VFD Long Lead Lengths - Correctly Calculating Voltage Drop
- Thread starter wonderdave
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Your calculations are for single phase!!!
winnie
Senior Member
- Location
- Springfield, MA, USA
- Occupation
- Electric motor research
As you can probably tell from my previous set of questions, I am sort of grasping at straws to see if there is something that I can understand that will direct to what you are looking for. I am not trying to diagnose the system, just trying to tease out if there is a situation that would result in greater than expected voltage drop.
I think that your original question has been answered: you use the exact same voltage drop equations for a VFD and for any other source. Where those calculations need to be adjusted for VFD use is in the actual frequency being used; the higher the frequency the greater the inductive impedance for a given inductance, so if you are running the motor at 400 Hz you can't use a 60Hz spreadsheet.
One _assumption_ built into the spreadsheet that you posted is the actual inductance of the circuit conductors. I believe the assumption is a standard 3 phase set in a single raceway.
It is perhaps worth continuing to dig, because you might find a reason for excessive voltage drop, or you might come up with a _wrong_ approach that gives the result that the other engineer came up with, which puts you in a position of understanding the error.
Question: how are the cables arranged for the run. You said 2x4/0 per phase, but you never said how the phases were arranged; are all the conductors in a single conduit, or do you have 2 conduits with a full three phase set in each, or do you have isolated phases, etc?
Here is a fun wrong approach: do a 'back of the envelope' for the inductance of a single wire 2200 feet long. Use that inductance in a voltage drop calculation.
When I did this I got an inductance of about 1.5mH, for an inductive reactance at 60Hz of 0.55 ohms. At 190A, 0.55 ohms puts you in the ballpark of what the other engineer got. As a reactive voltage drop you still need to do vector math to get the total drop, and I'm way too tired to figure it out.
You will note that the X value that I got was about 10x the one in your spreadsheet. The reason is that when you have all three phases in close proximity the inductance is greatly reduced. But perhaps you have 'isolated phases' in the non-metallic conduit, which would increase the inductance of the run.
-Jon
I think that your original question has been answered: you use the exact same voltage drop equations for a VFD and for any other source. Where those calculations need to be adjusted for VFD use is in the actual frequency being used; the higher the frequency the greater the inductive impedance for a given inductance, so if you are running the motor at 400 Hz you can't use a 60Hz spreadsheet.
One _assumption_ built into the spreadsheet that you posted is the actual inductance of the circuit conductors. I believe the assumption is a standard 3 phase set in a single raceway.
It is perhaps worth continuing to dig, because you might find a reason for excessive voltage drop, or you might come up with a _wrong_ approach that gives the result that the other engineer came up with, which puts you in a position of understanding the error.
Question: how are the cables arranged for the run. You said 2x4/0 per phase, but you never said how the phases were arranged; are all the conductors in a single conduit, or do you have 2 conduits with a full three phase set in each, or do you have isolated phases, etc?
Here is a fun wrong approach: do a 'back of the envelope' for the inductance of a single wire 2200 feet long. Use that inductance in a voltage drop calculation.
When I did this I got an inductance of about 1.5mH, for an inductive reactance at 60Hz of 0.55 ohms. At 190A, 0.55 ohms puts you in the ballpark of what the other engineer got. As a reactive voltage drop you still need to do vector math to get the total drop, and I'm way too tired to figure it out.
You will note that the X value that I got was about 10x the one in your spreadsheet. The reason is that when you have all three phases in close proximity the inductance is greatly reduced. But perhaps you have 'isolated phases' in the non-metallic conduit, which would increase the inductance of the run.
-Jon
wonderdave
Member
- Location
- Pittsburgh, pa
I'm going to try to digest this information and then add to this discussion. It would be great to have these formulas in a handy spreadsheet calculator.
http://www.skm-eleksys.com/2011/03/transmission-line-parameters-resistance.html
http://www.skm-eleksys.com/2011/03/transmission-line-parameters-resistance.html
Not a bad article but not a lot about voltage drop calculations which what the OP wanted.
Please go through web link in post#8.
The below web link shows how with the use of their product, the input current wave form is close to a sinusoid.
http://www.emersonindustrial.com/en...rochures/CTA/HVACR/WHP_HarmonicMitigation.pdf
- Location
- Massachusetts
Sahib, stop the nonsense, Besoeker designs VFDs.
There is usually no serious issue with the assumption that harmonics do not interfere with the performance based calculations of an induction motor connected to a VFD. It is just that. The special design motor is not usually affected by harmonics. But the connecting cable is not. The voltage drop in it may be affected by harmonics.
iwire, you are silly, because nobody has yet shown how it is nonsense.
Pay heed to what iWire posted.
Very few VFDs are active fron end. Do get a grip on reality.
Thank you for that.
Shameful that you got silly for doing so.
They are in accordance with BS7671.
I already indicated so in an earlier post.
- Location
- Massachusetts
You first, no link dumps. Present your calculations.
Och, no problem.
kW Volts pf effy Current
110 415 0.880 94.0% 185 A
Distance 200 m
Pick 150 mm^2
VD
per BS7671 3.03%
This was for our contract C9770, a pumping station in St Albans in UK.
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