In short. Paper, printing and packaging lines are linked processes, so backup power has to be planned for either full continuity or a controlled stop and restart, never for a vague middle. The minimum generator scope is the set of loads that keep a stopped line recoverable: agitators, vacuum, compressed air, chilled water, boiler auxiliaries and control power. Backing up the whole line is a cost decision made against sheet breaks, web waste and restart time.
What an outage does to a linked line
A paper machine, a web offset press and a corrugator share a characteristic: material is in motion through several machines at once, and stopping one section without the others creates damage. On a paper machine, loss of power breaks the sheet and lets stock settle in chests and pipes if agitators stop. On a web press, the web tears and the dryer must cool and reheat. On a corrugator, the hot plates and glue system lose temperature and the board in the machine is scrap. Flexo presses, die-cutters and folder-gluers are more tolerant individually, but they feed each other, so a partially restored plant may still produce nothing.
The first engineering question is therefore not how many kVA, but which sections must keep running, which must stop safely, and what has to be live for the restart to succeed. The answer differs between a continuous kraft mill in Andhra Pradesh and a two-shift sheet-fed printer in Hyderabad, and it should be written down before equipment is discussed.
Two valid scopes: full continuity and safe stop
Most plants end up with one of two architectures, and both are legitimate when chosen deliberately.
| Scope | What the generator carries | Typical rating against plant load | Suits |
|---|---|---|---|
| Safe stop and protect | Agitators, vacuum pumps, compressed air, chilled water, boiler feed and controls, lighting, IT, fire pumps | Often 20 to 35 percent of running load (illustrative) | Sites with short, infrequent outages |
| Full continuity | Everything above plus production drives, dryers, presses and finishing | Full running load plus starting margin | Continuous mills, export packaging lines, long or frequent outages |
| Hybrid | Safe-stop loads on standby set, selected lines on rental or a second set during planned work | Varies by line | Plants with one line that dominates revenue |
The safe-stop scope is undervalued. It is inexpensive relative to a full-plant set, and it converts a damaging outage into an orderly one. Full continuity is justified when outage frequency and the cost of broke, web waste and restart hours exceed the cost of a large set or a paralleled plant. The method for comparing them is on lifecycle cost.
Load characteristics that change the generator
Large motors and starting events
Refiners, pulpers, vacuum pumps and large compressors are the starts that stress a generator. A 200 kW refiner started direct-on-line can demand well over 1,000 kVA for a few seconds, which is why these loads either need soft starters or inverter drives, or need to be sequenced onto a set that has already picked up the base load. The generator’s transient voltage dip for each start should be checked against the limits the drives and contactors on the same bus will tolerate; see voltage dip on generators and large motor starting on diesel generators.
Sectional drives and regenerative energy
Paper machines and modern corrugators use sectional drives on a common DC bus. Under normal running they draw harmonic-rich current; under a web break or a controlled deceleration, some sections regenerate energy back towards the source. A diesel generator cannot absorb reverse power, and its reverse-power protection will trip. The drive system’s braking resistors or regenerative units need to be confirmed as active in generator mode, and the generator sized with the drive manufacturer’s harmonic data in hand.
Thermal and utility loads
Dryers on web presses, hot plates on corrugators and IR or UV curing units are large, often resistive and sometimes switched in steps by contactors or thyristors. The steps are electrically abrupt. Combined with chilled water, vacuum and compressed air, the utility load can exceed the drive load and should be listed separately, with its control method, so that the starting and step analysis is realistic.
S_gen = (P_run / PF) + S_step
In this expression S_gen is the indicative generator kVA, P_run is the running kW of the protected loads, PF is their power factor and S_step is the additional apparent power of the largest single load step after the base load is on the bus. For a safe-stop scope with 180 kW running at 0.85 PF and a 45 kW vacuum pump starting star-delta at roughly 2.5 times its full-load kVA, S_gen is about 212 kVA plus roughly 150 kVA, which points to a 320 to 380 kVA set depending on the acceptable dip (illustrative). The exact figure must be checked against the engine’s step-load capability and the alternator’s reactance.
Compressed air, vacuum and control power
Three utilities decide whether a line can be restarted at all. Compressed air operates pneumatic clutches, brakes, feeders and sheet transfer on presses and converting machines; if the receiver is empty the line cannot even be inched. Vacuum holds the sheet on a paper machine and feeds sheet-fed presses; without it, restart is impossible. Control power for PLCs, drive controllers and register systems should be on a small UPS so that recipes, counters and positions survive the transfer gap, otherwise each outage becomes a cold start with manual re-homing. These loads are small in kW and large in consequence, which is why they belong in every scope. The transfer gap itself is set by the AMF panel and is typically 10 to 15 seconds from loss of mains to generator on load, which the UPS must bridge; see AMF panels.
Temporary power during planned electrical work
Printing and packaging plants often need a rental set during a transformer replacement or an HT shutdown, and the job goes wrong when the electrical handback is confused with the process restart. Define in writing which circuits the temporary set covers, who may switch between sources, what the load limit is during the work, and how supply returns to the normal arrangement. Then separate the electrical handback, which is the electrician’s statement that the permanent supply is restored and the temporary connection is removed, from the production authorisation, which belongs to the plant’s own process team. Rental scope details are covered on rental power for planned outages, and the general approach on rental and temporary power.
Common mistakes
- Sizing the generator for the press or paper machine but forgetting the vacuum, air and chilled-water plant. The line cannot restart on generator because its utilities are dead.
- Assuming sectional drives behave like plain motors. Regeneration trips the set on reverse power the first time a web breaks on generator.
- Leaving PLC and register-control power without a UPS. Every outage becomes a cold restart with lost recipes and an hour of setting up.
- Starting a refiner or large compressor direct-on-line before the base load is on the bus. The voltage dip drops out contactors across the plant.
- Treating a rental generator’s handback as production approval. Supply returns before the process team has checked temperatures, glue systems and safety interlocks.
What to send us
Send the plant single-line diagram, a load list that separates production drives from utilities and thermal loads, the rating and starting method of refiners, pulpers, vacuum pumps and compressors, and a statement of which lines must run on backup and which only need a safe stop. If drives are involved, include the drive manufacturer’s harmonic and regeneration information where available. Send your paper or printing plant details.
Questions engineers ask
Can one generator run a whole corrugator line?
Usually yes if it is sized for the running load, the dryer and glue heating steps, and the largest motor start, and if the drive system’s regenerative behaviour is handled. Many plants instead size for safe stop plus one converting line and accept that the corrugator waits for the mains.
Why does our generator trip when the web breaks?
Sectional drives decelerating under tension control push energy back to the bus. On the mains this is absorbed; on a generator it triggers reverse-power protection. The fix is to make the drives’ braking resistors active in generator mode or to add a regenerative load, not to raise the trip setting.
What is the minimum backup a printing press plant should have?
Compressed air, chilled water, press and PLC control power on UPS, lighting, IT and any fire pumps. That set allows a safe stop and a quick restart and is a fraction of the full press load.
How long does the transfer take and what does it affect?
A standard AMF transfer is typically 10 to 15 seconds from loss of mains to generator on load. Motors stop, controllers without UPS reset, and the restart sequence begins from a standstill, which is why control power should ride through on a UPS.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.