Okay, let's break down this situation and diagnose the possible causes of the motor tripping the breaker. Here's a structured approach

Understanding the Problem
Symptoms

Motor runs briefly (3 seconds) and trips the breaker. High current draw (354 amps) before tripping. Wiring fault initially found (likely contributed).

Context

Motor was previously working on commercial 3-phase. Now powered by a 40 HP rotary phase converter. Voltage measurements indicate a slightly unbalanced phase converter output. Motor is correctly wired for 208-230V.

Possible Causes, Ranked by Likelihood


Phase Converter Issues & Voltage Imbalance (Most Likely)



Capacity

Is the 40 HP phase converter sufficient for the 20 HP motor? Phase converters need headroom, especially for high-starting-torque loads like vacuum motors. While a 40HP converter should work, the efficiency and quality of the converter matters.

Voltage Imbalance

The 208V on one phase and 120V on the other two phases to ground strongly suggests a significant voltage imbalance from the phase converter. This imbalance will cause

excessive current draw in one or more of the motor windings, leading to overheating and tripping. This is the most likely culprit.

Phase Converter Output Quality

Even with sufficient HP, a poor-quality phase converter can produce a "dirty" synthesized phase that stresses the motor. Some rotary phase converters may need adjustment for optimal performance.2.

Motor Damage (Highly Possible Due to Initial Wiring Fault and High Current)



Winding Damage

The initial wiring fault and the high current draw during your second attempt could have damaged the motor windings. The motor windings insulation could have been compromised from overheating, causing shorts and further increasing current draw.

Short to Ground

Even if you corrected the wiring, the initial issue might have weakened the insulation, potentially creating a short to ground inside the motor.3.

Control Circuit Damage (Possible, But Less Likely as Primary Cause)

While possible, damaged control circuits rarely cause such high current. The control circuit energizes the motor contactor coil. If the contactor isn't pulling in fully, it might cause voltage drop, but likely won't cause 354 amps of current draw. That said, inspect the motor starter (contactor) contacts for signs of arcing or damage.4.

Breaker Issues (Possible, but Easily Checked)



Incorrect Breaker Size

Verify the breaker is sized correctly for the motor's FLA (Full Load Amps) and the applicable motor starting code. The motor nameplate will specify the FLA. A general rule of thumb is to size the breaker to 225% of the motors FLA.

Faulty Breaker

It's possible the breaker is weak and tripping prematurely. It's relatively easy to test with another breaker of the same size that you know is good.5.

Motor Mechanical Issue (Less Likely)



Seized Bearing or Obstructed Fan

A mechanically jammed motor will draw excessive current. However, if you observed the fan spinning correctly, this is less likely. However, verify it is spinning freely.6.

Motor Incompatibility with Phase Converter (Least Likely)

Some motors, especially older ones, can be more sensitive to the "synthesized" phase produced by rotary phase converters. This is rare, but possible.

Troubleshooting Steps (in order)


Phase Converter Assessment (CRITICAL)



Measure Phase-to-Phase Voltages at the Phase Converter Output

With the motor OFF, measure the voltage between each pair of phases (A-B, B-C, C-A) directly at the phase converter output terminals. These voltages must be as close to balanced as possible (ideally within 2-3%). A large imbalance here indicates a problem with the phase converter itself. Also, check the voltage drop with the motor on.

Check Phase Converter HP Rating

Double-check the phase converter's HP rating against the motor's HP and starting torque requirements. Rotary phase converters can struggle with high-starting-torque loads.

Consult Phase Converter Documentation

Review the phase converter manual for any specific requirements for motor loads, voltage balancing adjustments, or troubleshooting procedures.

Capacitor Bank

Check to see if the phase converter has a capacitor bank and verify the capacitors are not bulged.2.

Motor Insulation Testing (Mandatory if Winding Damage is Suspected)



Megger (Insulation Resistance Test)

Use a megohmmeter (Megger) to test the insulation resistance between each motor winding and ground (the motor frame). This will identify any shorts to ground. Also test winding to winding.

Low Resistance Winding Check

Use an ohmmeter to check the resistance of each motor winding. Compare the readings. Significant differences could indicate a shorted or open winding.3.

Motor Starter/Contactor Inspection

Visually inspect the contactor contacts for pitting, burning, or signs of arcing. Use a multimeter to check the contact resistance when the contactor is engaged. High resistance indicates a problem.4.

Breaker Verification

Confirm the breaker is correctly sized for the motor's FLA. Temporarily replace the breaker with a known good breaker of the same rating to rule out a faulty breaker.5.

Mechanical Check

With the motor off and disconnected from power, try to spin the motor shaft by hand. It should spin freely and smoothly.

Next Steps (Depending on Findings)



If the phase converter voltage is significantly imbalanced

The phase converter is the problem. You'll need to troubleshoot the phase converter (consult the manufacturer or a qualified electrician specializing in phase converters).

If the motor fails the insulation test

The motor is damaged and will likely need to be rewound or replaced.

If the motor starter/contactor is bad

Replace the contactor.

If everything else checks out

Consider a soft starter to reduce the inrush current. However, a soft starter will only mask the underlying problem.

Important Safety Notes



Lockout/Tagout

Always disconnect power and use proper lockout/tagout procedures before working on electrical equipment.

Qualified Personnel

If you're not comfortable working with electrical equipment, consult a qualified electrician.

Regarding the Soft Starter

A soft starter might help reduce the inrush current and prevent the breaker from tripping if the problem is purely an inrush current issue and the phase converter can handle the reduced starting current draw over a longer period. However, a soft starter won't fix a voltage imbalance or a damaged motor. Address the underlying problems first.

In summary, the voltage imbalance from the phase converter is the most likely culprit, exacerbated by possible motor damage from the initial wiring fault and subsequent high current draw. Start with a thorough assessment of the phase converter's output and perform a motor insulation test.* Flag for review