How to Remagnetize a Generator: Causes, Diagnosis, and Fixes

Electric generators rely on magnetic fields to produce power, but over time, these fields can weaken, leading to low or no output. This comprehensive guide explores the science behind generator magnetism, common causes of loss, diagnostic steps, and practical remagnetization techniques suitable for DIY enthusiasts and professionals alike.

Understanding Generator Magnetism Fundamentals

At the heart of every electric generator is the principle of electromagnetic induction, where moving magnets or coils create voltage in nearby conductors. Most portable generators and permanent magnet generators (PMGs) use **permanent magnets** in the rotor to initiate this process. These magnets generate a residual magnetic field that induces initial current in the stator windings, building up to full output voltage.

Residual magnetism is the key starting point. Without it, the generator cannot self-excite, even if the engine spins perfectly. Factors like prolonged storage without load, high temperatures, or opposing magnetic fields from improper shutdowns can erode this field. In PMGs, neodymium or ferrite magnets are common, but they are not truly ‘permanent’ under stress—they can partially demagnetize over time.

• Self-Excited Generators: Use residual magnetism to bootstrap excitation via AVR (Automatic Voltage Regulator).
• PMGs: Directly employ rotor magnets to excite the main field, common in wind turbines and high-efficiency portables.
• Brushless Designs: Rely on exciter windings that need flashing if magnetism fades.

When output drops to a fraction—say, 6 volts instead of 120—the issue is often magnetism loss, as noted in real-world cases with stored Honda clones or industrial PMGs left idle for years.

Why Does Magnetism Fade in Generators?

Magnetism loss isn’t random. Permanent magnets consist of aligned magnetic domains. External influences misalign these domains, reducing field strength. Common culprits include:

• Thermal Demagnetization: Exceeding the Curie temperature (around 80-350°C for typical magnets) during overloads or hot restarts.
• Storage Without Protection: Without ‘keeper bars’ or shorting plates across poles, self-demagnetizing fields act over time, especially in high-speed rotors like 3000 RPM units.
• Opposing Fields: Short circuits or reverse currents during faults create demagnetizing forces along the hysteresis curve’s second quadrant.
• Vibration and Mechanical Shock: Jarring impacts disrupt domain alignment in ferrite magnets.
• Age and Material Fatigue: Neodymium magnets lose 1-2% strength per decade naturally, accelerated in harsh environments.

The BH hysteresis curve illustrates this: strong magnets have high coercivity (resistance to demagnetization), requiring significant reverse field to erase magnetism. Weaker ones saturate easily but fade quickly.

Diagnosing Weak or Lost Magnetism

Before remagnetizing, confirm the issue. Run the generator at operating speed (typically 3600 RPM for 60Hz portables) and measure AC output with a multimeter.

1. Check stator windings for continuity and shorts to ground—should read 10-50 ohms between leads, infinite to frame.
2. Measure residual voltage: Under 10-20V AC across output suggests magnetism loss.
3. Inspect AVR diodes and capacitors if equipped—failed components mimic low excitation.
4. Test engine speed and load—low RPM yields low voltage.
5. For PMGs, verify rotor visually if accessible; spin by hand to feel drag from field strength.

If windings test good but output stays low (e.g., 6V on a 120V gen), proceed to remagnetization. Always disconnect loads and wear safety gear.

Safe Remagnetization Techniques for Portable Generators

Remagnetization, or ‘flashing the field,’ realigns domains using a strong external field. For most Chinese/Honda-style portables, use the battery method—no disassembly required.

Battery Flashing Method (Easiest for DIY)

This pulses 12V DC into the stator to create a temporary electromagnet stronger than residual field.

1. Locate stator leads (consult schematic sticker, often on case).
2. Run engine to speed, measure output.
3. Connect 12V battery + to one brush or stator excitation wire (usually red), – to the other (black). Polarity matters—match generator’s field direction.
4. Momentarily touch leads (1-2 seconds); you may see sparks.
5. Disconnect battery, start engine—output should build to 120V.
6. Repeat 2-3 times if needed, tapping rotor lightly to aid domain movement.

Safety: Insulate connections, avoid sparks near fuel. Works because DC current exceeds coercivity, pushing BH curve past saturation.

Capacitor Discharge Method (For Stronger Fields)

For stubborn cases, charge a 100-1000µF capacitor to 200-400V DC, then discharge into field coils.

• Build circuit: Variac, rectifier, capacitor bank.
• Align rotor pole with stator if possible.
• Discharge pulse aligns domains permanently.

Ideal for magnetos or small PMGs; YouTube demos show rebuilt magnet chargers restoring spark on old engines.

Advanced Techniques for PMG and Industrial Rotors

Permanent magnet generators in wind turbines or large alternators need rotor access.

Stator Energization for Rotating Magnets

As in Eng-Tips forums, align rotor pole with stator, pulse DC through stator windings.

• Remove rotor if needed (bearings first).
• Position north pole facing stator coil.
• Apply 10-50A DC for 5-10 seconds per pole, reversing for alternates.
• Reinstall, test PMG output (should reach rated voltage for AVR).

Electromagnet Jig for Disassembled Magnets

Build a C-core electromagnet with 300+ turns of 12-18 AWG wire.

1. Wind coil on steel core.
2. Insert demagnetized magnet inside coil gap.
3. Apply increasing DC current to saturation (test with gaussmeter).
4. Tap gently, reverse polarity if needed for full hysteresis loop.

Achieves ~1 Tesla fields, restoring full strength.

Preventing Future Magnetism Loss

Proactive steps extend magnet life:

• Store with shorted output or dummy load to preserve flux.
• Install keeper plates on exposed poles.
• Avoid hot shutdowns; let cool unloaded.
• Upgrade to high-coercivity NdFeB magnets if rebuilding.
• Annual flashing during maintenance.

Frequently Asked Questions (FAQs)

Can all generators be remagnetized?

Most yes, especially self-excited portables and PMGs. Brush-type exciters may need winding checks first.

How long does remagnetization last?

Indefinitely if protected; reapply after 2-5 years storage.

Is it safe to DIY?

Yes with precautions: Disconnect battery, eye protection, no fuel nearby. Consult manual for wiring.

What if battery method fails?

Try capacitor pulse or professional shop with pulse magnetizer.

Does heat permanently kill magnets?

Above Curie point yes; below, partial reversible loss.