VFD Long Lead Lengths - Correctly Calculating Voltage Drop

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wonderdave

Member
Location
Pittsburgh, pa
I've been in the VFD industry for 20 years and understand the problems associated with dv/dt and long lead lengths. So please do not discuss the effect peak voltage has on the motor or best practice in motor lead length. I'd like this discussion to be very specific to voltage drop at the motor.

Recently I had a discussion with a senior application engineer that provided voltage drop calculations for a 480V 2,200' length of cable between the motor and the drive. The cable is double 4/0 and the motor load is 190 nameplate. There is a TCI dv/dt filer in the motor circuit as well. His calculations included some inductive reactance estimates that would add to the standard resistive voltage drop. Standard resistive voltage drop was about 15-19V for this application. His calculations were more like 70-100V drop.

Here is what I'm searching for.

For long lead lengths between the motor and VFD does anyone have a spreadsheet calculator, or website one, that would factor in the Inductive reactance to arrive at an overall impedance to get a worst case voltage drop?
I'm assuming that switching frequency would be a factor as well as cable conductor orientation.



Thanks for the assistance.
 
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wsbeih

Member
Location
USA
wonderdave said:
I've been in the VFD industry for 20 years and understand the problems associated with dv/dt and long lead lengths. So please do not discuss the effect peak voltage has on the motor or best practice in motor lead length. I'd like this discussion to be very specific to voltage drop at the motor.

Recently I had a discussion with a senior application engineer that provided voltage drop calculations for a 480V 2,200' length of cable between the motor and the drive. The cable is double 4/0 and the motor load is 190 nameplate. There is a TCI dv/dt filer in the motor circuit as well. His calculations included some inductive reactance estimates that would add to the standard resistive voltage drop. Standard resistive voltage drop was about 15-19V for this application. His calculations were more like 70-100V drop.

Here is what I'm searching for.

For long lead lengths between the motor and VFD does anyone have a spreadsheet calculator, or website one, that would factor in the Inductive reactance to arrive at an overall impedance to get a worst case voltage drop?
I'm assuming that switching frequency would be a factor as well as cable conductor orientation.



Thanks for the assistance.
Click to expand...

Well, a straight forward answer is voltage drop is dependent on the cable impedance (R+jX) and its length.
I would follow normal approach in voltage drop calculation for your application.
Mike holt free resources have a good Voltage drop tool. Check it out. http://www.mikeholt.com/freestuff-menu.php
 
L

LMAO

Senior Member
Location
Texas
majorminotaur said:
may I ask what the application was that the VFD was so far from the motor?
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Example: Drives that run ESPs (Electrically Submersible Pumps) positioned deep in the ground. There was a step up transformer at the output of a low voltage VFD to mitigate voltage drop.
 
Besoeker

Besoeker

Senior Member
Location
UK
wonderdave said:
Yes this is an area where explosive gas is present, so the vfd is far away from the motor.
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I would go with what Wsbeigh suggested in post #2.
Size the conductors as you normally would for the required current carrying capacity and voltage drop.

We've had some long runs, up to 350m/1150 ft for borehole pumps. Not in your league, I know but still quite a long run.
I'm in the UK and we use BS7676, "The Regs". The cable tables give volt dop as mV/A/m, (m being metre in this case).
That has worked for us for decades.

And as LMAO noted, VFD output current is a relatively good sinewave. PWM switching frequencies tend to be in the several kHz range so a fraction of a ms between pulses. That simply isn't enough time for the current to change in a motor inductance.
 
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wonderdave

Member
Location
Pittsburgh, pa
Thank you for your input. I really appreciate the time everyone has donated giving their thoughts to this issue.

My problem is that I've never heard of the cable inductance having a huge impact on voltage drop. If anyone has had this experience I'd like to know how they identified the problem or took steps to avoid it.

The application engineer at the outset suggested that we need to get a motor wound for 400V rather than 480 and size the VFD for the higher amp requirement, and possibly boost the input voltage to the VFD as well. This is all good, if we have some sort of engineering data to back this up, but I have found nothing other than the standard voltage drop formulas.
 
Besoeker

Besoeker

Senior Member
Location
UK
wonderdave said:
Thank you for your input. I really appreciate the time everyone has donated giving their thoughts to this issue.
Click to expand...
That's a kind comment. People like to help.

wonderdave said:
My problem is that I've never heard of the cable inductance having a huge impact on voltage drop.
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Nor have I. Your conductor is about the same as our 120mm^2. The inductive element adds about 20% to the voltage drop.
If I were to hazard a guess, I'd suggest that someone has factored in the PWM frequency or some proportion of it.

