remote mounted VFD vs. long cable run to motor

We are looking into installing a well water pump (480vac, 25hp, 40FLA) with a PowerFlex 755 VFD. The challenge is that the well is located about 700' from the nearest building. The well pump motor comes with a sealed cable which covers the distance in the well shaft (100-150').

Option 1 is to install a drive in an existing building and run roughly 700' of VFD cable out to a box where it splices to the sealed cable. Our vendor says we don't need to use bigger cable to cover the long distance/voltage drop - not sure I agree. He did spec a 25hp heavy duty rated drive & line/load reactors, which I think is proper.

Option 2 is to install the drive panel out in the middle of a field, running 480VAC 3-phase, as well as fiber for Ethernet/IP comms. This seems like more of a pain, having to account for weather, etc.

I prefer option 1 personally, but after reading old posts about VFDs with long motor leads, I'm beginning to wonder which is the better option (and what caveats exist with either option). Any light you can shed on this topic will be appreciated as this is brand new territory for me. Thanks in advance.

Option 1 for sure. Just do a voltage drop calculation to verify the conductor size requirements.


The issues with reflected waves is mostly solved by the load reactor and a motor with windings rated for the appropriate voltage. Check page 130 of this guide for max distance.


https://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf


Looks like if you can't get a motor with high rated windings, you'll need both the load reactor and a damping resistor.

RonJohn said:

Option 2 is to install the drive panel out in the middle of a field, running 480VAC 3-phase, as well as fiber for Ethernet/IP comms. This seems like more of a pain, having to account for weather, etc.

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I would only do this if you are comfortable with ethernet and how to troubleshoot networking issues.

TrendNet
https://www.trendnet.com/products/product-detail?prod=110_TFC-GMSC

I've used this and was brought into a new company (new to me) and they had issues with some of these. The group doesn't have management over switches etc. and to fix a long standing problem I needed to unplug and replug in the ethernet side. I'm pretty sure it got stuck on half duplex on 1 end and was full duplex the other.

It was easy to solve to me, but the site dealt with the problem for months.

Fiber sounds scary but it really isn't, get it certified when installed and the only problem I see is having temperature problems at the shack. (too hot or too cold, note the new PLCs from Rockwell don't like to be frozen for example)

you will need to use bigger cable for voltage drop.
if the pump motor is 25 hp, I would upsize the vfd to accommodate the 700 ft distance and reduce the strain on the system. I would go to 40 hp, but that's me.
james

rupej, thanks for the link. The reactor listed is the one they specified in the manual.

It seems like VFD cable has different ratings for volt drop compared to "normal" conductors, so I'll have to do some more digging there to figure out voltage drop. The vendor noted that frame size 3 drive terminals cannot handle anything above #6 awg, so I'm not sure how exactly to handle anything bigger than this.

VAN, I wouldn't say I'm an expert on networking by any means. I will check out that link in case it comes to that however. Thanks.

If you need a larger wire than the VFD terminals are made for, just put in a terminal block in the panel that does accept the larger wire, and use that as a junction to upsize.

It depends on the motor and the Carrier Frequency setting. For a 30HP 755 (25HP HD rated), the maximum tested distance from VFD to a VFD rated motor (NEMA MG1 Part 31 with insulation of at least 1488V), with a load reactor and the VFD set to 2kHz, is 1200ft. No reactor, it's 600ft, non-inverter duty motor (1200V insulation or less), 300ft.


Download and read this, the chart for PF755 480V is on page 130.
https://literature.rockwellautomation.com/idc/groups/literature/documents/in/drives-in001_-en-p.pdf


Voltage drop is another issue though. #8 will get you a 7.25% VD, probably not what you want, it will limit the motor speed. #6 is better, but still 4.56%. I would go with #4 at 2.87%. I use this calculator, I used 800ft because you said the motor is another 100ft deep.
https://www.calculator.net/voltage-...ce=800&distanceunit=feet&amperes=40&x=60&y=21

One aspect worth checking up on with the pump supplier is if they have any "gotchas" with the use of VFDs. Grundfos for example, require you to fit a sinus filter and ensure the pump doesn't run below 30Hz for more than a few seconds... at least the last time i bought one of their multistage deep well pumps. Something to do with premature thrust bearing failure i think.

