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Hello Alnerthyrn

Welcome to the forum.

I would expect a motor of this size, driving a fan, to require a start current of 450 - 500% Full load current.


The motor has a rating of 800KW, assuming a full l,oad power factor of 0.95 and a full load efficiency of about 95%, that would equate to a loading of about 890KVA. The start loading would be in the order of say 4.4MVA. If the transformer has an impedance of 4.7%, and the starter can operate at -15% (required by IEC standards) then I would suggest that you would need to look at a transformer of about 1.5MVA should suffice.

If the transformer impedance is higher than 4.7% or the starter can not tolerate 15% drop in voltage, then you would need to use a larger transformer.

I think that you need to discuss and confirm this with the soft starter manufacturer.

Best regards,

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Hello Marke,


MV motors are notorious for having very poor start performance, in fact with most machines I have come accross, the locked rotor torque has generally been less than rated torque.


I would expect a fan to require anywhere between 3.5 and 5 x FLC to achieve rated speed in an LV (or 400V) installation, however I would have expected almost DOL currents would be required to start these types of machines on MV supplies.


Hello Alnerthyrn,


Welcome to the forum. You mention on your second post that the current limit level is 4.2 x FLC. Has anyone conducted a start performance appraisal to ensure the motor will accelerate the fan to rated speed at the current?


I believe one of our fellow members 'theDOG' is developing a level of experience in the field of MV soft starter applications. Maybe we can get some input from him also.




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Yes GGOSS, it is common for large motors, not just MV, to have a very poor starting torque, but it is also common for large machines to have shutters or dampers to reduce the starting load.


Alnerthyrn. To estimate the voltage drop, you take the maximum load as a percentage of the transformer rating and multiply that by the transformer impedance.

For example, your machine has a rating of 800KW and a pf of 0.86 and eff of 0.961 => Full load = 800/0.86/0.961 = 968KVA

For a start current of 4.2 x, we have a starting KVA of 4.1MVA

If we apply this to a 1.5MVA transfomer, then the load on the transformer during start will be 4.1/1.5 = 2.733 times the rated output of the transfomer.

If the transfomer has an impedance of 4.7%, then the voltage drop wil be 4.7 x 2.733 = 12.85%.

(The impedance gives an indication of the voltage drop at full load)


Make sense??

Best regards,

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Yes these figures are not uncommon in very large machines, I have data on a number at around 60%LRT.

With a large machine such as this one, if the fan dampers are closed during start, then the starting load is essentially inertial and by the time the load begins to come on, the torque has risen. If you are trying to start a free ventilating fan, then that could be a different story as I would expect that there will be too much torque required at partial speed, typically in the 70% area.

It is difficult to fully comment without full curves for the motor and the fan etc. Back to the original question, I assume that these issues have been resolved and that the only unresolved is the size of transformer for that particular load. If the application design has not been done properly, then you are correct, there could be a much higher starting current required.

Best regards,;c;

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Based on this information you will have a voltage drop of 2.2% at full load (using marke's transfomer impedence example) - this seems a little high. Do you know what the actual transformer impedence is, or the fault level? I would guess that the actual transformer impedence would be higher than the 4.7% suggested by marke. Also, is the motor getting to full speed with the current limit set to 420%? What are the limitations on voltage drop from the utility? My initial reaction is to suggest that the transformer is too small. If you used a drive on the other hand.....(get the cheque book ready!!)



Generally you will find that a motor of this size is oversized for the load. ie. if the motor is 800kW, the fan may only draw 500kW for example. This will mean that the load can accelerate to full speed even with the poor charcteristics of the motor (I have seen motors with <50% LRT).


I am working on a project at the moment that has a 1320kW motor @ 3.3kV on a compressor that requires less than 25% of the motor torque at full speed! Just as well because the motor LRT is 50% of FLT. The motor current required to get the motor to speed is about 380%. Transformer size is 5MVA.


Keep us posted Alnerthyrn.

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Good information theDOG,


As you quite rightly put it, for MV applications such as this one, the motor may have been oversized to ensure adequate start torque is developed.


The drawback with this however is that the power factor at rated speed will be very poor, hence the addition of power factor correction will more than likely be necessary. As you would expect, this further complcates the electrical circuit and represents a substantial increase to the overall system cost.


Alnerthyn, you may wish to have a look at http://www.lmphotonics.com/pwrfact.htm to get a better handle on how to integrate power factor correction to your soft starter system.




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I wouldn't expect the power factor to drop significantly on a motor this size untill the load was very low. I would expect a good power factor down to about 60% load, so not necessarily a problem.

Best regards,

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I would have thought that efficiency of MV motors would have been significantly less than the numbers posted above, because of the large air gap required, coil turns etc. Can you please shed some light?

Also, in your post on 16/9 you mention that by starting a free ventilating fan it will be at maximum load. I would agree that a Centrifugal fan would exhibit these characteristics, but an Axial fan would be the opposite. Again, your thoughts??

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  • 2 years later...

Hey fellow forumers (?) (forites? forumnians?, forumidians?),

I know this is an old thread, but in case this subject ever comes up for anyone again, here is a very simplistic Excel spreadsheet tool available from our friends at GE that can be used to determine the required size of a transformer for starting a motor. It is set up for Autotransformer sizing, but can be tweaked to use with soft starting by determining the current requirements first, then altering the transformer details backwards.



It is pre-loaded with US NEMA sizes of motors, but if you click on "Motor Nameplate Data" it will allow you to enter anything else.

Get it now before GE loses it!

"He's not dead, he's just pinin' for the fjords!"
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