Genset voltage unbalance is the spread between the three phase voltages on one bus, written as a share of their average. Read how far the phases drift under load rather than at no load, and Tesla Power will return a quotation with the derating written as a line.
- Measure all three voltages at the same moment, because the reactive load on the bus moves them together.
- Divide the largest gap from the average by the average, which gives the figure the standard uses.
- Derate the frame before comparing prices, since a 2% unbalance costs more than the alternator frame it needs.
- Keep the survey sheet, because a warranty question about a burnt rotor starts with these three numbers.
What does genset voltage unbalance measure on a site?
It measures how far the three phase voltages sit from each other, and it is the figure that decides whether a frame carries its rating. A balanced load draws equal current from all three windings. An unbalanced one draws more from one, and the difference circulates in the rotor as heat rather than doing work.
The measurement is taken at the terminals under the site load rather than at no load. A set with nothing connected reads close to balanced whatever its winding, because there is no current to divide. The figure that matters appears when the single phase loads are on, and that is the moment the survey has to catch. Unbalance is a load condition, not a property of the machine alone. The instrument that records all three phases at once is the one that settles an argument about a hot rotor.
How is the figure calculated from three readings?
Genset voltage unbalance for a three phase bus comes from three readings taken at one moment. Add them, divide by three, then take the largest difference from that average and divide again. Three readings of 400 V, 396 V and 392 V give 400 + 396 + 392 = 1,188 V, and 1,188 ÷ 3 = 396 V average. The largest gap is 4 V, so 4 ÷ 396 × 100 = 1.01% of unbalance.
Three readings taken one after another are not the same measurement, because the load moves between them. The arithmetic looks trivial and the consequence is not. The standard the alternator is built to allows 1% before any derating, and the derating beyond that point is steep. A site that reports 2% has a machine running outside its condition even though every individual reading looks acceptable. The standard names the limit rather than the derating curve. The IEC rotating machine standards set the negative sequence limit behind that figure.
| Unbalance | Effect on the frame | What the survey should note |
|---|---|---|
| 0 to 1% | No derating | Normal for a mixed load |
| 1 to 2% | Derate about 4% | Single phase load on one phase |
| 2 to 3% | Derate about 12% | Load moved or split before order |
| above 3% | Frame not suitable | Redistribute the single phase load |
Worked example: 400 V, 396 V and 392 V on one bus
Take a 400kVA frame at 400 V serving a workshop with a large single phase welder. The survey reads 400 V, 396 V and 392 V under load, which is the 1.01% worked out above. The frame then carries about 96% of its plate figure, so 400 × 0.96 = 384 in kVA terms rather than the 400 on the plate.
Move the welder to a different phase pair and the readings become 398 V, 398 V and 396 V. The average is 397 V and the largest gap is 1 V, so the unbalance falls to 0.25% and the derating disappears. That single change recovered about 4% of the plate figure without touching the machine. Tesla Power Equipment asks for the phase of each large single phase load on the survey sheet for that reason.
Why does a small unbalance heat the rotor?
Because the negative sequence component of an unbalanced current turns against the rotor rather than with it. A current that rotates backwards cuts the rotor at twice the synchronous speed, and the losses in the surface of the rotor rise with the square of that current. A 1% voltage unbalance produces a negative sequence current several times larger. The rotor surface is where that loss lands.
The heat is not visible on the stator winding, which is why the failure surprises a site. A frame running at 60% load with 2% unbalance can be hotter in the rotor than the same frame at full load on a balanced bus. Most genset manufacturers state the limit as 1%, and the figure is a rotor limit rather than a winding limit, so a stator temperature reading gives no warning. A balanced bus at the same load runs cooler in the same frame.
- Read the negative sequence current if the analyser provides it.
- Compare the three phase currents as well as the voltages.
- Note which single phase loads were running during the survey.
What does NEMA allow before derating?
A genset in a plant with a large single phase load meets the American standard, which allows a small unbalance before any derating and a steep curve after it. Most export tenders quote the same 1% figure. Above that point the alternator is derated by a share that grows faster than the unbalance does. A frame rated 400kVA at 1% is not a 400kVA frame at 3%. The rating definitions behind both figures come from the ISO 8528-1 rating classes.
