Question About VFD/VSD AC Drives Input

Think it through a little more. In a 240V single phase circuit, current flows from L1 to L2. In your scenario, where is current flowing?

Hint: there is a flaw in your statement "the two 220v legs would be 180 degrees out of phase with each other". There is no "phase" relationship, they are 2 ends of the SAME phase, offset from each other by 180 degrees.

Thanks! So the "current" flows from L1 to L2 through the entire the VFD circuitry...I thought that the 120V AC signal was chopped into DC the moment it hits the rectifier and did not need to complete a circuit.

Sorry newbie here...

I think a more accurate description in lay terms would be that the current flows from L1 to L2 through the rectifier. The output of the rectifier then goes to the rest of the VFD circuitry.

In the USA and its 60Hz dominions....what you said is true....

The OP has not said where he lives!!

In the UK and Europe, the 230VAC is between a single phase Lx and the neutral......where "x" = 1, 2 or 3!!

I live in the U. S. of A...where we use things like pounds,miles,gallons, etc. when the rest of the world uses the metric system. Said VFD can take the European 230VAC circuitry as well as the 230VAC we produce here or three phase 230VAC...it will take anything....

Question: If I have two 120V generators side by side (A/B)and I stick my meter into a single prong socket of Gen. A and one from Gen. B at the same time, what will the meter read VAC?

Theoretically, it will read nothing, but minor leakages may produce a smallmeter movement.....

What is thepoint of measuring like that?

I read your previous post and the best way is to take the big generator. Bite the bullet!!

Per JRaef:

Note that neutral is not connected to any VFD input pin. So your external 120 volt generators would typically not be outputing power of the 120VAC (each) that is 180 degree timed to create the 240VAC.

Is this what your question is?

Either get a generator with a 240 output, or use a 1:2 step up transformer rated for the motor plus losses that the VFD will drive.

Your previous posts were "Please Help Me Build An Electric Boat Drive!" dated 5-21-2012. Is this a continuation of this question?

No this is not a continuation of that question about the boat...this is about whether the voltage coming from the hot leg of a generator can be rectified to DC by a VFD with no neutral input.

The two ~120V wires that make up Single Phase 240V are "HOT"...If I touch them individually I will feel it (I think) with no neutrals involved. I am asking if I fed the "hot" leg from a generator into a VFD's rectifier circuit would it produce DC by itself or would neutral have to be wired in somehow to COMPLETE the circuit?

So let's say I wired it up this way...

Gen#1 Hot to input 1 of VFD

Gen#2 Hot to input 2 of VFD

Gen#1 Neutral tied to ground

Gen#2 Neutral tied to ground

Do I need a fire extinguisher or will the VFD see it similarly to the two ~120v leads of standard ~240V single phase?

Your still missing the fact that the 2 generators aren't necessarily 180 degrees apart in their phase relationship.

They could be, or they could be in-sync with each other, in which case their is no difference of potential between "Gen#1 Hot to input 1 of VFD", and "Gen#2 Hot to input 2 of VFD".

Add to this that Gen #x wants to have a path to ground (through the load), and there is no ground hooked to the VFD; only hot legs from both gen-sets.

The short answer is "No".

The "regular generators" that you have will not be synchronous (to each other) and will be hard wired directly to each other.

Think about what will happen when they are out of phase with each other and then again when they are in phase with each other. Will they see the other generator as "dead short" load when out of phase and thus fry the cables? Even when in phase, what would be the impact of the voltage waveform seen by the inverter? (It will not be sinusoidal.)

In my humble opinion, Don't go there!

You would have to parallel the generators and use a transformer to step up the voltage...cheaper to just get the proper generator...you would probably need a regulator control from the VFD to the generator as well.....VFD's are not tolerant of varying voltage supply....

What kind of generators are they and what are their power ratings?

Most portable generators that are more than a few KW/KVA that puts out 120 VAC has 240VAC as an optional output configuration.

To run a 5 HP VFD at full power you would need a roughly 5000 watt source of which if it was a single generator would be rare to not have a 120/240 output system.

Super Quiet Honda inverter generators 2KW a piece@120V...Yes, I think a 5HP VFD would require a ~3700 Watt generator. 746x5

Honda makes a 1000W, 2000W($1000 X 2), 3000W,and a 6500W ($4000.00)

Needing 3700 watts would force me to get a 6500W @$4000 unless I could buy another 2000W and run them both thru the VFD to get the desired results...

I wish I could get two from Australia or Europe that are 230VAC with a parallel kit.

