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Which Voltage to choose (and why)

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Which Voltage to choose (and why)

Which Voltage to choose (and why)

podobing

(Electrical)

(OP)

17 May 17 09:41

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 4160 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

RE: Which Voltage to choose (and why)

FreddyNurk

(Electrical)

17 May 17 10:23

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

RE: Which Voltage to choose (and why)

GroovyGuy

(Electrical)

17 May 17 14:14
- 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

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 LVI 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 Switchgearfor 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 (1847-1931)

RE: Which Voltage to choose (and why)

cranky108

(Electrical)

17 May 17 17:53

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

4160 is a standard voltage, but most business don't use that much energy to justify it. You also have a standard voltage of 2400, but it is not used as much as 4160.

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.
4160 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 6900, 4160, 2400, and 480, at different plants, and there seems to be a upper current limits at 2000 and 3000 amps, of where the cost of the gear increases.

RE: Which Voltage to choose (and why)

unclebob

(Electrical)

17 May 17 19:57

With that kind of motors and the distance to be covered, I'd go for medium voltage (4160V) and 12 or 18 pulse drive, to take care of the harmonics.
I'm curious about comparing the sizes of a 600 HP 480 V motor versus its equivalent at 4160V.

RE: Which Voltage to choose (and why)

EdStainless

(Materials)

17 May 17 22:34
When we got up over 400hp we went to 2400V (or higher).
This kept support equipment and cables smaller.
But then our voltages were each unique to the motors since they were purpose wound.

We used to run motors on VFDs with long leads, say 14,000ft.When we got up over 400hp we went to 2400V (or higher).This kept support equipment and cables smaller.But then our voltages were each unique to the motors since they were purpose wound.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Which Voltage to choose (and why)

waross

(Electrical)

18 May 17 00:20
There are not many 2400 Volt systems left.

One of the early distribution systems was 2400 Volt Delta. Many of those systems were changed over to 4160 Volt Wye in the 40s and 50s.There are not many 2400 Volt systems left.

Bill
--------------------
"Why not the best?"
Jimmy Carter

RE: Which Voltage to choose (and why)

jraef

(Electrical)

18 May 17 01:05

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 4160V 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

RE: Which Voltage to choose (and why)

edison123

(Electrical)

18 May 17 11:17
OP

Is there any specific reason to go in for VFD's for these pumps like flow control, power savings etc.?

jeff

4160 V submersibles? Hearing it for first time.

OPIs there any specific reason to go in for VFD's for these pumps like flow control, power savings etc.?jeff4160 V submersibles? Hearing it for first time.

Muthu
www.edison.co.in

RE: Which Voltage to choose (and why)

podobing

(Electrical)

(OP)

18 May 17 13:51

Guys,
I like the responses so far.

EdStainless - In a previous life, I have seen large HP VFD's used at both 480 volts and 4160 volts. I worked in the mines, and in the mine itself, 480 volts was used for both soft start and vfd applications for conveyor belt (for example), regardless of HP. On the surface facilities at that same mine, on any conveyor belt, 4160 volts was used for any motor greater than 300 HP, regardless of the starting method.

In my present life, I am trying to establish a recommendation as to motor voltage for people starting with a clean sheet of paper.

edison123 - Flow control is probably the biggest reason for the VFD's, although there may be some other advantages realized by using VFD's.

jraef - These are not submersible pumps, they are vertical turbine pumps. The pumps will be in a building, so running either conduits or cable tray is acceptable, at least at this time. No, I am located in the Morgantown WV, area. This is a totally different problem (opportunity?), but we may find that we have similar solutions.

Regards,
Podobing

RE: Which Voltage to choose (and why)

ScottyUK

(Electrical)

18 May 17 15:51

European perspective would be to make the jump to MV at around 250kW, so over here you're well into the MV drive range.

Low voltage motor and drive could be a possibility if you provided a captive dry-type transformer as part of the drive package, although the feeder to the drive package would still be at the higher voltage.

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\$\begingroup\$

A simple model for a DC motor is \$V= R*i + e\$, where \$V\$ is the terminal voltage, \$R\$ is the motor resistance, and \$e\$ is the back-emf voltage.

R can be measured as I said above in a comment and which I'll repeat here. Resistance can vary on a DC motor due to brush contact. The best way to measure resistance is to take several measurements and average. If possible, lock the rotor and then apply a small current to the terminals. Measure voltage and current and calculate R = V/I. Typically you this test would be done at ~25% rated current. Repeat several times and average. There is a dynamic test, too, that can give even better results - do the same as I just said, but instead of locking the rotor, back drive the motor. 50 RPM would be sufficient speed at which to back drive the rotor.

\$e\$ can be determined from \$e = K_b* \omega\$, where \$K_b\$ is the back-emf constant (units of V/(rad/sec) or V/RPM) and \$\omega\$ is the speed in the same units as \$K_b\$.

You've already found \$K_b\$. It is just \$\frac{16 V}{570 RPM} = 28.07 \frac{V}{kRPM} = 0.268 \frac{V}{rad/sec}\$. As somebody else mentioned, the torque constant of a motor is equivalent to the back-emf constant, so \$K_t = 0.268 \frac{Nm}{A}\$.

At locked rotor, you know that \$V = R*i\$ because \$e=0\$. If you solve for \$i = \frac{V}{R}\$, you can find the current when the rotor is locked at different voltages. And from that current, \$i\$, you can solve for the locked rotor torque at different voltages: \$T_{lr} = K_t*i = K_t*\frac{V}{R}\$.

You can also determine the maximum speed of the motor at different voltages using \$V= R*i + e\$. If you assume \$i=0\$ when there is no load, that equation becomes \$V=e\$, which becomes \$V= K_b* \omega\$, which becomes \$\omega = \frac{V}{K_b}\$.

Once you have the stall torque at different voltages and the maximum speed at different voltages, you can plot them on a graph with speed on one axis and torque on the other axis. Connect the lines and you have various speed-torque curves at different voltages.

There are a lot of assumptions in what I wrote above. The 2 main assumptions you should be aware of are 1) that the motor stays relatively cool (so the resistance doesn't change) and 2) that the no-load current is zero (in reality it won't be).