The curve matters because a site rarely knows its unbalance before the survey. A buyer who orders on the plate figure and measures 2% on site owns a frame that is 12% smaller than the order, and Tesla Power prices the frame on the measured figure. Write the measured figure in the enquiry, or ask for a survey before the frame is chosen, and the harmonic distortion notes cover the second half of the same problem.
How does single phase load cause the problem?
Because a single phase load draws from one pair of phases and leaves the third carrying less. A 40kW welder on a 400 V bus draws near 100 A from two phases and nothing from the third, and the winding that carries it runs hotter than the other two. The alternator then delivers a voltage that sags on the loaded pair.
The fix is distribution rather than hardware in most cases. Split the single phase loads across the three phase pairs so that the total on each is close, and the unbalance falls without any change to the set. Tesla Power asks for the circuit list with the enquiry for that reason. A genset for a workshop with a dozen single phase circuits is usually balanced by moving circuits rather than by buying a larger frame, and the survey sheet shows which circuits to move.
Which readings belong in the site survey?
Four readings turn a survey into a document a supplier can quote against. The three phase voltages under load, the three phase currents under load, the list of single phase loads and their phases, and the time of day the reading was taken are the four. A reading taken at midnight on a workshop that runs one shift answers a question nobody asked. A single reading describes one operating condition and nothing more.
The survey also belongs beside the load list, because a balanced total can hide an unbalanced distribution. A site with a balanced total of 250kW spread evenly reads differently from one with the same total and half of it on one pair. The two sheets are read together, and the frame is chosen on the phase figure rather than on the total. The two sheets are compared before the frame is fixed.
What goes wrong when the load is not balanced?
The first symptom is a set that runs hot at part load, and the second is a rotor that fails after two or three years. Neither shows on the panel, because the stator temperatures stay inside their band while the rotor carries the negative sequence current. A site that adds a large single phase load to an existing set is the usual case, and Tesla Power asks for that list first.
The other failure is a frame chosen on the total kW with the phase distribution left to the electrician. A 400kVA frame serving 250kW spread evenly carries the load with room to spare, and the same frame with 150kW on one phase pair runs outside its condition. Most genset manufacturers will read a survey sheet on request, and Chinese genset suppliers often quote from the total alone, which is why the sheet is worth sending. The range of frames we build lists the single phase capacity of each size.
How is the frame checked against a survey?
The survey figure is applied to the plate, and the derated figure is what the site may carry. A frame at 400kVA with 2% unbalance carries 400 × 0.88 = 352 in kVA terms, and a site that needs 95% of the plate needs the next frame rather than the same one. The check takes one line of arithmetic and it belongs before the order rather than after commissioning.
A frame that fails the check on site cannot be fixed by adjustment, because the winding is what it is. The options are to redistribute the load or to add a frame, and both are cheaper before the set is shipped. The ambient derating notes show how the two corrections are applied together. Tesla Power applies the survey figure to the quote, so the derated rating and the price are read on the same line.
Frequently asked questions about genset voltage unbalance
No, and the current figure is the larger of the two. A 1% voltage unbalance produces a current unbalance of several percent, because the negative sequence impedance of the machine is low. Read both if the analyser provides them, and treat the voltage figure as the one the standard uses.
It can, up to the single phase capacity the maker states for that frame. A frame built for a mixed load carries a share of its rating on one phase, and a frame built for a balanced load carries less. Tesla Power states that share on the datasheet.
Once before the frame is chosen and once after the site load is complete. A workshop that adds equipment over two years changes its balance without anyone noticing, and the second survey catches the change before it becomes a rotor failure.
Only if the panel displays all three phase voltages and currents. A panel that shows one voltage and one current reads balanced whatever the other two are doing, and that is the panel most sites have. Take the survey with a separate instrument.
It changes the negative sequence capability, which is why two frames of the same rating can carry different unbalance. A frame wound for a high negative sequence capability costs more and carries a single phase load better. Ask for that figure rather than for the plate rating alone.
Then the frame is chosen on the derated figure rather than on the plate. A site with a fixed single phase load and no way to split it needs the next size up, and the derating belongs in the enquiry so the price reflects it.
Write the three voltages, the three currents and the single phase list on one page, and send it to our export desk. Yangzhou Tesla Power returns the derated figure as a line on the quotation. The genset voltage unbalance CE marking requirements sit in the technical file rather than on the plate, and conformity assessment for the assembly follows the same route as for a balanced frame, so one file covers either duty.