Chas, this is a great question and gets at the heart of generator AVR retrofitting. Here's a breakdown of the considerations and a (cautious) assessment of feasibility

Understanding the Problem
Your Generator's Original System

The bridge rectifier/varistor setup is a very basic, rudimentary voltage regulation system. It's essentially relying on the engine's RPM stability to maintain voltage. The varistor is a surge protector; the rectifier likely provides DC voltage for the exciter field.

AVR Advantage

A proper AVR actively controls the excitation field based on the output voltage. This allows the generator to maintain a more stable voltage output, even under changing load or RPM conditions. The AVR dynamically adjusts the current to the exciter field, which then controls the output voltage.

Why This is Not a Simple "Swap"

While theoretically you could connect an AVR to the exciter field and the main output of your generator, here's why it's complicated:1. AVR Compatibility (Critical)

AVRs are not universal. They are designed to work with specific generator types and excitation systems. Critical parameters include:

Excitation Voltage/Current

AVRs have specific voltage and current ranges for the excitation field. The AVR needs to be compatible with the existing exciter field winding. Too much voltage or current from the AVR will damage the exciter winding; too little and it won't generate sufficient voltage.

Sensing Voltage

The AVR needs to sense the output voltage of the generator. Your existing generator will need to provide the voltage range that the AVR is expecting.

Frequency

AVRs are also designed to regulate at a specific frequency (60Hz in your case).

Physical Mounting

Less critical, but still important.2.

Wiring Changes

You'd need to rewire the excitation field circuit. The AVR would need to connect to the exciter field windings, and the sensing voltage connections.3.

Unknown Winding Parameters

You need to determine the specific voltage and current requirements of the exciter field winding in your existing generator. This is often not readily available without detailed specifications or testing.4.

Potential for Damage

If the AVR is not compatible or wired incorrectly, you could damage the AVR, the generator windings (especially the exciter winding), or both.5.

Exciter Winding Resistance Your existing generator was designed to provide a certain current to the exciter winding through the existing bridge rectifier and varistor. The AVR will need to be able to compensate for this resistance, and the existing value may need to be calculated/determined to allow for selection of an appropriately designed AVR.

How to Approach This (Cautiously)

If you're determined to try this, here's a methodical approach:1. Research, Research, Research



Find Generator Models with AVR

Focus on finding the exact later B&S 5500 watt model that uses the AVR you are considering. Get the model number.

Schematics

Obtain the wiring diagrams (schematics) for both your original generator and the AVR-equipped model. This is essential to understand the wiring differences and AVR connections.

Parts Lists

Compare the parts lists to see what other components changed in the AVR-equipped model. This could reveal other necessary modifications (e.g., different stator windings).

AVR Specifications

Try to find the exact specifications for the AVR used in the later model, including input voltage range, output voltage/current to the exciter, and sensing voltage requirements.2.

Identify Exciter Winding Characteristics



Resistance

Carefully measure the resistance of your exciter field winding with a multimeter. This is important for selecting an appropriate AVR.

Voltage Test (Careful!)

Only if you are experienced with electrical testing: While the generator is running at 3600 RPM (with no load), carefully measure the DC voltage being supplied to the exciter field winding from the bridge rectifier. This will give you a clue about the voltage the AVR needs to supply.3.

AVR Selection



Choose a Compatible AVR

Based on your research, select an AVR that closely matches the specifications of the AVR used in the later B&S model (if you found one). Pay close attention to the excitation voltage/current, sensing voltage, and frequency. If you can't find an exact match, err on the side of an AVR that has slightly lower voltage/current output capability for the exciter.

Adjustable AVR

Consider an AVR with adjustable voltage and frequency settings. This will give you more flexibility in fine-tuning the system.4.

Wiring and Installation



Follow the Schematic

Use the schematic from the AVR-equipped model as a guide for wiring. Disconnect the existing bridge rectifier and varistor completely.

Double-Check

Carefully double-check all wiring connections before applying power.

Start with Low Load

After connecting the AVR, start the generator with no load connected. Monitor the output voltage carefully.5.

Fine-Tuning (If Possible)



Adjust the AVR

If the AVR has adjustable settings, use them to fine-tune the output voltage to 120VAC at 60Hz.

Load Testing

Gradually increase the load on the generator and monitor the voltage stability.

Important Considerations



Safety First

Work on generators only when they are disconnected from the power source and have cooled down.

Professional Help

If you are not comfortable with electrical wiring and testing, consult a qualified electrician or generator technician.

Risk of Damage

Be aware that there is a risk of damaging the AVR or the generator during this process.

My Overall Assessment

While possible, retrofitting an AVR is

not a simple task. It requires careful research, detailed schematics, compatible components, and a good understanding of generator operation. The biggest hurdle is finding the exact specifications of the exciter field winding in your existing generator and matching it to a suitable AVR.

Alternative Approach (Possibly Easier):

Consider a digital voltage regulator (DVR) or electronic governor for your existing system. These devices monitor the generator's output frequency and adjust the engine speed to maintain a more stable output. While not as sophisticated as an AVR, they may provide some improvement in voltage regulation without the complexity of rewiring the excitation system.Good luck, Chas! I hope this detailed breakdown helps you make an informed decision. Flag for review