In short. When a UPS that is perfect on mains misbehaves on the generator, neither machine is usually faulty. The UPS is doing its job: it is refusing a supply that, by its own settings, looks out of tolerance. The frequency moves too fast, the voltage is distorted, or the bypass cannot be trusted for a synchronised transfer. The cure is almost always in the interface — UPS generator-mode settings, the restart sequence, and the generator’s governor and voltage regulator — and it can be proved with a one-hour site test.
Who this page is for: For facility managers, IT and electrical teams whose UPS behaves on mains but misbehaves on the generator: find which side is the cause, what to change, and how to prove the fix with a proper site test.
Key facts
- The UPS judges the generator by three things: voltage window, frequency window and how fast frequency changes. A generator can be perfectly healthy and still fail all three for a few seconds after a load step.
- Most UPS units have a separate generator mode or input profile, switched by a “generator running” signal. Many sites never commission it.
- “Bypass not available” on DG is common and often acceptable — but then a UPS overload or inverter fault means a break, not a seamless transfer. Decide this deliberately.
- Battery autonomy must cover the worst case, not the normal case: a failed first start, a retry, and an ageing battery.
- A limited charger on DG is correct engineering, but on sites with several outages a day it can leave the battery half-charged when the next outage arrives.
The engineering behind generator sizing for UPS loads — rectifier types, harmonic ratios and the input window — is set out on our UPS & Non-Linear Loads page. This page starts where that one stops: the UPS is already installed, the generator is already installed, and something goes wrong when they meet.
Symptom finder
Find the line that matches what you see on the UPS display or in its event log. The middle column is what the UPS is actually reacting to.
| What you see | What the UPS is seeing | First checks | Usual fix |
|---|---|---|---|
| UPS stays on battery after the generator has started and changed over | Input voltage or frequency outside its acceptance window, or the rectifier is waiting for its delay timer | Read DG voltage and frequency at the UPS input terminals, not at the alternator. Check the UPS input-delay setting. | Correct DG voltage setting or cable volt drop; set UPS generator-mode window; check that the “generator running” contact actually reaches the UPS. |
| UPS cycles between battery and rectifier every few seconds or minutes | Frequency swinging in and out of the window as the rectifier loads and unloads the set — a feedback loop | Watch DG frequency while the UPS walks in. Check governor type and stability; check rectifier walk-in time. | Longer rectifier walk-in, stable isochronous governing, governor tuning; widen frequency tolerance only as far as the UPS maker allows. |
| “Bypass out of tolerance”, “sync lost” or “inverter not synchronised” alarm on DG | DG frequency moving faster than the inverter’s slew limit, so the inverter stops tracking the bypass | Note the alarm time against load events: lifts, chillers, pumps starting. | Stagger large motor starts; tune governor; agree a generator-mode slew setting; decide the bypass policy on DG. |
| Batteries never fully recharge on long DG runs, or autonomy is short at the second outage of the day | Charger current deliberately reduced in generator mode, or rectifier power limit active | Check charger limit in generator mode and battery state after a long DG run. | Set the charger limit to what the DG can genuinely carry, not the minimum; size the DG to include some recharge. |
| Generator voltage oscillates or AVR hunts when the UPS is on, steady without it | Rectifier current distortion interacting with an AVR that senses a distorted waveform | Measure voltage distortion at the alternator with the UPS loaded. Check AVR sensing and excitation type. | Three-phase sensing or PMG-supported AVR, input filters on the UPS, or a larger alternator — see our UPS & Non-Linear Loads page for ratios. |
| Generator trips on over-voltage when the UPS is lightly loaded | Capacitive input filters giving a leading power factor the alternator cannot absorb | Check DG power factor at low UPS load (night time, weekends). | Switch out input filters on DG where the maker allows, keep a minimum lagging load on the set, or review AVR settings. |
| UPS transfers to battery every time a big motor starts on the same DG | Voltage dip and frequency dip from the motor start | Record dip depth and duration during the motor start. | Soft starter or VFD on the motor, start the motor before the UPS walks in, or feed the motor from a separate set. |
| Earth-leakage, neutral or “N-E voltage” alarm only on DG | The neutral-to-earth bond is in a different place on DG supply, or the changeover does not switch the neutral correctly | Measure neutral-to-earth voltage at the UPS on mains and on DG. | Correct the bonding arrangement and changeover pole count with your electrical consultant. |
Why “sync” matters more than people think
An online UPS keeps its inverter running in step with the bypass supply. That is what allows it to hand the load to bypass without a break if the inverter is overloaded or fails. On mains, the bypass frequency hardly moves, so the inverter follows it easily. On a generator, frequency dips and recovers every time a load is switched. The inverter can only follow at a limited rate — its slew rate — so the UPS protects itself: it stops following, runs on its own internal clock, and declares the bypass unavailable.
That is not a fault. But it changes the risk:
- With bypass available: an inverter fault or overload transfers the load to the generator seamlessly.
- With bypass unavailable: the same event either drops the load or transfers with a break that most IT equipment will not ride through.
