inverters & buck and boost Xfrmr
- Thread starter Ponchik
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Almost any BB transformer raises the 120 vac source impedance above the 0.5 ohm exhibited by the avg. wall outlet, so then the inverter may not start properly.
Just my two cents. It's a power supply 'stiffness' issue. Motors also need a stiff power supply.
Please help me understand, so if i have a load connected to the AC side of the inverter that has 10ohms the inverter will not start properly????
jaggedben
Senior Member
- Location
- Northern California
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- Solar and Energy Storage Installer
What kind of an inverter are we talking about?
The simply answer is yes, the AC output of an inverter can be raised (or lowered) by a buck boost transformer.
However, depending on the inverter and application, it may not accomplish anything, may not be tolerated by the inverter, or may not be allowed. (If it is a grid-tied solar inverter it will likely fit in the last category.)
The simply answer is yes, the AC output of an inverter can be raised (or lowered) by a buck boost transformer.
However, depending on the inverter and application, it may not accomplish anything, may not be tolerated by the inverter, or may not be allowed. (If it is a grid-tied solar inverter it will likely fit in the last category.)
gar
Senior Member
- Location
- Ann Arbor, Michigan
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- EE
121113-2111 EST
An unloaded transformer will produce an output voltage that is proportional to the input voltage. This will occur if core saturation does not occur (this is defined by the volt-time integral, turns, and core area), the signal is within the bandwidth capabilities of the transformer, and the signal is not too small.
If there is a load on the transformer, then there is a slight modification of the output waveshape as a result of the transformer internal impedance.
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An unloaded transformer will produce an output voltage that is proportional to the input voltage. This will occur if core saturation does not occur (this is defined by the volt-time integral, turns, and core area), the signal is within the bandwidth capabilities of the transformer, and the signal is not too small.
If there is a load on the transformer, then there is a slight modification of the output waveshape as a result of the transformer internal impedance.
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Misread your post. Sorry.
If there is excessive resistance in the DC source it may not start properly if the AC side is already loaded.
The AC output impedance of the inverter is probably already high unless the output is electronically regulated, so adding a BB transformer may not make a difference,
mike_kilroy
Senior Member
- Location
- United States
not enough info for answer
not enough info for answer
without more description of the inverter & load and how it is to be used, in my opinion a reasonable answer cannot be given. too many variables.
not enough info for answer
without more description of the inverter & load and how it is to be used, in my opinion a reasonable answer cannot be given. too many variables.
>Can the AC side of an inverter be boosted by a BB transformer?
>in my opinion a reasonable answer cannot be given
Flipping a coin will give you the right answer half the time.
With just the info given I'd say the odds are 3/4 that this will work.
What's the worst that can happen if it doesn't work?
>in my opinion a reasonable answer cannot be given
Flipping a coin will give you the right answer half the time.
With just the info given I'd say the odds are 3/4 that this will work.
What's the worst that can happen if it doesn't work?
What other info do you need?
It is vehicle mounted inverter, 1750 watts, once in while I use it for tools, charging batteries, and work light.
I think the worst that can happen is that the output regulation (change in output voltage/change in output current) of your inverter will be degraded.
Almost certainly this arrangement cannot damage your inverter.
Post the spec's to the intended buck/boost 'frmr and your inverter.
Almost certainly this arrangement cannot damage your inverter.
Post the spec's to the intended buck/boost 'frmr and your inverter.
hurk27
Senior Member
- Location
- Portage, Indiana NEC: 2008
First a buck/boost transformer is nothing but a high voltage to low voltage transformer configured in a auto transformer configuration, the secondary of the transformer is put in series with the primary in either an additive (boost) or subtractive (buck) so if the primary is 208v and the secondary is 24v then adding the two will result in a boost to 236 volts, the only load will be the transformer losses and the inrush if you were to connect the transformer after the inverter is powered.
Most inverters have what is called soft start when powered up or after an overload if it has the automatic restart feture, starting the inverter after the transformer is connected will take care of the inrush, but you will have to account for transformer loss when calculating how much load you can now apply to the inverter.
Many inverters will claim an output voltage of 110vac, but the true RMS voltage will be in the range of 89-95vac this is because many inverters output a modified wave (square wave) this modified wave will have a much lower average RMS voltage and many universal type motors (AC/DC brush motors used in drills and other hand tools) will run much slower, as well as some switching type power supplies don't like it.
