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Small 3-phase inverter


stoic

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Hey ppls.

I have 2 boreholes that i had to take off my solar system as i switched from three phase to single phase for 99% of the plot.

The two boreholes are .75kw and .5kw respectively. (3P)

I would like to get a small three phase inverter and 3x300w panels. so that i can run those boreholes during the day off solar. 

What is the cheapest i can get away with?

 

What would be nice, is if the inverter was grid tied.

 

i will also consider 3 single phase inverters that can be placed in parallel

 

Edited by stoic
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28 minutes ago, phil.g00 said:

The pumps won't last forever, do you really want to buying another 3Ph pump because you now have a VFD next time?

I suppose that would make sense if the damn borehole pumps was not that expensive. .. i am looking at R14k for two new borehole pumps. for that price i might as well get a 3p inverter.... or three 1p inverters....

 

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  • 2 weeks later...
On 2019/10/14 at 1:47 PM, stoic said:

Maybe another options is to get a VFD that i can then hook up to my single phase inverter system and run the two boreholes off that?

I do not know enough about VFD's though.

Good idea but it depends on the wiring of your 3 phase borehole motor.

Some motors have the 3 windings separately terminated to 6 terminals. This allows you to connect the motor in star or delta configuration.

Each winding is (usually) 220V so if connected to 380V the motor must be configured in star. (phase to star/neutral is 220V)

If you use a VSD on your single phase supply it will generate 3 phase but only at 220V. So you need to configure the motor in delta.

If you are able to do this I can recommend a 0.75 kW VSD for R1200..

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19 hours ago, Richard Mackay said:

If you use a VSD on your single phase supply it will generate 3 phase but only at 220V. So you need to configure the motor in delta.

My understanding is that the incoming 220 V gets rectified, and you get a ~ 310 VDC bus. This allows the inverter to generate three wonky sine waves of about 220 VAC with respect to an imagined neutral, which when you look at the difference between outputs, you get 380 VAC three phase sine waves. They have to be wonky sine waves with respect to the imagined neutral point, otherwise you end up running out of DC bus voltage and the peaks will get clipped.

[ Edit: I was thinking of 3-phase input inverters, see next 2 posts. Sigh. ] 

So you only get three wires to the motor (the "star point" or "neutral" isn't even generated), but the windings have to be suitable for 380 V phase to phase, not 220 V phase to phase. (Three phase voltage is always measured phase to phase, unless clearly stated otherwise, of course.) If they are 220 V windings, they'll have to stay in star. If you wire 220 V windings in delta, then you'll be overvoltaging the windings. Sometimes you may want to do that if you want to run at higher than rated speed. I would think that for a borehole pump, you would not want to overspeed/overvoltage it, since the mechanical side of the pump won't be expecting it, nor be designed for it.

Edited by Coulomb
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3 hours ago, Coulomb said:

So you only get three wires to the motor (the "star point" or "neutral" isn't even generated), but the windings have to be suitable for 380 V phase to phase, not 220 V phase to phase. (Three phase voltage is always measured phase to phase, unless clearly stated otherwise, of course.) If they are 220 V windings, they'll have to stay in star. If you wire 220 V windings in delta, then you'll be overvoltaging the windings. Sometimes you may want to do that if you want to run at higher than rated speed. I would think that for a borehole pump, you would not want to overspeed/overvoltage it, since the mechanical side of the pump won't be expecting it, nor be designed for it.

image.jpeg.cdbe21642ce825e851d10b01064e2ce6.jpeg

These a the star and delta configurations.

If your VSD is only generating 3 phase 220V you need to connect the motor windings in delta. (The motor has no power if wired in star!)

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8 hours ago, Richard Mackay said:

If your VSD is only generating 3 phase 220V... 

Ah. My bad. A single phase to three phase inverter isn't going to generate the same voltage phase to phase at the output as the input voltage, duh. Unless it has a boost converter, that might be practical for very small (<~1 kW) power inverters.

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