We have a customer that has 120240 3ph4w service and they just got a new generator. The customer apparently told sales all the Hvac and shop equipment operate in 3phase. The generator was installed but During start up it's determine the Hvac isn't 3phase but single phase 240 and the shop doesn't necessarily operate their equipment full time so.the high leg at generator doesn't work unless some is working the equipment. Apparently the generator does not operate well if all 3phases are not used efficiently and will damage generator. I suggested putting the hvac 240v 1ph equipment and spread all 4 Hvac condenser and air handlers on 1phase 240 but using the high leg. Anyone see a problem with this approach? Or add some type of 208v resistive load using on highleg to keep generator running efficiently during outage.
3 Phase Generator
- Thread starter mannyb
- Start date
LarryFine
Master Electrician Electric Contractor Richmond VA
- Location
- Henrico County, VA
- Occupation
- Electrical Contractor
It sounds like the 3ph equipment is behaving like an RPC.
The 1ph equipment should be on the two non-high-leg phases,
The 1ph equipment should be on the two non-high-leg phases,
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
This is most likely nonsense being told you by somebody.
#1, the unbalance is only a problem if the generator is operating near its rated maximum. If it is lightly loaded, do whatever you want. This sounds like the situation you have.
#2, the out of phase windings still carry 1/3 of the 240 volt load. Many generators are connected this way from the factory in order to provide single phase and the third leg is ignored and not brought out to the panel.
The bottom line is, as long as your load does not exceed the current rating of any individual leg, you are good to go.
One possible problem I have observed is, if you load the main winding near full capacity, and the genset voltage regulator is sensing all 3 phases, main winding will go down as it averages all 3. I saw something like 220. If it is not sensing all 3 phases, then the stinger leg will rise quite a bit above spec.
I generally agree that spreading straight 240 loads around the delta is preferable. But if your poco has an open delta, it might be problematic to them. They want all 240 loads on the main winding.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You only have 1/3 the nameplate rating if only utilizing one side of the delta source.
So if for instance you had a 30kVA generator but only put loads on the 120/240 side you can only run 10 kVA of load before you begin to overload that section of the supply. Placing straight 240 volt loads on the other phases will utilize more of the generators capacity without overloading anything. If utility supply is from open delta this can create loading issues to the utility transformer though if it was only designed to handle a limited amount of load on the high leg.
The misinforation about all the HVAC running on three phase made whoever sized the generator to size it too small. They probably had correct kVA but was assuming more balance of loads. This one will need to be oversized to handle things as they were originally connected.
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
Read the link in my previous post. The out of phase windings will carry some of the current and support the in phase winding. It is never derated to 33%.
Most often it derates to something like 83% because the current available is 2 out of 3 legs, but a 3 phase machine has an 80% power factor, therefore 66% x 1.25x = 83%. This is basic generator 101.
Sometimes the single phase rating is even higher because the actual copper generator head may be a little oversized and have more current possible than the nameplate may show.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
If you only connect loads between A-N-C side of the delta what current of any significance is going to pass through the other two windings? You only have about one third the total capacity of the generator if you only connect to that side.
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
Yes, 1/3 of the total load will pass through the other two windings.
You have seen a double delta connection no doubt. The current flow is the same here. So long as no majorly unbalancing 120V loads are present.
The derating is generally the same as if you had connected it as dogleg. 67% of gen head rating which usually equates to 83% of total genset rating after making the 80% power factor 100% for single phase.
I actually commissioned one exactly in this situation last year after the buyer ordered 3 phase delta for a single phase job. It was determined that it was not worth restrapping it, and physical testing with a load bank bore it out.
I have load banked other units this way also, and amp clamped the individual windings, it performs exactly as described.
Basically you are saying if the 120240 3p4w didn't use the high leg running under generator everything would be fine. In your experience.
Customer mentioned generator assessment that they had 3phase hvac and has 3phase equipment that will utilize generator 3phase power during outage. The issue after installation it was determined the hvac doesnt have 3phase hvac and the equipment has limited usage especially during outage and the concern is that the generator alternative will malfunction during operation because its no being utilized is the issue/ comcern
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
Yes, I had a similar situation last year. Only mine was all single phase, not a 3 phase facility. And they had bought a delta generator for it.
Read the paper from Kohler and it basically states you can run up to the nameplate amps on single phase, which because of the power factor means 83% of total generator KW, USUALLY! Don't assume.
