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Centrifugal Compressor


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#1 anu_rags

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Posted 30 June 2006 - 05:59 AM

Hello All,

My customer has a centrifugal compressor application. Motor connected is 6.6kv, 750kw, 76Amps, 2pole rated. Presently the problem is that customer is not getting required flow for his production. Presently the motor is running DOL at 49.5 Hz. Compressor data sheet shows that compressor can be loaded upto 51.5 Hz to get more flow at same head. Presently compressor efficiency is 65% and motor is getting loaded upto 67-68 Amps only at 49.5Hz.

Customer wants to install VFD and increase the speed of motor and thereby get more flow. Our assumption is that because presently the motor is underfluxed at nominal conditions, we can increase the frequency of motor and still be able to get required torque to run the compressory at higher speed and thereby increase flow.

Any suggestions how best we can do this to not let the motor get saturated and get maximum possible speed. I will appreciate reply to this by all.

Regards,
Anu_Rags

#2 GGOSS

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Posted 30 June 2006 - 06:14 AM

How is the motor coupled to the compressor?

If it is not directly coupled, changing pulley ratios might provide the result you are looking for at a fraction of the cost associated with purchasing/fitting a variable speed drive.

Regards,
GGOSS

#3 anu_rags

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Posted 30 June 2006 - 06:36 AM

The motor is directly coupled to the compressor.

#4 jraef

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Posted 30 June 2006 - 03:39 PM

Are you saying that the compressor motor is designed for 51.5Hz at 6.6kV then? Seems odd to me. More likely that it's a 50Hz motor and they meant that the compressor curve goes up to the equivalent speed of the motor running at 51.5Hz. The problem with using a VFD to do this is that although you can increase the frequency, you cannot increase voltage with it, you will still be limited by your maximum line voltage. On the other hand, 1.5Hz is not much out of spec even if the motor was rated for 50Hz. But how much extra flow can you get with that >4% speed increase? As GGOSS said, that VFD is going to be extremely expensive at 6.6kV. Keep in mind also that nobody really makes one at that voltage any smaller than around 1500kW, even if they market it as a smaller drive. So you will be paying for more VFD than you need.
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#5 anu_rags

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Posted 01 July 2006 - 04:06 AM

Hello,

Yes the motor is 50Hz rated only and the compressor curve says that it can go upto atleast 51.5Hz equivalent to motor speed. With this increase from 49.5Hz to 51.5Hz we can get around 8% increase in flow as per compressor data sheet. Infact we want to try and go upto 54 Hz if possible with motor and compressor to achieve the best possible.

Also we are not considering at 6.6kv drive for this application considering the cost and hence want to go for High-Low_High solution using transformer.



#6 mariomaggi

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Posted 01 July 2006 - 08:58 PM

Pay attention, anu_rags, to select a good inverter without big dV/dt on the output.
The output trafo could introduce further hazardous overvoltages that will be transferred to motor windings.
Regards
Mario

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