In short. A factory generator is sized from how the production line starts, runs and stops, not from the sum of motor nameplates. The decisive inputs are which machines and shared utilities must continue, the starting method and sequence of the motors, the voltage dip the drives and controls will tolerate, and how long the plant must run on diesel. Done properly, the set is often smaller than a nameplate total suggests and far more likely to hold the load.
Production continuity is a sequence problem
In a manufacturing plant, power failure stops output, scraps work in progress and can leave machines in a state that takes hours to recover. The useful first question is not how many kVA the plant has installed but what the plant needs to keep doing during an outage. Some units in the industrial belts around Hyderabad, Medchal and Patancheru need the whole line on diesel because the DISCOM supply is interrupted for hours. Others need only a safe shutdown: hydraulics released, furnaces held, extraction running, controls alive. These are different generator projects with different costs, and the plant owner decides which one applies. The sizing and selection method we use is set out in generator sizing.
Shared utilities complicate the picture. Compressed air, cooling water, chilled water, dust extraction and the plant network serve many machines at once. A CNC cell with power but no compressed air is as idle as one without power, so the utilities that the protected machines depend on must be on the essential bus with them.
Motors set the generator size
Induction motors draw five to eight times full-load current for the first second or so of a direct-on-line start, at a low power factor. A generator that carries the running load comfortably can still see a voltage dip of 25 to 30 per cent when a large motor starts, enough to drop contactors, fault VFDs and reset PLCs. The starting method and the order of starts therefore matter as much as the running total.
| Starting method | Typical starting current | Effect on generator | Where it is found |
|---|---|---|---|
| Direct-on-line | 6 to 8 times full-load current | Largest kVA step and voltage dip | Small motors, compressors, pumps, old machines |
| Star-delta | About 2 to 2.7 times | Lower first step, but a second transient at changeover | Fans, pumps, mixers up to about 100 kW |
| Soft starter | Typically 3 to 4 times, adjustable | Controlled ramp, still reactive | Conveyors, compressors, crushers |
| Variable frequency drive | Near full-load current or less | Small step, but harmonic current and possible regeneration | Extruders, machine tools, process fans |
Starting kVA = P_motor × (I_LR / I_FL) × k_starter / (η × PF)
P_motor is the motor rating in kW, I_LR / I_FL is the locked-rotor to full-load current ratio from the nameplate code, k_starter is the reduction factor for the starting method (1 for DOL, about 0.33 for star-delta), and η and PF are the motor’s full-load efficiency and power factor. A worked example (illustrative): a 75 kW compressor motor started DOL with a ratio of 6.5, efficiency 0.94 and power factor 0.87 demands about 75 × 6.5 / (0.94 × 0.87), close to 600 kVA for the start. On a 500 kVA set already carrying 200 kW this would cause an unacceptable dip; the same motor on a soft starter limited to 3.5 times needs about 320 kVA and may be acceptable if it is the first motor started. The full method, including alternator reactance and AVR response, is in motor starting and starting kVA.
Drives, controls and power quality
Modern plants pair heavy rotating loads with sensitive electronics. VFDs, servo drives and switch-mode power supplies draw non-sinusoidal current, and on a generator, whose source impedance is several times that of the utility transformer, the resulting voltage distortion is larger than the plant has ever seen on mains. Symptoms include drive over-voltage trips, flickering displays, nuisance tripping of electronic overloads and overheating of the alternator. The remedies are an alternator sized with margin for the non-linear share, line reactors or active filters on the largest drives, and PLCs and plant servers on a UPS so that a transient during transfer does not reset the line. See harmonics and THD.
Load steps and sequence
The AMF panel or plant PLC should restore loads in a planned order with timers between steps: utilities first, then the largest motors one at a time, then production cells. The first step should stay within the engine’s load acceptance, typically 50 to 70 per cent of rating for turbocharged sets. Starting everything at once is the most common reason a correctly sized factory generator trips on under-frequency.
Standby, prime or both
A set that runs a few hours a month carries a standby rating. A set that carries the plant through daily load relief or a weak rural feeder for hundreds of hours a year needs a prime rating, with a lower continuous output and different maintenance intervals. Many Telangana plants fall between the two, and the honest answer is to log the hours for a season before specifying. Where several sets share the load, paralleling with load-dependent start and stop keeps each engine above the 30 per cent loading where wet stacking begins. Fuel autonomy should be stated in hours at the plant’s measured demand, not at the generator rating, and new sets up to 800 kW must be CPCB IV+ compliant. See prime power generators and paralleling.
Testing and handing back to production
A no-load weekly run proves the engine. It does not prove that the line restarts. Before a plant depends on the set, a load bank test to full rating and a staged transfer test with the real production sequence should be recorded, including transfer time, lowest voltage and frequency, and any drive or PLC that faulted. The power scope should also say clearly where electrical restoration ends and process restart begins. Returning voltage to a busbar is an electrical acceptance; confirming that the extruder temperatures, hydraulic pressures and quality checks are back in range belongs to production. Planned maintenance on the set should be scheduled against production windows with a rental bridge where the plant cannot be without cover; see rental power for planned outages.
Common mistakes
- Adding motor nameplates and applying a single multiplier. The set is either too large and runs lightly loaded for years, or too small for the first compressor start.
- Restarting the whole plant in one step after transfer. The combined inrush exceeds the engine’s acceptance and the generator trips, extending the outage.
- Leaving compressed air and cooling water off the essential bus. The protected machines have power and still cannot run.
- Ignoring harmonic load from drives when choosing the alternator. The generator carries the kW but the drives trip on distorted voltage and the alternator runs hot.
- Buying a standby-rated set for a plant that runs it daily. The engine wears out early and the warranty position is weak.
What to send us
Send the motor list with kW, starting method and the order in which machines must restart, the shared utilities the protected machines depend on, a single-line diagram or at least the main switchboard arrangement, and the hours per month you expect the set to run. For an existing generator, add a nameplate photograph, the controller event log and a description of what trips and when. Send your application details and we will come back with the starting sequence and rating we would propose.
Questions engineers ask
How do I size a generator for a factory with many motors?
List the motors with their starting method, decide the order in which they start on generator, and size the set for the running load plus the largest single start at an acceptable voltage dip. The largest DOL motor started first usually governs, and soft starters or VFDs on that motor often reduce the set by a frame size.
Why does my generator trip when the compressor starts?
The compressor’s starting current, five to eight times its running current on DOL, causes a voltage and frequency dip beyond the set’s limits, or the compressor starts loaded against pressure. Staging the start, adding a soft starter or fitting an unloader valve usually cures it.
Can VFDs run properly on a generator?
Yes, if the alternator is sized for the harmonic load and the drives’ under-voltage and over-voltage settings tolerate generator regulation. Line reactors on the drives and an oversized alternator are the usual measures.
Should the plant have one large set or two smaller ones?
Two paralleled sets give maintenance cover and keep each engine better loaded at partial plant demand. One large set is simpler and cheaper to install. The choice depends on how much of the plant must run during maintenance of the generator.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.