VFD Volt Drop Question

[andyb]

I need a help from those on here with experience with 3 phase motors and VFDs.

I have been asked to install the cable to supply a couple of pumps that will be controlled using a variable frequency drive.
As they are submersible pumps they come pre wired with 12 core 2.5 rubber cable, 10 cores used, 3 pairs for the pump and 2 for a float switch in the pump 1 for a sensor and earth.

The pumps will be installed 45m from the control panel so I need to calculate the volt drop.
My question is, does the variable frequency have any bearing on this?

All the other pumps I've wired for have been dol starters so no problem with calcs.

Thanks in advance.

 

[Darkwood]

Effective screening of the motor cables will be needed from drive to pump so pre-design on this a must as you have obstacles most other installs don't ?

Other issues with remote drives is harmonics - you need to read the drive install guides to see max length of motor cables before additional equipment is required to surpress Harmonics etc (EG - In line reactors).

Just a few questions to answer before we tackle your query.

 

[andyb]

Effective screening of the motor cables will be needed from drive to pump so pre-design on this a must as you have obstacles most other installs don't?

Other issues with remote drives is harmonics - you need to read the drive install guides to see max length of motor cables before additional equipment is required to surpress Harmonics etc (EG - In line reactors).

Just a few questions to answer before we tackle your query.

Thanks for your reply

The pump does not have screened cable, it's come with about 5m of 2.5 H07 12 core cable.

I've asked the company doing the install for the cable requirements and I'm awaiting their reply. I'm just trying to understand what's involved.
They are telling me that the cable needs to be SY/CY or SWA and will let me know the size re volt drop.
The problem I have is that the pump will be pumping sea water and the area is extremely damp because of the sea water so SY/CY or SWA is not ideal. Does the cable need screening or is there another way?

 

[Darkwood]

The requirements are going to be hard to achieve tbh as you will need an effective screening that will comply grounded at both the drive end and to the motor case although i do believe the water itself can act as a very effective screen.... I haven't a lot of involvement in this area of screening submersibles but I'll look into it.

Why is their a need for these to be on a drive, why are the cores paired up (is this to increase CSA of the motor supply cores).

There are many questions to ask but I sense a lack of knowledge in the design stages to the requirements of a VSD and the issues with long lengths of load cabling requiring line reactors and sine filters.

Fitting drives to submersibles is acheivable but has a lot of pre-design requirements... you cannot just buy a pump, a drive and connect them.


Found a link - read this guide from ABB then at least you know the requirements and bounderies before asking the questions to the team

http://www.abb.co.uk/cawp/seitp202/0ab6c25f1b7da067c1257707004c0668.aspx

 

[andyb]

I'm starting to suspect you're right re lack of knowledge.
In my simple electrician way I could not understand why it was that the type of cable factory fitted to a pump could not be the same when extending it.
I have to admit to not understanding harmonics but assumed that if it was a problem then the cable connected to the pump would be suitable.
So now if I run SWA for example I will need to connect it to the rubber cable. Would the lack of a screen matter for the last few meters? I'm only asking out of interest, I'll do what the designers say and leave them with the responsibility.

 

[Darkwood]

I'm starting to suspect you're right re lack of knowledge.
In my simple electrician way I could not understand why it was that the type of cable factory fitted to a pump could not be the same when extending it.
I have to admit to not understanding harmonics but assumed that if it was a problem then the cable connected to the pump would be suitable.
So now if I run SWA for example I will need to connect it to the rubber cable. Would the lack of a screen matter for the last few meters? I'm only asking out of interest, I'll do what the designers say and leave them with the responsibility.

I edited my post after you copied it so please re-read as I found a perfect link for you - even I learned a little here too lol ...see edit to last post for link , it turns out the water can act as an effective screen so maybe ok with the pre-flexed cable.

 

[andyb]

I edited my post after you copied it so please re-read as I found a perfect link for you - even I learned a little here too lol ...see edit to last post for link

Thanks for that Darkwood, I'll read through it though it'll take me a lot longer than it took you as I'm starting form a position of zero knowledge.

 

[Darkwood]

Thanks for that Darkwood, I'll read through it though it'll take me a lot longer than it took you as I'm starting form a position of zero knowledge.

Drive set-up is crucial here too, submersibles are usually running at full voltage/speed ... you may find issues at certain speeds and a good chat with the suppliers of the motor will be a good move and will also confirm there suitability to be run off a drive, if you are not familiar with drive installs then please don't do this off the cuff, someone with knowledge of both submersible pumps and drives should oversee your work here, mistakes could be costly to the life and efficiency of these pumps and they are not cheap in my experience, they can cost several times that of a standard similar sized motor.

 

[andyb]

Drive set-up is crucial here too, submersibles are usually running at full voltage/speed ... you may find issues at certain speeds and a good chat with the suppliers of the motor will be a good move and will also confirm there suitability to be run off a drive, if you are not familiar with drive installs then please don't do this off the cuff, someone with knowledge of both submersible pumps and drives should oversee your work here, mistakes could be costly to the life and efficiency of these pumps and they are not cheap in my experience, they can cost several times that of a standard similar sized motor.

I won't be doing anything, just running cables and terminating as directed by the company that designed it and are making the control panel.

From what you've told me and from what I've now read I'm not sure about their competence.

The problem I have is suitability of the equipment I install for the environment. I'm using plastic throughout. Plastic tray, fixings, enclosures etc. Now it seems I have to run SWA.

Thanks again.

