Puppeteer

Author Topic: What makes a generator single phase or three phase?  (Read 12116 times)

Eadie McCreadie

  • Full Member
  • ***
  • Posts: 153
    • View Profile
What makes a generator single phase or three phase?
« on: December 05, 2008, 07:52:08 AM »
And specifically, is the ST5 genny single phase?

Or is the phase unrelated to the model number or output, and merely something set by some other component inside the machine?

Thanks
6/1 oid.

rcavictim

  • Certified Generator Head and Grand Master Sparky
  • Hero Member
  • *****
  • Posts: 1827
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #1 on: December 05, 2008, 08:30:03 AM »
This is a bit of a challenge to describe but I shall try.

A single phase and a 3 phase alternator are vastly different in the way the internal poles are arranged and the number of coils and wires leading to the outside world.  Basically a single phase unit will have two output wires and for every revolution of the shaft you will see either a single 360 degree sine wave come out (2 pole 3600 RPM head) or two 360 degree sine waves in the case of the more common 4-pole 1800 RPM heads discussed here.  The single phase output winding is often center tapped to make 120-0-120 volts available from three wires.

The 3-phase head has three output windings instead of just one of the single phase head. Each winding of the 3-phase head will also make either a single 360 sine from each 'pair' of output terminals in a 2 pole 3600 RPM head, or two sines waves per rev in a 4 pole, 1800 RPM head, BUT the sine wave will be seen to begin on phase one winding, then 120 degrees of rotation later another sine wave will begin to be generated on the second set of wires and 120 degrees later yet a 3rd sine wave starts on the third winding.  If you look at only one winding output it will resemble that of the single phase machine output exactly.  In the three phase you have 3 simultaneously generated sets of sine waves that are each delayed or time shifted 120 degrees to the output seen on the other windings.  The 3 separate coils create sinewave peaks can be traced back to coincide with 3 separate points, 120 degrees apart around the revolution of the armature in a 2 pole machine.  This is repeated twice in a single revolution for a 4 pole machine where one 360 degree physical shaft rotation actually represents 720 electrical degrees.

Confused yet?   ;D

Maybe this will help to form the image.  You can generate 3-phase power IF you have three identical single phase heads and arrange their shafts to be phase locked to each other through a gear system or perhaps toothed timing belt.  The keyway on each shaft can be used as a physical phase reference.  The keyway on the first would be set at 12:00 o'clock.  The key of the second would be set at 120 degrees later or say 4:00 o'clock.  The key of the third shaft would be locked 240 degrees later or at 8:00 o'clock.  As the prime mover rotated these three heads they would always have the same 120 degree phase or timing separation between their armature shafts.  The six total single phase output wires from all three heads would give you true 3-phase power.  Wire them all in series would give you 3-phase Delta.  Wiring them all with one lead common makes a neutral and three live wires.  This is three phase Y or Wye.  In 3-phase Wye, the common 4th wire is used or not depending on the application.
« Last Edit: December 05, 2008, 08:42:26 AM by rcavictim »
-DIY 1.5L NA VW diesel genset - 9 kW 3-phase. Co-gen, dual  fuel
- 1966, Petter PJ-1, 5 kW air cooled diesel standby lighting plant
-DIY JD175A, minimum fuel research genset.
-Changfa 1115
-6 HP Launtop air cooled diesel
-Want Lister 6/1
-Large DIY VAWT nearing completion

Stan

  • Guest
Re: What makes a generator single phase or three phase?
« Reply #2 on: December 05, 2008, 09:37:24 AM »
kind of like trying to write instructions on how to tie your shoe laces, isn't it?
Stan

Eadie McCreadie

  • Full Member
  • ***
  • Posts: 153
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #3 on: December 05, 2008, 10:25:41 AM »
That's why I wear slip-ons.
6/1 oid.

