around zero 'pole angle'.

I will either find that recording, or make another one.

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Maybe you're right I did let it out of the bag too soon.  

But I still haven't revealed my explanation (no, not really mine... Krauss' explanation although I'd be happy to take the credit).

The assumption built into the torque speed curve?

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

The assumption, as I see it, is that it describes a stationary solution.

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Yes, that's right.  I would call it a "steady state" assumption.  The torque speed curve gives relationship between torque and speed for steady state operation but is not necessarily valid during transients.

Attached are some excerpts from Krauss'   "ANALYSIS OF ELECTRIC MACHINERY AND DRlVE SYSTEMS" 2nd ed.

In slide 1 you see the characteristics of the motors studied for "free" (unloaded) acceleration.  Curves of actual torque vs speed during acceleraiton are given for 50 hp motor (Slide 2), 500hp motor (slide 3), and 2250hp motor (slide 4).

On slides 3 and 4 the steady state torque-speed curve is plotted on the same axis.  If we imagine the transient progressing vs time, the actual torque is lagging behind  the steady state torque... it never reaches the breakdown torque and it is slow to return to 0 (has not returned to 0 yet when we reach sync speed).

The reason is that the rotor current decays with a rotor L/R constant which is relatively long for high horsepower motors (designed to have good efficiency and therefore low rotor resistance).     If we are steady state at sync speed we have zero rotor current and 0 torque.  But if we rapidly accelerate to sync speed, rotor current has not yet decayed to 0 and there is still accelerating torque even at sync speed.

I have to admit I was very surprised by this also – never heard of it until today.   The torque speed curve is so familiar that I guess I forgot about the steady state assumption built into it.

I think it is relatively common in motor starting studies to simply read the torque off of the torque speed curve (that's what was done at our plant).   That approach in my mind is similar to a "quasi-steady-state" approximation because it borrows steady state results to apply to transient simulation.  The error is small if it is a small motor because with the high L/R the actual torque reaches steady state torque very quickly.   I think the error is also smaller for loaded start than unloaded start because the speed changes slower.   But for unloaded start of a large motor the steady state torque curve is a pretty bad approximation of actual torque
 

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Correction in bold:

Quote:

The error is small if it is a small motor because with the low L/R the actual torque reaches steady state torque very quickly.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Thank you for the continuing education, Pete and Gunnar. I am sure that the first time you saw this Gunnar, you didn't accept it until you had rechecked your instrumentation.
Thanks again.
Bill

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Well, Bill. We were quite a lot of people that were surprised to see this. And the instrumentation to see the phenomenon was not even there. The tell-tale sign was an unstable induction motor. It is a long story, that I shall keep as short as possible.

I was a commissioning engineer at Siemens back in the early seventies. At that time, there were no fast frequency inverters, so the test rig for fast compressors that I was starting up had Ward-Leonard systems that fed a DC motor with a multi-pole three-phase generator. The WL system excitation was controlled by a PWM DC amplifier which was controlled by a standard speed controller.

I was fairly young at that time. But had been working with variable drives in the Swedish Army, at ASEA (now ABB) and Siemens for almost ten years, so I thought that I had seen it all already (little did I know).

Anyhow. I couldn't make that drive stable. It oscillated constantly with a low amplitude and a rather high frequency - something like 10 Hz, if I remember correctly.

Nothing could stabilize it. All the standard tricks were of no use. I finally replaced the speed controller with a fixed voltage source that excitated the WL directly. Oscillation still there. To rule out the suspicion that the WL was unstable 'in sich' (sorry, German company), we got ourselves a set of forklifter batteries and ran the DC motor from that. Oscillations still there. Munich was now engaged (the system had been designed there) and a Doktor-doktor plus a staff of technicians arrived with a van full of instrumentation.

A very thorough test followed. One interesting test was to record the speeds of the generator and the induction motor. We clearly had an unstable system. It was ringing loud and clearly and the 'steady state' assumption inherent in the speed/torque curve of the induction motor was scrutinized in detail. Hey! This was research - and I loved it.

The quick fix this time was to add inductance in the only available path, the connection between generator and induction motor. The inductance consisted of three 35 mm2 Cu cables that were laid in coils with 50 turns and around 600 mm diameter. Not that we really could prove mathematically that it would work. It was what was available and it worked the first time.

