We Buy Your Generator
- Finance Available, on all new generators.
A generator needs a magnetic field to produce electricity, but that field does not always come from a permanent magnet. Many commercial and industrial alternators create their main magnetic field with an electrically energised rotor winding. Some smaller machines use permanent magnets, while others use a separate permanent magnet generator (PMG) to supply the excitation system.
These arrangements serve different purposes. Knowing which one is fitted matters when selecting a replacement alternator, investigating low voltage or identifying a component. The answer depends on the alternator’s design, so use its manufacturer, model and specification rather than the generator’s fuel type or size alone.
Bellwood supplies alternators for commercial, industrial and standby generator sets. If you’re identifying an alternator or dealing with a generator fault, the information checklist below will help you prepare for an enquiry.
An alternator produces voltage through electromagnetic induction. When the magnetic field around a conductor changes, a voltage is induced in it. In an alternator, the rotor turns and creates a changing magnetic field relative to the stator windings. That interaction induces voltage in the stator.
The generator engine provides the mechanical power to turn the alternator. The alternator’s design determines how its magnetic field is created and controlled.
A magnetic field is essential to the process; a permanent magnet is only one way to produce it. In many synchronous alternators, current in a rotor field winding creates the main field. An excitation system supplies and controls that current.
The main distinction is whether the field comes from a magnet that retains its magnetism or from a winding that becomes magnetic when current flows.
|
Field arrangement |
How it works |
Where it may be used |
|
Permanent-magnet rotor |
Permanent magnets on the rotor provide the magnetic field without a separate field current to create it. |
Some smaller generators and permanent-magnet alternators. |
|
Wound-field rotor |
Current flows through a rotor winding to create the magnetic field. The excitation system controls the field current. |
Many synchronous alternators are used in commercial and industrial generator sets. |
|
PMG excitation source |
A separate permanent-magnet generator produces power for an alternator’s excitation system. |
Some brushless alternator designs; the PMG supports excitation and is not necessarily the source of the generator set’s main output. |
These are general arrangements, not a specification for every machine. Alternator manufacturers use different excitation designs, and some machines include several components in the excitation system. Check the wiring diagram and technical manual for the exact model.
“Diesel generator” describes the engine and fuel; it does not identify the alternator’s magnetic design. A diesel engine turns an alternator, which needs a magnetic field to generate electricity. That field may be created by a wound rotor, a permanent-magnet rotor or another arrangement specified by the alternator manufacturer.
A diesel generator may also have a PMG as part of its excitation system. That does not mean permanent magnets produce the generator set’s main electrical output. The distinction is important when ordering parts: components such as the AVR, exciter and rectifier assembly must match the alternator’s actual design.
Low or unstable voltage does not, by itself, show that a generator has “lost its magnets”. Depending on the alternator design, possible causes may include issues with the AVR, excitation circuit, wiring, connections, rotating rectifiers, windings, or operating conditions. A fault needs to be tested against the manufacturer’s procedure before a component is identified as the cause.
Some self-excited alternators rely on residual magnetism to begin building voltage. The checks and recovery procedure, if applicable, vary by model. A PMG-excited alternator has a different excitation arrangement, so a procedure intended for one design may be unsuitable for another.
Do not attempt to re-magnetise or adjust an alternator based on a generic online instruction. Incorrect testing or adjustment can damage equipment and expose people to live electrical parts or rotating machinery. Follow the manufacturer’s manual and use a suitably qualified engineer.
If you’re asking Bellwood to help identify an alternator or investigate a fault, provide as much of the following as you can:
Do not remove guards or open electrical enclosures to obtain this information while the generator is running or connected to a live supply. If the nameplate or component label is inaccessible, leave it for a qualified person to inspect.
This information helps narrow down which alternator design is fitted and which documentation or parts may apply. It does not replace electrical testing or confirm a diagnosis on its own.
The correct alternator or replacement component depends on the generator set and its electrical specification. Bellwood supplies generator alternators and provides generator repair services. Share the nameplate details and fault symptoms with the team so they can advise on the next step.
No. Generators need a magnetic field to induce voltage, but many alternators produce the main field with an electrically energised rotor winding rather than with permanent magnets.
No. A PMG is a smaller permanent-magnet generator that may supply power to an alternator’s excitation system. The main alternator produces the generator set’s electrical output.
No. Alternators use different excitation arrangements. Check the alternator model documentation to confirm whether it has a PMG and which AVR is specified.
That symptom alone cannot confirm a magnet fault. Low voltage can have several causes, including faults in the excitation or voltage-regulation system. The alternator should be assessed using the manufacturer’s diagnostic procedure.
Check the alternator nameplate, wiring diagram and technical manual for its excitation system. If you need help identifying a unit or sourcing a compatible replacement, contact Bellwood with the generator and alternator details.