If your generator voltage is normal at no load but drops when load is applied, first check the actual load and watch the frequency at the same time.
In our generator testing, this is usually the fastest way to narrow down the problem. If voltage and frequency both drop, I normally check the load and engine speed first. If frequency stays stable but voltage falls, I move to the AVR sensing, excitation system, rotating diodes, and alternator.
Do not replace the AVR just because the voltage drops under load. The AVR is only one possible cause.
What Should You Check First?
When a generator cannot maintain voltage under load, I normally check it in this order:
- Confirm the no-load voltage and frequency.
- Apply the load gradually, if it is safe to do so.
- Watch voltage and frequency at the same time.
- Check the actual current, kW, or kVA against the generator rating.
- See whether the voltage drops with all loads or only with a specific load.
- If frequency remains stable, move to the AVR sensing and excitation system.
I prefer this order because it rules out the simple external causes before we start testing electrical components inside the alternator.
Quick Diagnosis: What Does the Voltage Drop Tell You?
| What You See | Likely Direction | What to Check Next |
|---|---|---|
| Voltage and frequency both drop | Overload or engine speed | Reduce load and check RPM/frequency |
| Voltage drops but frequency stays stable | AVR or excitation | Check AVR sensing and excitation system |
| Voltage dips during motor starting, then recovers | High starting current | Check motor starting demand and generator sizing |
| Voltage drops and stays low under load | Generator cannot support the load correctly | Check load, engine, AVR and excitation |
| One phase drops more than the others | Load imbalance or electrical fault | Compare phase currents and voltages |
| Generator voltage is normal but load-side voltage is low | Cable or connection problem | Check cable size, length and connections |
1. Is the Generator Overloaded?
This is one of the first things I check.
A generator may produce the correct voltage at no load but fail to maintain it when the connected load becomes too high.
Do not judge the load only by the kW rating of the equipment. Check the actual generator current, total kW or kVA, and power factor where possible.
Motors, compressors, pumps, and similar equipment can also draw much more current during starting than during normal operation.
This means a generator may be large enough for the running load but still have difficulty starting a large motor.
What to Do
Reduce the load and test again.
If the voltage returns to normal after part of the load is removed, I would investigate overload, starting current, power factor, or generator sizing before touching the AVR.
If the generator voltage is already low before any load is applied, that is a different troubleshooting path. See Why Is My Generator Producing Low Voltage?.
2. Does the Frequency Drop at the Same Time?
I consider this one of the most useful checks for this fault.
Do not watch voltage alone.
When the load is applied, look at the generator frequency at the same time. Frequency is directly related to engine speed, so it gives us another clue about what is happening.
If voltage and frequency fall together, I normally investigate the engine and load side first.
Possible causes include:
- Generator overload
- Engine RPM dropping
- Governor response problems
- Insufficient engine power
- Fuel or air supply problems
- A large load being applied too quickly
The AVR can respond to changing load, but it cannot fully compensate for an engine that is losing too much speed.
What to Do
Check whether the engine reaches the correct rated speed at no load and whether frequency drops excessively when load is applied.
If frequency falls together with voltage, solve the speed or overload problem first.
This is why, when someone tells me, "The generator voltage drops when I add load," my first question is usually:
What happens to the frequency?
That answer can save a lot of unnecessary electrical troubleshooting.
3. Does the Voltage Drop for a Moment or Stay Low?
These are not always the same problem.
A short voltage dip can be normal when a large motor, compressor, pump, or other high-inrush load starts.
The important question is whether the voltage recovers after the equipment starts.
If voltage drops briefly and then returns close to its normal value, I would first look at:
- Motor starting current
- Starting method
- Generator transient response
- Size of the load step
- Generator sizing
If the voltage drops and remains low while the load is running, the problem needs more investigation.
In that case, overload, engine speed, AVR response, excitation, or alternator condition becomes more relevant.
I would not treat every short voltage dip as a generator fault.
4. What If Frequency Is Stable but Voltage Still Drops?
This changes the direction of the diagnosis.
If the engine maintains normal speed and frequency, but the output voltage falls significantly as the load increases, I start looking more closely at the voltage regulation and excitation system.
The main areas are:
- AVR sensing
- AVR output
- Excitation system
- Exciter
- Rotating rectifier and diodes
- Alternator internal condition
This is where the frequency check becomes useful.
