The combined voltage does not NEED to be zero. Nothing would happen if that were the case. Most likely the cheap step up don't work because they just can't handle the current your device needs.

Frankly, if you've got reasonable wiring already in place, why not change the circuit to 240V. It may take little more than a new circuit breaker.

The goal is to run a 240v pump off of a 120v generator.

A well pump? See if the pump has 120 volt connections for the motor. Jet pumps often do, submersible pumps typically do not.
For a 120 volt generator and a 240 volt motor, a transformer is the only choice. I suspect that a 120 volt generator will not operate a very large pump.

Here is a link that might be useful: Autotransformer

BY FAR the easiest thing would be to get a genny that provides 240v.

It's a submersible pump.

Sounds like there is no easy answer. Thanks for the replies erveryone.

Most generators have a plug for 120/240V operation.

What are the specs for the generator? What are the specs for the pump? (V and A)

You will need a pretty decent sized generator, especially for the starting current on the pump.

A transformer large enough is not going to be inexpensive.

The generator cannot output 240v. I've gone the 2:1 xfmr route and it didn't work - and it had enough headroom as does the generator (2.8kw) for a 1/2 hp pump, I believe.

I really think that there's something different between using both 120v legs vs. just bumping up 120v to 240v with a transformer. Maybe someone more knowledgeable than me can explain the difference.

I was hoping someone would know of a solid state device that could emulate the 2 legs. Perhaps the only answer is to go back to a 120v sub pump (yes, I know that a 240v pump has advantages over a 120 pump).

I have reservations about both the capacity of your set, and your level of expertise.

But if you are determined to do this, you need a transformer with a 240v center tapped winding, not a simple 240v winding. The "combined voltage of the two legs" is 240V, not zero. Each "leg" will read 120V to neutral.

"I really think that there's something different between using both 120v legs vs. just bumping up 120v to 240v with a transformer. Maybe someone more knowledgeable than me can explain the difference. "

There actually is not.

The way the two 120 V legs are created is by using a high voltage (7.2 kV) primary and a 240 V secondary with the secondary center tapped and grounded.
That center tap is what splits the 240 V into two separate 120 V lines 180 degrees apart from each other.

A 1/2 Hp pump will be at least 1000 V-A to start.
While the generator may be able to support the load, the transformer must be able to also.

If the transformer is not large enough (V-A rating) its core will saturate and the 240 V output will drop down preventing the pump motor from starting.

Without actually measuring the pump peak starting current under load, a 2 kV-A transformer would be in the right ballpark.

Gee, Brick, thanks for confirming exactly what I told him in the first post.

Also, if by cheap chinese transformer he means something like the sevenstar, it's also got an internal fuse he probably burned the first time he tried to start the motor, but if he didn't, you're right the motor can't get enough current to start.

Brickeyee, I would like to believe there is no difference. The seller says it won't work: See faq #3 http://www.voltageconverters.com/faq.htm#1

However I had one of these step-up trasnformers at one time & tested it from grid power. It did indeed power the pump, but there was noticable hum through the piping which led me to believe that the motor was not getting what it needed. They're not expensive - a 3000w is just around $120. I have my doubts about the quality since the 2000w unit I tested was using 16 guage wire if I recall on the 120v input.

So if the two legs are 180 degrees apart, then at no time are both legs 0v at the same time, correct? 120v or 240v steped-up is at 0v 60 times per second. Wouldn't that mean the 240v taken from 2 legs produces more power than 240v stepped up from 120v?

"Brickeyee, I would like to believe there is no difference. The seller says it won't work: "

The seller is simply wrong.

"So if the two legs are 180 degrees apart, then at no time are both legs 0v at the same time, correct? "

Wrong.

The two legs are exactly synchronized and both pass through 0 volts at the exact same instant.

They pass through their peak at the same instant also, though one is +120 V while the other is at -120 V relative to neutral.

Running a load between the two legs is exactly the same as running it from a single 240 V line to neutral.

The two legs are the ends of a single transformer winding.

The frequency problem (50 Hz in Europe vs. 60 Hz in the US) means you can use a 50 Hz appliance on 60 Hz, but NOT the other way if it contains induction motors (universal motors are fine).

A 50 Hz motor has more iron to carry the magnetic field created.
It will turn faster on 60 Hz but will run fine.
A 60 Hz motor may not have enough iron and the core will saturate at 50 Hz.
This decreases the power output and leads to overheating and can burn the motor windings up.

Perhaps Wayne's point is key. Perhaps these inexpensive transformers are of the simple winding type that don't work (well), and perhaps Brickeyee is refering to the center tapped type that will work?

Any idea where the center-tapped type can be purchased?

For this application, the center tap would be unnecessary. The reference was included in an attempt to explain how the power companies deliver the dual voltage from one transformer.
There is no low-cost way to do what is desired in this case. A 120 volt submersible pump would experience much greater voltage drop in a deep well than would the 240 volt model. The real answer is to buy a 240 volt generator of suitable wattage.

You do not need a center tap, just a plain old 120 V to 240 V transformer that is large enough.

I really doubt the ratings on that site also.

They may be using an auto-transformer winding style, and these can have problems with large loads.

An auto-transformer would need to be seriously oversize for an induction motor load.

There are a number of sites that seem to think US 240 V is some weird thing.

We use an Edison circuit to limit the voltage to 120 V to ground, while still having 240 V available for larger loads.

It is NOT two phases.

It is single phase power.

Anyone that thinks the two legs are phases is using sloppy jargon or does not understand how the system works.
Two phase power has bee tried.

The phases are 90 degrees apart and it requires four wires.
Three phase power has the phases 120 degrees apart, and can get by with just 3 wires (delta connection) but often uses four (Y connection) in case the phase currents do not sum to zero. The forth wire can carry the mismatch between the phase currents and prevent circulating currents that can play havoc with distribution ransomers.

3-phase power is especially good for running large induction motors.
No start windings, start capacitors, start switches are required, or flywheel effect to keep the motor turning.

A 3-phase motor has a large starting torque as soon as power is applied, unlike a single phase motor.

how much does the transformer you tried weigh?
I have a pair that would handle your pump load. together they are about 80 lbs. maybe 90 lbs..

have you check your generator windings..do you have 2 windings wired in parallel? can these be split from a parallel setup into a serial setup?

if ther're parallel now and you're getting 120v. if you can series the windings( pay attention to connections to chassis), then you get 240v..if I'm talking greek..DON'T consider this option.

-dkenny

Bus Driver posted a link (autotransformer) above that I glossed over on first pass but have now reviewd and I think I like what I see. It appears to be better quality (albeit more $$) than the Chineese transformers. It weighs 30-40lbs and has real specs. Anyone actually see one of these first hand, or have an opinion about it?

If not, thanks to all for your ideas and knowledge sharing.

"I have a pair that would handle your pump load. together they are about 80 lbs. maybe 90 lbs.. "

This sounds about right.

Heavy duty transformers weigh a LOT.

Between the copper wire and the steel core they are NOT small light things.