Okay, a high leg (also known as a wild leg, stinger leg, or bastard leg) on a rotary phase converter's output is a common issue. Let's break down why it happens and how you can address it to get a balanced 3-phase output suitable for testing sensitive equipment.

Understanding the High Leg Issue

Rotary phase converters create a synthetic third phase. They don't magically conjure power; they use capacitors and inductors to shift the phase of the single-phase input and create a semblance of a 3-phase system. This process is inherently imperfect, leading to voltage imbalances, especially with light or no loads.The high leg is the phase whose voltage to neutral is significantly higher than the other two phases. This is because the generated phase often has a voltage added to it, which creates a voltage higher than the typical 120 V to Neutral.

Why a High Leg is Bad for Sensitive Electronics Overvoltage

Sensitive electronics are designed to operate within a specific voltage range. A high leg introduces a voltage that can exceed these limits, leading to damage or malfunction.

Uneven Loading

Unbalanced voltages cause unbalanced currents in the connected equipment. This can lead to overheating, reduced efficiency, and premature failure of motors, transformers, and other components.

Control Circuit Issues

Control circuits or power supplies that rely on a specific voltage may not function properly, causing erratic behavior or shutdowns.

Solutions for Balancing the 3-Phase Output

Here's a breakdown of solutions, in order of typical effectiveness and complexity:1. Load Balancing



The First and Most Important Step

The primary way to balance the output of a rotary phase converter is to apply a balanced load to the three phases. A rotary phase converter performs best when all three phases are drawing similar amounts of current.

Resistive Load Banks

A resistive load bank is a tool specifically designed for this. It consists of a set of resistors that can be switched in and out to apply different loads to each phase. You can adjust the resistance on each phase until the voltages are balanced. This is the best way to balance the three phase system.

Practical Load Simulation

If you know the typical load characteristics of the equipment you'll be testing, try to mimic that load during the balancing process.

Don't Overload

Make sure you don't exceed the rated capacity of your rotary phase converter when adding load.2.

Capacitor Adjustment/Tuning (Advanced)



Only if you know what you are doing

Most Rotary Phase converters will use capacitor banks to balance the three phase system. You will need to contact the manufacturer and ask what capacitors they recommend to reduce the High Leg. This option can be dangerous.3.

Buck-Boost Transformer (For Fine Tuning)

A buck-boost transformer is a small, single-phase transformer designed to either increase (boost) or decrease (buck) the voltage by a small amount (typically a few percent). It can be wired in different configurations.

How it Works

You could, theoretically, use a buck-boost transformer on the high leg to lower its voltage and bring it closer to the other two phases.

Downsides

You need to carefully select the transformer's kVA rating and voltage boost/buck settings. It only corrects a single phase and might not fully address imbalances under varying load conditions. Also this can damage your system if not applied properly.4.

3-Phase Autotransformer (Voltage Regulation) Function

Adjust the output voltage of each phase individually to compensate for variations.

Why it's Uncommon

More complex and expensive than other options. Requires manual adjustment or automatic control circuitry to maintain balance.5.

Solid-State Phase Converter



Explanation

Unlike rotary converters, solid-state converters use electronic components (like IGBTs or thyristors) to synthesize the 3-phase output. They provide much better voltage balance and regulation.

Benefit

Significantly better voltage regulation and balance compared to rotary converters. They also have better efficiency and power factor.

Cost

Usually more expensive than rotary converters.

Important Safety Considerations



Work with a Qualified Electrician

Dealing with 3-phase power is dangerous. If you're not comfortable working with electricity, hire a qualified electrician to help you with the balancing process.

Safety Equipment

Always use appropriate safety equipment, including insulated gloves, safety glasses, and a voltage tester.

Lockout/Tagout

Before working on any electrical equipment, always de-energize the circuit and follow proper lockout/tagout procedures.

Check the Rotary Converter's Manual

Consult the manual for your specific rotary converter model for any recommendations or warnings regarding voltage balancing.

In summary: Start with load balancing as that is the simplest and cheapest way to balance the system. Using transformers can be a solution, but are often overly complicated, and if not implemented correctly can damage your system. If you can, avoid using rotary phase converters all together and switch to solid state converters. They are much more expensive, but offer a higher quality product. Flag for review