Stable frequency tells us that the engine is probably maintaining speed, so there is less reason to start adjusting the governor or investigating an engine speed problem.
5. Can a Bad AVR Cause Voltage to Drop Under Load?
Yes, it can. But I would not replace the AVR without checking the surrounding circuit first.
The AVR monitors alternator output voltage and controls excitation as the load changes.
If the AVR cannot provide the correct excitation response, the generator may produce normal voltage at no load but lose voltage as the load increases.
Possible AVR-related causes include:
- Loose sensing wires
- Poor terminal connections
- Incorrect sensing voltage
- Incorrect AVR adjustment
- Damaged AVR components
- Incorrect replacement AVR
- Problems elsewhere in the excitation circuit
What to Do
Start with the wiring.
Check for loose terminals, damaged wires, poor contacts, or signs of overheating around the AVR and alternator terminal box.
If the AVR has adjustable settings, do not change them randomly. AVR settings and adjustment procedures vary between manufacturers and models.
If you suspect the AVR itself, see How Do I Know If My Generator AVR Is Bad? before replacing it.
In our experience, replacing an AVR without checking why the voltage is dropping can solve nothing. In some cases, the new AVR ends up working under the same fault condition as the old one.
6. Can Weak Excitation Cause Voltage Drop Under Load?
Yes.
As generator load increases, the alternator needs sufficient excitation to maintain its output voltage.
A weak excitation system may still be able to build normal voltage at no load. The problem only becomes obvious when more current is required from the alternator.
Depending on the alternator design, possible causes include:
- Exciter problems
- Poor excitation connections
- Rotating diode failure
- Rotating rectifier problems
- AVR output problems
- Internal alternator faults
What to Do
Before making excitation measurements, identify the alternator and AVR model.
Different alternators use different excitation arrangements, so there is no single test procedure that should be applied to every generator.
Some excitation tests also require access to live electrical circuits or rotating components. These should only be carried out by technicians who understand the specific alternator design.
I do not recommend field flashing, bypassing an AVR, or applying an external DC supply unless the correct procedure for that alternator is known.
7. Can Failed Rotating Diodes Cause Voltage Drop Under Load?
Yes.
Rotating diodes are part of the brushless excitation system used on many industrial alternators.
A diode fault does not always mean the generator will produce zero voltage. In some cases, the alternator can still build voltage at no load but cannot maintain normal excitation as load increases.
Other symptoms may include:
- Poor voltage regulation
- Low voltage under load
- Abnormal heating
- Unstable output
- Reduced load capability
What to Do
If the engine speed is correct, the generator is not overloaded, and the AVR circuit appears normal, rotating diode testing becomes more relevant.
I would put this check after the basic load, frequency, wiring, and AVR checks, not at the beginning.
Testing rotating diodes normally requires the generator to be stopped, isolated, and the diode connections accessed correctly.
8. Could the Alternator Windings Be the Problem?
Yes, but winding failure is not my first conclusion.
Possible internal alternator problems include:
- Damaged windings
- Shorted turns
- Insulation deterioration
- Loose internal connections
- Phase winding problems
A winding problem can become more obvious under load because current and temperature increase.
You may also see abnormal heating, phase imbalance, unusual smell, or one phase behaving differently from the others.
What to Do
Where applicable, compare the phase-to-phase and phase-to-neutral voltages.
Also compare the phase currents.
If one phase is carrying much more load than the others, correct the load imbalance before assuming there is an alternator winding fault.
If the imbalance remains, more detailed winding resistance and insulation testing may be required.
These tests should be carried out with the generator safely isolated and according to the alternator manufacturer's procedure.
9. Could the Load Itself Be the Problem?
Yes, and this is sometimes overlooked.
I always want to know what was connected when the voltage started to drop.
A resistive heater, induction motor, compressor, UPS, and nonlinear electronic load do not behave in the same way.
Large motors can demand several times their normal running current during starting. Loads with poor power factor can also require more current from the alternator than the kW figure alone suggests.
A generator that runs a resistive load without any problem may behave very differently with a large motor or other difficult load.
What to Do
Ask:
- Does the voltage drop with every load?
- Does it happen only when one machine starts?
