Generator Conversion 1

[macat]

I brought a 3 phase generator cheaply thinking it would suit me better than a single. However I have found that three phase equipment for domestic use is prohibitive and I would like to convert the machine to single phase:
a] Is this Possible?
b] If possible how would I proceed?

 

[ScottyUK]

1) If you have a 12-lead set you can usually get a single phase output from the machine by suitable interconnection of the windings. You won't get full rated output, but it is the best solution if you have the option.

2) Look for a Scott connected transformer.

3) Split your single phase loads into three reasonably balanced groups and run them that way. Might need your entire house rewiring though.

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Sometimes I only open my mouth to swap feet...

 

[waross]

In North america, the easiest way to go is to just use two phases. You should have 120 volts, but in place of 240 volts you will have 208 volts. This is a common voltage in residential apartment buildings in Canada. Hundreds of thousands of people are blissfully unaware that they are using 120/208 instead of 120/240 volts. The apartment that I am living in now is supplied with nominal 120/208 volts (Actually closer to 124/215 volts.
I have seen a couple of standby generator installations where the utility voltage was 120/240 and the generator voltage was 120/208. The owners were unaware of the difference.
If you want to change the generator to single phase, it is a simple conversion with a twelve lead generator. If it is a ten lead machine, changing to true single phase is problematic.
You will lose 1/3 of your KVA rating, but the power factor will rise from 80% to 100%.
For example, a machine rated at 50 KVA/40 KW, PF .8 on three phase, will rerate to 33 KVA/33 KW, PF 1 on single phase.
If you have twelve leads and wish to convert to true single phase, let us know and we will give you the connections.
There are two connections for single phase. Double delta and zig-zag. I recommend zig-zag for older machines. It is a simpler change with less chance of mistake. It may be converted safely with a continuity tester even if some or all of the lead identification numbers are missing. Not so with the double delta connection which requires all leads to be properly identified.
respectfully

 

[itsmoked]

Hi waross. I have seen a lot of 3ph gens that have no neutral and must be loaded with a balanced 3ph load. Can these generators be modified to single phase but with large output concessions or only 4 wire 3ph units?

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[waross]

Hi Keith;
It depends on how the leads are brought out, and what we mean by "modified" If you are willing to open the end of the machine and break open the internal connections, any reconnection is possible, usually without rewinding.
If both ends of each winding are brought out, they may be reconnected in star. (6 leads, 4 Wire three phase)
A star connection may be easily changed to a zig-zag connection.
All of the conversions from three phase to single phase impose a 1/3 reduction on the KVA rating.
The minimum KW capacity depends on the power of the prime mover, and the maximum KW will equal the maximum KVA.
When you connect a single phase load to a delta connected generator, two windings act similarly to an open delta transformer connection. You may think of it as an open delta winding pair in parallel with a single winding. If you do a little vector sketching, you will see that just as the open circuit voltages of the windings in the open delta pair equal the open circuit voltage of the single winding because of phase differences, the voltage drops of the open delta pair equal the voltage drop of the single winding.
This gives pretty good, equal sharing of the current between the open delta pair and the single winding.
A single phase load on a delta transformer should divide equally between the in phase winding and the open delta pair.

To convert from a wye (star) to single phase:
Step one.
If the line leads are not labled, label them L1-("A"), L2-("B"),and L3-("C")
Step two.
Open the neutral connections. Find the lead or leads that have continuity to L1. This will be L1-N. Remove these leads from the neutral terminal and re-install the other neutral leads on the neutral terminal.
Step three.
Connect L1-N to L2 and insulate the connection.
You will now have straight line single phase between L2 and L3. The neutral terminal will be the center tap.
The reason that I like the zig-zag connection for these conversions on older machines is that the leads may be arbitrarily L1, L2, and L3 without regard to phase rotation and the conversion will work.
The zig-zag will work on a six lead machine.
The other option is a double delta connection, but all leads must be accurately labeled and the double delta connection is only applicable to 12 lead machines. (this is also an instance of a single phase load applied to a delta winding without issues.)
In the old power plant, we had some 350 KW, delta generators which we paralleled with one or two 600 KW wye generators.
The load was often unbalanced three phase delta.
Other than that, I haven't seen many delta generators. (I assume we are both mainly concerned with below 500 KVA sets in this discussion.)
The odd sets that I have seen a lot of are the old telephone backup sets with a 10 lead alternator. They are wound as a 12 lead machine, but the 10,11,12 connection is made internally and brought out as #10. This makes them dual voltage machines, which may only be used as wye, three phase. If you need single phase, use 120/208 three phase. Consider increasing the voltage to 124/215 and 999 out of 1000 end users will never know, or notice.
Respectfully

 

[itsmoked]

Thank you sir!

