What causes a voltage drop in a diesel generator set?

When your diesel generator suddenly shows low or unstable voltage, it can stop work, damage tools, and cause expensive delays.

A voltage drop or instability in a diesel generator set often comes from loose wiring, overload, or mechanical issues inside the alternator, such as rotor–stator contact.

In my years of troubleshooting generators, voltage complaints are the top service call. The cause can be simple, like a loose bolt, or serious, like mechanical rubbing between moving parts inside the generator head. The first step is to measure the voltage properly to confirm if it’s truly unstable. Then, check the main winding output, mechanical condition, and load level before replacing any parts.

How do you check if generator voltage is stable?

Before repairing, you need to measure correctly.

Use a multimeter set to AC 750V to check the output while the engine is running under normal load.

I always start diagnosis with a proper voltage check. Set your multimeter to the AC 750V range. Connect it to the generator’s output terminals while the engine runs under a normal load. If the voltage reading fluctuates widely or changes with load, there’s a stability issue. If readings are consistent, the problem might be elsewhere, like the load equipment itself. This first test helps avoid unnecessary disassembly.

Step	Action	Tool Needed
1	Start generator under normal load	-
2	Set meter to AC 750V range	Multimeter
3	Measure output terminals	Multimeter
4	Note if voltage is steady or fluctuates	Notepad

A stable reading means the alternator output is likely fine, while a fluctuating reading needs deeper checks.

How do you inspect the generator’s main winding voltage?

If the external voltage is unstable, check inside.

Remove the alternator rear cover to measure the main winding voltage and inspect connections for looseness.

An overloaded generator causes both voltage and power to drop. The alternator cannot keep up with demand, and the engine slows slightly under strain. This drop is most obvious when heavy equipment starts. I always remind customers to match total load to the generator’s rated output and allow extra for startup surges. Using a clamp meter and wattmeter helps confirm if you’re exceeding the limit.

Cause	Effect	Solution
Overload	Voltage and power drop	Reduce connected load
Inrush surge	Short-term voltage dip	Use larger capacity unit

Running within limits keeps voltage stable and protects both engine and alternator.

What is “rotor–stator rubbing” and how does it cause voltage drop?

Some drops are due to mechanical faults inside the alternator.

Rotor–stator rubbing happens when the rotor touches the stator, often caused by loose stator fixing bolts, leading to sparks and power loss.

I’ve seen cases where voltage drops suddenly, and inside the alternator you can see sparks while running. This happens when the stator shifts out of alignment and touches the spinning rotor. The gap between them is very small, so any looseness can cause contact. The fix is to tightly re-secure the four stator fixing bolts, ensuring the rotor spins freely. If left unfixed, this rubbing can destroy both parts.

Symptom	Cause	Repair
Sparks in alternator	Loose stator bolts	Tighten and re-align stator ends
Voltage & power drop	Mechanical contact	Inspect gap, prevent rubbing

Always check for sparks or abnormal noise when diagnosing voltage drop.

Conclusion

A voltage drop in a diesel generator often comes from loose wiring, overload, or rotor–stator contact. Accurate testing and step-by-step checks lead to fast and safe repairs.