rcavictim

  • Certified Generator Head and Grand Master Sparky
  • Hero Member
  • *****
  • Posts: 1827
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #4 on: December 05, 2008, 11:18:50 AM »
Well if someone wants to learn how to tie their own shoe laces by written instructions I'll let someone else tackle that challenge.  I hope I didn't just go to all this trouble for Eadie just to keep my two typing fingers in shape 'cause I gotta tell ya', I had to clear out some brain cobwebs that maintenance hadn't dealt with properly composing that and in the process I had to fire some neurons.  :D
-DIY 1.5L NA VW diesel genset - 9 kW 3-phase. Co-gen, dual  fuel
- 1966, Petter PJ-1, 5 kW air cooled diesel standby lighting plant
-DIY JD175A, minimum fuel research genset.
-Changfa 1115
-6 HP Launtop air cooled diesel
-Want Lister 6/1
-Large DIY VAWT nearing completion

billswan

  • Sr. Member
  • ****
  • Posts: 439
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #5 on: December 05, 2008, 01:05:56 PM »
Great job rcavictim I solute you sir!!!!!
16/1 Metro  in the harness choking on WMO ash!!

10/1 OMEGA failed that nasty WMO ash ate it

By the way what is your cylinder index?

xyzer

  • Hero Member
  • *****
  • Posts: 1058
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #6 on: December 05, 2008, 03:48:09 PM »
rca,
I learned somthing today for a change!
Dave
Vidhata 6/1 portable
Power Solutions portable 6/1
Z482 KUBOTA

SteveU.

  • Sr. Member
  • ****
  • Posts: 384
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #7 on: December 05, 2008, 04:10:17 PM »
Rcavictim, my hats off to you sir.

So once the internal windings are wound as single or three phase they'ed have to be stripped out and re-engineered to convert the gen head.  Right?

SteveU.
Use it up. Wear it out. Make Do, or Do Without.
 Electrodyne 12vdc. AC MeccAlte 8.5kw
John Deere 950 w/Yammar 3cyl IDI; Peterson 21" sawmill w/20hp Kohler v-twin; four Stilh chainsaws,  Stilh weedeaters; various Kohler, Onan, Honda, Briggs, Tecumseh singles.

BruceM

  • Hero Member
  • *****
  • Posts: 3054
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #8 on: December 05, 2008, 04:21:51 PM »
Very nice primer on 3 phase, RCAVictim!

Yes, SteveU.

 I'll add this:

The original purpose of three phase power was solely to drive three phase induction motors;  the motors can be either more compact and/or have much larger HP ratings than single phase motors since those power pulses are now much closer together in time.  The rotor can be smaller and lighter for the same HP.  Single phase motors are limited to about 7.5 HP, max.  No limits on 3 phase.

Three phase also has extra value when generating DC, as the rectified output doesn't swing all the way to zero since a new "hump" is starting just when the last was starting to fade. So there is much less ripple voltage on the DC.   Auto alternators are set up this way for just this reason.

I hope this is somewhat helpful.  

listeroidsusa1

  • Sr. Member
  • ****
  • Posts: 285
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #9 on: December 06, 2008, 05:05:41 AM »
Also, all three phase generators can be used as a single phase generator but a single phase generator can never be used by itself to generate 3 phase power. It could theoretically be done with a capacitor bank, but then it would be used like one of the homemade phase converters.

Another thing, if you look inside the single phase generator you'll see that all of the slots in the stator are not used. Due to the decreasing effectiveness of the chord spacing and span of the single phase skein winding it will on average only be 69% as efficient as a dedicated 3 phase winding, which will use all of the slots ensuring efficient use of the iron.

Just a note from an old motor winder....

Mike

Eadie McCreadie

  • Full Member
  • ***
  • Posts: 153
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #10 on: December 06, 2008, 07:27:57 AM »
Thanks RCA - I've now established what I needed to know in order to obtaint the correct electricity meter - there are at least two types, single phase and three phase.

