Convert dc to ac without transforming the voltage

thierry · June 2019

1. I have 8 solar panels. 23 - 29 v output. 7,5 max amp, 175 W max output ...

2. I have an electric waterpump with an expansion barrel pumping up ground water ... The pump is installed below the expansion barrel below the ground surface so i cannot see it. But from experience i know it must be between 1000 and 1500 watt.

3. I have a large capacity of reservoirs to collect rainwater for gardening.

4. I only use the waterpump during the summer when we have exceptional drougth to fill up the reservoirs once in a while. Happens maybe 5 times a year. Maximum 10 times and i only need to pump a few hours max to fill all my reservoirs.

5. I do not like noisy generators.

6. So here is my reasoning ...

If i connect the solarpanels in series i have a voltage between 184 (8 × 23) and 232 (8 × 29) volt.

I think on 184 volts the pump will run slower but i do not care because time is not my enemy but much rather my friend.

The max output will not damage my pump.

The problem however is DC output from the solarpanels and the pump ofcourse needs AC. So i guess i only need to convert from dc to ac. No need to transform to another voltage.

I can find all kinds of converters and transformers combined on the internet but non of them who don t change the voltage.

I have two questions ...

1. Your opinion about my reasoning ... does it make sense or do i see it the wrong way ? I barely know something about electricity indeed.

2. If it makes sense i reckon it would not be that hard to make my own converter fit for what i want to do. How could i accomplish this ?

BB. · June 2019

What you ask for is not "that hard" to do (solar panels => AC inverter => AC loads)...

But, in general, most AC loads do not like brownouts and/or stopping&starting during the day.

A frequent answer is to use a VFD (Variable Frequency Drive)... These can be programmed/made to take energy from a solar array and provide 3 phase variable frequency (and voltage) to a pump motor.. I.e., when first starting low frequency to drive motor to get it spinning, then increase frequency to 100% RPM... And as clouds go overhead, or the sun starts to go down in the West, the VFD will reduce pump speed (keep pumping at a lower rate, so more total water is pumped in a day). However, generally these are designed for 3 phase motors.

There are also DC pumps which can be connected directly to solar, or through a Linear Current Booster... A DC motor needs lots of current, but not that much voltage to begin turning--Which an LCB provides--Then as sun rises, the LCB limits voltage the DC pump at 12 or 24 volts. LCBs tend to be expensive, brushed DC motor/pumps tend to be cheap--But brushed motors have a limited life (brushes/commutator). There are electrically commutated DC motors--But may not be cheap either (basically a VFD/Inverter internal to the motor, or through an external motor controller).

You can also directly connect a DC pump to a solar array--But this may have issues (solar panels are not anywhere near an "ideal battery").

You are now in a hole... There is lots of neat stuff out there, but most would require you to either do a full battery+AC inverter support for your present AC pump--Or instead buy new hardware, or design your own solar input AC inverter--Can be done, but not an "easy" project).

A 12 volt or 24 volt RV water pump would probably be a good solution plus a pair of 6 volt @ 200 AH golf cart batteries...

Or if you have 120 VAC within a few hundred feet, run a direct burial cable from the AC source to the pump...

And perhaps one of our members has found a batteryless solar panel=>AC pump solution (there are GT Inverters that output q120 VAC during sunny weather if the utility power failures--But they are not a cheap solution for your pumping requirements.

-Bill

thierry · June 2019

BB. said:

What you ask for is not "that hard" to do (solar panels => AC inverter => AC loads)...

But, in general, most AC loads do not like brownouts and/or stopping&starting during the day.

A frequent answer is to use a VFD (Variable Frequency Drive)... These can be programmed/made to take energy from a solar array and provide 3 phase variable frequency (and voltage) to a pump motor.. I.e., when first starting low frequency to drive motor to get it spinning, then increase frequency to 100% RPM... And as clouds go overhead, or the sun starts to go down in the West, the VFD will reduce pump speed (keep pumping at a lower rate, so more total water is pumped in a day). However, generally these are designed for 3 phase motors.

