Help needed in selecting an Inverter

[MidniteMike]

Hi,  I am looking for some recommendations for an inverter to go with a planned Off Grid System.  I have already chosen the  Midnight Solar Classic 150 for the charge controller.  I am after some thing similar to the following.

Around 2400 Watts pure sine wave output
240 or 230 Volts
50 Hz
Stackable to two or 3 units
Good Value for money and good reliability.

and the tricky one.....    No built in Battery Charger.

Simple field repair with replaceable boards would be nice too, as I have heard the Outback units have.

I have looked around the net for 2 days, and have not found a unit like this.  I intend to charge from a DC Gen set.  The closest I have found so far is the Apollo Solar TSW3224 True Sine Wave Inverter/Charger 3200W, 24V, but it still has the battery charger.

Thanks in advance for any help given.

Mike

[Halfcrazy]

That is going to be a tough nut to crack. I say go with the outback inverter it needs all your requirements and even has an added feature it has a battery charger. The Charger will not hurt anything and in the odd scenario where your dc gen set should crap out you can borrow or buy a cheap ac gen set to keep the battery's maintained. I would wager a guess any inverter you find with out a charger is not going to be field repairable.

[Robin]

There is no good reason to be looking for an inverter without the charger. The ones you find will be cheap stuff made in China. The battery charger in the OutBack does not add much to the cost or complexity. There is no other inverter to my knowledge that can be repaired in the field like an OutBack. I designed the mechanical package of that inverter just for this very reason!
Stay away from Apollo. I hear they may be out of business now? The Magnum Energy inverter is also a good choice, but I would not say it is field repairable by the consumer. A ten year old can watch the video on how to replace boards in an OutBack and do the repairs.

[MidniteMike]

Thank you Halfcrazy and Robin for your comments. That's 2 votes for the Outback Inverter so far.   That is the first system I looked at.  The extra current from the midnite charge controller piped the outback charge controller at the post.  I was just reading that other post regarding inverter choice, and may be I should go up to 48 V to get the extra value out of the Charge Controllers ? 

Thanks Robin for your input. It is nice to hear from the designers.  My original thought was to keep every thing in separate box's,  with duplicates of each unit for redundancy,  but since you say the Charge controller does not overly complicate the inverter design, and there aren't any stackable inverters with no chargers,  I will be happy to go with the combined unit. 

With regards to the choice between the Magnum and the Outback,  they both seem like good units, but I will probably go for the lightest to save on postage.  Thanks once again for your help.

Mike

[MidniteMike]

A quick question about mppt charge controllers efficiency, if you have a moment...  I understand that they are most efficient when the input voltage is closer to the out put voltage, but I am wondering if the efficiency "drops off" in a smooth curve as the input voltage rises against the out put voltage, or does it " step down " in a saw tooth type wave form,  possibly due to switching transformer winding's as an impedance matching device ?     My electronics is a little rusty, and " off the top of my head " I cant think of a way of converting the voltage / current in a continually smooth curve as in the first option.  Can any one put some light on this ?

Mike.

[boB]

A quick question about mppt charge controllers efficiency, if you have a moment...  I understand that they are most efficient when the input voltage is closer to the out put voltage, but I am wondering if the efficiency "drops off" in a smooth curve as the input voltage rises against the out put voltage, or does it " step down " in a saw tooth type wave form,  possibly due to switching transformer winding's as an impedance matching device ?     My electronics is a little rusty, and " off the top of my head " I cant think of a way of converting the voltage / current in a continually smooth curve as in the first option.  Can any one put some light on this ?

Mike.

Interesting question now that I think about it.

The efficiency will appear to drop off smoothly.   Yes, there are steps in the PWM change that adjusts the input voltage but it is pretty fine.
Any steps that would be there would be mixed in with the couple tenths of a volt (worst case) that the input Vis changing so would appear smooth.

As the input voltage gets closer to the battery voltage, the steps get finer and finer too so that curve gets even smoother.
We're talking a thousand steps or so between battery V and Voc V.

boB

[nigel]

I noticed that your looking for a European Spec 50 hz model, I assume your in Europe somewhere, the I would at the present time only consider the Outback as your choice. Magnum currently have no service facilities in Europe to the best of my knowledge. Where Outback do. I am in Spain and ship all across Europe the full Outback Power range and also stock all the Spare boards, although I dont sell many, usually only after lightning strikes when no surge arrestors were in place.

