Okay, let's break down this problem and explore potential solutions for your boat hoist. You've correctly identified the issue: the high Locked Rotor Current (LRC) of your motors is exceeding the capacity of your inverter, causing shutdowns. You need a way to limit the LRC long enough for the motors to spin up without losing essential torque.Here's a breakdown of approaches and considerations

1. Soft Starters - The Most Likely Solution
How They Work

Soft starters gradually increase the voltage applied to the motor during startup, reducing the current surge.

Suitability

Soft starters can work in this application if sized and configured properly. The 9 ft-lbs of LRT suggests torque should not be an issue, especially since it is a momentary surge. This is why VFDs will likely not be required.

Sizing

You need to consider both the LRC and the required torque.

Current Rating

The soft starter needs to handle the running current (SFA = 6.8A each motor, 13.6A total) and limit the LRC to below your inverter's 60A limit. The closer the better as there is no current foldback/limiter, the better. So a 20-30A starter seems appropriate.

Torque

Since you have some torque, especially if the hoist is designed to compensate for the weight, you may be able to limit to SFA to start. If not, the soft start will simply cause the motor to delay before fully running.

Location

This is critical. The soft starter MUST be installed downstream of your existing contactors. Here's why:

Existing Contactors Function

Your existing contactors provide the forward/reverse switching (Up/Down). They effectively change the motor's winding configuration to achieve the desired direction. The soft starter needs to control the voltage after that directional switching is established.

Implementation

The soft starter needs to be installed in the hot leg between each contacter and the motor.

Number of Devices

You will need two single phase soft starters, one for each motor. Soft starters work by gradually increasing the voltage to the motor. They are not designed to handle directional changes like the reversal of a phase.

Wiring

A simplified example:1. Power from Inverter panel2. "Orange" (Hot) -> Contact 1 for Up, Contact 2 for Down3. Contact 1 -> Soft Starter -> One motor.
4. Contact 2 -> Soft Starter -> Other motor.5. The neutral will run directly to each motor.

Programming

Soft starters have adjustable parameters for:

Initial Voltage

Start with a low value (e.g., 30-40% of line voltage) and increase it until the motor starts reliably.

Ramp-Up Time

Adjust the time it takes for the voltage to ramp up to full voltage. Start with a longer ramp time (e.g., 3-5 seconds) and decrease it until you find the shortest time that still avoids the overcurrent trip.

Important Considerations



Single-Phase Availability

Ensure the soft starter is explicitly rated for single-phase operation. Many are designed for three-phase only.

2. Potential Issue - Inverter Response Time

Some inverters, particularly those designed for solar/battery power, can be very sensitive to inrush currents and have a fast reaction time to overcurrents. If your inverter is shutting down extremely quickly, even a properly sized soft starter might not be fast enough. Try to obtain specifications, and if you can tune it.3. Why VFDs are Problematic (for your stated constraints)



Single-Phase In/Out VFDs are Rare

As you've discovered, they are not common, and often not designed for motor reversal applications like this.

Cost

Replacing with three-phase motors + a three-phase VFD would likely be more expensive.

4. Other Considerations



Inverter Upgrade

The most straightforward solution, but likely the most expensive, would be to replace the inverter with one that can handle the inrush current. If you are able to swap the grid with the battery power source for this load, that would be a preferred solution.

Capacitors

While you already have capacitor-start motors, adding larger or additional capacitors might help, but it's a tricky and unpredictable approach. Capacitor sizing is critical and requires careful calculations to avoid damaging the motor or creating other problems. This is usually only useful in situations where the motors run under a constant load.

Series Resistance

Adding resistance in series with the motor during startup is possible but is a rough (and not ideal) way to limit inrush current. This is how a clutch engages the motor.

Recommendations


Start with Soft Starters

This is the most promising avenue. Find a single-phase soft starter with an adjustable current limit and ramp-up time, sized to handle your running current (SFA) and limit the LRC to below 60A.2.

Proper Installation

Install the soft starters downstream of your existing contactors. This is critical for directional control.3.

Careful Tuning

Experiment with the initial voltage and ramp-up time to find the optimal settings that allow the motors to start reliably without tripping the inverter.4.

Monitor Inverter Response: If the inverter is tripping very quickly, even with the soft starter, you may need to consider an inverter upgrade.Good luck! I'm available to provide more advice as you work through this. Flag for review