There are also DC pumps which can be connected directly to solar, or through a Linear Current Booster... A DC motor needs lots of current, but not that much voltage to begin turning--Which an LCB provides--Then as sun rises, the LCB limits voltage the DC pump at 12 or 24 volts. LCBs tend to be expensive, brushed DC motor/pumps tend to be cheap--But brushed motors have a limited life (brushes/commutator). There are electrically commutated DC motors--But may not be cheap either (basically a VFD/Inverter internal to the motor, or through an external motor controller).

You can also directly connect a DC pump to a solar array--But this may have issues (solar panels are not anywhere near an "ideal battery").

You are now in a hole... There is lots of neat stuff out there, but most would require you to either do a full battery+AC inverter support for your present AC pump--Or instead buy new hardware, or design your own solar input AC inverter--Can be done, but not an "easy" project).

A 12 volt or 24 volt RV water pump would probably be a good solution plus a pair of 6 volt @ 200 AH golf cart batteries...

Or if you have 120 VAC within a few hundred feet, run a direct burial cable from the AC source to the pump...

And perhaps one of our members has found a batteryless solar panel=>AC pump solution (there are GT Inverters that output q120 VAC during sunny weather if the utility power failures--But they are not a cheap solution for your pumping requirements.

-Bill

Thanks for your reply Bill but to be clear .... my present pump aint no AC pump neither is it 3 phase (i think). Its a DC 220 v pump and currently i use a 5 kw generator to run it. In the past i have also ran it with a 2 kw generator. I have tried it with a 750 watt generator as well but that didn t work. This is why i know it must be somewhere between 1 and 1.5 kw.

BB. · June 2019

Sounds like Grundfos or similar DC pump with integrated VFD/Pump controller.

Yes, you could just simple series parallel solar panels to achieve (say 2,000 Watt array) with Vmp~220 VDC...

Some guesses:

220 Volts / 30 Volt Vmp (typical) less expensive GT panels = 7.33 panels, round up to 8 in series
2,000 Watts * 1/0.81 typical "hot panel" power derating = 2,469 Watt array
2,469 Watt array / 8 panels in series = 309 Watt panels (8x309 Watt panels in series)
309 Watt panel / 30 Volts Vmp = 10.3 Amps

That is very doable... You have to find the exact panels you need to do the # of panels and series/parallel connections.

https://www.solar-electric.com/silfab-sla-m-310-solar-panel.html (example panel with "close enough" specs).

It would be great to get the name plate rating/brand/model of motor to figure out its exact power requirements (again solar panels are very "poor" batteries and have very wide operating specifications.

Also, if you use an On/Off Switch, make sure it is rated for >220 VDC and >10 ADC ... AC switches on high voltage DC circuits can arc/fail.

-Bill

thierry · June 2019

BB. said:

Sounds like Grundfos or similar DC pump with integrated VFD/Pump controller.

Yes, you could just simple series parallel solar panels to achieve (say 2,000 Watt array) with Vmp~220 VDC...

Some guesses:
220 Volts / 30 Volt Vmp (typical) less expensive GT panels = 7.33 panels, round up to 8 in series
2,000 Watts * 1/0.81 typical "hot panel" power derating = 2,469 Watt array
2,469 Watt array / 8 panels in series = 309 Watt panels (8x309 Watt panels in series)
309 Watt panel / 30 Volts Vmp = 10.3 Amps
That is very doable... You have to find the exact panels you need to do the # of panels and series/parallel connections.

https://www.solar-electric.com/silfab-sla-m-310-solar-panel.html (example panel with "close enough" specs).

It would be great to get the name plate rating/brand/model of motor to figure out its exact power requirements (again solar panels are very "poor" batteries and have very wide operating specifications.

Also, if you use an On/Off Switch, make sure it is rated for >220 VDC and >10 ADC ... AC switches on high voltage DC circuits can arc/fail.

-Bill

Here is a link to the system on the website from the company that has installed it years ago for the former owner of my land https://www.clasal.be/zuigerpompen/hydrofoorgroepen . It is in dutch but the pictures can give you some idea i think. The strange thing is in my system the pump is not installed on top of the expansion barrel but below it and in the ground. I will call the company tomorrow to ask some explanation and specifications on the pump.

bill von novak · June 2019

thierry said:

1. Your opinion about my reasoning ... does it make sense or do i see it the wrong way ? I barely know something about electricity indeed.
2. If it makes sense i reckon it would not be that hard to make my own converter fit for what i want to do. How could i accomplish this ?

