Author: Site Editor Publish Time: 2026-01-28 Origin: Site
It is a scenario every facility manager dreads. You are running a routine monthly test, or worse, operating during a blackout, when the generator suddenly trips. The control panel flashes a critical alarm—often code 81O or "High Hz." The unit shuts down immediately, plunging the facility back into darkness or leaving you scrambling to reset the controller. This is not a random nuisance trip. It is a protective intervention designed to save your engine from self-destruction.
The root of this problem lies in the rigid physical relationship between engine speed and electrical output. Unlike voltage, which is regulated by an Automatic Voltage Regulator (AVR), the frequency of a generator is locked directly to the engine's RPM. If the engine spins too fast, the frequency spikes. While operators often blame "load dumping," the reality is frequently more complex, involving mechanical governor failures, fuel instability, or incorrect calibration.
Ignoring these trips is dangerous. They signal that your power system is losing control of its kinetic energy. In this guide, we move beyond basic theories to explore the mechanical and operational root causes of over frequency, how to diagnose them, and how to protect your critical downstream equipment from invisible damage.
RPM = Hz: Frequency is purely a function of engine speed. A 2-pole generator must spin at exactly 3000 RPM (50Hz) or 3600 RPM (60Hz).
The #1 Cause: Sudden "Load Rejection" (removing a heavy load instantly) is the most common operational cause, causing the engine to over-speed before the governor catches it.
Mechanical Culprits: If load is stable, the issue usually lies in "Governor Hunting," fuel line air leaks, or sticky actuator linkages.
Equipment Risk: High frequency causes magnetic saturation and overheating in transformers and motors connected to the system.
To diagnose a frequency fault, you must first understand the physics governing your equipment. Frequency is not an electrical variable you can adjust with a dial; it is a mechanical result of the engine's rotation speed. We define this relationship with a specific formula.
The physics are non-negotiable. When calculating the frequency of a generator, we use the equation: F = (P × N) / 120.
In this formula, F represents frequency (Hz), P is the number of magnetic poles in the alternator, and N is the engine speed in RPM. For a standard 4-pole generator to produce 60Hz, the engine must maintain exactly 1800 RPM. If the engine speed creeps up to 1900 RPM, the frequency rises to 63.3Hz. This mechanical lockstep means that any inability to control fuel flow directly results in a frequency error.
Not all high-frequency events are the same. Facility managers must distinguish between a temporary "overshoot" and a dangerous "runaway" situation. The table below outlines the critical differences.
| Feature | Transient Overshoot | Mechanical Runaway |
|---|---|---|
| Definition | A temporary spike occurring immediately after a large load is removed. | Uncontrolled acceleration where the engine continues to speed up regardless of load. |
| Duration | Lasts 1–5 seconds before stabilizing. | Continuous until safety shutdown or failure. |
| Typical Cause | Operational: Load dumping or slow governor response. | Mechanical: Stuck fuel rack, broken governor springs, or fluid ingress. |
| ISO 8528 Limit | <10% deviation is often acceptable for Performance Class G2. | Any sustained operation >115% of rated speed is critical. |
Overshoot is a reaction. When a heavy load drops, the engine has excess momentum and fuel. It revs up briefly before the governor cuts the fuel. Runaway, however, is a failure of the control loop itself. Modern controllers have an "Overshoot Trip" to catch this early, but a true mechanical runaway can lead to catastrophic engine disassembly if the air intake shutoff valves fail to activate.
When the alarm sounds, the immediate question is: "Did the load change, or did the engine fail?" We can categorize the causes into four distinct buckets.
This is the most frequent culprit. Diesel engines produce torque by injecting fuel. If a generator is operating at 80% capacity, the fuel rack is wide open to maintain speed against that resistance. If a large elevator bank stops, a chiller cycles off, or a transfer switch opens suddenly, that resistance vanishes instantly.