Outback now supply a GFX1448E with is 1400w  continuos and 2000 watts for 30 minutes as well as the FX2348ET both of which are substanially less expensive than there main VFX 3kw models. Both these models are available in 24v also

Please also remember shipping from USA to Europe is expensive you will be charged a fee for import duties dependant on goods and transit value, no longer a fixed fee Then  VAT or IVA on the goods + transit costs as well which in UK is 20%. The average costs incurred amount to between 25 to 28% of the total cost of goods and shipping, from USA  dependant on which courier you use. If you are in  Spain I have to charge IVA at 18% but if you are a registered EU VAT company I can ship in Europe with out VAT. I am also a stockist of Midnite Solar Products including the new Surge arrestors and E Panels.

Nigel  

[MidniteMike]

Thanks Nigel,  You raise a good point about the lightning strikes.  I better get that sorted before I install everything.  I live in the south of Australia, so will be purchasing the units locally, unless the local units are completely noncompetitive ( which they often are, due to the changing dollar ).  We have 240 Volts here with 50 hertz, but 230 is close enough for a remote site.

boB, I read some where that the Mppt controllers where "sweeping" the input from the solar panels.  How do they do this ?  and is it a step-less sweep, or more like sequentially testing multiple taps of a transformer ?  if it is a step-less process,  how does this work ?  I can't see how PWM would be of use here.  If any one cares to explain ? or may be it is an industry secret !

Mike

[boB]

boB, I read some where that the Mppt controllers where "sweeping" the input from the solar panels.  How do they do this ?  and is it a step-less sweep, or more like sequentially testing multiple taps of a transformer ?  if it is a step-less process,  how does this work ?  I can't see how PWM would be of use here.  If any one cares to explain ? or may be it is an industry secret !

Mike

The PV input change in voltage is done by a change in PWM duty cycle.  This is basically the case for any switching supply or typical buck converter used in an MPPT controller.   So it isn't a step-less process but the steps are pretty small, especially when the input voltage is nearer to the battery voltage (close to 100% PWM duty cycle). If the PWM is at 50.0 percent, then the input voltage is exactly twice the battery voltage.  50.1 % duty cycle would be just a little bit lower in input voltage and 49.9 % would be just a bit higher than twice battery voltage.  As the input voltage gets a lot higher than the battery voltage, the "steps" in voltage get bigger for every percent of PWM duty cycle change.  So, it's more "course" of an adjustment at really high input voltages.  It can get a bit tricky when you have a large ratio of input to output voltage, like, say, 250 Volts input and a 12V battery.   At the other end, the steps get really really small when the input is closer to the battery voltage.


There are different "sweep" styles to choose from in the Classic.   Slower, faster, full I-V curve or operate at a percentage of of the input's open circuit voltage (U-Set) and power curve driven for variable speed wind tracking mode.   They all change PWM percentage in different ways.

The industry secrets have to do with the MASS amounts of time and details of the software programming and hardware R & D.  It's very difficult and time consuming to get these things working properly and reliably.  The basic principle of how they work is fairly simple but the manufacturing can be very difficult, expensive and involve tricks and jumping through hoops.   It's sort of like a circus !

Is that kind of what you were wanting to know ?

boB


PS, if you  want to know more about the technical aspects, there is a lot on the web.
For starters, there's wikipedia...

http://en.wikipedia.org/wiki/Buck_converter

[MidniteMike]

Thanks for the link boB,   I had better do some reading... 

[nigel]

Opps just half a planet away from a pitch   ,  ;better try surf boarding 

[niel]

from boB,
"There are different "sweep" styles to choose from in the Classic."

boB,
do you think you can touch on these areas somewhat?

[Halfcrazy]

Niel here is a a definition of the Classic Mode's boB and I have been working on.