I think you will be _way_ better off with a DC pump and a pump controller like an LCB. Then you are using the pump the way it's designed to be used.

thierry · June 2019

bill von novak said:

I think you will be _way_ better off with a DC pump and a pump controller like an LCB. Then you are using the pump the way it's designed to be used.

You speak as if i can just simply replace the pump. The pump is builth into the ground and the weekendhouse is on top of that. Installing it cost the former owner several thousands of euros. And i am using the pump the way it is designed to be. It is a 220V AC pump and i power it with a 5kw generator. Would be nice however if i could replace that generator by the solar panels as a power source. Why ? No noise ... no fuel ... good for the environment ... good for my pocket ... good for nature surrounding my land.

mcgivor · June 2019

thierry said:

bill von novak said:

I think you will be _way_ better off with a DC pump and a pump controller like an LCB. Then you are using the pump the way it's designed to be used.

You speak as if i can just simply replace the pump. The pump is builth into the ground and the weekendhouse is on top of that. Installing it cost the former owner several thousands of euros. And i am using the pump the way it is designed to be. It is a 220V AC pump and i power it with a 5kw generator. Would be nice however if i could replace that generator by the solar panels as a power source. Why ? No noise ... no fuel ... good for the environment ... good for my pocket ... good for nature surrounding my land.

The simple way would to use something like this https://m.aliexpress.com/item/33007670299.html?pid=808_0000_0109&spm=a2g0n.search-amp.list.33007670299&aff_trace_key=aed1d0ac02fd4b43bc1caec53517db50-1560507214903-05141-UneMJZVf&aff_platform=msite&m_page_id=4246amp-ZXkLFFJjLTwwvWd9Q8WQJQ1560508224658
This particular one is 3 phase, single phase are probably available, this would allow series panels up to 390 VDC, no batteries required.

NANOcontrol · June 2019

Driving a motor with variable speed is a piece of cake with 3 phase. Not so with single phase. I doubt one exists. Just watched a video of some Colorado company with one, they wanted $4,000. Buy two or three at a minimum if getting one from China. You will never get it fixed.

mcgivor · June 2019

Here is a single phase one http://www.kewoinverter.com/1phaseinverter.html that accepts solar input, I must agree 3 phase is more common and simpler, Hitachi also has some single phase inverter controllers with MPPT input controller. Single phase may be single speed.

mike_s · June 2019

thierry said:

If i connect the solarpanels in series i have a voltage between 184 (8 × 23) and 232 (8 × 29) volt.
I think on 184 volts the pump will run slower but i do not care because time is not my enemy but much rather my friend.
The max output will not damage my pump.

Thing is, 200 VAC is 566 volts peak to peak. So, no, you can't simply/directly convert 200 VDC to 200 VAC. Simply chopping 200 VDC into a square wave would produce a bad approximation of only ~70 VAC.

NANOcontrol · June 2019

I don't think anyone would be happy with that single phase unless the array was way oversized. Single phase motors are terrible at very low speeds. I think there is a lot of fine print. Three phase you can slow to a creep. I've run 2 HP 480V three phase motors on a 220V 0.1KW

Tecnodave · June 2019

Mike_s,

The conversion ratio of RMS voltage to peak to peak is: RMS times 1.414 is peak to peak, therefore 200 volts RMS is 282.2 volts peak to peak.......and conversely conversion of peak to peak to RMS is peak to peak times .707 so 200 volts peak to peak is 141.4 volts RMS

MichaelK · June 2019

You speak as if i can just simply replace the pump. The pump is builth into the ground and the weekendhouse is on top of that. Installing it cost the former owner several thousands of euros. And i am using the pump the way it is designed to be. It is a 220V AC pump and i power it with a 5kw generator. Would be nice however if i could replace that generator by the solar panels as a power source. Why ? No noise ... no fuel ... good for the environment ... good for my pocket ... good for nature surrounding my land.

