2000hp, 4160v Motor with VFD

[Brian2903]

Hi all, I'm new to this forum, nice to meeting you.

 

My facility is looking for adding a VFD for a 2000hp, 4160v air blower motor; however, the price for those VFD's are around 250,000USD. So, one company contated me and suggested to use 4160-460v transformer + low voltage VFD setup instead.

 

Could anyone input some comment/advise on the setup above? Thanks in advance.

[marke]

Hello Brian

 

I have heard of this being done before. I believe that one of the suppliers used to do this regularly.

You could uase a transformer down to LV and use an LV motor, or you could use two transformers, one to step down plus one on the output of the ionverter to step up again. If you use a transformer on the output of the inverter, you should use a V/Hz inverter only. A vector type controller would probably give problems. - for an air blower motor, a v/Hz drive is all that you need.

 

Best regards,

[Brian2903]

Thanks Marke, a suggestion from an engineer the VFD company:

 

"The problem is that the amount of current that a 2000HP motor will require

at 460VAC, is to high for a single drive to handle it, so now you will need

to "gang" drives together to arrive at the proper current levels to run the

motor. When you "gang" drives together, you need to be very careful when

each of the drives fire their SCR's. One little miss-fire, and the entire

system will blow up!!!"

 

Any ideas/comments on that?

[marke]

Hi Brian

 

Yes that is a major problem, but there are drive manufacturers who have modules that are designed to be parallel connected. This is how they get larger drive sizes. I am not sure what the upper limit is.

I suspect that by the time you "build the system up of parrallel modules plus the cost of the transformer, you may not be saving anything. I would need to research that to comment further.

 

Best regards,

[Brian2903]

The quote for ABB drive are 210,000 for a 1750HP, 460VAC VFD is over 210,000; yet, the medium voltage VFD which fits my motors is around 250,000.

 

I'm not sure about 2000hp, 460VAC VFD, but I'm sure that would be a bit more expensive.

 

However, even the price is the same, I'd go for one with less maintenance.

[jraef]

Brian,

There is also the consideration of the transformer on the load side needing to operate at lower frequencies. Hysteresis in the transformer core can cause voltage instability at lower frequencies, but just having a low hysteresis core can cause other problems then at normal frequencies. The secondary of the VFD is also very rich in harmonics, something that is notusually a problem until you add a transformer that can overheat quickly. This requires a special transformer design and only a few manufacturers offer it, which translates to higher costs than you would expect in a similarly sized standard transformer. I know for a fact that many people who do this ignore that transformer issue, but they are asking for trouble. It can eventaully damage the drive and/or result in premature transformer failure.

 

Another issue is losses. With an additional transformer in the circuit, you have losses associated and those are continuous regardless of speed when compared to using a MV drive.

[Brian2903]

Thanks guys.

 

I just pull up the data sheets from air blower motor company, the efficiency for 50% load is 96.84, 75% is 96.75, 100% is 96.32, 115% is 95.97; also, the pf for 50% load is 84.4, 75% is 88, 100% is 88.5 and 115% is 88.1.

 

The motor usually runs for 50-75% load. According to those data, is it worth it to apply VFD even the % load is so low? and the pf is only 88% for 115% load, is that worth to do pfc? or is it possible to do that?

 

Thanks.

[marke]

Hello Brian

 

Those figures are not unusual. The VFD is not to improve the efficiency of the motor, it is to improve the efficiency of the system.

Depending on your current method of controlling the airflow, there will be system losses. You would need to study the blower curves of energy in for work done. In some situations, there is a lot of energy to be saved, in others it is considerably less, but the use ov VFDs is to save energy lost in the driven load, not the motor.

 

Best regards,

[Brian2903]

our system is running 4 single stage 27,000 standard scfm dresser-roots process areation blowers rated for 11.6psig. Each blower is driven by 2000hp, 4160v constant speed motor and is equipped wth inlet guide vanes to adjust the process aeration airflow rate.

 

By using VFD, we could control the motor speed (Input) instead of the output of the system.

[marke]

Hello Brian

 

Yes, that is correct. You control the airflow with the drive and leave the vanes fully open. This will increase the efficiency of the system, but you would ned to study the performance curves of the existing system to get an idea of the improvement.

 

Best regards,

[Brian2903]

Mark, could you give me some hints on how to get the performance curve. Should I take reading on kw VS % vane open?

 

And what're the reason why we like to control the input instead of output gate?

 

Thanks.

[marke]

Hello Brian

 

You really need to measure the actual work done at different settings as well as the input power. If you had the correct desing onformation, you may be able to calculate the work done and just measure the input power.

I would suggest that you talk to the suppliers of the equipement.

 

Best regards,

[Brian2903]

Besides VFD, I just found out another kind of ASC that maybe use instead of VFD:

 

http://www.magnadrive.com/md_asd/benefits_vfd.shtml

 

Any comments on that? Thanks

[marke]

Hello Brian

 

This looks similar to the eddy current couplings that have been around for a long time.

One issue is going to be the efficiency.

The page quotes that efficiency at above 90% speed as being high, but the efficiency below 90% speed will be low.

 

This "coupling" transfers torque through from the input to the output such that the output torque is equal to the input torque, but the output speed can be considerably lower.

input power = K x input torque x input speed.

output power = k x output torque x output speed.

 

efficiency = output power / input power

= k x torque x output speed / k x torque x input speed.

= output speed / input speed.

so at half speed, the efficiency will be 50%. You are probably no better off than you are now with the vane control.

 

Best regards,

[Brian2903]

Mark you told me that:

 

"... control the airflow with the drive and leave the vanes fully open. This will increase the efficiency of the system..."

 

Could you give me more thought on that? Such as the reason that the motor speed would decrase even if you only control the output.

[marke]

Hello Brian

 

I am sorry, I do not understnd your question.

If you use the vanes to control the airflow and leave the fan running at full speed, there will be a drop off in efficiency. The loss in efficiency heats the air being circulated by the fan.

If you reduce the fan speed, you can retain the efficiency of the fan for a given airflow.

 

Best regards,