In my case the I knew the facility would not draw near that much power at any time so I really didn't worry about it then.
I did load bank it both ways, on 3 phase it performed as expected, on single phase the voltages were quite imbalanced on a heavy load. So if you had 3 phase motors you would not want to run it real far out of balance.
In your case it sounds like you have other 240 loads you can spread around if necessary. But if fed off open delta you might not want to.
How much load is on this system and how many kw is the generator?
Read the paper from Kohler and it basically states you can run up to the nameplate amps on single phase, which because of the power factor means 83% of total generator KW, USUALLY! Don't assume.
In my case the I knew the facility would not draw near that much power at any time so I really didn't worry about it then.
I did load bank it both ways, on 3 phase it performed as expected, on single phase the voltages were quite imbalanced on a heavy load. So if you had 3 phase motors you would not want to run it real far out of balance.
In your case it sounds like you have other 240 loads you can spread around if necessary. But if fed off open delta you might not want to.
How much load is on this system and how many kw is the generator?
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
You do not have to load the high leg in order for the generator to function. You simply have less than total capacity if you don't balance the load on it. Whether that be 1/3 or 86% seems to be somewhat of a debate. Might be possible the specs could vary depending on actual design of the generator.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
You can think of a closed delta high leg as a superposition of a single phase A-N-C and an open delta A-B-C with AC open. Either of them can power 240V loads A-C. So clearly the combined A-C capacity is higher than 1/3 of the 3 phase capacity.
Cheers, Wayne
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
That is a good way to describe it.
In both theory and in my actual measurements, the out-of-phase windings carry 1/3 of the total current and the winding directly across the load carries 2/3 of the current. Probably because 2x the amount of winding in series is 2x the impedance.
If you somehow lost the main winding without fireworks, the generator would continue to operate as an open delta like usual, excluding 120V loads.
All manufacturers who specify delta configuration for single phase (Cummins prominently) allow the same 2/3 current rating as usual for the more common dogleg configuration.
2/3 the current usually equates to 83% the total generator capacity on single phase, since 3 phase generators are rated at 80% PF or 125% amp capacity as the rating, while single phase sets kW = kVA, not sure why they rate them that way.
wwhitney
Senior Member
- Location
- Berkeley, CA
- Occupation
- Retired
OK, so then if the generator has a rating of say 10 kW / 12.5 kVA (to pick an arbitrary number), the allowable winding current would be 12500/240V/sqrt(3) = 30A. And with 30A in one winding and 15A in the other two, that would provide single phase apparent power of 45A * 240V = 10.8 kVA.
So the single phase kVA would indeed be maximum 86% of the generator's kVA rating. [Note this result does not rely on the 1.25 ratio of kVA to kW.] But it seems to me that if the 240V loads are power factor 1, you could get the full rated real power out of the generator, as 10.8 kVA > 10 kW? Or is there some other factor I've overlooked?
Cheers, Wayne
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
Neglect any 120 loads for this example.
You have a 240 delta machine that is 10 kW / 12.5 kVA.
That is 30 amps per corner on 3 phase loads. Because each corner is being supported by both sets of windings. The individual windings carry less than the full 30 amps.
If you take single phase load from 2 corners, the max is still 30 amps due to total heat dissipation in the windings.
The in-phase winding will be carrying 20 amps, and the out-of-phase windings will be carrying 10 amps.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
So you still can only put one third of total generator rating across one side of the delta. Keep in mind you will also have overcurrent protection involved and it won't know the difference between 30 amps used for single phase vs 30 amps used for three phase loads, it just sees what passes through it.
120 volt loads do matter as well. You certainly can not put all 12.5 kVA across 1/6 of total windings of the generator, You have to keep load on that winding section to no more than 1/6 of total generator capacity even though there may be some current flowing all the way around the delta .
Birken Vogt
Senior Member
- Location
- Grass Valley, Ca
Where are you getting the claim that you can only draw 1/3 of the total power from one side of the delta?
Did you read the paper from Kohler above? It spells it all out directly.
https://resources.kohler.com/power/kohler/industrial/pdf/Unbalanced_3PhaseDeltaSystems_WP.pdf
Three phase generators are connected FROM THE FACTORY as delta with one side being loaded for single phase, with factory ratings exactly as I have described above, by manufacturers, every day.
It can save them from having to stock a dedicated 3 phase alternator end.
kwired
Electron manager
- Location
- NE Nebraska
- Occupation
- EC
Never mind, brain cramp. Is still early in the day