The DOL overshoot is something I could see on a much later occasion. I shall look for that recording. Or, if necessary, make another one. The instrumentation to do it is much more userfriendly these days. Problem: Finding an induction motor with an encoder that is DOL started (the motor, I mean).

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Pete.

Do you have a DOL induction motor with an encoder?

If so, I could send you my fast encoder decoder (works with reciprocal frequency measurement). It makes fast speed recording possible. I even was able to record longitudinal transients in a sheet of steel in a cutter with it.

There is a short description at http://www.gke.org/rapporter/files/Reciprok%20frekvensmetning%202.pdf
Sorry, only in Swedish, I really have to do something about that. The last picture shows step response. It is around 300 microseconds in this case. It can be made faster, if needed.

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Is the overshoot due to the accelerating inertia of the rotor ?

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

That is what I hastily thought. But I am not so sure any more. There is also an accelerating torque that depends on the low frequency rotor current, which doesn't (cannot) go to zero immediately as the synchronous speed is reached and thus creates an accelerating torque also after synchronous speed has been reached.

I think that Pete will find out using one of his math tools. Stand by!

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

We are all familiar with the phenomena of synchronous motors "Hunting".
Am I correct in visualizing an induction motor hunting, where the frequency excursions due to hunting are more than twice the slip frequency?
Thanks

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
edison - Inertia is not the source of the overshoot. As gunnar said in his latest post, a high-inertia load would overshoot less than a low inertia load if all else is equal (same motor and same load torque).  Inertia is a property which makes an object want to keep moving (rotating) at a constant speed unless acted on an outside force.  When you remove accelerating force, acceleration stops.

Rather than inertia being the source of the overshoot, rotor inductance is the cause of the overshoot.   Inductance is a property which makes a current want to keep on flowing at the same level unless energy is added/removed from the circuit.   If you plotted used the steady-state current-vs-speed curve and added to it a plot of actual current vs speed during the transient, the steady state  current would decrease faster than the actual current (in the neighorhood approaching sync speed) because the inductance limits how fast the actual current can decrease.

Gunnar - we don't have any multi-pulse-per-rev gear type encoders on any motor driven equipment (I think we have them on our main turbines for torsional montioring).  We do have a few large motor-driven machines with keyphasor (one pulse per rev) as part of Bentley Nevada vibration monitoring.  I never looked closely for speed indication output from those devices, but I will check.   We also have the ability to capture vibration time waveform at startup with high sample rate using our walkaround vibration data collectors.....that is a little bit less direct indication of speed, but I think I could estimate speed reasonably well from there.   We'll see what opportunities present themselves.

waross -
It is interesting to observe that the torque applied between the stator and rotor of an induction motor acts like a damper (related to rate of change of rotor position - and in fact directly proportional to rate of change of rotor position if we linearize about the operating point).  In contrast the torque applied between stator and rotor of a sync motor acts like a spring (proportional to rotor angular position).  

So an induction motor model would look as follows:
Stator  ===   Field/Damper === RotorInertia
If the entire machine inertia acts rigidly, there is no oscillating frequency inherent in the above "mechanical" system.  If you perturb it, it will just slowly settle toward equilibrium position rather than oscillating.

In constrast for sync machine the field plays the role of a spring.   Armotrisseur windings do play the role of a damper so we could call it a damped spring, but the damping factor is low.    So a sync motor model would be:
Stator =   Field/DampedSpring ===RotorInertia
If you perturb it from initial position it will experience decaying oscillations (assuming damping is less than critical damping).  Much different than induction motor.

Now there are plenty of other oscillations that can occur.  If coupling acts as a flexible spring, then the motor inertia and driven-load inertia can move in opposite directions and oscillation can occur.  Also if there is any controls associated with he power supplied to the motor (various types of vfd), new possibilities for hunting are introduced.

So there are a lot of possibilities for hunting.  At least one of them is eliminated due to the damping effect of induction motor and absence of any spring action between stator and rotor.  I tend to think in general induction motor is less susceptible than sync motor because that damping damps many different types of oscillation.  I don't have a lot of experience with any hunting problems... just my thoughts fwiw.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
I guess my discussion of induction motor as damper ignored  the behavior described in this thread.  Still I think it is important and interesting to note that as a first approximation the torque in induction motor acts like a damper and the torque in a sync motor acts like a spring.  There can be other effects when we analyse more closely.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

If the load inertia is very low, then the rate of acceleration can be very high.
The rotor circuit has a relatively long time constant and for a short period of time (relative to the rotor time constant), the rotor appears to have constant excitation.
If the rate of acceleration is higher than the rotor time constant, then as the rotor approaches synchronous speed, there will be a period where the rotor is "over excited" relative to the speed it is operating. It is thus able to behave like an over excited synchronous motor and ocsillate around synchronous speed.