Micro Hydro
This mode is intended for use with hydro systems but can be used with other sources as well.   When the Classic first turns on after the input voltage goes above battery voltage, it will sweep from that open circuit voltage down to battery voltage, finding the maximum power point voltage (MPP V). Then return the input to that newly found voltage. After the original turn on sweep, the Classic will do mini sweeps at user adjustable time intervals.  If the time interval is set to 0, the Classic will not do any mini sweeps but will stay on this first found MPP V until the user goes to the mode menu and turns it off and back on again.
Micro Hydro mode sweeps slightly slower than Solar mode and has 2 user adjustable settings. Sweep Interval is the time between mini-sweeps, in minutes, and sweeps around the present (i.e. the last found), MPP Voltage.  The range of this sweep is determined by the Sweep Depth user adjustment and is expressed as a percentage of Watts that the sweep started from.  For example, if in Micro Hydro mode, the Classic was outputting 1000 Watts and the Sweep Depth percentage was set for 20%, (200 Watts), the sweep will bring the input voltage DOWN until the output power drops down to 800 Watts, then will sweep UP in voltage until the power drops again down to 800 Watts and then go back to the newly found MPP Voltage, waiting for the next sweep.
This mode will NOT go to resting even when the input has been disconnected and will always stay awake so the turbine has the least chance of over-spinning.

Solar
This is the default mode for PV systems and has a very fast sweep (typically1/2 second or less) that will re-sweep at user adjustable sweep intervals, unless the Classic finds that it needs to do a sweep on its own because of changing conditions. The timed sweep interval is user adjustable and is in units of minutes.  SOLAR mode is typically best for PV systems, especially if there is partial shading at times during the day.  The Classic will show a message of "PV SHADE" if it thinks the PV array is partially shaded (if this feature is enabled).
SOLAR mode is best suited for shaded or un-shaded PV arrays that are at least one nominal voltage above the battery voltage. For severe partial shading or PV arrays with nominal voltage equal to battery voltage, you may also want to try Legacy P&O (Perturb and Observe)  MPPT mode.

Legacy P&O
Legacy P&O (Perturb and Observe) mode is a slow tracking mode similar to the Micro Hydro mode but with the difference that it is slightly faster and will shut off if the power source goes off.
It has 2 settings that are user adjustable. Sweep Interval is the time between mini-sweeps, in minutes, and sweeps around the present (i.e. the last found), MPP Voltage.  The range of this sweep is determined by the Sweep Depth user adjustment and is expressed as a percentage of Watts that the sweep started from.  For example, if in Legacy P&O mode, the Classic was outputting 500 Watts and the Sweep Depth percentage was set for 10%, (50 Watts), the sweep will bring the input voltage DOWN until the output power drops down to 450 Watts, then will sweep UP in voltage until the power drops again down to 450 Watts and then go back to the newly found MPP Voltage, waiting for the next sweep.
This mode is also useful for such items as DC generators or rectified ac alternators being driven by a fossil fuel engine. An Example would be a Lister diesel with an Other Power 3 phase alternator.  The AC output would be rectified and run into a Classic set for legacy P&O.

NOTE ON HIGH VOLTAGE-HIGH POWER INPUT SOURCES...  When using an unlimited power source, it is recommended that the input voltage not be much higher than 4 or 5 times the output (battery) voltage.  i.e.  Don't try to charge a 12V battery from an unlimited input power source sitting at, say, 200 Volts. The Classic is able to work with high power-high voltage sources but will usually require an input voltage dropping power resistor in series with the input voltage source. Please call a MidNite technical representative for advice on what size input resistor to add to the input for these special cases if you are unsure what to place in-line and we will try to help.

Wind Track
This mode uses a power curve that is either built by the user or one of the pre-installed graphs. The power curve consists of 16 set points that consist of output amperage and input voltage, allowing the user to custom build a curve for their Wind turbine. Please refer to the Wind section of the manual for full details on programming the curve as well as our video that will help in understanding how to adjust these curves using the wind graph editor.

Dynamic
This is typically used for PV (solar) input sources and tries to follow, on a slow dynamic basis, the changing conditions of the input source.  This mode has one user adjustment which is a forced sweep perturb trigger interval for times when the input condition changes do not trigger a dynamic sweep.  The interval is in units of minutes.

U-Set VOC%
This is a fully manual mode based on a percentage of VOC. The Classic will sweep based on the user set time in minutes and then park at a user set % of the VOC the Classic found on that sweep. This mode is useful for testing or constant voltage sources.