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3 Phase to Single Phase

Triple Jim

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In a house with the normal third leg being grounded to the buss bar, if the two phases are not separated they can cause problems.
I'm not sure I'm reading this as you meant it, but if you connect a 3-phase supply to a house with two phases on the two hot sides of the panel and connect the third phase to the neutral bus, you will have big problems. Normal household power has only two "legs" of 120v each, 240 between the two. The third is not a leg, but neutral, and is tied to earth ground at the entrance panel. When supplying with 3 phase power to a single phase panel, you connect two phases (legs) to the two sides of the single phase panel, and the neutral of the 3-phase to the neutral bus of the panel. The third phase of the 3-phase power does not connect to anything.

Like I said, I'm probably reading your post wrong.
 
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Guyfang

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Or you can spread the 208 loads around if they're not all wired together. There are three combinations that give you 208: 1-2, 2-3, and 1-3.
Too many people forget about the third leg here. I would personally spread the load over all three phases. That way no unbalanced load, not that I would worry too much about that. Maybe a distribution box, that is between the gen set and the house mains. Two legs to the house mains and the third to some plugs, for other things. As long as you wire it up right, there should be no safety problems.
 

jaws4518

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I have a MEP 003A. I believe most of the military generators have three power configurations.

120V single phase
120/240 single phase
120/208 three phase

You "must" select 120/240 single phase if you are hooking it up to a house or panel box "bus" wired for single phase. There are only three setting for the power selector switch which is located close to the main lugs on the front side (or control panel side) of the generator. You can run in 3 phase mode and supply 240 volts to some motor or device if you put a step-up voltage transformer of the right size for the application device you are needing 240 volts to run. (e.g., standard AC unit, or 220V stove or water heater etc...) However, this can be expensive unless you find a used transformer that came out of some piece of control panel equipment. A new transformer that will handle 50 AMPs is expensive to buy. A Lincoln AC stick welder might work for this, but I haven't tried it yet. I'm not sure it is the right type of transformer on the output side. It's secondary winding is probably not big enough for a load of more than 10-15 amps. Honestly, I don't know... Anyway, if you're really in to it, you can google search delta - y conversions, experiments etc... It will take some electrician skill level to accomplish this modification properly. It's not hard, but you need the correct size of transformer to cover the load requirements, and... you cannot exceed 80% of the output current between the two 208 phases that supply the 208/240v single phase transformer which is going to be around 30+ amps on a MEP-003a generator.

I hope my information is clear and correct on this. If not, someone else can correct my "bads". I'm listening.
 
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155mm

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I use a pair of halogen lamps, 1500watts on the loose leg, makes for a good yardlight until you shut down for the night, then it gets really dark. You only forget your flashlight once.
 

Triple Jim

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Buck/Boost transformers are much cheaper than standard stepup transformers because they don't have to supply all the power, just the difference. For example, if you need 5 kW at 240 (21 amps), and you have 208, the boost transformer supplies 32v at 21 amps = 572 watts. So you could be conservative and get a 3/4 or 1 kW boost transformer instead of having to buy a 5 kW transformer.

But as said earlier, most 240v devices will run fine on 208v. Obviously not all, but most, so check the labels.
 

doghead

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fwiw, the OP hasn't been back since he posted this thread.
 

DieselAddict

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I have a MEP 003A. I believe most of the military generators have three power configurations.

120V single phase
120/240 single phase
120/208 three phase

You "must" select 120/240 single phase if you are hooking it up to a house or panel box "bus" wired for single phase. There are only three setting for the power selector switch which is located close to the main lugs on the front side (or control panel side) of the generator. You can run in 3 phase mode and supply 240 volts to some motor or device if you put a step-up voltage transformer of the right size for the application device you are needing 240 volts to run. (e.g., standard AC unit, or 220V stove or water heater etc...) However, this can be expensive unless you find a used transformer that came out of some piece of control panel equipment. A new transformer that will handle 50 AMPs is expensive to buy. A Lincoln AC stick welder might work for this, but I haven't tried it yet. I'm not sure it is the right type of transformer on the output side. It's secondary winding is probably not big enough for a load of more than 10-15 amps. Honestly, I don't know... Anyway, if you're really in to it, you can google search delta - y conversions, experiments etc... It will take some electrician skill level to accomplish this modification properly. It's not hard, but you need the correct size of transformer to cover the load requirements, and... you cannot exceed 80% of the output current between the two 208 phases that supply the 208/240v single phase transformer which is going to be around 30+ amps on a MEP-003a generator.

I hope my information is clear and correct on this. If not, someone else can correct my "bads". I'm listening.
If you have a 803 I totally agree with your comment about how to set the phase switch. If you have anything larger than a 803 then there is no single phase option. Its 3 way (phase) or the highway.

Where I differ in your assessment is with the need for a transformer. If you look at the data plate on your AC it very likely says 208/230. Your dryer or any resistance based machine won't care in the least about 208v. The motors will care a little bit but they will simply pull more current to make up for it. That is unless the appliance motors are running off a hot-neutral split of the power supply and in that case they are happily getting the full voltage they are expecting.

