Engine hunting, which is essentially an oscillation in engine speed, happens almost always as a result of one of the following two factors: either the use of an improperly configured governor for the given application or Woodward governor components which have gotten out of alignment. While speed control may be all about having the right governor used in your equipment, it really involves more than that, because having a working governor does not mean much if it is not properly calibrated for the given load.
This article is about how Woodward governors should be calibrated in order to provide stable operation. It describes the principles of droop and isochronous mode functionality and explains the reasons of engine hunting and frequency instability resulting from improper calibration.
Why Droop Exists in the First Place
It’s good to know the fundamentals here first, because droop is not a design constraint; it is designed into the governor for stability purposes. In absence of any sort of droop, natural behavior of speed control will result in instabilities in the process: an increase in load leads to decrease in engine speed, governor compensates by injecting more fuel, engine speeds up, maybe even overspeeding, governor takes it back down again, and so on. In the single engine situation, poor control of such cycle results in oscillatory behavior, while in the multiple generator case, several governors fight each other on controlling the frequency of common bus.
Droop allows stable decrease in steady-state engine speed due to increase in load it’s proportional and predictable correlation of load and speed, instead of trying to maintain steady speed under all circumstances.
Droop vs. Isochronous: Getting the Mode Selection Right
This is among the top reasons for instability of multi-generator plants; it is a configuration issue rather than a calibration issue. With an isolated system (where the generators operate without connection to the utility grid), only one unit out of all should be configured for isochronous operation where frequency would be maintained with precise accuracy through continuous adjustment of fuel level in order to compensate the difference. All the remaining generators should be configured to work in the droop mode with speed decreasing proportionally to their share of the load.
If there are more than one generator working in isochronous mode on the same bus, both generators will independently try to compensate frequency, causing instability and potentially tripping off units due to the overspeed and underspeed protections being triggered. It is necessary to verify explicitly that the system is configured properly (only one unit is isochronous and the rest work in the droop mode) upon commissioning and after changing the control system.
Calibrating Droop Percentage Correctly
Having selected a mode, calibration of the droop percentage is crucial, and any mistakes made in that regard will cause issues.
If there is too little droop (too close to isochronous calibration), the system will become unstable, because small frequency differences result in too great a difference in load sharing between the machines. Industrial governor applications always use droop calibration in a range that does not lead to such instability but provides load sharing accuracy nonetheless.
Too high a percentage of droop will decrease the accuracy of load sharing and cause a large frequency deviation when the application is fully loaded, because the droop calibration by definition provides stability at the cost of accuracy.
Equalizing the droop percentage of all paralleled units is just as important as selecting an appropriate percentage value. Two generators paralleled on one bus may have different percentages of droop, meaning that their speed-to-load curves would be different and that the load will not be shared proportionally between those two units, one working overloaded and the other underloaded, although both will be calibrated and working correctly individually.
Calibration Procedure Fundamentals
Even though the process of calibrating individual Woodward governors may differ based on the type of the governor in question and on the system used to control it, there are certain aspects that stay consistent with all the governors using droop calibration:
First comes no-load speed calibration. While the engine is still unloaded, the no-load speed calibration sets a basis for droop speed reduction when the load gets added. It is crucial to calibrate this speed precisely, as droop works as a percentual reduction from this reference point.
Next step full-load speed calibration. Calibration at full-load gives an endpoint of a droop curve, therefore giving its slope. In the case of governors with electronic sensing of the actuator position, it actually means teaching the control system the actuator position for full load, which is why the full-load speed calibration value always should be higher than no-load speed calibration one.
Load sharing should be tested at multiple load points, and not only at full load. Once a system has been calibrated initially, testing load sharing at additional load points, such as 25%, 50% and 75% of system capacity, proves that the droop function operates proportionally during the whole range of its operation, and not only at the edges, which were used for its calibration.
Consider non-linear fuel/power characteristic when testing load sharing on diesel and heavy fuel engines. In many cases, the relationship between fuel position/gate and output is not strictly linear across the whole load range the same shift in fuel position will yield different output changes depending on whether the engine operates at low or high load. Non-linearity in this case may get integrated into the droop function, and lead to unequal load sharing precisely at low load levels.