The engine still has a large volume of fuel in the combustion chambers and the fuel lines. This energy has nowhere to go but into accelerating the flywheel. The engine spins up rapidly. If the governor is too slow to react, the RPM crosses the safety threshold (usually 55Hz for 50Hz units, or 66Hz for 60Hz units), and the breaker trips.
Sometimes the load is perfectly stable, yet the engine sounds like it is "breathing"—revving up and down rhythmically. This is called hunting.
Hunting occurs when the governor fights to find the correct speed but constantly overshoots the target. In electronic governors, this often points to incorrect PID (Proportional-Integral-Derivative) settings. If the "Gain" is set too high, the controller reacts too aggressively to minor speed changes, causing a pendulum effect. In older mechanical governors, worn flyweight springs or sticky ball bearings prevent smooth movement, causing the engine to surge erratically between 48Hz and 55Hz.
A stable frequency requires a stable fuel supply. If air enters the fuel lines through a loose fitting or cracked hose, the fuel density fluctuates. The engine controller tries to compensate for these "lean" pockets by opening the throttle further. When a slug of solid fuel finally arrives, the engine surges violently, spiking the frequency.
Physical linkages also play a role. If the rod connecting the actuator to the fuel injection pump is binding due to dirt or corrosion, it may not return to the "low fuel" position quickly enough when load is removed. This physical lag creates a dangerous delay in speed control.
In some baffling cases, the engine is running fine, but the controller thinks it is over-speeding. This is often due to RFI (Radio Frequency Interference) or EMI (Electromagnetic Interference). High-voltage cables running too close to the Magnetic Pickup Unit (MPU) wires can induce noise. The governor interprets this noise as extra speed pulses, falsely calculating a high frequency and tripping the unit.
Similarly, intermittent earth faults can cause torque shocks. Engineering forums frequently discuss cases where a ground fault creates a momentary heavy load that clears instantly, effectively simulating a load rejection event that confuses the governor.
Operators often reset an over-frequency alarm thinking, "At least the voltage didn't spike." This is a dangerous misconception. High frequency is a silent killer of facility infrastructure, even if voltage remains normal.
Transformers and induction motors operate based on a specific Volts-per-Hertz (V/Hz) ratio. They are designed to manage magnetic flux at a specific density. When frequency rises while voltage remains constant, this ratio is disrupted. However, if voltage also rises (which often happens during generator overshoot), the core of the transformer can enter "magnetic saturation."
Saturation causes stray magnetic flux to escape the core and heat up the steel laminations and surrounding tank. This results in rapid overheating and insulation degradation. A transformer subjected to repeated frequency spikes will fail prematurely, often months after the event.
The speed of any AC induction motor is dictated by the frequency of the power supply. A 10% increase in generator frequency results in a 10% increase in motor speed.
For a centrifugal pump or fan, the power required to drive the load increases with the cube of the speed. A 10% speed increase does not mean 10% more load—it requires roughly 33% more power. This can overload the motor windings, shear mechanical shaft couplings, or burst high-pressure water pipes that were not rated for the increased flow rate.
Data centers rely on Uninterruptible Power Supply (UPS) systems to bridge the gap during power failure. However, modern UPS units are highly sensitive. If the generator power is "dirty" with unstable frequency, the UPS will reject it to protect the servers. It will remain on battery power, eventually draining completely even though the generator is running. This defeats the entire purpose of having a backup generator.
Before you call a technician, you can narrow down the problem using this isolation framework. This logic helps determine if the issue is the machine or the building.
Action: Open the main output breaker so the generator is completely disconnected from the facility load. Start the engine manually.
Decision Logic:
If frequency is unstable without load: The problem is internal to the Generator. Focus your diagnosis on the Governor, Fuel System, or Filters.
If frequency is stable without load: The generator is likely healthy. The problem lies in the Facility Load Profile (move to Step 2).
Action: If possible, use a resistive Load Bank to apply load in controlled increments (25%, 50%, 75%).
Observation: Watch the frequency meter carefully. Does the unit trip only when load is removed?
