Hi All.

I have 8 solar panels of 330w - DC power. (No batteries required or wanted)
My 0.75kw borehole pump (ac) is 650 meters away from the 8 solar panels
Pump Amps Running 6.2
Pump Amps Startup 20
I have a VSD Drive that will convert the DC power to AC power .

I also have some protection...breakers that electrical installed.

What cable do I need to connect the panels to the pump? (can i run the positive and negative from the panels to the pump? ) if so what size? 4mm? 
Do I put the VSD (variable speed drive) and control box at the borehole or with the panels.

Thanks

On 2022/03/16 at 8:16 AM, stok said:

according to the Victron toolkit app, at 650m and a 10a load with a 50mm2 cable you will have a 9.5% volt drop at 48v dc.

with a 4mm2 cable you will have a 100% volt drop, all the power generated with the pv panels will be spent heating the cable up.

it will be a hundred times cheaper to just buy 8 new panels and install them at the borehole.

1300m of 50mm2 cable will cost in excess of R130 000

Not sure where you got the calculation from that gives a 100% volt drop. Any load connected to a cable produces a impedance at the end of the cable. This impedance is in series with the cable resistance and you will get a far greater voltage over the load than over the cable as the load will have a greater impedance than the cable.

Vdrop=I X Rcable

It stays a problem but powering equipment at the higher voltage = AC will have a lower current than the 48V DC and thus a 5 times lower voltage drop.

Hi Anthony, you mention the pump being AC. As in 220V AC?

If so, you need to stick an inverter at the panels and run 220VAC to the pump.

Check https://photovoltaic-software.com/solar-tools/voltage-drop-calculator-dc-ac to work out the cable loss.

Caveat: a .75kW pump probably draws a fair amount more

Either way, the cable will be damn expensive!

Cheers

Edited March 22, 20224 yr by Ian

On 2022/03/15 at 8:11 PM, Anthony H said:

I have 8 solar panels of 330w - DC power. (No batteries required or wanted)

I understand not wanting batteries due to extra cost, but with clouds, planes, flock of birds, etc moving over the panels, the  output will drop and put strain on the VSD and motor. Even on startup, the panels will battle with this, and batteries are normally used to supply instantaneous power.

As an example, I have an 8 kW inverter, and when I turn ON heavy loads, the inverter uses batteries for a short while, until the solar can catch up, IF I am running off grid.

On 2022/03/15 at 8:11 PM, Anthony H said:

Do I put the VSD (variable speed drive) and control box at the borehole or with the panels.

I would put the VSD as close to the pump as possible, seeing as it is controlling an inductive load (motor), and depending how deep the borehole is already you need it to be close.

650 meters of DC cable is also an expensive problem. Never mind the cable gauge needed, running the wires overhead will open the installation up to lightning damage, and the elements. Underground wires could be exposed to water damage. Underground or overhead, DC wires are extremely difficult to protect from corrosion. If there is a small nick or crack in the insulation, or the made off connector is not protected, the wire will corrode and fail.

I imagine the reason you don't want the panels by the borehole is due to theft problems?

If this was my problem, I would get an off-grid inverter, say 3.6 kW and couple the panels to this with a small lithium battery. I would forget the VSD control, and run the AC motor straight from the inverter. The 650 meters of AC wiring would still cost more than the rest of the setup. (Man, that's a long way)🤔

You must be on a farm.

If I was in this position I would install a R2700 Solis 700W grid tied inverter at the panels.  Run 220V AC from the panels to the VSR. This will mean during sun periods the panels provide the power. When heavy cloud you just draw from the grid. Also no need for an expensive battery. Also the peak starting current you just draw from the grid for 2 seconds. Only start pump during the day time.

 

This is only when in fact you do have a grid connection.

Current could be 5-6A for a 0.75kW motor as per this pic.

Edited March 23, 20224 yr by Scorp007

The cable size and price will be prohibitive if your supplying DC to the pump.

VFD's don't work well with long cable lengths because of reflected power, without going into technicalities they're usually limited to 50 meters or so (see manufacturers spec sheet).