Thanks again for detailed post.
6/1 oid.

oliver90owner

  • Hero Member
  • *****
  • Posts: 861
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #11 on: December 06, 2008, 08:48:52 AM »
all three phase generators can be used as a single phase generator

Not so.

To separate the phases it will need to be a 4 wire generator.  Three wire generators are 3 phase only - unless a deal of rewinding and modification is carried out......

The answer to the question is: The manufacturer.

Regards, RAB

rcavictim

  • Certified Generator Head and Grand Master Sparky
  • Hero Member
  • *****
  • Posts: 1827
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #12 on: December 06, 2008, 12:55:34 PM »
Very nice primer on 3 phase, RCAVictim!

Yes, SteveU.

 I'll add this:

The original purpose of three phase power was solely to drive three phase induction motors;  the motors can be either more compact and/or have much larger HP ratings than single phase motors since those power pulses are now much closer together in time.  The rotor can be smaller and lighter for the same HP.  Single phase motors are limited to about 7.5 HP, max.  No limits on 3 phase.

Three phase also has extra value when generating DC, as the rectified output doesn't swing all the way to zero since a new "hump" is starting just when the last was starting to fade. So there is much less ripple voltage on the DC.   Auto alternators are set up this way for just this reason.

I hope this is somewhat helpful.  

Bruce,

You have added some useful notes about the advantages of 3-phase over single phase power systems.  The fact that in a rectified 3-phase system the DC level never drops to zero, even when smoothing/storage capacitors are NOT employed is the reason that a rectified 3-phase welder (i.e. not speaking of any MG set welders here) makes such a much nicer DC arc to weld with and potentially less splatter. Some of us here may have noticed this.
-DIY 1.5L NA VW diesel genset - 9 kW 3-phase. Co-gen, dual  fuel
- 1966, Petter PJ-1, 5 kW air cooled diesel standby lighting plant
-DIY JD175A, minimum fuel research genset.
-Changfa 1115
-6 HP Launtop air cooled diesel
-Want Lister 6/1
-Large DIY VAWT nearing completion

oliver90owner

  • Hero Member
  • *****
  • Posts: 861
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #13 on: December 06, 2008, 01:07:58 PM »
Regarding '3 phase welders'.

I think you will find they only use two phases.  'Three' phase ones can drive the meter backwards, I have been told by an electricity board electrician.

Regards, RAB

rcavictim

  • Certified Generator Head and Grand Master Sparky
  • Hero Member
  • *****
  • Posts: 1827
    • View Profile
Re: What makes a generator single phase or three phase?
« Reply #14 on: December 06, 2008, 04:05:22 PM »
Regarding '3 phase welders'.

I think you will find they only use two phases.  'Three' phase ones can drive the meter backwards, I have been told by an electricity board electrician.

Regards, RAB

RAB,

I have a ~350 amp, Miller 3-phase welder here and it is a real 3-phase power supply (transformer followed by three bridges).  I have been into it and have the schematic so there is absolutely no doubt.  The only way I can run it off the mains here however since my mains service is only single phase is to use a homebrew rotary phase converter (7.5 HP induction motor).  I never saw my meter go anywhere except forwards at near light speed when I was running this equipment.  I could only use the Miller on low range. I do not use the rotary converter anymore since I now have real 3-phase from my DIY VW generator plant.  I can still only use low range but what an improvement!

I cannot see how a three phase transformer followed by rectifiers into a series reactor and electric arc (basically a resistor) as a load could set up a condition to run the meter backwards.  If it could there are a lot of industrial and commercial power supplies that would be saving their owners money, including radio station broadcast transmitters!
-DIY 1.5L NA VW diesel genset - 9 kW 3-phase. Co-gen, dual  fuel
- 1966, Petter PJ-1, 5 kW air cooled diesel standby lighting plant
-DIY JD175A, minimum fuel research genset.
-Changfa 1115
-6 HP Launtop air cooled diesel
-Want Lister 6/1
-Large DIY VAWT nearing completion