Well, that sounds extraordinarily foolish! All motors eventually fail. Maybe not next year, maybe not in 2025, but 10-15 years down the road, and you eventually WILL have to face pump replacement. Having a pump at 125 meters down fail myself was an expensive learning experience! In my own case, having constructed a room over the well-head, I constructed a "hatch" where the steel roofing could just be unbolted to allow a crane to pull out the pump. You can create a 230VAC system that will power your pump. I did. It will not however be good for your pocket. It does however allow me to completely silently pump water from 8am till 4pm.

NANOcontrol · June 2019

I just remembered talking to a guy that convinced a company to replace all of its motor controls wit VFD to save energy. Six months later they started having about two motors a month fail. These drives start creating standing waves at the transitions over 100 feet. These old motors did not have the insulation to deal with 2KV spikes continuously. You will find some motors mention inverter ready. Three phase line filters are especially recommended for long runs with submerged pumpa.

mike_s · June 2019

Tecnodave said:

Mike_s,

The conversion ratio of RMS voltage to peak to peak is: RMS times 1.414 is peak to peak, therefore 200 volts RMS is 282.2 volts peak to peak.......and conversely conversion of peak to peak to RMS is peak to peak times .707 so 200 volts peak to peak is 141.4 volts RMS

Nope. RMS times square root of 2 (1.414) is peak. Peak to peak is twice that. Diagram below is for 120 VAC, where peak to peak is 340 V. But thanks for bringing it to my attention, because I had the wrong numbers in my post. Fixed now, I originally worked it out for 240 VAC, then changed to 200 VAC to be in the middle of the OP's range, and didn't re-do the math to make it match. 200 VAC is actually 566 V peak to peak. The point remains unchanged.

Image: http://academic.pgcc.edu/ent/ENT%20171%20Online%20Project/rms.gif

Estragon · June 2019

I'm no kind of expert on AC, but is (+) peak to (-) peak (or trough) voltage relevant to much of anything?

I get that curve shape, timing, and capacitance/inductance matters. Just not sure how RMS peak x 2 matters.

mcgivor · June 2019

Estragon said:

I'm no kind of expert on AC, but is (+) peak to (-) peak (or trough) voltage relevant to much of anything?

I get that curve shape, timing, and capacitance/inductance matters. Just not sure how RMS peak x 2 matters.

Peak to peak is meaningless as it's never at both positive and negative potentials at the same time.

mike95490 · June 2019

mcgivor said:

Peak to peak is meaningless as it's never at both positive and negative potentials at the same time.

You have lived in DC land too long.

RMS voltage and P-P voltage are critical. I just read a long article about VFD drives, and how fixed speed motors, connected to VFD controllers, are being fried from P-P voltages induced in them. P-P voltage is why most household wire is rated 600V, otherwise, why not rate it to only 300 ?

mcgivor · June 2019

mike95490 said:

mcgivor said:

Peak to peak is meaningless as it's never at both positive and negative potentials at the same time.

You have lived in DC land too long.

RMS voltage and P-P voltage are critical. I just read a long article about VFD drives, and how fixed speed motors, connected to VFD controllers, are being fried from P-P voltages induced in them. P-P voltage is why most household wire is rated 600V, otherwise, why not rate it to only 300 ?

Quiet the contrary on being in DC land too long.

The rating for insulation is based on RMS of the peak voltage to ground, not peak to peak, typically the insulation value of low voltage conductors such as 600V is actually way in excess of its rated value. This discussion https://forums.mikeholt.com/showthread.php?t=127506 briefly discusses the topic. The problem with using frequency drives with motors not specifically designed for them, is the drives tend to produce spikes in the waveform, these have a tendency to break down the insulation of the windings creating shorts between them, leading to premature failure, something I'm very familiar with.

mike_s · June 2019

mcgivor said:
Peak to peak is meaningless as it's never at both positive and negative potentials at the same time.

What does that even mean? Can you be in two places at once?

mike_s · June 2019

Estragon said:

I'm no kind of expert on AC, but is (+) peak to (-) peak (or trough) voltage relevant to much of anything?

