Flashing a generator

Want to know how to flash a generator safely? This guide explains what flashing is, why it’s needed, and how to do it using diy methods.

February 26, 2026
BY BISON

Sometimes a generator’s engine runs perfectly, but it does not produce electricity. This is often due to lost residual magnetism—a small magnetic charge that the generator needs to create power. Without it, the generator can run but won’t provide electricity.

Flashing a generator means restarting it by manually feeding mechanical or electrical power. This can be done using a battery or a drill. Flashing restores the magnetic field so the generator can produce electricity again.

Want to know how to flash a generator safely? This guide explains what flashing is, why it’s needed, and how to do it using diy methods.

Understanding generator excitation

A generator produces electricity by converting mechanical energy from the engine into electrical energy through the interaction of magnetic fields and conductors. Inside the generator, a magnetic field moves across coils of wire, creating a flow of electricity. To start this process, the generator relies on a small leftover magnetic charge called residual magnetism. Without it, the engine may run, but the generator will not produce power.

Residual magnetization refers to the magnetization that remains after a generator has been in operation for a previous period. This residual magnetism is sufficient to generate a powerful electromagnet.

Understanding how a generator excites its magnetic field clarifies why flashing is sometimes needed. Generators use different excitation systems to create the magnetic field:

Self-excited generators – these use a portion of their own output to strengthen the magnetic field. If residual magnetism is lost, the generator cannot build enough magnetic field on its own, making flashing necessary.
Separately excited generators – these get their magnetic field from an external power source, so they are less affected by lost residual magnetism. However, if the external excitation fails, flashing or repairs may still be required.

Causes of loss of residual magnetism

Long periods of disuse – residual magnetism can fade if a generator sits idle for months or years.
Improper shutdown procedures – turning off a generator incorrectly or cutting power suddenly can reduce the magnetic charge.
Faulty voltage regulators – a broken or malfunctioning regulator can prevent the magnetic field from building properly.
Reverse polarity connections – connecting the generator’s terminals incorrectly can damage or remove residual magnetism.
Physical impacts – dropping or jarring the generator can disturb its internal magnetic field.
Running without a load – operating the generator for long periods without connected devices can weaken the residual magnetism.
Incorrect wiring or repairs – mistakes during maintenance or repairs can interrupt the magnetic field.

Signs that a generator needs flashing

The engine runs normally, but no or low voltage output.
The generator may produce some electricity, but the voltage is far below what your equipment requires.
When measuring the output voltage, the multimeter reading is zero or close to zero.
The circuit breaker did not trip when the generator was overloaded.

Preparations before a flash generator

Tools and materials needed for flashing a generator

Insulated jumper wires – for safely connecting the battery to the generator’s field terminals.
12v battery – provides the electrical charge needed to restore residual magnetism (for dc generators).
Multimeter or voltmeter – measures voltage output to check if flashing was successful.
Small lamp – can be used as a load for testing.

Safe operation

Disconnect appliances – unplug all devices from the generator before starting.
Turn off the engine – ensure the engine is completely off before flashing.
Disconnect the spark plug wire – prevents accidental engine startup.
Work in a dry, ventilated area – reduces the risk of electrical shock and ensures proper airflow.
Wear personal protective equipment (ppe) – safety gloves and goggles protect your hands and eyes.
Consult the generator manual – follow the manufacturer’s instructions and safety warnings. Some generators are easier to flash than others, and professional help may be needed if you are unsure.

How to flash a generator: step-by-step

At BISON, we want your generator to work properly when you need it most. There are two common methods to flash a generator, depending on the type:

Method 1: using a 12v battery (for brushed DC generators)

Locate the generator’s field terminals: check the manual for the correct terminals (usually labeled f+ and f–).
Remove exciter field leads from the voltage regulator: leaving them connected can damage the regulator.
Measure exciter field resistance: using a multimeter, check f+ to f–. A reading of some resistance confirms continuity; infinite resistance indicates an open circuit.
Connect the battery with insulated jumper wires: attach the positive (+) battery terminal to f+ and carefully touch f– to the negative (–) terminal for 3–5 seconds. A small spark may appear, which is normal and confirms the circuit is complete.
Disconnect the battery safely: remove the F– lead first, then the F+.
Reconnect the exciter leads to the regulator: restore f+ and f– to the regulator.

Method 2: using an electric drill (for brushless generators)

Over time, the magnets inside a brushless generator can lose their charge. The engine may run normally, but the generator won’t produce electricity. The following are the steps for using an electric drill to help restore the magnetic field:

Start the engine: turn on the generator as instructed in the manual.
Connect the drill to the rotor shaft: carefully attach the drill chuck to the generator rotor or shaft. Ensure it is secure and properly aligned.
Spin the rotor: slowly run the drill in the direction indicated in the manual. You may notice a slight change in engine sound or a small spark, which indicates the generator is regaining magnetism.
Remove the drill and test output: stop the drill, remove it safely, and check the generator’s voltage with a multimeter. The generator should now produce normal voltage.
Run the generator briefly: operate the generator under load for at least 5 minutes to stabilize the magnetic field and prevent it from losing magnetism again.

Verification and troubleshooting

Step 1: Verify voltage output

Use a multimeter to measure the generator’s output voltage.
Compare it to the rated voltage. Normal readings indicate the flashing was successful.

Step 2: Troubleshooting if flashing fails

Repeat the flashing process: Sometimes more than one attempt is needed to restore the magnetic field. Repeat the procedure if the voltage is still low.
Check polarity: ensure battery connections were correct; reversed wires can prevent proper flashing.
Ensure battery charge: A weak battery may not supply enough current.
Consider other generator issues: Problems with the avr, brushes, circuit breakers, or windings can also prevent the generator from producing power.

Step 3: call a professional

If repeated flashing fails or internal damage is suspected, contact a qualified technician. Attempting repairs without expertise can be dangerous and may worsen the problem.

Prevention: keeping your generator ready

Run the generator under partial load monthly: operate the generator with a small load for a short time each month to maintain the magnetic field.
Store in a dry, protected location: moisture, extreme heat, or cold can weaken the magnetic field and damage the generator. Keep it in a covered, ventilated, and dry area.
Unplug loads before shutdown: disconnect appliances and devices before turning off the generator to prevent voltage spikes and preserve residual magnetism.
Maintain the battery: keep the battery charged, avoid letting it run to zero, check for loose cables, and prevent exposure to corrosive liquids, extreme temperatures, or direct sunlight.

Summary

By following the step-by-step procedures and precautions outlined in this article to flash your generator, you can ensure it is reliable and always ready to use. Don’t worry if flashing is necessary—it’s a straightforward process that usually works within a few attempts. If your generator still isn’t producing power, it may require professional inspection or repairs.

For parts replacement or specialized generators, please contact BISON. As a generator manufacturer with many years of experience in the industry, we are dedicated to providing you with reliable power. We have a complete sales process, offering professional customer service from product manufacturing to after-sales support.

FAQs

What are the dangers of flashing a generator?

Flashing a generator can be dangerous if done incorrectly. Improper procedures, reversed connections, or interrupted processes can damage the generator or create safety hazards. Always follow manufacturer instructions, use proper safety equipment, and consult a professional if unsure.

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Murphy

Taizhou BISON Machinery Industry CO.,LTD

4+ years of expertise in generator export. A specialist in BSCI and international standards (EPA, EURO V, CE). Dedicated to providing professional solutions for generators, genest, and generator parts with a global perspective.

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