Which Voltage to choose (and why) | Eng-Tips

Hello,
We are in the very early stages of doing the electrical engineering on a pump project. The pumps will be used to pump water over a long distance (not much lift) for a gas processing operation. The people that engineered the hydraulics say that they need three 600 horsepower motors to do the job. The also want each motor to have its own VFD. The voltage choices are either 480 volts or volts. My question is which voltage is preferred for motors of this size using VFD drives? Is there an "accepted" industry practice for this application?

The company where I used to work had a practice that any motor greater than 300 HP was to be a medium voltage voltage motor, but this practice did not necessarily pertain to VFD applications.

Assume that wire sizing, voltage drop, utility interfacing, and transformer selection will not present a problem either way. The size of the drives may affect the size of the building, but that can be taken care of.

Also, is there a concensus as to whether a linestarter is needed in front of the VFD drive at either voltage level for safety or isolation or any other reason?

Thank you.

Regards,
Podobing I've got no coverage with 4.16kV as its not a standard voltage here, but I'd have thought part of the requirements would be operational, and whether different operators are needed for switching and maintenance for the higher voltage equipment. I know it has been a factor for some plant I've been involved in, but that's for different voltages and standards.



EDMS Australia Most companies have a statement that states all motors > 250 (or 300) hp are medium voltage. This can vary somewhat in practice, especially when VFDs are being considered. This is strictly for commercial reasons (ie a MV ASD is >$$ than a LV ASD). But that is just the cost of the drive. You also need to consider the cost of;
- the motor feeders
- the cost of cleaning up the harmonics left behind by a LV 6-pulse drive
- the cost of additional transformation / power distribution to LV

I would guess that as far as LV ASDs are considered, 600hp at 480V is as high as I would ever consider reasonable. If the motor feeders are short, you could likely build a case for LV ASDs.
As far as line-side protection for a drive;
​for LV ASDs: Use either; a LV CB either in a LV MCC or a power ACB in LV Switchgear
​for MV ASDs: Use a MV CB in metal-clad MV Swgr or a fused contactor in a MV MCC.

GG




"I have not failed. I've just found 10,000 ways that won't work." Thomas Alva Edison (-)

Using the larger voltage will result in the lowest voltage drop, which can be a concern.

is a standard voltage, but most business don't use that much energy to justify it. You also have a standard voltage of , but it is not used as much as .

Other concerns are that with 480 there is much more support, and it can be used for other plant uses, like lighting. But 480 tends to be in the higher arc-flash issue range.
gear will cost more, and likely take longer to order, but will have much less copper because of the lower currents (maybe it will not cost more).

We have many pumps, fans, and motors on each of , , , and 480, at different plants, and there seems to be a upper current limits at and amps, of where the cost of the gear increases.
The point raised about the type of equipment is very valid. I'm working on a project with a very similar scope and similar considerations (you aren't in san Francisco, are you?). here, a big issue is that these are submersible pumps, so the 360 ft (110m) distance is a serious concern for the weight of the cables to run these at 480V. I'm recommending V for that reason alone, but their big concern for that is that none of their staff are currently trained on working on MV systems, so they have an added cost of providing (and maintaining) that staff training over the life of the project. They religiously perform scheduled maintenance on the pumps, which means pulling them and de-coupling the cable connections every time. It's a valid concern, but one that can be, if necessary, overcome by using qualified contractors.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington