How to Choose Medium voltage electric motor?

Low-voltage motors are often a preferred choice due to familiarity with products and available services, as well as the typically lower cost of individual components. However, as horsepower (hp) increases, there can be advantages to moving to a medium-voltage motor. Low-voltage motors typically go up to 1,000 hp while medium-voltage motors can cover 250 hp and higher.

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Furthermore, in special variable frequency drive (VFD) applications, low-voltage motors can go up to or even over 5,000 hp. This high rating is preferably above the National Electrical Manufacturers Association (NEMA) low-voltage limit of 600 volts but still under International Electrotechnical Commission (IEC) low-voltage limit of 1,000 volts.

Knowing when to select the right motor for an application can save users time, space and money. Here are some areas to consider when choosing between low- and medium-voltage electric motors.

Cabling

In low-voltage motors, as the hp range increases, the size of cabling increases to handle the increase in amps. With conductors being a copper component, this increase in wire gauge can add cost, especially on longer cabling runs across a large facility or over a long distance to a remote pumping station. This increase in diameter also makes turn radii larger, which increases the difficulty in making connections within the terminal boxes. This can be time-consuming and introduce additional risk to the maintenance crew during initial setup of the motor.

A lower current in medium voltage motors allows for smaller cables (leads) even at higher hp. The use of smaller gauge leads reduces the cost per foot for those long-distance connections to remote pumping stations. Also, during the motor connection procedures, the small gauge wires are easier to work with and connect within the motor terminal box. This can reduce the maintenance crew's time in making the connections and reduce the risk of damage to the cables.

The cost of copper as a commodity and the difference in thickness of leads sized for low-voltage machines versus medium-voltage machines can be so large that this can be the primary determining factor in what voltage service is specified. The higher cost of medium-voltage equipment can easily be offset in applications with long cable runs from distribution.

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Size

When space is a consideration, more than motor size should be reviewed as the choice between a low- or medium-voltage motor that has an impact on the components in the entire system.

Low-voltage drives are smaller than medium-voltage drives when variable speed applications play a role in the motor selection. However, above 1,000 hp this ratio starts to flip, and drive size may be comparable or even smaller. Due to lower amps, medium-voltage motors also enable the use of smaller supply side switch gear, supply transformer and controls.

Windings

To prevent short circuits and preserve the longevity of medium-voltage windings, they are commonly produced using a form wound insulation system. The insulation system is sealed using a vacuum pressure impregnated (VPI) system, which fills the voids in the coils to protect from contamination. The coils are organized outside of the stator core to ensure the ideal spacing of turns, which allows for air flow around the coils to improve heat transfer. It is a more labor-intensive process but is well suited to the rigors of the voltage impulses of a medium-voltage system. Additionally, due to the smaller conductors used in the windings, there is the possibility of having more turns, so there is greater flexibility in the electrical design, making it possible to achieve specific performance characteristics.

In low-voltage motor windings with larger diameter conductors, there are more limitations to the electrical design but less need for the precisely ordered coils required to withstand medium voltage. Because of this, low-voltage machines can use a more cost-effective random or mush wound design with a thorough dip-and-bake in varnish that is often coupled with a vacuum impregnation of the winding to ensure that the insulating material fills all voids. The result is a low-voltage insulation system that is capable of exceeding industry standards for longevity while achieving the performance characteristics necessary for a broad range of applications.

Like all good questions, whether to pick a low- or medium-voltage motor for a pump system does not have an easy answer. There are several factors to weigh, including site and installation specifics that will impact what voltage service is best for a given project. When selecting a motor for an application, evaluating these three factors should provide the best all-around motor for the facility.  

Wayne Paschall is a product market specialist with ABB Inc., in the large machine and generator division. For more information, visit abb.com.

For more information, please visit Medium voltage electric motor.

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