Best regards,

Mark Empson
L M Photonics Ltd

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Yes, Marke. That is what we saw. An oscillating system where there shouldn't possibly be one.

Oscillation is undamped ringing. Put some velocity dependent losses in the system (first derivative) and you have a system that overshoots when subject to high acceleration.

Pete is the one most versed in math. Looking forward to an analytical explanation.

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
I agree with Mark's explanation - it is the same basic phenomenon but explains the oscillation a little better.  To the extent the current is slowly decaying it acts like dc.  And if you have dc current on the rotor it acts like  a sync machine.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Actually the sync machine is not a perfect analogy since it is not oscillation in phase but oscillation in frequency.  We would be slipping poles many times on a sync machine and that difference in speed doesn't generate any net restoring torque on a sync machine but it does on induction.

Another way to look at it is that we have a control loop with a target speed of 30hz.  The rotor current is part of the feedback comparing actual speed to target speed.  Since there delay (due to inductance) in the rotor current feedback, we have overshoot and oscillation.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Very interesting posts. I assume this does not happen in slipring motors or motors which have controlled start like wye/delta starting, VFD, soft starters etc ?

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
From my perspective reading Krauss,  it applies to large squirrel cage motors since they have high L/R ratio, especially in the rotor circuit.   It also applies during rapid changes  (in addition to DOL start, he also shows example of slight ringing on the large motor during rapid load change and during external fault which is quickly cleared).

It would not apply to smaller motors since they have lower L/R (for example see the 50hp startup curve in attachment above - no overshoot).   It would not apply to vfd start since the speed is slowly ramped.  It would not apply to slip ring motor if: 1 - speed is increased slowly, or 2 - motor is small.  If there is large slip ring motor started DOL, I suppose it's a possibility although I think rotor resistance is higher for slip ring motors than squirrel cage motors.

I was able to do a simulation which matched the results posted.  I will posts some results from there to show a slightly different view:  speed vs time and torque vs time.   The thing that confuses me a little bit is the time constants.  For this 2250hp motor with R2' ~ 0.02,  XL2' ~ 0.2, Xm ~ 13, which has L/R on the high (long) end of the spectrum, it still seems to have a fairly small/short time constant.  Using the the rotor leakage inductance:
Rotor L/R = L'/R' =  Xl' / R' /(2*PI*60) = (0.2/0.02) / 377 ~ 0.025 sec.
That seems relatively short in comparison with the transient and the oscillation a..........

Now if for some reason you included Xm in the ratio the time constant would be a lot longer (but it's not obvious that should be included)
Rotor L/R = L'/R' =  Xl' / R' /(2*PI*60) = (13/0.02) / 377 ~ 1.5 sec.

At this point it doesn't quite make sense to me - I can't reconcile the time constant with the transient.  Maybe my question will be clearer to readers when I post the plots vs time (maybe tonight).
 

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Correction in bold:

Quote:

Now if for some reason you included Xm in the ratio the time constant would be a lot longer (but it's not obvious that should be included)
Rotor +Magnetizing L/R  ~  (L2'+Lm)/R' =  [(X2'+Xm) / R'] /(2*PI*60) = (13/0.02) / 377 ~ 1.5 sec
[/quote]

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Looking forward to that, Pete. I knew you would do it.

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Interesting subject! I made some simulations with ATP. Results are shown in the attached picture. Speed is given in rad/s as a function of time. Motor has some "typical" parameters (that I picked up somewhere..), and two pole pairs. The frequency is 60Hz. The synchronous speed is 188.5 rad/s.

First picture: nominal parameters, normal start-up.
Second picture: inertia reduced by a factor of ten.
Third picture: Additionally, the rotor resistance reduced by a factor of ten. Has this kind of growing oscillations been observed with real motors?

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Hey! The third picture is what we got back in the early seventies when our induction motor started oscillating for no apparent reason (se post 5 Jul 09 0:08 in this thread).

