Why synchronous motor with VFD 3

[RRaghunath]

Synchronous motors are preferred at ratings of say, 10MW and above for better efficiency. But, when motor is driven through variable frequency drive, is there stil a reason to go in for synchronous design?
I have come across a 25MW motor with VFD, but synchronous design recently.
Appreciate your inputs. Thanks in anticipation.

 

[ePOWEReng]

VFDs for synchronous motors are very expensive and thus harder to justify due to large size of synchronous motors. The VFD will still provide the advantage of varying the synchronous speed of the motor, depending on the load being driven this can increase efficiency or may be necessary for the process.

 

[RRaghunath]

Speed variation should be possible even with induction motors, with V/f control, is it not?

 

[itsmoked]

Absolutely.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[RRaghunath]

so, still my initial doubt remains, why synchronous motor with VFD?

 

[slavag]

Hi.
It's intresting project, please see attached link:

http://www02.abb.com/global/seitp/s...c125737600424c86/$file/Ormen+Lange+drives.pdf

Regards.
Slava

 

[ScottyUK]

Synchronous machines can run at much higher speeds than induction machines because of the superior mechanical performance of the solid steel rotor forging.


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

 

[RRaghunath]

slavag,
Thanks for the link. It is interesting.
The document is about a 48MW compressor with VSD and Synchronous motor.
The max speed is 3750rpm. Probably, the high speed explains the reason for selecting synchro motor (Thanks to ScottyUK).

 

[dpc]

At one time, the only real option for large medium-voltage VFDs was the use of load-commutated inverters with synchronous motors. I'm not sure of the current state of the art, but I suspect for very large motors, it may still make sense.

 

[RRaghunath]

dpc, is it that load commutator inverters work better with synchronous motors? What could be the reason? I am not much familiar with VFDs.
Thanks.

 

[ScottyUK]

raghun,

LCIs are thyristor based inverters. The thyristor commutation relies on the ability of the synchronous machine to produce its own internal voltage due to the rotating field of the rotor. This internally produced voltage is fundamental to the commutation process where the continuous DC link current switches seamlessly from one stator phase to the next simply by gating the incoming thyristors at the correct moment in time when the rotor is in the correct angular position. I have tried to find a decent link on the net which shows the waveforms and I have failed so far, but this topology is well illustrated in many motor and drives texts. Have a look at the FAQ in the Power Engineering forum for a couple of leads.


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

 

[dpc]

http://search.abb.com/library/ABBLi...LanguageCode=en&DocumentPartID=&Action=Launch

This ABB link is hideously long - if it doesn't work, try searching the ABB site (shudder!). It has a block diagram and basic description of LCI drives.

 

[RRaghunath]

Thanks dpc. I got the ABB Megadrive Brochure and will go through.
Thanks once again. I am happy with the interaction.

 

[slavag]

Hi.
Brochure of Megadrive is O.K.
Before two years I see one ( I think for 25MW) in Turgi Swiss. It's product!!
Slava

 

[motorspert]

Hi
Those motors are Brush (FKI) synchronous motors I believe. At these ratings and speeds induction motors are limited mechanically. But there are examples of 4 pole inductiom motors up to 18MW out there, so this would be the cheaper option, if the drives are available. So, between 10 and 18MW i guess there is a case for either and a full costing would be needed.