How can I convert 120 Volts DC into 120 Volts AC???

Question

This is for a project I am working on. I need to run an small air compressor (120V - 15 amps). I know I could use a power inverter that converter 12V to 120V etc, but my friend said since I already have a 10 battery bank, I could build a 120DC to 120AC for a fraction of the cost of a big inverter. can someone tell me how to build one or give me and schematic?

Thanks

Answer 1

No offense, but I'm reading between the lines of your question and I'm guessing that you don't have a lot of expertise in this. (One of those "if you have to ask how, you probably shouldn't be doing it" things...)

A quick search and I found two 2000 watt 12VDC inverters (linked below) for $300 and $159, respectively. Why try and re-invent the wheel when you can buy the wheel for that cheap?

Plus, you have a lot less hazard working with your batteries when connected for 12VDC than you do for 120VDC. Have you ever dropped a wrench across 12VDC? Pretty impressive sparks. You drop it across 120VDC and it won't be so much sparks as it will a plasma ball! (And that's no exaggeration. Electrician's Mates in the Navy working on submarine batteries at 200+VDC have had their tools disappear in a flash and bang after touching across terminals.)  

Answer 2

The first question is does your device require AC?

Can it be changed to a DC motor easily? Then you need no converter.

An inverter uses a chopper circuit to convert dc to a square wave, Then some shaping if needed.  

Answer 3

You're going to be learning an awful lot of power electronics before you get an air compressor of that size running off batteries.  120v at 15 amperes is 1800 watts or 2.4 horsepower, a motor that's not going to forgive a poorly-shaped AC waveform, and you'll have interesting problems with power factor and the like.  

I can't imagine why your friend would have told you what he did.  You cannot just reverse the battery voltage 120 times per second and expect the motor to run properly, especially if it's an induction motor, which it very likely is.  I'd suggest that you change the sort of motor you're using and drive it directly off your DC.  

Answer 4

With solid-state electronics in your mind as a solution, you'll need to buy or design/build an inverter.

Or you could buy a 120 VDC motor and connect it's output shaft to the input shaft of a 120 VAC alternator.

Do whichever is within your means.

Answer 5

If you really want to run this compressor converting DC to AC you are going to run into electronics and effifiency problems in the way. However, the best way in my opinion, is to replace the 2.5 HP AC motor for a 96 VDC, 18 amps motor.You can rearrange your battery bank to get 96VDC output. I hope this helps.

Answer 6

A VFD is a variable frequency drive. It uses  electronics to rectify AC into DC, generate a frequency, then use output electronics to synthesize an AC waveform. With a battery bank, you need to do only the last part of this. I don't know if anyone makes just this module. AC motors are generally pretty forgiving of waveform, it doesn't need to be a perfect sine wave. They generally are made using a pulse width modulated output.

120VAC has 170V peak to peak. I expect your batteries need to be this, or half this, to work directly.

Google to find circuits used for VFD's. You can leave yours at 60Hz, don't need to change speed, though this may not make much difference. PIC microprocessor site and others have example VFD circuits. Their application note nicely explain the PWM schemes and tradeoffs. Yours could be fairly simple.

Some VFD's may also run on DC. Check their specs. A 3HP unit is about $300 USD, so probably not too much cheaper than the inverter. Why not start by checking pricing on a 2000 watt inverter powered by 12V or 24V? Should be able to find for about $200 new. Plus, if you get an inverter, it could be used to drive other loads if needed.

Maybe you can get a used inverter to use or VFD to experiment with on ebay, if you are comfortable working with transistors. I would use 96V, use a MOSFET bridge to allow positive and negative going waveforms, then set up a controller and output transistor to PWM the output. It's actually a fairly complex project, and the time required may be big compared to the money saved. Maybe your friend can help...

The voltages involved make it a little bit dangerous. Be safe, use caution.

Answer 7

be very careful that you do not accidentally cross +/-, you could blow up the air compressor, or just fry it out totally...ask an electritian, or your local hardware store for help with this one...