Using 120V generator as temporary 120/240 system backup?

[Al and Missy]

Originally Posted by mikeoxlong

Okay. I’m watching football as I try to save someone some aggravation.
In order for a 240v motor to work properly it requires the full sine wave. If you make both motor leads to the same branch circuit, you WILL have voltage present. But, no potential difference or current. So if you are saying that the motor won’t be damaged, you are correct. If you are implying that somehow the motor will work...you are wrong.

Sorry Mike, but you are mistaken or there is a semantics issue. The red wire has 120 volt to neutral sine wave at 0 degrees phase. The black wire has 120V to neutral sine wave at 180 degrees phase. So when the phase angle is 90 degrees the red wire has plus 120 and the black wife has minus 120, thus 240 line to line. At 0 degrees phase the red wire and black wire have 0 volts to neutral and 0 volts to each other.

In the OP's case, if the 240 volt loads have no reference to neutral or ground, then there will be no potential between the red and black and no harm, no foul. But the safest thing is to ensure they are all disconnected.

Al

[mikeoxlong]

I don’t think you read the post correctly. If you take the two hot leads for a 240v single phase motor (a rare thing these days) and connect both leads to 120v you will have 120v present. But it will in effect be an open circuit. No current flow. No motor movement. If you use the neutral, it comes from the center point (neutral) of the sine wave. If you try this you would be trying to use the motor at half of the required voltage. It wouldn’t have the torque needed to carry the load and would most likely not move at all. It would then become the equivalent of a dead short through the windings. Resulting in burned out motor windings or tripped breaker, whichever happens first.

[TG Twinkie]

240 volt single phase motors are NOT RARE.
My 80 gallon air compressor has a 5hp 240 volt single phase motor.
Most if not all residential irrigation pumps are 240 volt single phase motor powered.
My neighbor has a brand new 7.5 hp 240 volt single phase motor which powers his 300 gallon per minute irrigation pump to water his 5 acre alfalfa field.
There is no neutral involved in the operation of the aforementioned motors.
They run on the 2 hot legs L1 and L2.
The split air conditioner in my home run on 240 volt single phase power. No neutral involved. There is a 240 volt motor in both the evaporator and condenser units.

[TG Twinkie]

There is no such thing as the "neutral point or center of the sine wave"
You only get the 120 volt power by referencing the hot leg to a center tapped neutral. Either hot leg L1 or L2 will provide 120 volts when referenced to the neutral. Thus the reason for a 4 wire connection. L1, L2, neutral and ground.
Either L1 or L2 from a 240 volt generator or power source can provide the 120 volt power required by most RV's. The RV uses either L1 or L2 as the hot leg along with a neutral and ground.
The typical RV is not wired for 240 volts. Some with 2 or more AIR CONDITIONERS will use both L1 and L2 along with a neutral to provide 120 volts on 2 seperate circuits.

[mikeoxlong]

None of us seem to be on the same page here.
1)the question was about his home, not his RV.
2)other than an air-conditioner compressor, homes RARELY have a 240v pump motor. Yes, in other situations they are common.
3)the center tapped neutral in your home panel IS the center of the 240v sine wave. You achieve 240v by forgoing the neutral and using L1 and L2. If you think there is no sine wave, you are thinking of DC power. If you think there are two separate sine waves...you are incorrect.
4)if you are unwilling or unable to understand what I am saying, I don’t really care. This thread has devolved into a bunch of pointless sniping. The original purpose was to save a fellow ‘airstreamer’ some heart ache. If you guys want to put 120v to your 240v motors, go right ahead, have fun.

[KK4YZ]

Originally Posted by mikeoxlong

...... If you guys want to put 120v to your 240v motors, go right ahead, have fun.

Who here advocated doing that? Do you know what a sine wave looks like? Do you know what it is?

Do your lab homework and report back

[mikeoxlong]

[mikeoxlong]

Please read the info. There is no excuse for ignorance

[mikeoxlong]

One last illustration to help explain.
I suppose it’s possible that you understand all of this, but it seems more like you are being a belligerent troll. In any case, I am now unfollowing this thread.

[TG Twinkie]

Note the sine waves on the 120 volt side of L1 and L2. They are 180 degrees in the opposite direction. This is what I pointed out earlier.
This is accomplished by the way the secondary of the transformer is wound.
This is how the 240 volts is developed. It is still single phase as you can see by the sine wave next to the 240 volt.
I think we have beat this subject to death.

[KK4YZ]

Originally Posted by mikeoxlong

Please read the info. There is no excuse for ignorance

Absolutely. So where do you see “half of a sine wave” in either of your illustrations? Those are all full sine waves.

[KK4YZ]

Originally Posted by TG Twinkie

I think we have beat this subject to death.

Man, that’s for sure.

[Mollysdad]

Originally Posted by JRF0st3r

I have a Champion 3800 Watt, 30 amp, portable generator that wants to temporarily back up a 5500 watt, 30 amp, 120/240V system via transfer switch while the 120/240 generator is in the shop. Of course I would have to disconnect or turn off the two 240V pumps so only 120 V loads remain.

Before we veer off into Mr. Wizard world, the original question was about hooking up a 120v. generator TEMPORARILY while the big unit was in the shop. AND the OP was already willing to turn off/disconnect all 240v. appliances.
Put away your slide rules.

[Protagonist]

Originally Posted by Mollysdad

Put away your slide rules.

I still have my slide rule from college. But the last time it saw any use at all was in 1999, when I mounted it in a picture frame at my office with the caption "In case of Y2K, break glass."

[KK4YZ]

Originally Posted by Mollysdad

Before we veer off into Mr. Wizard world, the original question was about hooking up a 120v. generator TEMPORARILY while the big unit was in the shop. AND the OP was already willing to turn off/disconnect all 240v. appliances.
Put away your slide rules.

Too late. We’ve been in orbit in mr wizard land for quite some time.
I, too still have my old college slide rule. That was when the HP35 just came out and almost none of the students could afford them.

Haven’t framed it, though. Still floating around in my desk drawer somewhere...

Anyway, isn’t it a tried and true tradition for these threads to launch into orbit at some point? �� what fun would it be otherwise?

[OTRA15]

For those wanting to go beyond a normal orbit into The Argue Thread's lost in the ozone . . .

Happy New Year!



http://www.airforums.com/forums/f161...ead-60297.html

[KK4YZ]

Originally Posted by OTRA15

For those wanting to go beyond a normal orbit into The Argue Thread's lost in the ozone . . .

Happy New Year!



http://www.airforums.com/forums/f161...ead-60297.html

😮 there IS an Argue Thread. I thought that was just a nickname people put on the “gas vs diesel” or “best hitch” threads....cool
Happy New Year!

[Mollysdad]

Originally Posted by Protagonist

I still have my slide rule from college. But the last time it saw any use at all was in 1999, when I mounted it in a picture frame at my office with the caption "In case of Y2K, break glass."

Ha! I kept mine for a long, long time. It had a leather case with a belt clip for that extra geeky look.
I once took a test where calculators were banned, and your slide rule was checked to make sure you didn't have crib notes in the groove.

Originally Posted by KK4YZ

Anyway, isn’t it a tried and true tradition for these threads to launch into orbit at some point?

Indeed. A person asks if he can power a toaster from a Honda 3000, and before long people are arguing about the Space Station's solar panels!

[rmkrum]

I still have the K&E slide rule I used in college in the 60's, the orange scabbard, plus a copy of the book that came with it...just in case the phone batteries die and I need to calculate something...