In short. In electronics and semiconductor manufacturing, power that is merely available is not the same as power the process can use. The generator sits behind a UPS for tools and controls, and in front of cleanroom air handling, process exhaust, cooling water and vacuum, all of which must recover in a defined order. Design for the UPS input characteristics, hold voltage and frequency within tool limits during transfer, keep a clean earthing architecture, and prove it with measured tests.
Why this sector is unforgiving
A reflow oven, a wire bonder, a wafer prober or a lithography tool does not just stop when voltage sags; it may abort a lot, scrap work in progress, and require a qualification run before production resumes. Many semiconductor tools are specified to ride through only brief sags, in line with the SEMI F47 approach that defines voltage dips a tool must tolerate for fractions of a second. Anything longer than that is an interruption as far as the process is concerned, which is why the generator alone is never the continuity device for the tool. It is the long-duration source that keeps the UPS batteries from running flat and keeps the environment within limits.
The second characteristic is the environment. Electronics assembly needs controlled temperature and humidity for ESD and solder quality; cleanrooms need continuous air changes and pressure cascades; wet benches and furnaces need exhaust and scrubbing for safety. These look like building services, but here they are process loads with safety consequences, and they belong on the generator with defined restoration times.
The continuity architecture: UPS, generator and utilities
| Subsystem | Sensitivity | Support during outage | Restoration target |
|---|---|---|---|
| Process tools and controls | Milliseconds | Double-conversion UPS, generator behind it | No break |
| Facility monitoring, gas detection, EMO circuits | Milliseconds | UPS, often dual-fed | No break |
| Process exhaust and scrubbers | Seconds, safety-related | Generator, first in sequence | Within the transfer time |
| Cleanroom make-up air, fan filter units, humidity control | Minutes before pressure and particle limits are lost | Generator, early in sequence | Typically under a minute |
| Process cooling water, vacuum, compressed dry air, nitrogen | Minutes | Generator, sequenced | Before tool restart |
| General lighting, offices, canteen | Hours | Shed or restored last | After process stable |
The table reveals a sequencing requirement that the AMF or master controller must execute: safety exhaust first, then air handling, then process utilities, then anything else, with the UPS carrying tools throughout. Where the site is large, more than one generator on a common bus with load management is usual, and the critical architecture choices are covered on UPS critical power architecture and load management.
Generator and UPS interaction
The UPS is the generator’s most demanding load and the one most often misjudged. Three things need to be checked with the UPS data sheet in hand.
- Input current distortion. Older six-pulse rectifiers draw heavily distorted current and have historically required the generator to be rated at 1.5 to 2 times the UPS kVA. Modern IGBT rectifiers with input THD below 5 percent allow a ratio closer to 1.1 to 1.25, but the figure must be confirmed against the specific UPS and alternator data.
- Frequency and slew-rate windows. A UPS synchronises its output to its input only while input frequency stays within a window, often plus or minus 2 to 5 percent, and while the rate of change is below a set limit. A generator with a slow or hunting governor can push the UPS onto battery even though it is running, and the batteries then drain while the engine idles.
- Walk-in and battery recharge. After transfer the UPS ramps its input over several seconds, then adds battery charging current to the load. Both must be in the load profile.
The practical guidance is on UPS and nonlinear loads on generators and UPS plus DG integration.
S_gen,min = k × S_UPS + S_mechanical + S_largest-start
Here S_gen,min is the indicative minimum generator kVA, k is the manufacturer’s UPS-to-generator ratio for the rectifier technology, S_UPS is the UPS rating, S_mechanical is the apparent power of air handling, exhaust, cooling and vacuum that must run at the same time, and S_largest-start is the transient kVA of the largest motor start in the sequence. For a 300 kVA modern UPS with k of 1.2, 350 kVA of mechanical load and a 75 kW make-up air fan started through an inverter drive, S_gen,min is about 360 plus 350 plus a modest transient, which points to an 800 kVA class set or two 500 kVA sets in N+1 (illustrative). The split choice depends on how much the site can afford to lose when one set is out for service; see N+1 generator sizing.
Power quality, earthing and protection
Voltage regulation, frequency stability and harmonic distortion are measured quantities with tool-specific limits, and the generator’s AVR and governor settings should be tuned against them during commissioning rather than left at factory defaults. Where many drives and UPS units share the bus, a harmonic study decides whether the alternator alone is enough or whether filtering is required; the screening approach is on harmonics and THD.
Earthing deserves its own drawing. Sensitive equipment wants a TN-S arrangement with the neutral earthed at one point only, separate protective and functional earth conductors bonded at the main earth terminal, and no circulating neutral current through the structure. When a generator is added, the location of its neutral-earth link relative to the utility transformer’s link decides whether the transfer switch must switch the neutral. Getting this wrong produces nuisance earth-fault trips and noise on instrumentation. See earthing and neutral grounding.
Protection should remain selective on generator supply even though the fault current is far lower than on the mains, which may require a review of relay settings in generator mode.
Evidence-based acceptance
Agree the acceptance tests before the equipment is ordered. A sensible minimum set is a load-bank test proving rating and step-load response, a transfer test with the UPS and mechanical loads connected while voltage, frequency and THD at the critical bus are recorded, a retransfer test, a UPS-on-generator test confirming the UPS stays synchronised and recharges, and a simulated generator failure during an outage to confirm the UPS autonomy covers it. Tool qualification remains with the process owners, but the electrical evidence should be in one report they can rely on. The structure of such a programme is set out on integrated systems testing.
Common mistakes
- Sizing the generator at the UPS rating. The rectifier’s distorted current overheats the alternator and the UPS rejects the generator as a source.
- Leaving cleanroom air handling and exhaust off the standby bus as “HVAC”. Pressure cascades collapse, the room is out of specification, and restart requires a requalification.
- Tuning nothing after installation. Factory governor and AVR settings leave the UPS cycling between mains and battery while the generator runs.
- Earthing the generator neutral without checking the utility arrangement. Circulating currents trip earth-fault relays and inject noise into measurement equipment.
- Accepting the installation on a no-load start test. The first real outage is the first integrated test, and it is run without instruments.
What to send us
Send the single-line diagram, the UPS make, rating and rectifier type with its input THD and generator compatibility notes, a list of mechanical loads with ratings and starter types, the tool voltage and frequency tolerance where known, and the sequence in which the facility team wants systems restored. Photographs of the existing generator nameplate, control panel and main earth terminal help if the site is a brownfield one. Send your electronics facility requirement.
Questions engineers ask
How much bigger than the UPS should the generator be?
For modern IGBT-rectifier UPS units a ratio of about 1.1 to 1.25 is common; for older six-pulse units it can be 1.5 to 2. The UPS manufacturer’s generator compatibility note and the alternator’s nonlinear rating decide, and the mechanical loads are added on top.
Why does our UPS go to battery even when the generator is running?
Most often the generator frequency or its rate of change is outside the UPS synchronisation window, or the generator voltage is distorted beyond the UPS input limit. Governor tuning, a wider UPS window where the tools allow it, or a larger alternator usually solves it.
Should cleanroom HVAC be on the generator?
Yes. Make-up air, fan filter units and humidity control hold the room within specification, and exhaust and scrubbers are safety systems. They should be among the first loads restored after transfer.
What should the acceptance test record?
Voltage, frequency and total harmonic distortion at the critical bus through a full transfer and retransfer with the real UPS and mechanical loads connected, plus confirmation that the UPS stayed synchronised and recharged on generator. A load-bank test alone does not prove this.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.