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

[electricpete]

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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[itsmoked]

I'm guessing NO.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[electricpete]

On what basis?

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[itsmoked]

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

 

[waross]

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

 

[electricpete]

Are there any assumptions built into the torque speed curve?

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[electricpete]

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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[Skogsgurra]

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

http://www.gke.org

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[electricpete]

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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[Skogsgurra]

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

Gunnar Englund

http://www.gke.org

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[electricpete]

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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[electricpete]

Correction in bold:

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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[waross]

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

 

[Skogsgurra]

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

http://www.gke.org

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[Skogsgurra]

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 frekvensmetning 2.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

http://www.gke.org

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[edison123]

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

 

[Skogsgurra]

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

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

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

 

[electricpete]

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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[electricpete]

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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