- Does the voltage recover after starting?
- What is the starting current?
- What is the power factor?
- How much load is applied in one step?
- Is the generator correctly sized for this load?
If the voltage drops only with one specific machine, I would investigate that load before replacing generator components.
10. Can Cables and Connections Cause Voltage Drop Under Load?
Yes.
Undersized cables, long cable runs, loose terminals, damaged connectors, or poor contact points create more voltage drop as current increases.
At no load, the voltage may look completely normal because very little current is flowing.
Once the generator is loaded, the voltage loss becomes much easier to see.
What to Do
Compare the voltage at the generator output terminals with the voltage measured at the load.
If the generator terminal voltage remains normal but the load-side voltage is low, the problem is probably downstream of the alternator.
Check:
- Cable size
- Cable length
- Terminal tightness
- Circuit breakers
- ATS connections
- Busbar joints
- Damaged cables
One thing I would not do is increase the AVR voltage setting just to compensate for excessive cable voltage drop.
That may raise the generator terminal voltage too high, especially when the load is removed.
11. What If the Voltage Drops and Fluctuates?
A steady voltage drop and unstable voltage are different symptoms.
If the voltage repeatedly rises and falls after load is applied, I would also check:
- Unstable engine RPM
- AVR sensing
- Loose wiring
- Excitation problems
- Rapidly changing loads
- Poor electrical connections
For that condition, see Why Does Generator Voltage Fluctuate?.
Separating these two symptoms helps avoid treating an unstable regulation problem as a simple overload condition.
How We Normally Diagnose Voltage Drop Under Load
When we test a generator, I prefer to work from the outside inward.
My normal sequence is:
1. Confirm the no-load voltage and frequency.
Make sure the starting condition is correct.
2. Apply load gradually.
Watch when the voltage begins to drop.
3. Watch frequency together with voltage.
If both fall, check load and engine speed first.
4. Check the actual load.
Look at current, kW or kVA rather than guessing from the connected equipment.
5. Check the load characteristics.
Motor starting, power factor and large step loads can make a big difference.
6. Check cables and connections.
Especially if generator terminal voltage and load-side voltage are different.
7. Check AVR sensing and excitation.
This becomes more important when frequency remains stable but voltage does not.
8. Check rotating diodes and alternator internal components.
Move to these tests after the simpler causes have been ruled out.
I prefer this sequence because it reduces unnecessary adjustment, disassembly, and parts replacement.
Can a Load Bank Test Help?
Yes. For this particular problem, a load bank test can tell us a lot.
If the generator runs normally at no load but voltage drops with the actual site equipment, we need to know whether the problem follows the generator or the load.
With a controlled load bank, we can increase the load step by step while watching voltage, frequency, current, and engine response.
If the generator carries the controlled load normally but the voltage drops with the actual site load, I would look more closely at the site equipment, starting current, power factor, cables, and distribution system.
If the voltage also drops during the controlled load bank test, then the generator itself needs further diagnosis.
This is why we use load testing not only to confirm generator capacity, but also to help separate generator problems from load-side problems.
Learn more about diesel generator load bank testing.
When Should You Stop Testing?
Stop the generator and investigate further if you see:
- Severe or sudden voltage drop
- Excessive current
- Abnormal alternator heating
- Burning smell or smoke
- Abnormal noise
- Severe phase imbalance
- Repeated breaker trips
Live voltage and excitation measurements can expose technicians to dangerous electrical energy.
If diagnosis requires opening the alternator, accessing rotating components, measuring excitation circuits, or working around live terminals, the work should be done by a qualified technician familiar with that alternator and AVR system.
Still Cannot Find the Cause?
If the generator produces normal voltage at no load but still cannot maintain it under load, I would go back to three basic questions:
Did the frequency drop? Is the generator actually overloaded? Does the same problem happen with a controlled load?
Those three answers often tell us where to look next.
If a generator passes a controlled load bank test but the voltage drops with the actual site load, the problem may not be inside the generator at all. The load characteristics, starting current, power factor, cables, and distribution system also need to be considered.
If you are still unable to isolate the cause, you can send us the generator model, alternator model, AVR model, no-load voltage, loaded voltage, frequency, load current, and approximate load. With that information, we can usually give you a clearer direction on what should be checked next.
