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Quasi variable frequency soft start method!


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#1 Igneous

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Posted 05 September 2003 - 12:05 PM

Who got any info about this method??? I know that TRIOL corp. use it for heavy loaded induction motor start. Are any other manufacturers do the same? I know that SOLCON, Motortronics, AB, do not use this start mode.
There is no changes in a power schematics, only in software algorythm... Why no one do not realize this starting mode in their production???

#2 jraef

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Posted 05 September 2003 - 05:41 PM

Never heard of it.

You cannot change the output frequency without changing the power structure. Phase-angle fired SCRs are gated on, but are self commutating so the frequency output will always be the same as the fundamental. Anything else would be a VFD.

If what you are thinking of is a GTO (Gate Turn Off ) thyristor system, we refer to those as 6-step or Current Source inverters. Load Commutated Inverters are frequently used for soft starting large loaded motors, but again, we consider those to be Drives, not soft starters.

If you know of something new, let us know! We would love to here it. I see nothing in their website (English version) that mentions this. Being that most of us are Anglophiles, we would love to see a translation.
"He's not dead, he's just pinin' for the fjords!"

#3 marke

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Posted 05 September 2003 - 08:38 PM

This is a reduced version of the old cycloconverter system. By turning on different patterns for different cycles, you can effectively create subharmonic waveforms. If you have a lot more SCRs, you can create a greater number of subharmonics than when you have only six.

I think that AB were using this technique for their creep speed option some years ago, not sure, just a suspiscion.

Best regards,

#4 Igneous

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Posted 06 September 2003 - 10:14 AM

Hi all!!! Take a look at this power schematic
user posted image
And one more image... This is quasi VF start method phase voltage diagramm

user posted image

As we see the current frequency can be chanhed by fixed values... 1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 17, 25 Hz... while voltage frequency is still 50 Hz...

I think that it is usefull method... I am waiting your comments!!!
;f;

#5 marke

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Posted 06 September 2003 - 10:31 AM

Hello Igneous

Yes, this is a form of cycloconverter as in my last post.

If you go a stage further and add SCR from each phase input to all phase outputs, i.e. 9 sets of SCRs, you have a full blown cycloconverter and can have a much better control over the pseudo frequency. The frequency is reduced by this method, but it has effectively a 50Hz carrier so ther frequncies that can be synthesised are limited, especially when you only use three SCR sets.
Best regards,

#6 jraef

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Posted 06 September 2003 - 09:43 PM

I agree with Marke, this is a variant of the cyclo-converter system. This is exactly what AB does for their Pre-set Slow Speed feature, as well as using it in a reverse pattern for their "Smart Motor Brake". Several manufacturers including Benshaw, Motortronics and others offer it as a Jog-at-Set-Speed option but not as a complete starting ramp as suggested in the original post. It was explained to me that it tends to heat the motor windings more (per unit of torque increase) than conventional soft starters. As a result it has more limitations on starting cycles than most users are willing to accept. It is however widely used on large synchronous motors for both starting and speed control, since they are usually started less often and/or separately cooled anyway.
"He's not dead, he's just pinin' for the fjords!"

#7 GGOSS

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Posted 08 September 2003 - 01:23 AM

Hello jraef & marke,

Not sure if they will ever get this one off the ground. Take a look at;

http://www.acdrives.com/tec.htm

The same message has been posted for over 12 months now. No joy in obtaining further information.

Regards,
GGOSS




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