EEVblog Electronics Community Forum

Electronics => Power & Renewable Energy => Topic started by: Robert Smith Eco Warrior on June 17, 2021, 09:45:47 am

Title: How difficult would it be to make a simple 5kw inverter?
Post by: Robert Smith Eco Warrior on June 17, 2021, 09:45:47 am
Hi All,
We live off grid and I use two Victron 3kw inverters.
The sun is out today so I have our water pump going and water distiller going. My inverter fans are roaring away keeping them cool while water is being pumped and distilled.
The water pump is just a motor (about 400w, The label fell off) and the distiller has a heater element and a cooling fan so there are no clever electronics to be too concerned about.
I am wondering how easy it would be to make a 5+kw inverter (5kw to take the motor start up load) to convert 50v DC into 240v AC in a quite roughly approximated sine wave?

Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Seekonk on June 17, 2021, 02:43:34 pm
I think the answer to a problem is stepping back and thinking about what you want to accomplish.

I live off grid and have fridge, hot water, dishwasher, clothes washer and everything else with only have a car battery. Why would you ever use an inverter to heat water?  You can run that right off the panels with a power point controller and not even need a fan. Use power when it is there. Heating water controller is well within the capabilities of electronics neophyte. Energy management is the future.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: fordem on June 17, 2021, 07:01:23 pm
Get a water pump with a DC motor to match your battery voltage, and eliminate that as an AC load, you should also be able to find "solar stills" that use no electricity, they require only the heat of the sun to operate.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Robert Smith Eco Warrior on June 17, 2021, 09:05:53 pm
The pump is 300m down the field so 240v AC is far less current than 48v DC would be. I don't want to buy 300m of very thick copper cable.

I also have other motor driven projects I wish to do. An automated wood chunker for processing our biomass willow and maybe other projects. That wood chunker would be 3hp or a bit more and also 100m from the batteries so wanting to get the voltage up and the current down. I am not wanting to drag our biomass willow to the machine and want the machine near the willow.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: dietert1 on June 17, 2021, 09:07:49 pm
Why don't you buy one? A 5 KW inverter is about US$ 700 and you can't make one for that money.

Regards, Dieter
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: ucanel on June 17, 2021, 09:15:35 pm
Topic: How difficult would it be to make a simple 5kw inverter?
...
If it is simple it could not be difficult.

If it is not for hobby or educational purposes it is not a good idea to make one.
Just buying the components would be much more expensive than buying ready one.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Robert Smith Eco Warrior on June 17, 2021, 09:17:06 pm
Why don't you buy one? A 5 KW inverter is about US$ 700 and you can't make one for that money.

Regards, Dieter

This is what I am wondering. I don't know how plausible it would be. I can't afford US$700 but cheaper chinese ones might be ok for this job as I am not too interested in getting a perfect sine wave.
I am just wondering if anyone has built a fairly inexpensive tough, simple inverter.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: james_s on June 17, 2021, 09:38:40 pm
It's certainly something that is possible to do, you just need a beefy regulated boost converter and then a H-bridge capable of handling the current and you can get a "modified sine" wave which is a square wave with some dead time. You're not going to be able to compete on cost with China though, and if you value your time at even minimum wage you'll spend more than the cost of a top quality commercial inverter.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: ucanel on June 17, 2021, 09:58:18 pm
As always you will get what you paid for.
Cheap ones will cost you more.
I think you are in a remote location if it fails you will pay more for repair costs.
While it is in repair you will lost usage time.
Cheap ones more likely to be not efficent, your source is limited why you would spend precious energy with inefficent converter.
"Robert Smith Eco Warrior" if that Eco means Ecology
the cheap garbage ones carbon footprints can be add to the equation.
That cons list can be made longer...

And there is transformer based simple ones you may make yourself relatively easily but the above problems apply this one too.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: NiHaoMike on June 18, 2021, 12:08:37 am
Why don't you buy one? A 5 KW inverter is about US$ 700 and you can't make one for that money.

Regards, Dieter
Prius inverters go for about $100 on the surplus market and they can be hacked to operate as general purpose inverters. They're good for at least 10-20kW continuous and on the order of 50kW peak.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: dietert1 on June 18, 2021, 08:31:29 am
What is the meaning of "simple 5kw inverter"? Did you find a youtube video that you want to follow? Do you mean using an audio amplifier with a toroid? There is nothing simple at 5 KW. To start with you need to wire your generators/buffers for higher voltage than 50 V.

Regards, Dieter
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Faringdon on June 18, 2021, 05:28:44 pm
Here is your basic inverter, simulated in the free LTspice sim programme.