I have used a 1 to 1 transformer to achieve a true sine wave from the fly wheel effect, but doing this does not bring up the voltage, by using a BB in a boost configuration you could, but you will loose the over all capacity of the inverter.
With a inverter that has an output of right around 90 volts RMS you will need a boost transformer that has a secondary of 24vac and a primary of 90vac, but putting 90 volts into a 120 volt winding should give you about 108vac out, a BB transformer with a 120 volt primary and a 36 volt secondary (if you can find one) will give you around 117 vac out, but I have never seen one.
I know you already have the inverter, but it is what it is, I have had a few that you could adjust the output voltage up to around 120vac, but not all can do this.
I have a true sine wave inverter that works well with hand tools as well with electronics, the only way to fly.
I think the problem is going to be trying to find a BB transformer with the right voltage ratings, use the ratio of the transformer in boost to find the right one, and use one only big enough to handle the largest load you will use, and this will place the least amount of losses on the inverter.
Also keep in mind with an output of 1750watts and an eff. of around 60% will result in a 12 volt load on your batteries of 243 amps at full load of the inverter, hope you have enough batteries and alternators for that kind of load, there are some high amperage alternators out there with modified mounting to fit, I had a 285 amp on my van to power a 3600 watt true sine wave inverter salvaged from a totaled RV system, it took 10/ 12 volt batteries to give me a good day of running it at 60% of capacity, a bull dog auto start programed to start the van at 11vdc kept the batteries from going dead, trust me it started quite often.
Although I did remove that system when my 2007 van was totaled and its in my garage less the batteries, but right now I have an 1800 watt with welder cables and a couple of 500 amp battery clamps to just hook it to my battery on the fly, if I get any where near 1500 watts it will kill my 2005 van in less then two minutes even with the engine running, (tested with a 1500 watt space heater) that is a 208 amps load on my puny little 104 amp (much less at idle) alternator and one factory battery, so keep this in mind.
Most inverters have what is called soft start when powered up or after an overload if it has the automatic restart feture, starting the inverter after the transformer is connected will take care of the inrush, but you will have to account for transformer loss when calculating how much load you can now apply to the inverter.
Many inverters will claim an output voltage of 110vac, but the true RMS voltage will be in the range of 89-95vac this is because many inverters output a modified wave (square wave) this modified wave will have a much lower average RMS voltage and many universal type motors (AC/DC brush motors used in drills and other hand tools) will run much slower, as well as some switching type power supplies don't like it.
I have used a 1 to 1 transformer to achieve a true sine wave from the fly wheel effect, but doing this does not bring up the voltage, by using a BB in a boost configuration you could, but you will loose the over all capacity of the inverter.
With a inverter that has an output of right around 90 volts RMS you will need a boost transformer that has a secondary of 24vac and a primary of 90vac, but putting 90 volts into a 120 volt winding should give you about 108vac out, a BB transformer with a 120 volt primary and a 36 volt secondary (if you can find one) will give you around 117 vac out, but I have never seen one.
I know you already have the inverter, but it is what it is, I have had a few that you could adjust the output voltage up to around 120vac, but not all can do this.
I have a true sine wave inverter that works well with hand tools as well with electronics, the only way to fly.
I think the problem is going to be trying to find a BB transformer with the right voltage ratings, use the ratio of the transformer in boost to find the right one, and use one only big enough to handle the largest load you will use, and this will place the least amount of losses on the inverter.
Also keep in mind with an output of 1750watts and an eff. of around 60% will result in a 12 volt load on your batteries of 243 amps at full load of the inverter, hope you have enough batteries and alternators for that kind of load, there are some high amperage alternators out there with modified mounting to fit, I had a 285 amp on my van to power a 3600 watt true sine wave inverter salvaged from a totaled RV system, it took 10/ 12 volt batteries to give me a good day of running it at 60% of capacity, a bull dog auto start programed to start the van at 11vdc kept the batteries from going dead, trust me it started quite often.
Although I did remove that system when my 2007 van was totaled and its in my garage less the batteries, but right now I have an 1800 watt with welder cables and a couple of 500 amp battery clamps to just hook it to my battery on the fly, if I get any where near 1500 watts it will kill my 2005 van in less then two minutes even with the engine running, (tested with a 1500 watt space heater) that is a 208 amps load on my puny little 104 amp (much less at idle) alternator and one factory battery, so keep this in mind.