We look after a few municipal installations with multiple water bores feeding one treatment plant, and generally we have a local outdoor cabinet (or a small shed housing the switchboard) which houses the VFD, low water protection relay, flowmeter transmitter and a local PLC or RTU. As soon as you have any instrumentation it becomes more attractive to have a local panel with controls to avoid long cable runs in close vicinity of that VFD cable.

The low water protection relay is definitely a must, bore pumps will destroy themselves very quickly running dry.

Thanks folks for all of your input. It is greatly appreciated.

Good call, James Mcquade. Once cable charging current was factored in, we were forced into the 40hp ND drive.

Thanks for the links, jraef. The VFD cable brand quoted to us calls for #2 AWG based on their volt drop spec, so I guess we'll be safe.

Saffa, we are using a Grundfos pump (how'd you know ) and I saw the 30Hz minimum referenced in their literature. No mention of a sinus filter in their doc but we are using a line & load reactors, which according to Post #7 in another forum by some guy named jraef makes me think we should be covered.

RonJohn said:

Thanks folks for all of your input. It is greatly appreciated.

Good call, James Mcquade. Once cable charging current was factored in, we were forced into the 40hp ND drive.

Thanks for the links, jraef. The VFD cable brand quoted to us calls for #2 AWG based on their volt drop spec, so I guess we'll be safe.

Saffa, we are using a Grundfos pump (how'd you know ) and I saw the 30Hz minimum referenced in their literature. No mention of a sinus filter in their doc but we are using a line & load reactors, which according to Post #7 in another forum by some guy named jraef makes me think we should be covered.

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I really like it when somebody takes the time to provide feedback!

I went to look for where I'd read the mandatory sinus filter thing and I can't find it. Maybe it was something the local rep had told me, hoping I'd buy a Grundfos branded inverter and filter.

But the manual for the motors does recommend it, but it seems a load reactor like you have should be fine.

Ramp times: Maximum 3 seconds for start and stop.
Depending on the type, the frequency converter may
cause increased acoustic noise from the motor.
Furthermore, it may expose the motor to detrimental
voltage peaks. This can be compensated by installing
an LC filter or even better a sine-wave filter between
the frequency converter and the motor.
For further details, please contact your frequency
converter supplier or Grundfos.

Just also check the motor minimum cooling velocity is met (i.e. flow past the motor is high enough) at your minimum speed. I have no idea where i got the idea of it being bad for the thrust bearing to run lower... head is getting too full of alternative facts these days! ��

For 700 or 800 feet I'd go with a dv/dt filter. I have had motor insulation problems over 500 feet with load reactors. Perhaps if you have insulation issues, it my be worth it to check them out.



Not that you have this situation, but over 1000 feet I'd upgrade to a sine wave filter, rated for FLA of the motor. I've got one running a 350 HP motor at about 1500 feet, and a 20 HP drive running 4 motors at 3 hp each, at over 2000 feet. These are not on Rockwell drives, but the carrier frequencies are similar, and a PWM drive is a PWM drive when you are running in V/Hz mode.

Ask the VFD manufacturer their opinion?

janner_10 said:

Ask the VFD manufacturer their opinion?

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^ This

For what it is worth, we had noise (RW) issues with 30' VFD Leads, I would for insurance try to get something from the Distributor or VFD mfg in writing on the installation.

This mistake costed a lot more later to correct, than if we just paid a little more upfront talking to people that eat drink and sleep VFDs and grounding.


There was a saying at Chrysler by all the Electricians, you can pay a little now or a lot more later.