So the real question for a site is not “how do we clear the alarm?” but “what do we want the UPS to do on generator supply?” Common, defensible choices are:
- Track the generator with a wider slew setting, accepting slightly larger output frequency movement, so the bypass stays usable. Suits loads that tolerate small frequency changes.
- Free-run on DG with bypass inhibited, accepting that a UPS fault on DG means a break. Suits sensitive loads, where a clean inverter output is worth more than bypass availability.
- Keep the bypass, but make the generator steadier — electronic isochronous governing, staged load restarts, motors on soft starters — so the window is rarely exceeded.
Whichever you choose, write it down and set both machines for it. Most “mystery” incidents happen on sites where nobody ever made the choice.
Battery minutes: plan for the bad day
The battery has to carry the load from the moment mains fails until the UPS rectifier has fully taken load from the generator. On a good day that is well under a minute. Batteries should be sized for a bad day.
| Stage | Good day (illustrative) | Bad day (illustrative) |
|---|---|---|
| Mains failure confirmed by the AMF panel | 3 s | 5 s |
| Start delay (to ignore short blips) | 2 s | 5 s |
| Crank, fire and run up to speed | 6 s | First attempt fails: 3 attempts of about 10 s cranking with 10 s rests ≈ 50 s |
| Voltage and frequency stable, changeover closes | 5 s | 10 s |
| UPS input delay and rectifier walk-in | 15 s | 20 s |
| Restart attempt after a hunting or sync problem | — | 60 s |
| Total on battery | about 30 s | about 2.5 min |
Illustrative timings to show the method — take real values from your AMF panel and UPS settings.
Then add what happens if the set does not start at all: someone has to notice the alarm, walk to the set and either start it manually or shut down the IT load in an orderly way. A common target for commercial sites is 10 minutes or more at end-of-life battery capacity — set it from your own response time. Battery capacity falls with age and heat, and a battery is commonly treated as end-of-life at around 80 % of its rated capacity, so a new battery should give comfortably more than the target.
Recharge matters as much as autonomy. Where outages are frequent, check how long the battery takes to recharge on generator supply with the charger in generator mode. If the next outage often comes before the battery is back to full, the real autonomy is less than the label says. The fix is to let the charger take a larger share of the generator — which has to be allowed for when the set is sized.
UPS settings to agree for generator operation
These settings exist on most modern UPS units under different names. They should be set by the UPS maker’s engineer, with the generator’s real behaviour in front of them — not left at factory mains values.
| Setting | Why it matters on DG | Who decides |
|---|---|---|
| Generator-mode input voltage and frequency window | Lets the rectifier accept the normal dips of a generator without going to battery | UPS maker, within what the load and rectifier can tolerate |
| Bypass tolerance and inverter slew rate | Decides whether the bypass stays available on DG | Site owner (policy) with UPS maker (values) |
| Rectifier walk-in time | Ramps load onto the generator gradually instead of as one step | UPS maker; longer is gentler on the DG |
| Staggered restart for parallel or modular UPS | Stops several rectifiers walking in at the same instant | UPS maker |
| Charger current limit on DG | Trades generator loading against recharge time | Site owner with generator and UPS engineers |
| “Generator running” input | The signal that switches the UPS into its generator profile — useless if it is never wired | Installer — wire it from the AMF panel |
| Eco or high-efficiency mode | Puts the load on raw bypass supply; on a generator that usually means raw DG transients reach the load | Normally disabled on DG supply |
Generator-side checks
- Governor: electronic, isochronous governing holds frequency far better than mechanical droop governing. If the set has a mechanical governor and a large UPS load, expect sync issues.
- Voltage regulator sensing: an AVR sensing one phase of a distorted waveform can misread the voltage. Three-phase sensing and PMG-supported excitation are steadier with rectifier loads.
- UPS share of the set: if the UPS input is a large share of the alternator rating, distortion rises. The ratios on our UPS page show when a larger alternator is the honest answer.
- Load sequence: chillers, precision air conditioning, lifts and pumps should restart in timed steps after the UPS has walked in, not together with it.
- Voltage at the UPS terminals: long DG cables can drop voltage that the alternator reading never shows.
- Neutral and earthing: check the neutral-earth bond arrangement on DG supply matches the design.
The site test: prove it, do not assume it
A monthly “DG test” with the UPS still on mains proves nothing about the interface. This test does. Plan it for a low-risk window, with the IT owner’s agreement, a UPS engineer and a generator engineer present, and a power-quality recorder on the UPS input.
Pass criteria (agree the exact values before the test)
| Check | Pass |
|---|---|
| Total time on battery | Within the planned good-day time, with margin against end-of-life autonomy |
| UPS transfers to battery after it has walked in | None during any load step |
| Bypass status | As per the written policy — available, or inhibited by design |
| Generator frequency and voltage during steps | Inside the UPS generator-mode window, recovering within the time the UPS allows |
| Generator alarms | None (no over-voltage, under-frequency or overload pre-alarms) |
| Retransfer to mains | Clean, no latched alarms, charger back to normal mode |
Repeat the test after any change to the UPS, the generator, the AMF panel or the major loads — and at least once a year.
