Why a DG set and a UPS argue
The generator starts, the changeover closes, and the UPS stays on battery as though nothing happened. This is the most common complaint on sites running both, and it is almost never a fault in either machine.
A UPS is a load that judges its supply. A diesel generator is a supply that objects to being judged. Put them in series and there are three specific places they disagree, all of which are settable rather than structural.
Frequency, and the first thirty seconds
A DG set takes several seconds to reach stable speed, and until the governor settles the frequency wanders. A UPS with a tight synchronisation window watches this, decides the supply is unqualified, and stays on battery — correctly, by its own rules.
The battery then discharges through the entire period the generator is running, which is the opposite of what the installation was designed to do, and the first anyone knows is when the bank collapses during a long outage.
Rectifier walk-in, and why it matters more on a DG
When a UPS accepts a supply it also begins recharging its battery, and a depleted bank will draw the maximum the charger allows. On the grid that is unremarkable. On a generator sized close to the load, a UPS demanding full load plus full charge current at the moment of changeover can pull the set into overload and trip it.
The fix is walk-in: the rectifier ramps its draw over a set period — typically ten to thirty seconds — instead of taking it all at once. Where a UPS trips the DG on changeover and the arithmetic says the set is big enough, walk-in is almost always the setting that has not been enabled.
| What you see | What it is | Where to look |
|---|---|---|
| UPS stays on battery while the DG runs | Frequency outside the sync window | Widen input frequency tolerance |
| DG trips on overload at changeover | Charger drawing full current instantly | Enable and lengthen rectifier walk-in |
| UPS accepts then rejects repeatedly | Governor hunting, or an undersized set | Log DG frequency; check set sizing |
| Loud alarm, no transfer, everything fine | Bypass unavailable on DG | Check bypass sync window separately |
Sizing the generator for a UPS load
The old rule of oversizing the DG by two to three times against a UPS load came from an era of six-pulse rectifiers with poor input power factor and heavy harmonic distortion. Those genuinely did require a much larger set than the kW suggested.
A modern unit with active power factor correction — ours run above 0.90 — draws a much cleaner current, and the multiplier drops accordingly. Somewhere between 1.25 and 1.5 times is usually defensible, though the number depends on what else the set is carrying and how much of it starts direct-on-line.
Anyone quoting a fixed multiplier without asking about the input power factor is applying a rule of thumb from thirty years ago, and it will cost you a generator size you did not need.
Ask for this at the enquiry stage
Give us the DG rating, the make of the controller and whether anything else starts on the set. Frequency window and walk-in are configured before dispatch, and getting them right on the bench is easier than diagnosing them during a monsoon outage.
The case for a wide input window
The single most useful specification on a site with a generator is the input voltage and frequency range. Ours accepts 330 to 480 V and 48 to 52 Hz, which covers most of what a settling DG does. A narrow window looks identical on a datasheet and behaves entirely differently at three in the morning when the set is cold.
Questions we get asked
Anything not covered here, call us — most of it is faster to answer on the phone.
Why does my UPS stay on battery when the generator is running?
Because the frequency is outside its acceptance window. A DG wanders while the governor settles, and a UPS with a tight window judges that supply unqualified and keeps running from the battery. Widening the input frequency tolerance normally resolves it — this is a setting, not a fault, and it can be changed on site.
How much bigger should the DG be than the UPS?
Between 1.25 and 1.5 times the UPS rating for a modern unit with active power factor correction. The old advice of two to three times came from six-pulse rectifiers with poor input power factor, and applying it today buys a generator size you do not need. Confirm the input power factor before accepting any multiplier.
What is rectifier walk-in and do I need it?
It ramps the UPS input draw over ten to thirty seconds instead of demanding full load plus battery charging the instant the supply returns. On a generator sized close to the load it is the difference between a clean changeover and a tripped set, so yes — on any site with a DG it should be enabled and its duration matched to the generator.
Can the UPS damage the generator?
Not damage, but it can trip it and it can make it work badly. A rectifier drawing distorted current forces the alternator to supply harmonics it was not designed for, which shows up as heating. Active power factor correction largely removes that, which is why the input power factor figure matters more on a DG site than a grid-only one.