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[ccjersey]

Waross,
I have always wondered why the delta 3 phase 4 wire generator connection is not recommended by manufacturers for single phase 120/240 using A and C phases as L1 and L2 instead of the zig-zag or double delta. It seems to be equivalent electrically and the only caveat would be "stay off the "B" phase". In fact B phase wouldn't be brought into a single phase transfer switch at all. I believe it would be a much better situation than using a 3 phase wye connected set for single phase as regards voltage (240 instead of 208)and kVA capacity (all 3 phases are in use instead of only 2 in the wye set). Am I missing something ?

The trouble with reconnecting some generators is getting the regulator and exciter to function when reconnected for voltages not on the nameplate. This can be impossible on some sets, and not a problem at all on others. With modern packaged regulators and brushless exciters, unless you change voltages so the correct regulator supply is no longer available, it would be ok.

The old brush type sets with high amperage exciters aren't always so easy to work with. Power-tronics seems to be the only manufacturer that makes a fairly small packaged regulator and compatable static exciter module that can handle the amperage. I would be very interested in knowing of other manufacturers that make this type of equipment for small generators.

In my limited experience with used generators, 10 lead dual voltage wye sets are very common on the used equipment market. I believe Caterpillar didn't make anything else for many years and still may not make many Caterpillar brand sets that aren't 10 lead. Onans seem to be capable of ONLY what is on the tag, while larger, old Kohlers are frequently 12 lead no matter what the spec tag says. Regulator/exciter are a problem with the Kohlers though.

 

[waross]

A couple of comments;
I grew up and spent many years working in an area where the standard connection for three phase was wye.
After years of exposure to 120/208 volts, I tend to favor wye connections.
I believe that the double delta and zig-zag connections give better voltage balance with heavily unbalanced 120 volt loading than the 4 wire delta connection. I may be corrrected on this.
Another possible issue with a larger sets using a 4 wire delta connection to supply single phase is the issue of the limited available fault current of a small generator compared to a similar size transformer. The 4 wire delta connection may further limit the available fault current so that the breaker trips rather slowly on the thermal trips rather than instantly on the magnetic trips. This is probably not an issue with smaller sets. (less than several hundred KVA)
My personal rule of thumb for transformers is:
Large single phase loads combined with small three phase loads, use 4 wire open delta.
Large three phase loads combined with small single phase loads, use wye connected transformers.
Special case and constrictions, consider 4 wire delta, open or closed.
On generators, we don't have the luxury of being able to specify the KVA of each phase as we do with transformer banks.
The 4 wire delta connection is only possible with a twelve lead set. I wouldn't use this connection myself, but if I encountered it in the field, I would not waste time changing it.
I was once able to solve a voltage regulator supply voltage problem with a small (100 VA) dry type transformer. The generator was 480 volts and the only available regulator was 240 volts. There was an issue with using the neutral and 277 volts. A small community of about 10,000 people had been waiting for a day and a night for me to fly in and get the power back on. Just a little pressure and no time to have the proper regulator flown in.
I don't know of a connection from three phase to single phase that doesn't lose 1/3 of the KVA capacity.
Respectfully

 

[ccjersey]

Waross (and others),

I have been thinking about the 3 phase 4 wire delta supplying single phase vs the double delta and zig-zag single phase connections.

If you look at the diagram of the generator coils and connections in each case, the "open" delta of the 3 phase connection is 240V across the whole center-tapped phase where the double delta has an "open" delta with 120V across each half of the single phase winding. The zig-zag has the 120 V open delta making one half of the split phase supply

Would this make the L-N voltages more stable if one 120V side was heavily loaded? Make any difference?

 

[waross]

Hi ccjersey;
I have wondered that myself. For many years, I was used to seeing the zig-zag connection to extract single phase from a three phase generator. About 15 years ago, I started seeing a lot more generators, and all the new ones I saw used the double delta connection.
As far as voltage stability (not the best word in this context, regulation may be a better word.) with heavy single phase loading, this will be an issue no matter what connection is used.
Where a transformer may have a regulation of a few percent, the voltage regulation of a generator with fixed field excitation may be as poor as 40% or 50%. When the loading is on the same connection as the Automatic Voltage Regulator, the AVR will compensate and the voltage regulation will be quite good. With the AVR sensing connected across 240 Volts and a heavy 120 volt load, the AVR will maintain 240 volts, but the voltage division between the two 120 volt winding groups will be poor. The loaded voltage will drop and the unloaded voltage will rise. The sum will be 240 volts (within the limits of the AVR capability).
AVR sensing of a 120 volt winding group will be even worse.
With the AVR on the unloaded group at 120 volts, the voltage may drop to 60% on the loaded group.
With the AVR on the loaded group at 120 volts, the voltage may rise to over 160%% on the unloaded group.
All you can do is try to keep your loading balanced and have the AVR connected across full voltage.
If you are having serious 120 volt loading issues you may feed problematic 120 volt loads from 240 volts with a 240:120 volt transformer.
respectfully
With