Failed Capacitor/resistors

[T.M.Haja Sahib]

The physical design deliberately ensures that impedance between the bucket and the IGBT switching elements is negligible for entirely practical reasons.

Even when there is a output filter?

 

[Besoeker]

Even when there is a output filter?

Unnecessary complication and equivocation.
What happens at t>0?

 

[T.M.Haja Sahib]

Unnecessary complication and equivocation.
What happens at t>0?

I only wanted to stress that the presence of output filter implies that the leads from AFD output are long and so their impedance is not negligible.
But still if you consider it negligible, please answer your own question.

 

[Besoeker]

I only wanted to stress that the presence of output filter implies that the leads from AFD output are long and so their impedance is not negligible.

That's a different scenario.
My post #63 where this little discussion started:
One example of where you do get high rates of change of voltage (dv/dt) is theoutput of a variable frequency. Fitting PFC there is an absolute no no because of that and the peak currents that would flow.
With an output filter and long leads it is no longer the output of the IGBT inverter bridge.


But still if you consider it negligible, please answer your own question.[/QUOTE]
The physical design deliberately ensures that impedance between the bucket and the IGBT switching elements is negligible for entirely practical reasons.
I'm sure that your grasp of simple is, or perhaps ought to be, sufficient for you to work out what happens to the current at t>0.

 

[T.M.Haja Sahib]

If you say the impedance between the capacitors is negligible, the voltage difference between the two capacitors should also be negligible. This follows from Kirchoff's voltage law. But if you still insist that the voltage difference is not negligible, it means that you committed a design mistake by choosing wrong size capacitors and so do not close that switch; heavy current would flow.

 

[Besoeker]

If you say the impedance between the capacitors is negligible, the voltage difference between the two capacitors should also be negligible. This follows from Kirchoff's voltage law.

It doesn't. The capacitor on the LHS, C1, is the DC bucket so will be at the voltage of the rectified AC. Possibly around 600Vdc. Until you close SW1, C2 would most likely be uncharged.

But if you still insist that the voltage difference is not negligible, it means that you committed a design mistake by choosing wrong size capacitors and so do not close that switch; heavy current would flow.

The mistake is not in terms of design. It would be in connecting a capacitor across the output terminals of the inverter which is why I wrote, all those posts ago, that doing such a thing is an absolute no no and maybe, just maybe, you are now beginning to understand why.
Heavy currents would indeed flow taking the IGBT beyond its SOAR curve and blowing its arse off.
That's why, in a nutshell, you cant connect capacitors at the the output of a variable frequency IGBT inverter.

More education for you. Still no charge. But I'm beginning to wonder if I ought to review that practice......

 

[T.M.Haja Sahib]

I am willing to learn from you too. But do not be evasive. Consider this: high dv/dt from AFD could cause damage to the driven motor winding but a capacitor across its terminals can smooth it out safely. But owing to high harmonic content, use of capacitor on the output side of AFD would damage it. My plan is to use a filter to take care of harmonics so that the capacitor could be used to reduce the dv/dt to a suitable value across the driven motor terminals.......

 

[Besoeker]

I am willing to learn from you too. But do not be evasive. Consider this: high dv/dt from AFD could cause damage to the driven motor winding but a capacitor across its terminals can smooth it out safely.

I despair.
You can't do that. It is an absolute no no.

 

[T.M.Haja Sahib]

I despair.
You can't do that. It is an absolute no no.

The basic building blocks diagram of ASD is given at page 8 of link below and note presence of capacitor on the output side of ASD for function I already mentioned.
http://pes-psrc.org/Reports/Final_J...plication_and_Issues_Working_Group_Report.pdf

 

[mivey]

The basic building blocks diagram of ASD is given at page 8 of link below and note presence of capacitor on the output side of ASD for function I already mentioned.
http://pes-psrc.org/Reports/Final_J...plication_and_Issues_Working_Group_Report.pdf

You will notice that this filter does not result in a capacitor in parallel with a capacitor. You may also note from page 16:

3. If there are capacitors on the bus feeding the ASD, such as power factor correction capacitors, resonances can occur which can be damaging to electrical equipment on the system. Capacitors located between the drive output and the motor terminals should be avoided.

 

[T.M.Haja Sahib]

You will notice that this filter does not result in a capacitor in parallel with a capacitor. You may also note from page 16:

Agreed. But my point is this

it is the high harmonic content in the output of VFD that does not permit use of any capacitor on the output side.

to

Consider this: high dv/dt from AFD could cause damage to the driven motor winding but a capacitor across its terminals can smooth it out safely. But owing to high harmonic content, use of capacitor on the output side of AFD would damage it. My plan is to use a filter to take care of harmonics so that the capacitor could be used to reduce the dv/dt to a suitable value across the driven motor terminals.......

 

[mivey]

Agreed. But my point is this

It seems like you have said more than one thing, but I may have missed a post or two. Do you have a diagram of what you plan to insert between the drive output and the motor?

 

[T.M.Haja Sahib]

Do you have a diagram of what you plan to insert between the drive output and the motor?

Yes. It is the same as the diagram on page no. 8 of web link in post#89. Nothing more.
Only the position of components change. The reactor playing the role of filter and the capacitor now connected across the motor terminals to reduce dv/dt just to see if it gives better performance.

 

[Besoeker]

Even when there is a output filter?

This was our your response to my post:

The physical design deliberately ensures that impedance between the bucket and the IGBT switching elements is negligible for entirely practical reasons.

The presence or absence of an output filter has no bearing whatsoever on the impedance between the bucket and the IGBT switching elements.

 

[Besoeker]

The basic building blocks diagram of ASD is given at page 8 of link below and note presence of capacitor on the output side of ASD for function I already mentioned.
And, as I already mentioned in post #84, that is a different scenario. In Fig. 2, The capacitor is NOT connected to the output side of the variable frequency inverter.
And I keep telling you that can't do that.

 

[T.M.Haja Sahib]

I keep telling you that can't do that.

I never told you it could be done. I am afraid you misunderstood.

 

[Besoeker]

It is better to start another thread to discuss further because it is neither above nor high dv/dt that causes failure but it is the high harmonic content in the output of VFD that does not permit use of any capacitor on the output side.

It absolutely is the high dv/dt.
The fast switching of the IGBTs.
i=Cdv/dt

 

[Besoeker]

I never told you it could be done. I am afraid you misunderstood.

Here is what you posted;

The current would, of course, rise. But if the switching is very fast as in IGBT, the rise in current may not be too high to blow a fuse.

That seems to at least suggest it could be done without detriment.
It can't.

 

[T.M.Haja Sahib]

If you give your remark on the suggestion in post#93, it would be nice.

 

[Besoeker]

If you give your remark on the suggestion in post#93, it would be nice.

Try it and report back.