In short. A cement plant’s installed load runs to tens of megawatts, but its emergency load is a few hundred kilowatts to a few megawatts: the kiln auxiliary drive that must turn the kiln within minutes of a stoppage, cooler drives and fans, lubrication, instrument air and the control room. The generator is sized for that list and its timing, built to breathe clean air in a dusty plant, and tested as the kiln’s last line of protection.
Why the kiln sets the brief
When a cement plant loses supply, the raw mill, cement mills, main kiln drive and large fans stop, and production resumes when the grid returns. The rotary kiln is different. A kiln that stands still while hot deforms under its own weight within a short time, bending the shell and damaging refractory and tyres, and the repair can take the line out for weeks. Every plant therefore has an auxiliary or inching drive, typically a diesel engine or an electric motor of some tens to a few hundred kilowatts through a high-ratio gearbox, that turns the kiln slowly until it is cool. If that auxiliary drive is electric, the emergency generator exists first to run it, and everything else on the emergency bus is arranged around that duty. The limestone belt from Nalgonda across to Kurnool and Guntur has many such plants, and in our experience the kiln drive, the cooler and the control room decide the generator size and its start sequence.
| Emergency load | Purpose after an outage | Time it must be live | Typical character |
|---|---|---|---|
| Kiln auxiliary or inching drive | Prevent shell deformation while hot | Within minutes, intermittently for hours | Motor with high starting torque, or diesel engine with electric controls |
| Kiln shell cooling fans | Cool the shell evenly during slow rotation | Minutes | Several medium fans, staged |
| Cooler grate drive and first cooler fans | Protect the grate from clinker heat | Minutes | Drives and large fans; often only part of the cooler |
| Lubrication units for kiln, mills, main gearboxes | Prevent bearing damage during coast-down and slow turning | Seconds | Small pumps, must be the first step |
| Instrument air compressor | Keep dampers, valves and bag filter purging alive | Minutes | Compressor with starting load |
| Control room, DCS, CCR UPS input | Visibility and control of the sequence | None | UPS with generator behind |
| Preheater and cooler emergency lighting, lifts, fire systems | Life safety and access | Seconds | Small, steady |
Sizing from the sequence, not the total
The emergency loads do not all start together, and they should not. Lubrication pumps and the UPS input come first, then kiln shell fans and the auxiliary drive, then the cooler items and the air compressor, each step separated by a few seconds so the engine recovers frequency before the next. The auxiliary drive motor is the awkward one: it starts a kiln that may be stationary under a heavy, hot, uneven charge, so its starting torque and current are high and the plant often specifies a soft starter or a VFD for it. A VFD helps the generator because the current is limited, but it adds harmonic load to a small bus. The method is in generator sizing and large motor starting on diesel generators.
kVA_step = kVA_running_before_step + kVA_starting_of_new_motor
kVA_running_before_step is the load already on the generator when the motor is called, and kVA_starting_of_new_motor is the motor’s starting demand for its starting method. A worked example (illustrative): with 180 kVA of lubrication, fans and control loads already running, a 110 kW auxiliary drive on a soft starter limited to 3 times full-load current adds roughly 110 × 3 / (0.94 × 0.86), about 410 kVA, for the start. The set must accept a step to about 590 kVA with an acceptable voltage dip, which points to a set in the 750 to 1,000 kVA range rather than the 400 kVA that the running total suggests. The exact figures depend on the motor data and the alternator’s transient reactance.
Dust, heat and the generator room
Cement dust is fine, abrasive and everywhere. A generator that draws combustion and cooling air from a dusty yard will choke its air filters, coat the radiator core and lose output within weeks. The room should be positioned upwind of the main dust sources where possible, pressurised with filtered air, and fitted with a radiator that can be cleaned without dismantling. Two-stage or cyclonic pre-cleaners on the engine intake are worth the cost. Ambient temperature near the kiln and cooler is high, so derating for intake temperature applies on top. Fuel storage must be covered and vented through filters, because dust in diesel is a direct route to injector and pump failure. See generator room ventilation and generator cooling systems.
Fitting the generator into a medium-voltage plant
Cement plants distribute at 6.6 kV or 11 kV with 415 V substations around the line, and the emergency loads are scattered across several substations. There are two common arrangements. A medium-voltage generator connected to an emergency MV bus feeds the relevant substations through their own transformers, which suits larger plants with emergency loads above a couple of megawatts. An LV generator at a dedicated emergency board, with the emergency loads cabled back to it, suits smaller plants and avoids energising transformers on a small source. Transformer inrush on a generator is a real constraint in the first arrangement and must be studied. Where the plant has a captive power plant or waste heat recovery unit, the diesel set may also be the black-start source for its auxiliaries, which adds boiler feed pumps and fans to the list. See medium voltage generators and black start.
Testing and readiness
The kiln protection duty is needed perhaps a few times a year, and only when the plant is already in trouble. Readiness therefore matters more than anything. Weekly automatic test runs, a monthly transfer test that actually turns the kiln auxiliary drive under generator power, annual load bank testing to rating, and a fuel quality check each quarter are a reasonable minimum. The emergency sequence should be written in the plant’s DCS and rehearsed, with the time to first rotation of the kiln recorded. Where a single set carries this duty, its overhaul window needs a rental set connected and tested before the permanent one is opened. Our structure for this is in annual maintenance contracts.
Common mistakes
- Quoting the generator from the plant’s connected load. The number is absurd, the project is shelved, and the kiln remains unprotected.
- Sizing from the emergency running total alone. The auxiliary drive start pulls the frequency down and the set trips while the kiln is hot.
- Placing the set in the open yard beside the clinker stockpile. Filters and radiator block with dust and the set loses output or overheats within a season.
- Energising several substation transformers at once from an MV generator. The combined inrush trips the set or collapses voltage before the loads even start.
- Testing the engine but never turning the kiln on generator. The auxiliary drive’s starter, interlocks or cable route fail unnoticed until the real event.
What to send us
Send the emergency load list with motor ratings and starting methods, the kiln supplier’s requirement for time to first rotation, the single-line diagram showing the emergency bus and the substations involved, and the proposed generator location with its ambient and dust conditions. For an existing set, add the nameplate, controller event log, filter change history and a description of any trips during past outages. Send your application details and we will reply with a proposed step sequence and rating.
Questions engineers ask
What size emergency generator does a cement plant need?
Usually between a few hundred kVA and a few megawatts, depending on whether the kiln auxiliary drive is electric, how much of the cooler must run, and whether a captive power plant’s auxiliaries must be black started. The step sequence, not the running total, fixes the rating.
How quickly must the kiln start turning after a power failure?
The kiln supplier states the limit for a hot kiln, and it is measured in minutes. The generator start, transfer, lubrication step and auxiliary drive start must fit inside that time, and the sequence should be rehearsed so the plant knows its actual figure.
Should the generator be medium voltage or low voltage?
If the emergency loads are spread across several substations and exceed roughly two megawatts, an MV set feeding an emergency MV bus is usually cleaner. Smaller emergency loads are better served by an LV set at a dedicated board, which avoids transformer inrush on a small source.
How do we keep dust out of the generator?
Site the set upwind of dust sources, pressurise the room with filtered air, fit intake pre-cleaners, choose a radiator that can be cleaned in place, and shorten the filter and radiator cleaning intervals to match the plant’s conditions rather than the manufacturer’s clean-site schedule.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.