In short. Frequency dip is the momentary fall in generator speed, and therefore output frequency, when real power is applied faster than the engine can increase its torque. It depends on the size of the kW step, the engine’s load acceptance, turbocharger response, rotating inertia and governor tuning. A turbocharged diesel set typically accepts 50 to 70 percent of its rating in one step within ISO 8528-5 class G2 limits; larger steps must be staged or the set sized for them.
What frequency dip is and why it is an engine problem
The alternator’s output frequency is locked to its shaft speed: 1,500 rpm on a four-pole machine gives 50 Hz. When a block of load is applied, the electrical torque on the shaft rises instantly, but the engine cannot increase its fuel and air supply instantly. For a fraction of a second the extra torque comes from the kinetic energy of the rotating assembly, which slows down. The governor senses the speed error, opens the fuel rack and the engine recovers. The depth of the excursion is the frequency dip; the time to settle near 50 Hz is the recovery time.
Voltage dip is mostly an alternator and excitation matter driven by reactive current. Frequency dip is mostly an engine matter driven by real power. A motor start produces both; a resistive block load such as a heater bank or a UPS taking load produces mainly a frequency event.
What decides the depth of the dip
- The size of the real-power step in kW relative to the engine’s rating, which is the dominant factor.
- Whether the engine is naturally aspirated or turbocharged; a turbocharger needs exhaust energy to spin up, so a turbocharged engine at light load lags before it can burn extra fuel cleanly.
- The engine’s load before the step. An engine already at 50 percent load has its turbocharger spinning and accepts a further step more readily than one idling.
- Rotating inertia of the engine, flywheel and alternator, which buys time for the governor.
- Governor type and tuning. Mechanical governors are slower and have inherent droop; electronic isochronous governors respond faster but hunt if poorly set.
- Fuel system response, ambient temperature and altitude, which together set how quickly the engine can take fuel without smoke.
Performance classes and typical acceptance
ISO 8528-5 defines four performance classes for generating sets, each with limits on transient deviation and recovery time. A specification should name the class rather than invent a percentage.
| Class | Transient frequency deviation on load increase (typical) | Frequency recovery time (typical) | Typical application |
|---|---|---|---|
| G1 | Up to about 15 percent | Up to 10 seconds | Simple resistive and lighting loads |
| G2 | Up to about 10 percent | Up to 5 seconds | General industrial and commercial loads, most Indian sites |
| G3 | Up to about 7 percent | Up to 3 seconds | Telecoms, UPS-fed IT, sensitive drives |
| G4 | Agreed between manufacturer and customer | Agreed | Specialised, very sensitive loads |
The figures are indicative and must be confirmed against the current edition of the standard and the manufacturer’s declaration. The class applies to a defined load step, not to any load the user chooses to apply. Manufacturers also publish load acceptance, the largest single kW step the engine can take from a stated initial load while staying within class. For modern turbocharged engines this is commonly 50 to 70 percent of standby rating from no load, and sometimes less for highly rated CPCB IV+ engines. Naturally aspirated engines, now mostly small sets, often accept 100 percent. The turbocharging guide explains the mechanism.
Why it matters to the load
Most motors, heaters and lighting are indifferent to a brief frequency excursion. The loads that care have a frequency window. A UPS will transfer to battery if the input frequency leaves its window, often plus or minus 2 to 5 percent, or changes faster than 1 to 3 Hz per second, so every generator load step cycles the battery. Synchronising relays and some medical and process controllers behave similarly, and paralleled sets cannot be synchronised while frequency is moving. Sites with these loads should specify G3 and set the UPS input window to the generator’s declared performance rather than the grid’s.
Managing block loads
The practical response is to control the size and sequence of load steps rather than buy a very large engine. A transfer switch that closes the whole building onto the set in one step applies a block load equal to the entire running demand, often far above the engine’s acceptance, which is why sets that tested well on a load bank trip on the first real transfer. Splitting the load into two or three sections closed a few seconds apart, with the largest motors last, keeps each step within acceptance; the load management page describes the controls. Jacket water heaters improve acceptance because a cold engine responds slowly. Where the load is inherently one large step, such as a UPS carrying a data hall, size the set on load acceptance rather than running kW.
Testing and recording the event
Frequency response can only be proved with a step test. Apply a defined load step with a resistive load bank, or the actual plant load section, from a defined initial condition, and record frequency with a recorder fast enough to catch the minimum. Note coolant and ambient temperature, initial load, step size, minimum frequency, recovery time to within 1 percent of nominal and any overshoot on load rejection. Repeat for the largest step the site will actually produce, not only the manufacturer’s standard step, and agree the acceptance values beforehand. Smoke on load application shows a turbocharger that has not caught up. The load bank testing service covers this procedure.
A worked example (illustrative)
A hospital in Hyderabad has a 500 kVA, 400 kW standby set feeding, among other things, a 120 kVA UPS for theatres and ICU. On mains failure the ATS closes the essential bus onto the set in one step of 250 kW, about 62 percent of engine rating, on a warm but unloaded engine whose declared acceptance is 60 percent. The dip reaches about 10 percent, frequency falls to around 45 Hz, and the UPS, with its input window at plus or minus 3 Hz, transfers to battery on every outage. Splitting the transfer so that lighting and HVAC close first and the UPS bus follows five seconds later reduces the second step to about 25 percent on an engine whose turbocharger is already spinning. The dip falls to around 4 percent, the UPS stays on generator and the batteries stop cycling, with no change to the generator.
Related photographs and videos
Common mistakes
- Transferring the whole site load in one step, which applies a block load above the engine’s acceptance and trips a set that passed its load bank test.
- Specifying a dip percentage with no reference to ISO 8528-5, so there is no agreed basis for acceptance when the result is disputed.
- Leaving the UPS input frequency window at grid settings, so the UPS cycles to battery on every generator load step and the batteries age early.
- Testing only at the manufacturer’s standard step and not at the site’s largest real block load, so the problem appears on the first outage instead of at commissioning.
What to send us
Send the generator nameplate and engine model, how load is transferred to the set and in how many steps, the kW of the largest single step including any UPS and its battery recharge, the UPS make and its input frequency settings, and any alarm logs from previous outages. If the set is yet to be bought, send the load inventory with loads marked by transfer group. Send your application engineering enquiry.
Questions engineers ask
How much load can a diesel generator take in one step?
Typically 50 to 70 percent of the standby rating for a turbocharged engine from no load within class G2, and closer to 100 percent for a naturally aspirated engine. The exact figure is declared by the engine manufacturer and depends on the initial load and the ambient.
Why does the frequency dip but the voltage stay stable?
Because the applied load is mostly real power, such as heaters, UPS or a motor that has already reached speed. Real power loads the engine, which cannot respond instantly, while the AVR holds voltage comfortably because the reactive demand is small.
Will an electronic governor cure frequency dip?
It shortens recovery and removes droop, but it cannot make the turbocharger spin up faster or create torque the engine does not have. Load acceptance is set by the engine; the governor determines how cleanly it returns to 50 Hz.
Why does my generator smoke when load is applied?
The governor has added fuel before the turbocharger has supplied enough air, so combustion is briefly incomplete. A short puff on a large step is normal; sustained smoke suggests the step is beyond the engine’s acceptance, the air filter is restricted or the engine is derated by heat or altitude.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.