Yes, it does exist IRL. Only, our oscillation didn't grow limitless. It stayed within a few percent amplitude.

This thread is getting better and better!

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
I got busy last night, never got to post my simulation. Will try again tonight.  It was a damped oscillation.

The oscillation will certainly be larger if you decrease rotor resistance and/or inertia.  But continually growing oscillations seem impossible to me (I'll bet if you continue the simulation it will level out and eventually start decreasing and if long enough return to steady state).   

This conclusion is based on looking at the motor start event in the synchronous ref frame (which is a very interesting and informative excercize).   In the sync reference frame, we have similar a pair of stationary (sync frame) magnets representing the stator field.  Initial conditions: at the moement of start we know in our normal reference frame the rotor is at rest, but in the sync ref frame the rotor is rotating at 3600rpm (2-pole motor) the moment before start.    Then the magnetic fields induce circulating bar currents which will slow down the motor.    Conservation of energy tells us that by the time the rotor comes to rest in the sync frame (up to sync speed in our normal ref frame), the I^2*R energy dissipated by the rotor must equal the initial kinetic energy of the rotor (correspondin to 3600rpm).   (This also correctly predicts the amount of energy dissipated in an unloaded motor during start by the way).   Once the energy is dissipated in the rotor resistance, there is nothing to sustain the oscillation and it must eventually disappear (otherwise we have a perpetual motion machine).

By the way, what is ATP?

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
I should clarify my conclusion is based on DOL start of unloaded motor connected to a pure sinusoidal power supply.  More complicated power supply or load can of course cause sustained oscillations.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

ATP means Alternative Transient Program. It is similar to ETAP, only a lot cheaper (free).

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Attached are results of a simulation of the 2250hp motor example discussed above.  Simulated in Matlab using d-q variables (current, voltage, flux linkage) in the synch reference frame as outlined in Krauss page 150.  

Slide 1 are machine parameters.
Slide 2 is torque vs speed – same as was shown in Krauss.  
Slide 3 is speed vs time
Slide 4 is Torque vs time
(oscillations in speed and torque occur at 7-8hz and decay quickly)
Slide 5 is stator flux linkage (lambda)  along the q axis in sync ref frame
Slide 6 is stator flux linkage (lambda)  along the d axis in sync ref frame
Slide 7 is rotor flux linkage (lambda)  along the q axis in sync ref frame
(This is the only one of the 4 flux linkages that shows the oscillation!)
Slide 8 is rotor flux linkage (lambda)  along the d axis in sync ref frame
Slide 9 is stator  current along the q axis in sync ref frame
Slide 10 is stator current  along the d axis in sync ref frame
Slide 11 is rotor current  along the q axis in sync ref frame
Slide 12 is rotor current  along the d axis in sync ref frame
(oscillation shows in both stator and rotor current in q axis only)
Slides 13-15 are stator currents transformed to the normal (stationary) reference frame

I think there are at least three interesting things to talk about.

ITEM 1:  OVERSHOOT OF SPEED AND SUBSEQUENT OSCILLATION
(the main subject of the thread).
Krauss makes the statement that these are the result of a rotor electrical transient.  He "proves" this in the chapter on "reduced order models" by doing a reduced-order simulation which NEGLECTS stator transients but includes rotor transients.  The overshoot and subsequent speed oscillation is still there in the results of that simulation and therefore not a result of stator transient and must be a result of rotor transient.   The reduced order model can be solved for eigenfrequency of about 7-8hz.

If I look at the results of my simulation slides 5-8 are 4 flux linkages in the q and d axes for the stator and rotor.  The oscillation only shows up in one of the four – the rotor q axis flux linkage.  I guess this is consistent with the fact that the overshoot originates from rotor transient.  I'm not sure what we learn from the fact that it's associated with the q axis as opposed to d axis.    When we look at stator currents (slides 9 through 12), we see the oscillation shows up in the q axis current of both stator and rotor – I guess the stator current oscillation is a result of the transient rather than an initiator.   Although when we look at the normal stationary-frame abc stator currents in slides 13-15 it's very tough to see the oscillation there.

ITEM2:  SLOW OSCILLATING DC ADDED TO LRC ENVELOPE DURING THE  FIRST SECOND
See slides 13 – 15.  Look at the top and bottom of the envelope and you see they move up and down together indicating some added dc component. The envelope of the LRC slowly varies up and down.
Krauss says it is due to interaction of stator electrical transient and rotor electrical transient (whatever that means... hard to visualize... don't see it in the flux linkages).   If anyone can provide a better explanation I'd be interested to hear.

