Contactor pole frequency limits 4

[thinker]

We plan to install AC contactor between VFD inverter and PM motor.This contactor would be opened only under no-load conditions, so we have no concerns regarding arc breaking issues. However, when contactor is closed and the system is running, the inverter output frequency could reach 800 Hz (fundamental). The contactor pole specification (Telemecanique, type LC1F630, 630 A) limits the frequency to 200 Hz. Is this limitation based on additional losses in closed contacts? If we operate above the limit of frequency, is there a chance of contacts welding? Please advise.

 

[ScottyUK]

Hi Pete,

It wasn't an accusation - but the general discussion between you and jraef above appeared to be hinting that the PWM voltage output dropping to zero was advantageous when interrupting current.

If you want a quote, have a look at your post timestamped 11 May 11 19:26 which, while correct in itself, applies to a different set of conditions to those which exist in a PWM motor, something you acknowledged yourself a little later on.


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

Well Scotty, I appreciate you chiming in. If you are inclined to try to correct any misunderstanding that I have, then I would invite you to always continue to do that, because there is certainly plenty I can learn from you. In this particular case, everything you said is already well known to me and there seems to be some misunderstanding between what I said and what you think I said. (I wrote the comments, I understand the context, they all make perfect sense to me... and none of them had anything to do with voltage zero crossing) I will accept blame for that misunderstanding due to the wandering nature of my comments.

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(2B)+(2B)' ?

 

[electricpete]

It makes sense. There are 2 halves of the race: the actual (recovery) voltage and the dielectric withstand.I focused on the actual (recovery) voltage, which suggested more severe duty at high frequency.I assumed no change in dielectric withstand with frequency, but in fact it is apparently the dominant effect. After the contact parts, an arc occurs for duration of up to 0.5 cycles before the first natural current zero occurs. The time duration of that 0.5 cycles is much longer for the low-frequency case and therefore causes more ionization... and it will take longer for the dielectric strength to recover from that more-ionized state.Thanks Mobius and Jeff for explaining that.

If you want a quote, have a look at your post timestamped 11 May 11 19:26 which, while correct in itself, applies to a different set of conditions to those which exist in a PWM motor, something you acknowledged yourself a little later on.

I have no idea what you're getting at. What's wrong with my 11 May 11 19:26 post? The frequency referred to in that post is the frequency of the fundamental. In what way does it no apply to PWM? And what the heck leads you to believe it has anything to do wtih zero crossing?

I am torn between moving on and figuring out what the heck you're looking at. Hope I didn't make the wrong choice.

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(2B)+(2B)' ?

 

[electricpete]

Correction:
And what the heck leads you to believe it has anything to do wtih zero crossing?
should've been:
And what the heck leads you to believe it has anything to do wtih voltagezero crossing?
Note I have talked about voltage many times in the context of return of voltage after contact opening (which is relevant), not in terms of voltage zero crossing.

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[ScottyUK]

Hi Pete,

I had replied to this at work, but I seem to have forgotten to click 'submit'. Damn computers. So prone to user error!

If you take the post I referenced above with Jeff's post "Well, that interestingly is the gist of the argument my colleagues had actually. Like I said I don't agree based on the specifics of that application being on the PWM output of VFDs, because technically, the individual pulses are going to zero in between each one. They don't CROSS zero and go the other way, but to me, that's an irrelevant distinction. Zero is zero." I think I can explain where the misunderstanding has come from.

When you were talking about how the relationship between recovery voltage and dielectric strength changes with frequency I had interpreted that as being the difference between the behaviour at 'low' (motor output) frequency compared to the behaviour at 'high' VFD carrier frequency. I now wonder if you meant that the behaviour changes between 'low' motor output frequency and 'high' motor output frequency. If you read the posts in order and note how Jeff's comments are in between a couple of your posts you can possibly see why I picked up on VFD carrier frequency voltage zeros.

Hopefully harmony is restored to all


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If we learn from our mistakes I'm getting a great education!

 

[electricpete]

No worries. Sorry for making a mountain out of a molehill. I was following the original thread subject of varying the fundamental frequency ("line frequency" as I called it... maybe not the best term), up until my comments to Jeff 11 May 11 19:56.

I do believe the carrier frequency voltage would degrade interrupting capacity if it were to appear across the contacts during interruption. Apparently, it does not appear across those due to the low pass filtering effect. The proof of the low-pass filtering effect is imo obvious for current, but not so obvious for voltage across those contacts.

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(2B)+(2B)' ?