Thanks Hurk,
I understand it a lot better now.
How did you come up with 243 Amps? and can you please explain EFF of around 60%
Also keep in mind with an output of 1750watts and an eff. of around 60% will result in a 12 volt load on your batteries of 243 amps at full load of the inverter"
I understand it a lot better now.
How did you come up with 243 Amps? and can you please explain EFF of around 60%
Also keep in mind with an output of 1750watts and an eff. of around 60% will result in a 12 volt load on your batteries of 243 amps at full load of the inverter"
broadgage
Senior Member
- Location
- London, England
The input current of a large inverter can be substantial, but not IME as great as posted above.
An efficiency of 60% is unduly pessismistic with many units reaching 90%
A crude guide for inverter input current from a 12 volt battery is to allow one amp of input current for every 10 watts of line voltage output. I would therefore expect the input current of a 1,750 watt inverter to be about 175 amps at full load.
If the inverter was as inefficient as suggested then the losses which appear as heat, would be comparable to the output of a large portable space heater. Is the inverter actually big enough and well ventilated enough to put out nearly 1,500 watts of heat ?
An efficiency of 60% is unduly pessismistic with many units reaching 90%
A crude guide for inverter input current from a 12 volt battery is to allow one amp of input current for every 10 watts of line voltage output. I would therefore expect the input current of a 1,750 watt inverter to be about 175 amps at full load.
If the inverter was as inefficient as suggested then the losses which appear as heat, would be comparable to the output of a large portable space heater. Is the inverter actually big enough and well ventilated enough to put out nearly 1,500 watts of heat ?
mike_kilroy
Senior Member
- Location
- United States
enough info now
enough info now
So it indeed is a simple square wave or at best modified sine wave output 1750w inverter for use at one steady 60hz output freq to run variable loads, most of the time much smaller than 1750watts. If you check specs on typical models of this size you will find they are typically over 90% efficient (so 160amp max input but that was not your question).
If it were a pure sinewave output inverter then there is no problem putting an autotransformer on the output side - we do that all the time on much larger units for running spindle motors. It simply changes the voltage to the load, being an autoxfmr has very very low added impedance, so looks almost totally transparent to the drive. We have never had a drive bothered in any way from doing so.
The thing is kva rating of the xfmr must be significantly larger than the watts of your inverter due to the really crappy square wave pulses going thru it. It will act like a series inductor and will smooth out the current, thus generating significant heat. I have never put an autoxfmr on the output of a square wave generating device, but know on a good 10kh pretty near pure current sinewave drive we often double the kva rating and mount it where the heat won't bother other stuff. So my guess is your 1750w inverter should have about a 5kva bb transformer to not overheat if you plan on pulling anywhere near that max rating.
enough info now
So it indeed is a simple square wave or at best modified sine wave output 1750w inverter for use at one steady 60hz output freq to run variable loads, most of the time much smaller than 1750watts. If you check specs on typical models of this size you will find they are typically over 90% efficient (so 160amp max input but that was not your question).
If it were a pure sinewave output inverter then there is no problem putting an autotransformer on the output side - we do that all the time on much larger units for running spindle motors. It simply changes the voltage to the load, being an autoxfmr has very very low added impedance, so looks almost totally transparent to the drive. We have never had a drive bothered in any way from doing so.
The thing is kva rating of the xfmr must be significantly larger than the watts of your inverter due to the really crappy square wave pulses going thru it. It will act like a series inductor and will smooth out the current, thus generating significant heat. I have never put an autoxfmr on the output of a square wave generating device, but know on a good 10kh pretty near pure current sinewave drive we often double the kva rating and mount it where the heat won't bother other stuff. So my guess is your 1750w inverter should have about a 5kva bb transformer to not overheat if you plan on pulling anywhere near that max rating.
The only time I have ever done this was on an installation of four 800kW variable frequency inverters for motors. We needed 690V for the motors and the inverters available then (quite a few years ago) were generally 400V so we needed step-up autotransformers. These were built with a gapped core to prevent saturation if there was asymmetry in the inverter output waveform. I do remember being a bit nervous about it. It was something we hadn't done before and I didn't know of any other installations where anyone else had. Getting it wrong would have been embarrassing not to mention very costly.
But it worked fine.
I suppose your application will work but you might need to consider inrush current on the transformer.
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