The interesting thing is I have observed similar oscillations in recorded motor start information on a very similar motor (2500hp, 1800rpm, different voltage=13.2kv).  (If anyone wants me to post that let me know).  I have wondered about it before - now I know it is nothing abnormal -  it is expected.  This particular item is apparently not limited to large /  high-efficient motors – it also shows up in Krauss' simulation of a 3hp motor start.

ITEM 3:  60HZ TORQUE OSCILLATION IMMEDIATELY AFTER START (NEAR ZERO SPEEED).
This is shown in slides 2 and 4  (also a slight speed ripple shows up in slide 3).    Krauss says this is due to the transient dc decaying offset of the stator currents.   In the chapter on reduced order models he simulates the start with stator transients nelgected, and this initial 60hz torque oscillation is mostly absent (which sort of proves it is caused by stator electrical transient).    Now it's interesting to observe that the initial stricly-decayind dc offset is gone within the first 0.1 seconds  (slides 13-15), but yet the 60hz torque oscillations last to at least 0.7 seconds (slide 5).  I guess the relevant dc is associated with that weird slowly-oscillating dc of item 2?  But it's strange because the envelope of the 60hz oscillating torque doesn't show any of the similar oscillation as shown in item 2... the 60-hz oscilating torque envelope just decays very smoothly to 0.  Pretty bizarre.   

I can't say I have ever seen evidence of 60hz oscillating torque during start although I wouldn't know what to look for.   If there were a 60hz torsional resonance it could be briefly excited.    I guess this is not so bad as the sync motor startup torque oscillations which vary in frequency from 120hz down to 0hz and can excite every torsional resonance in between.
 

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
I should clarify what is meant by stator and rotor transients and neglecting stator and rotor transients.

The induced voltages in any reference frame can be expressed as a sum of a speed voltage and a d/dt(Lambda).  Transients are associated with the d/dt(Lambda) term.  Reduced order simulation discards the d/dt term - sort of like a quasi-steady state analysis.  Good enough for most purposes and significantly reduces the compuation time.

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

Great that you did this, Pete!

I have been looking at the frames and your comments. The reason why you do not see any oscillation in Id is, I think, that there is no energy exchange involved between Id and the pole angle oscillation. Simply because there is no torque produced by Id, only EMF (remember our discussion about Back EMF?).

The oscillations shall only show in Iq, because that is where the "spring", i.e. elastic coupling, between rotor and stator flux is and that "stretching/relaxing" the spring influences the current.

A great thread, this is. I will look further and try to understand what is happening. Did you ever try ATP? I think you should. I registered and received the package. But it takes some time to get started with it. And I don't ever seem to get that time sad

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

RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
Thanks for those comments Gunnar

The applied stator voltage in my simulation was on the vq axis in the sync ref frame.  So it would make sense to me that  Iq is in-phase with applied voltage and therefore associated with real power (associated with non-zero average torque) and Id is associated with changes in stored magnetic field energy.  Is that correct?     

Even if that is the case I don't see that this necessarily proves why Id is not involved in transient torque.   Look at the instantaneous torque equation:
Te = (3/2) (Poles/2) * (Lamba_q*I_d - Lambda_d*I_q)
I believe the steady state torque is associated with Lambda_d*I_q.   But the transient torque also includes contributions from Lambad_q*Id.   So it is not immediately obvious that Idr  would be necessarily be excluded from torque oscillations.

For my present purposes, I am happy using Matlab or excel for solution of O.D.E. initial value problems.   It is a very general solution method applicable to a very wide variety of problems and I am already familiar with it.    I may eventually look at ATP if I find the need for a very large simulation or if I need to double check results of a model.

I noticed ijl's simulation also shows small speed ripples during the early acceleration - evidence that his model also predicts those line frequency torque oscillations immediately after start (item 3 above).
 

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RE: quiz - can a DOL-start unloaded induction motor "overshoot" sync speed

(OP)
I recreated ijl's scenario of increasing oscillations by decreasing R2 by factor of 10 and decreasing J by a factor of 10.  Unexpectedly, the oscillations came to steady state and never showed any sign of decreasing even though the simulation ran for 300 sec.  That's a surprise to me - I have no explanation.  

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