Everything You Need To Know To Find The Best low voltage 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

The cheap DC motors that one can buy don't generally have manufacturer's part numbers or available technical datasheets. And there can be several variations of motor with the same physical size.

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This Mabuchi web page has useful info about sizes but I suspect most of the cheap motors on the market are chinese copies.

For powering 00 Gauge model trains the N20 size motors are a convenient size and only cost about £2 each. However I have several variations on the theme and {A} it can be difficult to know if the one in my hand today is the same as the one I had last week and {B} whether motor X is more powerful than motor Y.

It occurred to me a few days ago to measure the no-load current of the motors when powered with a controlled voltage (which happened to be 3.3v because that is what I had set my regulator at for another purpose).

I discovered that some of my motors consume 12mA, some 20mA, some 29mA and some 60mA. I did not measure the rotation speed but, in general, I think the higher current also signalled a higher speed.

Is it reasonable to assume from this simple test that the motors with the higher current will have higher torque and, if I was offered other N20 motors could I safely assume that one which consumes (say) 29mA is pretty much identical to the 29mA motors that I already have?

I also have some 130 size motors which are physically bigger. If I was to measure the no-load current (at 3.3v) of one of them what (if any) conclusion could I draw about it in comparison to the N20 motors.

...R

Thanks. I should have mentioned the magnets, and I forgot. And I have no means to know what magnets might be inside the motors. However the N in N20 signifies a rare-earth magnet so I am content to assume they will be similar.

The winding resistance will depend on the length of wire and the diameter of the wire. I have been assuming that the lower current corresponds to more turns and thinner wire.

I need to think more about the impact of the speed - higher speeds mean a higher back-emf. How does the back-emf vary with the number of turns on the coil?

I think your comments do point out that comparisons across different sizes of motor would not be meaningful.

I wonder how practical it might be to build a tiny dynamometer ?

...R

I wonder if I started off with the wrong question ... maybe I should have been thinking about power rather than torque.

If all of the motors are powered at 3.3v then the 12mA motor consumes about 39mW and the 20mA motor about 66mW. Assuming they are roughly equally inefficient then the higher current clearly implies higher power.

And I am inclined to think that that would hold true for comparisons between completely different types of motor - always keeping in mind that I am not looking for great precision.

I guess motors with rare-earth magnets should be more efficient because they generate more torque and hence achieve their power at a lower speed.

CHANGLI ELECTRIC MOTOR Product Page

However AFAIK the quality of the commutator - brush interface has a large impact on the efficiency of very small motors.

...R

If all of the motors are powered at 3.3v then the 12mA motor consumes about 39mW and the 20mA motor about 66mW. Assuming they are roughly equally inefficient then the higher current clearly implies higher power.

Yes, in concept but that's a huge leap of faith. Are these new, clean motors? If not, those differences could just be frictional losses. Small, cheap motors are not known for their excellent shafts, bearings and brushes. If your dealing with an unknown motor, the only thing that would give any useful info is a dynometer. Measure the torque and speed, then you know.

I guess motors with rare-earth magnets should be more efficient because they generate more torque and hence achieve their power at a lower speed.

A definite maybe. They can create a stronger field for a given area. Field strength plus armature current determine the torque. That can be used to generate a higher torque per amp in the same volume but it might be used just to make a smaller motor.

However AFAIK the quality of the commutator - brush interface has a large impact on the efficiency of very small motors.

Absolute fact since cheap small motors use spring wire rather than carbon brushes. I doubt efficiency is even documented for 'toy'motors.

There are only a few significant specifications for electric motors. The electric bike websites seem to have a good handle on this.

Electric bike hobbyists have many options for different specs inside basically identical motor casings. With a fixed case size, there are only a few variables to play with. If you want more turns of wire on the coils then the wire must be thinner. Thinner and longer equals more resistance.

The RPM per volt is an important figure. Each motor has a maximum speed for a given input voltage. Poor bearings or poor lubrication will slow it down a little but it is still a good figure to measure. If you are testing many motors on a constant voltage, you can measure RPM with an optical sensor and a dot of paint on the shaft.

But motors with high speed will have less torque for the same case size. If you can also measure speed with a known load that will give you a lot more information. Maybe test the time it takes to wind up a fishing weight on a given pulley size?

Gearing is also important. Geared motors can look very much like pure motors but they bring the thousands of RPM down to a speed that's useful for normal things like wheels and robot arms.

MorganS:
But motors with high speed will have less torque for the same case size. If you can also measure speed with a known load that will give you a lot more information. Maybe test the time it takes to wind up a fishing weight on a given pulley size?

That's not really the case - size (specifically rotor volume) and cooling determines the maximum torque
available, only mechanical strength and bearing ratings limit the speed. Its possible to get enormous
power from a small motor if run at very high speed (the hard part is reducing losses). For instance the
exotic motors from Celeroton: https://www.celeroton.com/en/products/motors.html.

More prosaically cordless drill motors achieve high power density by having a high speed (20krpm or so),
and efficient fan cooling.

Practical DC brushed motors are often limited by the commutator - which will arc and wear too rapidly
at very high speed as well as having high friction losses. Brushless DC motors often run significantly
faster for the same size, have no commutator, and have a lifetime mainly dictated by the bearings.

The link of torque to rotor volume is from the basic physics of the situation and the properties of
copper and iron. The torque depends on the radius times the cylinder area, which is equivalent to
volume. The limits of copper and iron determine the limit for the force per unit area of the rotors
cylindrical surface - iron alloys admit upto a max of about 2.2T of field strength, copper's resistivity
places thermal limits on the current density that react with the field. Moving to superconducting
windings or liquid cooled windings can lead to much greater continuous torque ratings by overcoming
the limitations of copper.

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