In short. The main alternator of a diesel generator is a brushless synchronous machine whose replaceable parts are the bearings, the rotating rectifier diodes and surge suppressor, the exciter stator and rotor, the terminal hardware, temperature sensors, anti-condensation heaters and, when a winding fails, a rewind or a complete stator or rotor. Parts are identified from the alternator nameplate, frame size and serial number, never from the generator kVA alone.
What is inside a brushless alternator
Almost every diesel generator above a few kVA sold in India uses a brushless synchronous alternator. On the shaft sit the main rotor, which carries the field winding, the exciter rotor with its three-phase winding, and the rotating rectifier assembly that converts the exciter output into DC for the main field. Around them sit the main stator, which produces the 415 V output, and the exciter stator, whose small DC field is controlled by the AVR. Where fitted, a permanent magnet generator on the tail of the shaft feeds the AVR independently of the main output. Bearings at one or both ends support the rotor, a shaft-mounted fan moves cooling air, and the terminal box holds the output connections, current transformers and sensing wires.
The regulation side of this machine, the AVR, PMG and exciter, is treated in detail on AVR, PMG and exciter parts. The architecture itself is explained on generator alternator architecture. The present page covers the mechanical and winding parts that fail with age, heat and vibration.
Parts, symptoms and the evidence that identifies them
| Part | Typical symptom when it fails | Evidence needed to supply the right item |
|---|---|---|
| Non-drive-end bearing | Rising noise, heat at the end bracket, vibration, grease discolouration | Alternator make, frame, serial; bearing number from the old bearing or the manual; sealed or regreasable type |
| Rotating diodes | Low or unstable voltage, AVR running at full output, overheated exciter | Forward and reverse polarity count, current rating, stud or button style, heat-sink pattern |
| Surge suppressor or varistor | Repeated diode failure, often after a short circuit or lightning event | Voltage rating and mounting; replace with diodes as a set |
| Exciter stator | No build-up, open or shorted field, visible burn | Resistance of the old winding, frame and serial, number of leads |
| Main stator or rotor | Insulation resistance low, earth fault trips, burnt smell, unbalanced phase voltages | Winding connection code, voltage and kVA from nameplate; rewind quotation needs the machine inspected |
| RTD or thermistor | Spurious high-temperature alarms or no reading | Sensor type (PT100, PTC), location, number fitted |
Bearings: the part that fails most often
Most bearings in machines up to several hundred kVA are sealed deep-groove ball bearings with a nominal grease life measured in tens of thousands of hours at the design temperature. In a Hyderabad generator room that runs well above 40 °C in May, grease life falls sharply, and a bearing specified for 30,000 hours may need attention well before that. Larger machines use regreasable bearings with a nipple and relief, and overgreasing them is as harmful as neglect because churned grease overheats. Follow the manufacturer’s grease type and quantity, and record every greasing in the log.
When a bearing is replaced, the flexible coupling discs and the fan should be inspected at the same time, and the rotor should be checked for rub marks on the stator bore. A bearing that has run to destruction may have let the rotor touch the stator, and that damage decides whether the job is a bearing change or a rewind. Our alternator service covers this inspection.
Rotating diodes and surge suppressors
The rotating rectifier is a three-phase bridge, usually six diodes, three forward and three reverse polarity, mounted on heat-sink plates that spin with the rotor. The varistor across the DC side absorbs the voltage spikes that occur when load is thrown off or a short circuit is cleared. Diodes fail short or open. A shorted diode draws heavy current through the exciter, overheats it and often takes the AVR with it. An open diode leaves the field under-supplied, so the AVR runs at maximum output and the terminal voltage sags under load.
Testing is simple: isolate each diode and check forward conduction and reverse blocking with a multimeter. Replace all six as a matched set together with the varistor, because the survivors have been stressed. Always confirm polarity orientation on the heat sinks; a diode fitted the wrong way round shorts the bridge on the first start.
Windings, insulation and the decision to rewind
Winding insulation ages with temperature, moisture, vibration and contamination. Standard industrial alternators use class H insulation with a class F temperature rise, which gives margin at the 40 °C ambient the nameplate assumes and progressively less margin as the room gets hotter. Dust and oil mist in open-type sets settle on the end windings and hold moisture, which is why insulation resistance often falls during the monsoon on sets that run rarely.
IRmin ≈ (kV + 1) MΩ, corrected to 40 °C
Here IRmin is the lowest acceptable insulation resistance to earth in megohms and kV is the rated line voltage in kilovolts, a widely used industry rule of thumb that gives about 1.4 MΩ for a 415 V machine. In practice a healthy new or dry winding reads far higher, typically hundreds of megohms, so a reading near the minimum is a warning rather than a pass. (Illustrative) a 250 kVA alternator that read 500 MΩ at commissioning and reads 5 MΩ after a wet August should be dried with its heaters or a low-voltage current before the next load run, and the trend should be recorded. Manufacturer acceptance values take precedence over the rule of thumb.
A rewind is justified when a winding has failed to earth, between turns or between phases, or when insulation resistance cannot be recovered by drying. The decision between rewinding and replacing the complete alternator depends on frame size, availability and the condition of the rotor; see alternator rewinding for how we scope it. A rewound stator should be supplied with a test record showing insulation resistance, winding resistance balance and a high-potential test.
Identification and compatibility
Every enquiry should begin with the alternator nameplate: make, model, frame size, serial number, kVA, voltage, connection and insulation class. The same generator model is often built with alternators from two or three manufacturers, and the same alternator frame is wound to different voltages and connections, so the generator nameplate alone cannot select a diode kit, let alone a stator. Where the plate is missing, the serial number is frequently stamped on the frame or the end bracket. We confirm every match as described on parts compatibility verification.
Common mistakes
- Replacing the AVR for a low-voltage complaint without testing the rotating diodes; the new AVR fails within weeks because a shorted diode is still overloading it.
- Changing a single failed diode; the stressed survivors fail one after another and the set makes three service calls instead of one.
- Ordering bearings or diodes by generator kVA from a catalogue; the parts do not fit because the alternator frame was never confirmed.
What to send us
Send photographs of the alternator nameplate and the generator nameplate, a picture of the open terminal box and of any damaged component, and the number from the old bearing or diode where you have it. Describe the symptom, including whether voltage is low, unstable or absent, any noise or heat, and the last insulation resistance reading with the date. Send a spare parts enquiry.
Questions engineers ask
Should bearings be replaced at overhaul?
Yes, as a rule. Bearings are inexpensive compared with the labour and downtime of opening the machine twice.
How do I know whether my alternator is single-bearing or two-bearing?
Look at the drive end. A single-bearing machine is bolted directly to the engine flywheel housing with flexible steel discs on the flywheel and no bearing housing on that side. A two-bearing machine has a visible bearing end bracket and a separate coupling between engine and alternator.
Can rotating diodes be tested without removing them?
Each diode must be electrically isolated from the others to get a meaningful reading, which means lifting at least one lead per diode. With the leads lifted, a multimeter diode test shows conduction one way and blocking the other for a healthy device.
What insulation resistance is acceptable for a 415 V alternator?
A common minimum is about 1.4 MΩ corrected to 40 °C, but a healthy winding normally reads far higher. Treat a falling trend as the real signal and dry the machine before it reaches the minimum. Follow the manufacturer’s acceptance figures where they are published.
Technical content reviewed by Srinivas Reddy, Managing Partner, Reddy Generators. Last updated 19 October 2026.