Electro-mechanical resonance frequency 4

[InspectorGadget]

Has anyone got information on Electro-mechanical resonance? I have seen some info about this with servo motors but this concerns a standard AC induction motor. I think that if this resonant frequency coincided with VFD firing frequency or a integer multiple it could cause severe torsional vibration. What do you think? Does anyone check for this potential problem?

 

[electricpete]

In the previous discussion linked on the other board, you may have seen links to some word files which were very hard to read. I have re-created the last word file in very different format as a pdf file which has much easier to read equation formatting (although still may not be straightforward depending on whether you have used a similar computer program to Maple).

The link is here:

http://www.brazosport.edu/~pschimpf/forums/engtips/TwoMassSpringInductionSimpleRev1.PDF

It solves a system of motor rotor inertia, compressor rotor inertia, coupling spring, coupling damping, and induction motor field which acts as dampener. The exciting force is assumed to come from the load end. The variable Xd (d for differential) is examined which presents differential position between motor rotor and compressor rotor i.e. torsional flexing of the coupling.

There is one root at zero frequency (I provide some discussion of interpretation with respect to response to step change in load at the end). This zero frequency root I associate with the motor field. There is one resonance at w=14 which is very close to that computed from compressor mass and coupling spring. The frequency of this resonance may be altered some by the damping effect of the motor field. imho, there is no new resonance created by the motor field because there is no spring action associated with the motor field.

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[jbartos]

Comment on the previous posting: The posting link does not have addressed electrical variables or parameters. Therefore, the modeling is strictly mechanical. The original posting addresses "Electromechanical resonance." Clarifications are requested.

 

[electricpete]

The effect of the transient motor torque (identified above as T1(t) above) is captured in the damping element C1. The justification for this approach is identified in my 2nd post today 3/26.

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[electricpete]

Sorry, it is identified in my 3rd post today.

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[electricpete]

I think 2 points deserved to be mentioned again.

1 - If we were discussing a sync motor, there would be a spring corresponding to the field and an associated resonance frequency. Not so for induction motor.

2 - In a vfd there may be harmonic torques which will serve as excitation. The resonance excited by those torques (for induction motor) would be mechanical.

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[jbartos]

Suggestion: Visit

http://www.ipst.org/TechPapers/2001/IPST01Paper240.pdf

For "Perceptions about new kinds of subsynchronous resonances":
The numerical calculation of different types of drives brought an important insight: also in systems with induction machines an electromechanical resonance can occur, especially during running up. Consequently the torque in the connection components between motor and main engine can reach values much bigger than the maximum stationary breakdown torque (see fig. 8). This highly depends on the mechanical-geometrical and the electromechanical attributes of the entire system. Also the inner mechanical damping has to be considered. As a result, no global predictions about the endangerment of a drive system can be made and must not be made.

 

[electricpete]

The article linked above by jbarots discussing oscillating torque during startup of an induction motor.

In the article I references Engineering Dynamics Paper “Torsional Analysis of VFD’s”, this oscillation is not discussed under system stiffness on pages 2 & 3. It is discussed on page 7, more in context of a driving excitation. Whether or not you choose to call this an electromechanical resonance is a matter of terminology. I can see support for both viewpoints, and I previously have accepted the comment that electromechanical resonances are present.

One logical question is why does the model I showed above not predict this behavior. The model I showed is based on torque proportional to slip. We know this is true at steady state when speed is within a few percent of syncronous speed. It is reasonable to expect it to be approximately true for small transients from steady state condition. It is not reasonable to expect it to hold for faster transients or starting from zero speed. Below breakdown torque, the torque vs slip curve no longer resembles a proportional relationship and the torque no longer represents a damping effect.

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[jbartos]

Comment: There is a record of generator shaft cracks due subsynchronous resonances caused by the series in-line capacitors in the transmission lines. This is what is normally considered electromechanical resonance in electrical power engineering. Of course, there are strictly mechanical resonances caused by mechanical dynamics linked to the load behavior, etc. I see that many above postings are strictly addressing mechanical dynamics and associated resonances. It is necessary to address the discussed problems in broader terms. It appears that this thread is slowly heading in that direction.

 

[electricpete]

jb - I have never heard any reference to induction machine torsional failure associated with power system interaction as you describe. What you describe is associated with syncronous machines.

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[InspectorGadget]

Everyone, thanks for the input. This is really fascinating stuff.

Epete,
Please explain why you say this phenomenon only exists in synchonous machines. It may be my misunderstanding of how they work. I will go do some study. Don't VFD's have the potential to make torque pulses? Isn't the ouput wave actually made up of lots of square waves?

 

[InspectorGadget]

I'm wondering are there any motor/drive manufacturer engineers out there? Is this a concern for you?

 

[electricpete]

IG - Yes vfd's can make torque pulses, which can excite mechanical resonances in induction machines. I was referring to the discussion of subsyncronous resonance as a result of interaction with the power transmission system. This applies to only to syncronous machines in my opinion.

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[jbartos]

Comment on electricpete (Electrical) Mar 29, 2004 marked ///\\The article linked above by jbarots discussing oscillating torque during startup of an induction motor.
///Yes. Beside that, the paper also addresses:
1.
II. USUAL SSR PHENOMENA
The usual SSR phenomena caused by compensated transmission lines can be manifested in three forms: Induction generator effect (IGE), torsional interaction (TI) and torque amplification (TA). These phenomena may occur isolated or simultaneously, and when they occur they can cause damaging oscillations. Hazardous levels might be reached within 0.1 seconds.
2.
For calculation of the electrical resonance, the structure of the electrical system according to fig. 1 can be represented by a reduced model. The resonance frequencies depend on the short circuit capacity of the main system (fig. 4). For the analysis of SSR it is necessary to evaluate electrical frequencies close to:
for the steam turbine: fn - fm= 50 Hz - 18 Hz = 32 Hz
for the gas turbine: fn - fm = 50 Hz - 12 Hz = 38 Hz
where fn is the electrical frequency and fm is the mechanical resonance frequency.
3.
etc.
However, the generators are often considered for the subsynchronous (subharmonic) resonances, since the transmission lines are not directly connected to large induction motors. The impact of subharmonics would be somewhat similar to the detrimental impact on the generator rotors.\\

 

[jbartos]

Comment on electricpete (Electrical) Mar 29, 2004 marked ///\\jb - I have never heard any reference to induction machine torsional failure associated with power system interaction as you describe. What you describe is associated with syncronous machines.
///Yes, that is possible since, as I mentioned in my previous posting, the transmission lines are not directly connected to large induction motors. The impact of subharmonics would be somewhat similar to the detrimental impact on the generator rotors.\\