I'd do a pricing exercise where you use a cheapie inverter to convert the DC solar output to 230VAC and use the VFD you already have at the pump end to soft start the motor a reduce the start current. I'd then look at using a step-up transformer at the inverter end to just under 1kV so you don't run foul of the LV electrical regs or voltage rating of most SWA cable then do the volt-drop calcs at 950 volts and run an appropriately sized SWA cable to the pump where a step down transformer supplies the motor. If the load is permanently connected and by nature unlikely to vary much you can wiggle and juggle by making the step-up and step-down transformers slightly asymetrical to compensate for volt-drop but only within limits because you'll just end up robbing Peter to pay Paul. ie the step up could be 220v-950v and the step down could be 920v-220v and this migh allow you to use a cable one size smaller which could be a considerable saving.

The start current will self regulate depending on the volt drop in the supply cabling and if the cable is undersized and the volt-drop too high the start current will be reduced and likewise the starting torque will be reduced causing the duration of the start current to be increased. In short, if you skimp on the cable size the motor will be slow to start and will prematurely fail.

I've attached a pdf below that gives you info on the first page so you can do some cable calcs.

Just for example, taking your suggestion of 4mm at 230 volts, if you run a 4mm SWA cable it will have an impedance of 5.52 ohms per Km. A 650 meter cable means the conductor length is 1.2km (live + neutral) so the total cable impedance will be 7.18 ohms and at 5 Amps the volt-drop will be 35.88 volts. This means the pump will be supplied with 194.12 volts.... which is not good and this voltage will be even lower for the duration of the start current.

Cable-Wire-Catalogue.pdf

So, if you have access to mains power supply. My question would be then why limit it just to power to the borehole, surely it would make more sense to attach a grid-tied inverter to the main DB board where it can offset power to the whole property and save money on power going to other things when the pump is not running? 

22 minutes ago, Marv said:

The cable size and price will be prohibitive if your supplying DC to the pump.

If the load is permanently connected and by nature unlikely to vary much you can wiggle and juggle by making the step-up and step-down transformers slightly asymetrical to compensate for volt-drop but only within limits because you'll just end up robbing Peter to pay Paul. ie the step up could be 220v-950v and the step down could be 920v-220v and this migh allow you to use a cable one size smaller which could be a considerable saving.

 

Just for example, taking your suggestion of 4mm at 230 volts, if you run a 4mm SWA cable it will have an impedance of 5.52 ohms per Km. A 650 meter cable means the conductor length is 1.2km (live + neutral) so the total cable impedance will be 7.18 ohms and at 5 Amps the volt-drop will be 35.88 volts. This means the pump will be supplied with 194.12 volts.... which is not good and this voltage will be even lower for the duration of the start current.

Cable-Wire-Catalogue.pdf 1.27 MB · 0 downloads

I like the step-up and step-down approach where one can compensate for the 35V voltage drop.

@Sc00bsThe 700W grid tied was only based on the OP asking for a pump solution. Having the whole property on a larger grid-tied would also be what I would do. Suggested inverter can take panels over 700W as it would throttle itself at the maximum.

 

 

 

@Anthony HDid you come up with a plan/design for this install yet?

@Anthony H i am interested in the solution you pick as i am in the same boat

10 hours ago, MPS1 said:

@Anthony H i am interested in the solution you pick as i am in the same boat

Do you have existing equipment already installed, like pump, motor cabling etc. or are you setting up from scratch?

Good evening guys .

I Have a three phase 2,2 kw borehole pump at a depht of 70 m the flow is 9000l per hour . Lets call the 2,2 kw borehole 1 .I also have a 1.1 kw three phase pump about 300 m away from borehole1 lets call the 1,1 kw borehole 2 . But I control both pumps from borehole 1 .

My plan is to get a inverter with one set of solar panels strong enough to start both pumps but not at the same time .

I will need a swich over controler . To give power to the 2,2 kw if i need to use it as borehole 1 . I use it to pump water for my house 10 000l once a week for 1 hour .

Borehole 2 the 1.1 kw motor i use daily to pump water for the cattle . The pump runs for about 3 hours just to fill all the resiviors . During the summer months i will make use of 2 x5000l water tenks as resivours to keep up with the cattles water intake .

I am not an electrician. So i see things from a farmers point of view .

Supply the inverter with 380v from the pannels and use the sunlight to drive the motors if needed.

Please note i will not use both at the same time . My store room is about 50 m away from borehole 1 from where i control both pumps . I want to put the solar pannels about 50 m away on the shed . I am afraid of theft .

Please advice on the cable thickness and do i need 100 m of cable ?