 

[Besoeker]

Drive set-up is crucial here too, submersibles are usually running at full voltage/speed ... you may find issues at certain speeds and a good chat with the suppliers of the motor will be a good move and will also confirm there suitability to be run off a drive, if you are not familiar with drive installs then please don't do this off the cuff, someone with knowledge of both submersible pumps and drives should oversee your work here, mistakes could be costly to the life and efficiency of these pumps and they are not cheap in my experience, they can cost several times that of a standard similar sized motor.

Over the years we have put variable frequency drives on quite a few submersibles in a number of pumping stations. The only installation I recall was one supplied and installed by others who should have known better.

In short, the problem was excessive dv/dt. No output reactors had been fitted and there was excessive rate of rise of voltage at the motor (3kV/us in some cases). This was motor killing the winding insulation. The average operational life of the motors was about four weeks.

 

[andyb]

Over the years we have put variable frequency drives on quite a few submersibles in a number of pumping stations. The only installation I recall was one supplied and installed by others who should have known better.

In short, the problem was excessive dv/dt. No output reactors had been fitted and there was excessive rate of rise of voltage at the motor (3kV/us in some cases). This was motor killing the winding insulation. The average operational life of the motors was about four weeks.

Thanks Besoeker

I think I've read enough to realise that I'll do as instructed and any problems will be others responsibility.
I'll be keeping all correspondence and instructions.

 

[Darkwood]

Over the years we have put variable frequency drives on quite a few submersibles in a number of pumping stations. The only installation I recall was one supplied and installed by others who should have known better.

In short, the problem was excessive dv/dt. No output reactors had been fitted and there was excessive rate of rise of voltage at the motor (3kV/us in some cases). This was motor killing the winding insulation. The average operational life of the motors was about four weeks.

TBH - lengthy runs isn't something I entertain alot, my area of experience is machinery where I'm quite flexible with design and I like to keep drives local to motors for these reasons plus its allows full sections of the machine to be removed or upgraded without disruption the the main panel control as they can be used as independent units with little adaption.

Only issue I ever had was bearing pitting leading to failure due to circulating current down the shaft through the bearings, took some digging in them days to realise what was happening but its a known issue now with some motors and drives, I always ensure insulated bearings are requested where this issue could arise.

 

[Besoeker]

TBH - lengthy runs isn't something I entertain alot, my area of experience is machinery where I'm quite flexible with design and I like to keep drives local to motors for these reasons plus its allows full sections of the machine to be removed or upgraded without disruption the the main panel control as they can be used as independent units with little adaption.

Only issue I ever had was bearing pitting leading to failure due to circulating current down the shaft through the bearings, took some digging in them days to realise what was happening but its a known issue now with some motors and drives, I always ensure insulated bearings are requested where this issue could arise.

A couple of points if I may.
I wouldn't consider 45m a particularly lengthy run. Some VSD manufacturers recommend output filters if it is likely to be 200m or more.

Bearing problems I know of but haven't experienced. Gambica have done a reasonably good technical article on this. The copy I have is a few years old now but here is a link that may give more up to date information:

Variable Speed Drives Group

 

[davesparks]

It strikes me that if the sal****er environment calls for plastic/GRP enclosures fixings and even cable tray then SWA or SY is almost certainly not going to be up to the job.
I'd be looking in the direction of cables designed for a marine environment for a soloution for this.

 

[Besoeker]

It strikes me that if the sal****er environment calls for plastic/GRP enclosures fixings and even cable tray then SWA or SY is almost certainly not going to be up to the job.
I'd be looking in the direction of cables designed for a marine environment for a soloution for this.

You may well be right on the money. Uncoated steel and salt water don't make good bedfellows.

We've supplied drives for the marine environment but, as a rule, we don't often get involved at the installation phase.

 

[andyb]

Thanks for all the input guys, I'm just going to concern my self with the cable installation side of things as instructed.

That still leaves me with the problem of suitability. I defiantly will not be going down the SY route. If using swa does the armour need earthing at both ends? I mean in terms of for screening properties. If not then at the wet end I could terminate into a plastic enclosure using IP68 plastic glands.

Besoeker what do you mean by excessive dv/dt ? and what does the us in 3kV/us stand for?

I've heard back from the people doing the install to inform me that 6mm is required, which brings me back to my original question regarding VFDs and volt drop.
These are 5.2Kw 10.4a pumps so I'm assuming that the variable frequency has an efect on vd.

 

[andyb]

Just re read the email from the company doing the work and they inform me that they will be supplying some screened Belden cable to be run from the pump back to the control panel. No problem with that, but they then go on to ask me how long the run is so they can supply the right amount.

If they do not know the length of the run how could they calculate the volt drop of the pump cable?

Also, if I'm using armoured for the pump and the Belden is screened, would there be an issue with emc?

 

[Darkwood]

You require an effective screening from drive to pump, usual methods are a screened cable grounded to the motor case and usually a specified ground on the drive... your situe differs somewhat though so you may need to look into alternatives here.... like the link mentions I posted e expresses in that the seawater itself provides a screening effect.

Like besoeke mentions in a reply to me you really don't need to worry about fitting in line filters, chokes or reactors for your length but always check with the drive install instruction to confirm this and to see what screening, earthing arrangements are required etc.

If all your doing is running cable from A to B then most of the design issues don't fall in your court.

You say they have supplied a cable for the submersible, now in my experience the pumps are pre-flexed and sealed pumps so not something you would be able to replace yourself unless it has a user IP68 terminal block?

 

[sysrq]

Examples of shielded cables recommended by IEC 60034-25