This one is open loop, so only works for this case of fixed load.
------------------------------________________________________
In fact, the way to do it is to  first have a sinusoidal reference.
This  represents what you want  your inverter to give as an output voltage. So you increase the PWM duty cycle to the bridge FETs as the sinusoidal reference increases. A big inductor in the bridge will smooth out the current.
You need fast overcurrent limiting  so that the current never goes above your maximum allowable. You also need average current limiting which is obviosuly much slower acting.
In order that your output voltage never goes above 339Vpk....make sure your DC supply is exactly 339VDC. Then enjoy PWM'ing away.

Remember every 10ms to swap from one set of bridge diagonal fets to the other set....then you get your bipolar voltage.

So its easy because you are not grid tied......just increase the pwm duty cycle as the 50Hz sinusoidal reference increases its amplitude, and let the current take care of itself...just have fets overcurrent limiting....and your slow average current limiting

So to get increasing pwm duty cycle as your sine reference  increases.......use the old trick of a repeating  ramp (up and down) into one pin of a comparator...and into the other pin...shove your reference.....then your comparator output will have a duty cycle which increases as your sine reference increases.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Siwastaja on June 19, 2021, 07:50:03 am
Seeing you already have inverters, just a tad too small to run all the loads including the heaters, I would take the simplest and largest load - the resistive heating elements - out of the equation.

You can supply the heaters with a simple boost converter boosting your 50VDC to whatever power level the heaters require, up to 240VDC, but as a side effect, you would get smoother control here, i.e., if the elements currently run with a thermostat turning them on and off, you can instead just boost to lower voltage. Remember though that mechanical switches and fuses rated for 240VAC are not necessarily suitable for 240VDC.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Seekonk on June 20, 2021, 11:43:19 am
Even simpler. Take the boost out of the circuit and run with DC right from the panels.  You should only be making water when the sun is shining and there is an excess of power.  Panel voltage will be higher which makes heating element simpler.  Then PWM from a capacitor bank to keep the panels at power point.   I do this and it makes the entire solar system efficient by removing the conversion losses.  I love free hot water.  Even my clothes washer has its own 40 gallon tank and all cycles use hot water.  This easily does a KW or more depending on voltage. It shows the simplicity of the circuit.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Siwastaja on June 20, 2021, 11:52:05 am
Even simpler. Take the boost out of the circuit and run with DC right from the panels.  You should only be making water when the sun is shining and there is an excess of power.

This is obviously simplest. You won't get MPPT but the simplicity makes up for it.

But I kinda assumed the need for heating is decoupled from the sun shining and there is some significant battery storage involved. If not, I'm all for the simplest possible solution.

PV voltage being too low for existing heating elements could be a problem though.

More information is needed, i.e., a full schematic of all components in question, including the panel string voltage and battery capacity and voltage, if any.

Maybe I'm misunderstanding your circuit, but I have seen some similar claims before so I'll comment about it even if it doesn't apply to your circuit. "PWM from capacitors to get power point and not have conversion losses" doesn't make any sense. You are likely having significant losses in capacitor ESR although it might kind-of work with parasitic inductance. You really need an inductor in the system for voltage conversion (and voltage conversion is needed to track MPPT while supplying loads). Buck, boost or any other power conversion topology works, and you can't completely get rid of conversion loss but properly designed converter can easily run at 98% efficiency. A ghetto converter which lacks important components like the inductor likely isn't as efficient even if the first instinct is that removing the inductor removes inductor loss.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Seekonk on June 20, 2021, 04:52:33 pm
I see MPPT as implemented in many solar controllers as a running joke and very easily get lost. No, you don't need an inductor to be efficient. A decent design for a capacitor bank is needed keeping the delta V under 3%. Pseudo tracking can be done via a temperature tracking of a small metal plate exposed to similar sun conditions.  This allows it to be used in parallel with a MPPT charge controller.   With dropping voltage on a fixed resistance you have pretty much lost it. As these tend to be self powered very early morning tends to be the most dangerous time for these circuits. Under voltage is a serious design concern many ignore. Lots of designs oscillate like crazy. This design has been used successfully around the world. I fully agree that that most people heating water have no idea what they are doing.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: Siwastaja on June 21, 2021, 01:33:29 pm
Assuming "heating water" doesn't mean extreme temperatures (hot steam), using air-to-water heat pump would be a great idea given the limited power source. At dT=50degC, i.e., going from +20degC to +70degC or from -10degC to +40degC would typically result in COP around 2.5 to 3, meaning you get 2.5 to 3 kW of heat out of 1kW of electric power.
Title: Re: How difficult would it be to make a simple 5kw inverter?
Post by: G7PSK on June 21, 2021, 02:41:42 pm
The usual thing is to dump excess power straight to an imersion heater, a standard imersion heater works you may need more than one in the tank to use all the power. Have a \look at this \mans videos as he has been at the echo/green thing for years.https://www.youtube.com/results?search_query=andy+reynolds (https://www.youtube.com/results?search_query=andy+reynolds)  He even runs classes or did before the CCP plague.