Lets look at the AC unit. If you have a unit that pulls about 20a at 230v then it will pull say 22-25 amps at 208. That unit will be on a 30a circuit minimum so you are still under the 80% rule for breaker loading. You can't apply straight ohms law here because motors are inductors. The math gets a little more complicated but it can be calculated. There is a good chance the manufacturer has already done that and its on the data plate.

If in doubt look at the data plates for the appliances in question. In most cases there will be absolutely zero problem running single phase 240v equipment on 2 phases of 208.

Now back to unbalanced loading.. Why is it a problem to run unbalanced loading on a generator or 3 phase transformer? Coil heating? Nope. The electron gods will be unhappy?? nope. Its neutral current. You can burn the neutral in two if you have a high phase imbalance in a system (split phase or 3 phase). Specifically with these generators you can damage the Y bonding point with excess current. You're allowed by code to undersize the neutral. Something that in practice I never do.

How much of a threat is this in reality with these machines? In my opinion it is quite low but certainly not zero. From the practical side if you have a 804 for an ordinary house its much less if a problem than you think because how often is the energy demand in the house 10kw? Or even 8kw? I have to TRY to get the load that high at my house. Even if you do hit that level I doubt you'll sustain it for any length of time.

To add another layer here.. When you wire a 3 phase generator in a zig-zag pattern like what happens the the 802 and 803 machines its not a free lunch. You aren't making a 3 phase generator single phase. You are increasing circulating currents in the coils but the net sum of those currents equal split phase 120/240. You are creating a permanent hard wired imbalance between the coils that will cause more heat rise versus when you run it in 3 phase mode at the same power levels. BUT.. How many of us set our 803s to single phase then load them to the hilt? Anyone burn up an alternator? I've not heard of any. Anyway, its not the magnitude of problem that is commonly made out to be.

In a install where these generators were the prime source of power forever I'd put a lot more effort into balancing loads. Since they are used in standby service I would worry less about load balancing and more about keeping the mice out of them and the fuel clean. That stuff is much more likely to bite your rear end.
 

boatman69

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Couldn't agree more myself. The dryer is not going to get as hot and the hot water tank/oven isn't going to heat as quickly. When properly sized and wired, pumps and motors should not care.
 

Triple Jim

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I line dry almost everything, clothes last twice as long and I like my jeans nice and crispy.
I'm with you there! My wife knows to dry my towels on the line because I like them nice and scratchy. She prefers hers done in the dryer so they're soft. The worst is the dryer with dryer sheets. My eyes burn if I'm anywhere near something dried with that perfume on them.
 

jaws4518

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I like the idea of running in the native three phase mode for maximum generator efficiency, but is that really a factor here, or now days? So, I know I'm good running at 120/240 single phase 99% of the time, but lets say I have have some motor loads in the shop requiring 208v or 220v exclusively? I have a split-pack HVAC unit which can be configured for 220 or 208, then I have my welders which are 220v.

I guess my question is... being safe too... Is it more of a hassle to run the MEP 003a in three phase mode and transform the voltage up (if this was absolutely necessary for some application), or just run it on 120/240 single phase? Like I mentioned, I do have some 3 phase motors and equipment, but they can all be configured for 240 single phase. I'll post that article about transforming 208v to 220 if anyone is interested. Actually, there is a bunch of how-to methods floating around. I guess I would conclude that the method all depends on the application and the need. I'll check out the Buck/Boost. Thanks!
 

Triple Jim

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Like I mentioned, I do have some 3 phase motors and equipment, but they can all be configured for 240 single phase.
How is that possible? Are you referring to using a 3 phase to single phase converter? Or maybe they're run from a VFD that can be configured for either(?) I'm not aware of any 3-phase motors that can be configured for single phase, but I realize I'm not aware of everything. :)
 

DieselAddict

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How is that possible? Are you referring to using a 3 phase to single phase converter? Or maybe they're run from a VFD that can be configured for either(?) I'm not aware of any 3-phase motors that can be configured for single phase, but I realize I'm not aware of everything. :)
Thats a trick I want to see too.
 

tobyS

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When you run most transformer based welders on 208 you will stick the rods a lot from low voltage. Some motors do not have the 208 volt rating as well. Refrigerator compressors don't like 208 if they are not rated for it.

I do work in jails that are all 208/120Y. I use a three phase transformer 240-> 208 and just use one leg for the welder as a boost up. Works great. I'd consider doing that in your case but to do it right, you might want your transfer switch to feed a 3 phase panel and then run 3 single phase 240v as needed, keeping the load as balanced as you can.

There are some 3 phase welders which I think are using diodes to power a DC buss...mine are all single phase. Even my big Hobart running on 480v is single phase.

I worked in engineering at Square D 74-77 and have had my electricians license 43 years.
 

tobyS

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Do you have commercial refrigeration? Most home refrig's run on 120v. If your running a 120v machine on 208 (coming from 2 legs and not one leg to neutral), then it's getting too much voltage.
 
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