A diesel engine coupled to an alternator, generating electricity when the grid supply fails.
01 / Definition
What a DG Set (Diesel Generator) is
A DG set is a diesel engine coupled to an alternator. The engine runs at a fixed speed — 1500 rpm for a four-pole alternator — because output frequency is tied directly to shaft speed. Hold the speed and you hold 50 Hz. A governor adjusts fuel to keep that speed as load changes, and an automatic voltage regulator adjusts the alternator's excitation to hold voltage steady.
The changeover gear is what makes it useful. It senses the grid failing, signals the set to start, waits for speed and voltage to stabilise, then transfers the load across. On the way back it waits to confirm the grid is genuinely stable before returning the load and letting the engine run on to cool down. Most DG failures are not the engine at all: they are a battery that could not crank it, or fuel that had degraded while waiting.
Also called
02 / Anatomy
The parts that matter
The components a technician reaches for when this machine stops working.
Diesel engine
The prime mover. Starting reliability depends on batteries, fuel condition and whether it is exercised under load.
Alternator
Converts shaft power to electricity. Its insulation resistance falls with moisture and age, especially on a set that sits idle.
Governor
Holds engine speed as load changes, which is what holds output frequency at 50 Hz.
AVR
Adjusts excitation to hold output voltage steady through load steps.
Starting batteries and charger
The most common single cause of a set failing to start when needed.
Changeover gear (AMF/ATS)
Senses grid failure, starts the set, transfers load, and reverses it all when the grid returns.
03 / Applications
Where it is used
Standby power for plants, hospitals, data rooms and continuous processes that cannot tolerate an outage.
04 / Failure modes
How it fails
What actually goes wrong on Indian shop floors, and what each fault looks like to the operator.
FM_01
Fails to start on demand
Flat or sulphated batteries, a failed charger, or stale fuel. The most common DG fault by a wide margin.
FM_02
Starts but will not take load
Governor or AVR fault, a fuel supply restriction, or the changeover gear not transferring.
FM_03
Frequency or voltage wandering
Governor hunting, a failing AVR, or injectors needing attention.
FM_04
Wet stacking
Long running at very light load leaves unburnt fuel in the exhaust, fouling injectors and the turbo over time.
FM_05
Overheating under load
Blocked radiator core, failed fan belt, low coolant, or insufficient ventilation in the DG room.
05 / Onsite
What our technician checks
A methodical path to root cause, not a scattergun of part swaps.
- 01Test battery condition under cranking load, not just resting voltage, and check the charger output
- 02Check fuel condition and the state of the filters, and look for water in the tank bottom
- 03Run an on-load test and confirm voltage and frequency hold within limits through the load step
- 04Watch the full changeover sequence in both directions, including the cooldown run before shutdown
- 05Check coolant level, radiator core cleanliness and fan belt tension
- 06Measure alternator insulation resistance, particularly on a set that has stood unused in humid conditions
06 / Get it fixed
Related services
The MachineryFix services that cover this equipment.
07 / Questions
Frequently asked questions
Why did the DG set fail to start during a power cut?+
In most cases the batteries. They sit on float charge for months and sulphate quietly, and resting voltage looks fine until the starter actually draws current. Test under cranking load, and check the charger is working rather than assuming it is.
How often should a standby DG set be run?+
Regularly, and on load rather than off load. Running it on no load does not properly exercise the engine and contributes to wet stacking. A periodic load test also proves the changeover gear still works, which is the part nobody discovers has failed until it matters.
What is wet stacking and why does it matter?+
Running a diesel for long periods at very light load leaves unburnt fuel and soot in the exhaust system. Over time it fouls injectors, the turbo and the exhaust, reducing output and eventually requiring cleaning. Load banking or a properly sized set prevents it.
Why does output frequency drift when a big load comes on?+
The governor is not holding engine speed through the load step — frequency follows speed directly. It may need adjustment or service, or the load step may simply be too large a fraction of the set's rating for the engine to absorb cleanly.
Written & reviewed by MachineryFix — last reviewed — questions? Call +91 70133 33007