You've been in this field for two decades and have never seen this "problem" before. Myself, likewise. Interesting.

It would perhaps, be informative to have sight of your application engineer's calculations...........
 
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winnie

Senior Member
Location
Springfield, MA, USA
Occupation
Electric motor research
Is there any possibility that this motor is going to be run at higher frequency, eg. 400 Hz?

Is there any possibility that the motor will (for whatever reason) be required to run at poor power factor? Since the VFD can adjust voltage, I would think that you would always want to run the motor with a good power factor.

Is there any possibility that the motor will be run at full torque and low speed, where the operating voltage will be lower and the % voltage drop higher?

Is voltage drop being calculated for 'across the line starting'?

As I see it, any harmonic component in the drive output current will see greater voltage drop than the fundamental, but this will not increase the _fundamental_ voltage drop.

-Jon
 
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wonderdave

Member
Location
Pittsburgh, pa
winnie said:
Is there any possibility that this motor is going to be run at higher frequency, eg. 400 Hz?

Is there any possibility that the motor will (for whatever reason) be required to run at poor power factor? Since the VFD can adjust voltage, I would think that you would always want to run the motor with a good power factor.
NO

Is there any possibility that the motor will be run at full torque and low speed, where the operating voltage will be lower and the % voltage drop higher?
It is a fan/blower application so the answer here is no as well.

Is voltage drop being calculated for 'across the line starting'?
NO

As I see it, any harmonic component in the drive output current will see greater voltage drop than the fundamental, but this will not increase the _fundamental_ voltage drop.

-Jon
Click to expand...

See my answers in the quote above.
 
Besoeker

Besoeker

Senior Member
Location
UK
wonderdave said:
Yes I have asked for just that, but have been denied. Not sure the reasons, but it must be closely guarded.
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It's not easy to see why disclosing his calcs to you should disadvantage him unless of course...........well, call me a cynical old git.
 
Besoeker

Besoeker

Senior Member
Location
UK
winnie said:
As I see it, any harmonic component in the drive output current will see greater voltage drop than the fundamental, but this will not increase the _fundamental_ voltage drop.

-Jon
Click to expand...
The current harmonic contribution is very low. I think that was covered earlier in the thread.
 
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smoothops10

Member
Location
FL
Occupation
EE
I would imagine the VFD mfr provides a maximum output cable distance. What justification are you going to use for exceeding this? If it were a 200HP Powerflex 753, the maximum length recommended would be 800-1200' depending on output filter, motor VFD rating, and carrier frequency. http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf pg. 130. That manual seems to the imply limitation is based on the reflected waves and charging current on each pulse from the cable capacitance.

This may be a dumb question but if voltage drop were really the only concern and the drive operates on a constant V/Hz ratio. Could that ratio simply be increased by a small factor so the voltage across the motor would be within tolerance. For example assuming a 60V drop to the motor - could the V/Hz ratio = 460/60 = 7.67 be increased by 15% to 8.82 and at full load you would thus see 460/60 across the motor with 529V at the output of the drive?
 
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wonderdave

Member
Location
Pittsburgh, pa
smoothops10 said:
I would imagine the VFD mfr provides a maximum output cable distance. What justification are you going to use for exceeding this? If it were a 200HP Powerflex 753, the maximum length recommended would be 800-1200' depending on output filter, motor VFD rating, and carrier frequency. http://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf pg. 130. That manual seems to the imply limitation is based on the reflected waves and charging current on each pulse from the cable capacitance.

This may be a dumb question but if voltage drop were really the only concern and the drive operates on a constant V/Hz ratio. Could that ratio simply be increased by a small factor so the voltage across the motor would be within tolerance. For example assuming a 60V drop to the motor - could the V/Hz ratio = 460/60 = 7.67 be increased by 15% to 8.82 and at full load you would thus see 460/60 across the motor with 529V at the output of the drive?
Click to expand...


Just to be clear, I didn't design this system with the long lead length. It is already in place designed by a "credible" engineering firm. They just happened to ignore or disregard the manual. I agree this is outside of the mfg recommendation.

After a two years in operation issues started to come up with this drive. I'm not asking for a discussion here of what is correct or good practice or help troubleshooting the drive system. I'm searching for more information on Inductive Losses in Motor Cables on VFD applications. That is my golden nugget here. It was brought up by a trusted senior application engineer and I'm hunting for any details on how to calculate correctly for it in any application. Long leads or not.

Your question is not dumb. I made the same suggestion, only limiting the top frequency to 50Hz or less because we can't get more voltage out than what is put in. But the application requires 60Hz operation much of the time. And your suggestion would require a 530V input to the drive, therefore requiring a 575V drive and line voltage.
 
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