Root Cause: If the trip happens specifically during load removal, your facility is dumping too much load at once. This is an operational issue, not a mechanical failure.
If the unit failed the No-Load test, perform these physical checks:
Check MPU (Magnetic Pickup Unit): Unscrew the speed sensor from the flywheel housing. Wipe off any metal shavings. Re-install it and ensure the gap distance is adjusted to manufacturer specs (usually backing off 1/2 to 3/4 turn after contact).
Check Linkages: Disconnect the actuator arm. Move the fuel rack by hand. It should feel smooth with no binding, gritty spots, or friction.
Check Static Frequency: Is the base setting simply too high? A no-load setting of 53Hz (for a 50Hz unit) leaves very little room for error. Adjusting it down to 51.5Hz gives the system more headroom to absorb spikes.
Once you have identified the source, the remediation strategy depends on budget and severity. Use this guide to decide your next move.
If the issue is load rejection, you don't need engine repairs; you need load management.
Load Stepping: Reprogram your Automatic Transfer Switches (ATS) or Building Management System (BMS). Instead of bringing all elevators and chillers online (or offline) simultaneously, stagger them. shedding load in stages reduces the "kick" the engine receives.
Base Idle Adjustment: If the generator is mechanically sound but trips on tight tolerances, lower the "No-Load" frequency setting slightly. Ensure it stays within the lower limit of the specification (e.g., ISO 8528 Class G2) to allow a larger buffer for overshoot.
If the governor is hunting, professional maintenance is required.
Governor Tuning: A qualified technician can connect a laptop or use a multimeter to tune the PID loop. Adjusting the "Gain" and "Stability" potentiometers can smooth out the response, eliminating the oscillation that triggers alarms.
Fuel System Bleeding: Replace all fuel filters and water separators. Bleed the high-pressure lines to remove trapped air. This often cures the erratic "surging" behavior found in older diesel units.
For chronic issues on critical sites, hardware upgrades offer the best protection.
Mechanical to Electronic Conversion: Older generators with mechanical flyweight governors react slowly. Upgrading to an electronic actuator and controller system provides millisecond-level reaction times, drastically reducing overshoot during load changes.
Flywheel Sizing: In specialized industrial applications with highly variable loads (like rock crushers or large welders), standard generators struggle. Increasing the flywheel mass adds inertia, physically dampening speed fluctuations and keeping frequency stable through sheer kinetic energy.
Generator over frequency is rarely a random ghost in the machine. It is almost always a symptom of a mismatch between the engine's fuel reaction time and the facility's load changes. Whether it is caused by a sticky actuator, air in the fuel lines, or a massive load rejection event, the physics remain the same: excess RPM equals excess Hertz.
Do not simply reset the alarm and walk away. Frequent over-frequency trips indicate a system struggling to maintain stability. Over time, this instability will damage expensive downstream motors, overheat transformers, and potentially destroy the engine itself. We recommend conducting a professional Load Bank Test to verify governor response. Diagnosing the issue now is far cheaper than replacing a melted transformer or a thrown connecting rod later.
A: Standard generators operate at 50Hz or 60Hz depending on the region. However, under "Droop" control, the frequency is typically set slightly higher (e.g., 51.5Hz or 61.5Hz) at no-load and settles to the rated frequency at full load. A variance of 3-5% is generally considered normal operation.
A: Directly, no. A bad battery usually causes starting failures. However, if the DC voltage supplied to an electronic governor controller is unstable or fluctuating, it can cause the controller to behave erratically, potentially leading to unstable speed control and frequency hunting.
A: This phenomenon is called "Hunting." It occurs when the governor is trying to find the correct speed but is over-correcting. It is usually caused by incorrect "Gain" or "Stability" settings on the governor controller, or worn springs and sticky linkages in mechanical systems.
A: They damage different things. Under frequency (low RPM) often causes the engine to stall or overheat due to poor cooling airflow. Over frequency (high RPM) is generally more dangerous to the load, causing transformers to overheat and motors to spin dangerously fast, risking mechanical bursting.