What is the 5 HP electric motor going to be powering? That's what will determine the overall design.

$4000 buys a lot of other drive systems that would be far simpler and more efficient along with some pretty impressive muffler systems that can make most any engine super quiet.

Given your quest to build a IC powered something of other that requires a variable speed output of zero to ~5 mechanical HP there are a lot of options other than generators VFD units and electric motors out there to work with that will certainly beat this design in terms of cost Vs efficiency..

As far as two independent AC generators placed in series no they will not give you a stable output voltage. As the two alternating current sources change their respective phase alignments your output voltage will likely appear to be a slowly changing DC output that cycles from zero to around 340 volts DC.

Thanks for your comment! I never said I would put two generators in series...I'm merely questioning the output from the VFD...it has capacitors etc and circuitry that will give a stable output despite some variations by converting the AC to DC then back to a clean AC sine wave.

It will power an air mover.

To power an air mover as in a big fan?

If so a simple choice of the right gearbox or belt set to match your engines RPM range to your fans operating range is all that's needed. No need to over complicate a simple process.

Your VFD will accept a single phase input, but will produce a 3 phase AC sine wave which will require a motor to take the 3 phase voltage @ 120' out of phase for a 360' In phase. You can not use a dryer on a single phase from the output from the 3 phase inverter. It's just physics. It will fault out and shut down.

Tell us what you are trying to do, or is this just a theory question?

It sounds quite dangerous to life and limb to me....

Thanks for all of the responses! I want to know if it is theoretically possible to feed two separate 120VAC sources into one VFD that converts it to three phase 240VAC. I read somewhere that VFD cares not about the phase relationship of the individual input sources as long as it sees a usable voltage and it will rectify that single source to DC and then later convert both of them to 3 phase AC. (BTW This VFD can take a 3 phase or single phase input.) I was wondering if this was possible at all. If so how?

This is to run a 3 phase motor...and yes I know I can buy an genny with 240v. :-) This is purely theoretical.

Thank you all!

I would say that you cannot connect two AC generators except possibly synced. I cannot see how you would sync them, using what equipment?

I have to admit that I have never synced, or heard of syncing of single phase generators, only 3 phase ones many, many times.....

I will stick my neck out and say a) dangerous b) totally pointless c) not possible...

Maybe someone else here has experience.....

DC Generators are far more forgiving and easier to run in parallel.....

lectricboat

1. Your simple statement of the VFD's components is correct--input voltage rectified to DC (with filtering and capacitors there), then inverted back to the desired output voltage and frequency. So far, so good.

2. With only a few exceptions, the VFD will fault out if the input voltage drops below a specified percent of its nameplate rating. No VFD is designed to output a voltage more than ~5-10% higher than its input, so when the input drops too far, then the VFD is unable to do the job it was designed/programmed to do.

3. What many others have been trying to say is that two 120-volt generators, connected together via their neutrals have an undefined voltage between their two "hot" legs. Assuming that they are at exactly the same frequency, the phase relationship of their "hot" legs can be 0, 15, 90, 180, or any number of degrees apart. If 0º, there would be 0 volts between them. At 90º, it would be about 170 volts; at 180º, it would be 240 volts. Without special (and expensive) controls, the typical generator is not designed for the precise control needed to force them into perfect synchronization with a phase relationship of 180º.

4. Therefore your VFD would have an unpredictable input voltage and would fault. Many others have suggested using a generator's 240-volt output is one way to avoid this.

5. However, nobody mentioned a different way (although one that is not easy). VFD's are also manufactured with a DC input and AC output (no rectifier front end, and typically used on battery-powered vehicles, etc). If each of your generators were to be supplying a rectifier and capacitor bank, with the two capacitor banks connected in series, now you would have the required DC voltage for the input to such a VFD. Such a connection eliminates the question of phase angle relationship or frequency relationship between the two generators, because you are only working with DC (zero frequency).

6. I do not advise you to do this, unless you have lots of space, good advice and help from professionals in this type of work, and appropriate protective equipment. Or, you could purchase two DC power supplies that have 120VAC single-phase input and ~165-170VDC output and wire them as described above.

--John M.

Thanks John M. for your thorough explanation and at least considering this question! Also thanks for an alternate method....you spoke to the science behind it in lay terms and didn't so much question why I would even want to try this.

With that being said earlier in the day I experimented with this "theory". Using the one generator I have I tied my residential 120V neutral to my generators neutral and ground and to the VFD's ground(E). The "hot" from the house was wired to one input and the generator hot to another input.(R,S,T) The VFD powered up!!!!