Common Sources of Hunting Beyond Droop Misconfiguration
Not all hunting problems stem from the droop calibration process. Here are a few more things that may lead to instability in speed, despite the fact that everything else is configured perfectly.
Worn Woodward governor components. Wearing of some governor component, like a flyweight or linkage or hydraulic part, may cause delay or lack of consistency when governor is responding, effectively nullifying proper droop calibration. This explains why the problem may be related both to calibration issues and mechanical wear, but should be solved in completely different ways.
Electrical interference in speed signal lines. If the governor uses magnetic pickups to receive speed feedback, it will be vulnerable to any interference on those lines, causing governor to react to non-existent changes in speed, thus generating a hunting that has nothing to do with calibration at all.
Lack of stabilization or correction of droop settings. Many governor designs have a special stabilizing system for dampening of correction, and if it either doesn’t work anymore or was not properly adjusted during commissioning, it may cause hunting despite all other aspects being perfect.
Calibration Checklist for Stable Load Management
Proper droop calibration is essential for stable load sharing between paralleled generator units. The following checks help confirm that each unit is correctly configured, calibrated, and communicating reliable speed-feedback signals.
| Calibration Step | What to Verify |
|---|---|
| Mode Configuration | Exactly one unit is configured for isochronous operation, while all other units are set to droop mode |
| No-Load Speed Calibration | Establish an accurate no-load speed reference before making droop adjustments |
| Full-Load Calibration | Set the full-load reference correctly above the no-load reference to define the required droop slope |
| Droop Percentage Matching | Confirm that all paralleled units use the same droop percentage |
| Load-Sharing Verification | Check load sharing at multiple points — 25%, 50%, 75%, and 100% — rather than only at full load |
| Signal Integrity | Inspect magnetic pickup and feedback wiring for electromagnetic interference or signal noise |
| Documentation | Record final droop, no-load, and full-load settings for each generator unit |
Why Documentation Matters as Much as the Calibration Itself
An undocumented calibration is a calibration that in due time needs to be done again from scratch, often in conditions even more unfavorable than in the first case. Noting down the final droop percentage, the reference value in no-load mode and full-load mode, along with the date of calibration for each unit, gives your maintenance personnel something to compare when troubleshooting later on, and also simplifies the process of confirming whether or not all of the settings are synchronized across the units, should one of them require servicing.
Sourcing Woodward Governor Parts for Calibration Work
Calibration is partially dependent on having the Woodward governor parts that function properly at the specified parameters in the first place the governor might have some worn-out parts inside that would make it hunt, even if the calibration process itself was done perfectly. Jawaid Enterprises provides you with the genuine and properly certified Woodward governor and actuator components that you would want to perform the calibration on.
Frequently Asked Questions
What is the distinction between droop and isochronous governor operations?
The isochronous operation keeps the engine speed at exactly the same level irrespective of the load, while the droop mode allows speed to drop proportionally when the load increases. Multi-unit generation systems have one isochronous unit among many running on droop mode to prevent the conflict over the frequency control.
Why do several generators have to have equal droop percentages?
With unequal droops, the slope of speed versus load characteristics of different units will be different and result in an uneven distribution of the load across the units because one generator will bear more load than others despite being properly calibrated.
Can the wear of mechanical parts lead to hunting if the generator is calibrated correctly?
Yes. Wear and tear of mechanical components, such as flyweights, connections, or hydraulic pistons, can distort the mechanical process, leading to improper functioning of the governor even if it is properly calibrated.
How often should governors’ calibration be checked?
It needs to be checked when the system is commissioned, as well as during the repair or replacement of components and within regular maintenance.
Should I try calibrating the governor myself, or get a technician involved?
Calibration has a direct impact on engine safety and stability of the electrical system, which is why it should be done by a technician who understands the workings of your particular governor, according to the procedure provided by the governor manufacturer for this particular unit.
Get Stable Load Management With Properly Calibrated Governor Parts
Efficient multi-engine load balancing is dependent not only on proper calibration but also on appropriate governor parts that physically can maintain it in real operating conditions. Jawaid Enterprises can supply you with reliable Woodward governor parts needed for this purpose. Just submit your system information for a simple parts suggestion.