Worked example (illustrative)
Illustrative case built to show the method. An office building has a 160 kVA generator feeding a 60 kVA six-pulse UPS for its server room, plus about 30 kW of air conditioning. On mains everything is fine. On DG, the UPS goes to battery for a few seconds every time an air-conditioning compressor starts, and the bypass shows “out of tolerance” for most of the DG run.
- Recorder results: each compressor start pulls frequency down by about 1.5 Hz for around 2 seconds. The UPS mains-profile window is narrower than that, and the “generator running” signal had never been wired, so the generator profile was never used.
- Interface fixes: the generator-running contact is wired from the AMF panel, and the UPS engineer sets the generator profile with a wider window and a 20-second walk-in.
- Load fixes: the air conditioning restart is delayed until 60 seconds after changeover, and staggered by unit.
- Generator fixes: the governor is tuned; the set already had an electronic governor, so no hardware change was needed.
- Bypass policy: the owner chooses to keep bypass available on DG, so the slew setting is widened within the UPS maker’s limits.
- Re-test: no battery transfers during load steps; bypass available throughout; total battery time about 35 seconds.
Nothing was replaced. Two settings, one wire and a restart sequence solved a problem that had been blamed on the generator for two years.
Situations that need more care
- Parallel and modular UPS systems: several rectifiers walking in together look like one big load step. Stagger them.
- Two or more generators in parallel: load sharing and synchronising transients add to the picture; test with the real number of sets running.
- Old six-pulse UPS without input filters: distortion may be the root problem; the honest fix may be an input filter, a newer UPS, or a larger alternator.
- Data centres: the interface becomes part of the facility’s documented design and test regime; see our data-centre pages.
- Small single-phase UPS in shops and homes: these often have one fixed tolerance setting. Choose a generator with good voltage and frequency regulation, or a model the UPS maker states is generator-compatible.
Interface record sheet
| Item | Your site |
|---|---|
| UPS rating, type and rectifier type | |
| UPS generator-mode profile set? (yes/no, date) | |
| Generator-running signal wired? (yes/no) | |
| Bypass policy on DG (track / inhibit) | |
| Walk-in time | |
| Charger limit on DG | |
| Battery autonomy at end of life (minutes) | |
| Generator governor type | |
| AVR sensing / excitation type | |
| Largest motor on the same DG and how it starts | |
| Last full interface test: date and result |
Keep this sheet with the UPS log book and the generator service record, so the next engineer on either machine starts from facts.
Common mistakes
- Testing the generator monthly with the UPS still on mains, so the interface is never proven.
- Never wiring the ‘generator running’ signal, so the UPS generator profile is never used.
- Leaving eco mode active on generator supply.
- Buying a larger generator before measuring frequency and distortion during walk-in.
What to send us
Send the UPS rating, type and rectifier type, the generator’s kVA and governor type, the exact UPS alarm or event-log entries on generator supply, battery autonomy, and the other large loads on the same set. Use the application engineering enquiry, or WhatsApp the details and photographs.
Questions engineers ask
Why does my UPS go to battery even though the generator is running fine?
Because the UPS judges the supply against its own voltage and frequency window. A generator dips and recovers on every load step, which can be outside a window set for mains. Commissioning the UPS generator-mode profile and wiring the ‘generator running’ signal usually solves it.
Is ‘bypass not available’ on generator a fault?
Not necessarily. It means the inverter has stopped following the generator’s frequency. It is acceptable if you have decided to inhibit bypass on DG, but you must know that a UPS fault on DG supply would then cause a break.
How many minutes of battery do I need with a generator?
Enough for a bad day: a failed first start, retries, and time for someone to respond. A common target for commercial sites is 10 minutes or more at end-of-life capacity, set from your own response time, even though the normal transfer takes well under a minute.
Should we use UPS eco mode on generator supply?
Usually not. Eco mode feeds the load from the raw bypass supply, so generator transients reach the load directly. Most sites disable it whenever the generator is running.
Can a bigger generator fix UPS problems?
Sometimes, where distortion is the cause. But most UPS-on-DG problems are settings and sequencing problems. Test and adjust first; buy a larger alternator only if the measurements show it is needed.
How often should we test the UPS on generator supply?
At least once a year with a real simulated mains failure, and after any change to the UPS, the generator, the AMF panel or major loads.
Related pages
- UPS & Non-Linear Loads (sizing and interface engineering)
- UPS + DG Integration
- UPS Battery Autonomy & Runtime Engineering
- Generator Controller Alarms Explained for Site Staff
- Will Your Breakers Trip on Generator Supply? A Field Check for Fault Current, Trip Settings and Cable Length
- Generator Transient Response
- Harmonics & THD
- AVR & Excitation Service
- Why DG Sets Trip When the APFC Panel Is On
- Generator Hunting
Technical content reviewed by D. Srinivas Reddy, Managing Partner and service engineer. Last updated 3 November 2026.