I get that curve shape, timing, and capacitance/inductance matters. Just not sure how RMS peak x 2 matters.

It matters if you're trying to simply/directly change DC into AC. That's what this thread is about.

mcgivor · June 2019

Back to the thread topic, here is a Danfoss 230VDC to 230VAC inverter, it would seem it can accept up to 380VDC, https://www.alibaba.com/product-detail/inverter-12kw-danfoss-vlt-2811-inverter_60679221182.html
Just a reference link, contacting Danfoss directly would probably be a good idea, Alibaba specs are for multiple units and often misleading, getting lost in translation.

Estragon · June 2019

mike_s said:

Estragon said:

I'm no kind of expert on AC, but is (+) peak to (-) peak (or trough) voltage relevant to much of anything?

I get that curve shape, timing, and capacitance/inductance matters. Just not sure how RMS peak x 2 matters.

It matters if you're trying to simply/directly change DC into AC. That's what this thread is about.

Seems to me there's nothing simple in changing DC into (grid quality) AC. If it was, we'd all be fine with cheap hardware store MSW inverters. Especially with motor loads, it gets complicated pretty fast.

mike_s · June 2019

Estragon said:

mike_s said:
It matters if you're trying to simply/directly change DC into AC. That's what this thread is about.

Seems to me there's nothing simple in changing DC into (grid quality) AC. If it was, we'd all be fine with cheap hardware store MSW inverters. Especially with motor loads, it gets complicated pretty fast.

It's simpler if you start with the full voltage swing you need, then you just have to modulate it. Go back and re-read the OP and what it was asking.

NANOcontrol · June 2019

In the typical full H bridge, the peak voltage is not doubled. One side or the other is switched to the common negative at the desired frequency and the other side is pulsed to positive with a variable duty cycle at a much higher frequency. An inductor averages out those pulses to make a sine wave. The motor can be thought of as an inductor. In three phase there are three half bridges.

thierry · July 2019

How about ...

1. Connecting the 10 solarpanels in parallel. This would give me 28 V dc 75 amp output.

2. Converting this into 230 V ac by use of a converter that is sold for truckdrivers who want to watch television in their cabin etc when sleeping on the road.

So instead of feeding the converter with power from batteries directly feeding him solarenergy ...

Good idea ? Foolish idea ?

I have found such converter for a reasonable price. It outputs 230V ac max 1500 Watt and it needs an input of in between 20 and 31 V dc.

BB. · July 2019

Here is "thierry's" pump link through google translate (to English):

https://translate.google.com/translate?sl=auto&tl=en&u=https%3A%2F%2Fclasalpumps.com%2Fnl%2Fzuigerpompen%2Fhydrofoorgroepen

I don't see any motor specifications at all.. It looks like a standard induction motor--And I would guess there are configuraiton options (voltage, frequency, 1/3 phase, etc.).

Yes, it is possible to put a bunch of solar panels in parallel and run an "off the shelf" AC inverter. One vendor did that, but I cannot find their product... It appeared to be a voltage controlled switch. When the DC input fell below the AC inverter's input voltage, it would power off/on the inverter when the voltage was restored (many AC inverters will "lock off" when the DC input falls below LVD--low voltage cutoff--The voltage controlled switch cut power to reset the inverter and allow it to start again).

I still think that a VFD (variable frequency drive) would be a better fit--But there is a lot of research needed to figure out what motor you have on the pump and match it with a "solar capable input" VFD to get everything working.

10 years ago, a solar VFD setup was not common--These days, there are a bunch of vendors (including China based) that are seliing these VFDs (from what I can see).

-Bill

thierry · July 2019

What i know about the pump through experience is that it doesn t work on a 750 w generator. But it does work with a 1500 w generator ...

thierry · July 2019

and al the generators i have used output 230V ac 50 hz. No 3 phase connections on it.

thierry · July 2019

https://www.dwe-oss.nl/product/24v-naar-230v-omvormer-2000w/

and here is the convertor i have in mind. Its dutch again but on the right you can see the technical specifications.

Convert dc to ac without transforming the voltage

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