In short. A rice mill is a chain of motor-driven machines, elevators and fans that must start in order and often restart loaded with grain after an outage. The generator is sized from a process-linked load register and the loaded restart, not from total horsepower, with attention to whiteners and polishers, fans, the parboiling plant and the sensitive colour sorters. On the rural feeders that serve most mills the set runs long hours and must be rated for it.
A mill runs on a sequence, not a single motor
Paddy moves through intake, pre-cleaning, parboiling and drying where applicable, dehusking, paddy separation, whitening, polishing, grading, colour sorting, and packing, and between every stage there are elevators, conveyors and aspiration fans. Each machine is driven by its own motor, and the plant control starts them downstream first so that nothing receives grain before it is running. In the milling clusters around Miryalaguda, Nizamabad and the Godavari districts, a 4 to 8 tonne per hour mill typically has 150 to 400 kW of connected motor load spread over thirty or more drives, nearly all started direct-on-line. On a weak rural feeder, for several hours a day the generator is the process’s only supply. The sizing method we apply is described in generator sizing and load inventory.
Build the load register around the process
A customer who says the mill has 250 horsepower of motors has given useful information, but not a generator specification. The register should list every significant drive with its kW, starting method, process stage, duty and the other drives that run with it. From that register the running demand, the simultaneous groups and the largest starting events fall out, and the generator is selected from those.
| Machine group | Typical motor range | Starting behaviour | Notes for generator design |
|---|---|---|---|
| Pre-cleaners, destoners, paddy separators | 2 to 10 kW each | DOL, light start | Many small motors; count them as a group |
| Rubber roll shellers and huskers | 5 to 15 kW | DOL, moderate start | Aspiration fan started with it |
| Whiteners and polishers | 15 to 75 kW each, often several in series | DOL, heavy and loaded restart | Usually the governing starts; sequence them |
| Aspiration, husk and dryer fans | 5 to 55 kW | DOL or star-delta, high inertia | Several fans together produce a large transient |
| Elevators and conveyors | 1 to 7.5 kW each | DOL, may be loaded with grain | Downstream first; loaded restart torque |
| Parboiling plant: soaking pumps, boiler fans and feed pumps | 5 to 30 kW | DOL or star-delta | Boiler controls on UPS; husk-fired boiler needs fans continuously |
| Colour sorters and their air compressor | 1 to 3 kW electronics plus 7.5 to 22 kW compressor | Electronics sensitive; compressor DOL | Sorter on UPS or stabiliser; compressor unloaded start |
Empty start and loaded restart are different events
On a normal morning start the machines are empty and come up one at a time. After an unplanned outage the whiteners hold rice under pressure and the elevators and conveyors are full of grain. Restarting a loaded whitener or a full elevator demands far more torque, and therefore current, than its empty start, and some machines must be emptied by hand before they can be restarted at all. The generator’s transient capability and the mill’s recovery procedure both have to be written for the loaded case. A set that starts the plant comfortably each morning can trip on under-frequency when a loaded polisher is restarted after an outage.
kVA_start ≈ P_motor × k_start / (η × PF)
P_motor is the motor rating in kW, k_start the ratio of starting to full-load current for the starting method, and η and PF the motor’s efficiency and power factor at full load. A worked example (illustrative): a 55 kW whitener started DOL at 6.5 times full-load current, with efficiency 0.92 and power factor 0.86, draws about 55 × 6.5 / (0.92 × 0.86), close to 450 kVA, for the start. If the generator is already carrying 120 kW of fans, elevators and the first whitener, the set must accept a transient near 600 kVA while holding voltage within the limits the contactors and sorter electronics accept. A 250 kVA set that carries the mill’s running load with ease cannot do this; a set in the 320 to 400 kVA range, or a soft starter on each whitener, is the usual answer. See motor starting and voltage dip.
Fans, sorters and capacitors
Aspiration fans run throughout the plant, and parboiling mills add dryer fans and the fans of a husk-fired boiler. Fans are high-inertia loads with long acceleration times, and several started together produce a transient as large as the biggest whitener. The dryer and boiler need their fans continuously, so the step sequence should restore them before the milling section, with the boiler’s combustion controls on a UPS or stabilised circuit.
Colour sorters, electronic weighers and the plant PLC are small loads but intolerant of the dips that DOL starts cause on a generator. Feed them through a UPS or at minimum a servo stabiliser, and sequence the big starts away from them. Most mills also have power factor correction capacitors on the main board, and these must be disconnected when the generator supplies the plant; a lightly loaded alternator feeding a fixed capacitor bank sees a leading power factor and loses voltage control, so an interlock on the changeover is the fix. Husk and bran dust is combustible and abrasive, so the generator room belongs outside the milling building, drawing clean air. See power factor and generator room ventilation.
Duty rating and running hours
Where the feeder fails for three to six hours a day in season, a mill’s generator may accumulate 1,500 hours a year or more. That is prime duty, and a standby-rated set used this way will need overhaul early and will run outside the manufacturer’s terms. Sets in this duty should be prime rated, serviced by hours run, and fuelled from a clean, filtered storage tank rather than barrels, because new CPCB IV+ engines are more sensitive to fuel quality than those they replace. Mills that add a line should revisit the generator rather than assume it will cope. See prime power generators.
Common mistakes
- Converting total horsepower to kVA and ordering a set. The running load fits but the first loaded whitener restart trips the generator on every outage.
- Restarting the mill upstream first after a trip. Grain floods machines that are not yet running and the plant jams for hours.
- Leaving the capacitor bank connected on generator. The alternator runs at a leading power factor, voltage climbs and the sorter electronics are damaged.
- Running a standby-rated set for prime hours. Oil consumption rises, the engine wears early and the warranty does not apply.
- Housing the generator inside the dusty mill building. Husk dust blocks the radiator and filters, and the set overheats during the hottest, busiest months.
What to send us
Send the drive list with motor kW, starting method and the stage each belongs to, the daily running pattern and outage hours in season, whether the mill has parboiling and drying, and the rating of the capacitor bank on the main board. For an existing set, add a nameplate photograph, the hours run per month and a description of when it trips. Send your generator requirement and we will come back with a restart sequence and the rating we would recommend.
Questions engineers ask
What size generator does a rice mill need?
For a typical 4 to 8 tonne per hour mill the answer usually lies between 250 and 500 kVA, but it depends on the largest whitener or polisher start, how many fans start together and whether the restart is loaded. The drive list and the restart sequence decide it, not the total horsepower.
Why does the generator trip when we restart after a power cut, but not in the morning?
Because the machines are full of grain. A loaded whitener or elevator needs far more starting torque and current than an empty one, and the step exceeds the set’s capacity. Empty the critical machines before restart, sequence the starts, or fit soft starters on the largest drives.
Do we need to switch off the capacitor bank on generator?
Yes. A fixed capacitor bank on a lightly loaded generator pushes the alternator into leading power factor, which destabilises voltage and can damage the AVR and sensitive electronics. Interlock the bank with the changeover so it is disconnected automatically.
Can the colour sorter run directly on the generator?
It can run on generator supply, but it should be fed through a UPS or a good servo stabiliser so that the dips caused by motor starts do not reach its electronics. Its air compressor should start unloaded and early in the sequence.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.