Okay, let's break down this situation and see what might be happening. It sounds like you have a single-phase to three-phase VFD (the ATV11HU4) that's now causing problems when connected to a larger load. Here's a summary of what we know: Problem

The VFD is tripping a 30A breaker and displaying an "SCF" (Short Circuit Fault) when connected to a 2.5HP mixer.

Setup

ATV11HU4 VFD: Single-phase input, three-phase output, rated for 3HP. Previously working on a 1.5HP mixer. Now connected to a new 2.5HP mixer. Using ATL contactors for starting/stopping the mixer. (Implying motor control through external contactors triggered by the VFDs dry contacts)

Symptoms

Trips the 30A breaker. Displays "SCF" fault. Works fine when reconnected to the smaller 1.5HP mixer.

Timing

This happened on Monday.Here's a breakdown of possible causes and what to investigate:

Possible Causes1. Overload/Inrush Current

The most likely cause is that the 2.5HP motor is drawing a significantly higher inrush current on startup than the 1.5HP motor, especially if this mixer has a heavier load or higher starting torque requirements. While the VFD is rated for 3HP, the surge current the input side can handle is a limiting factor. The VFD might not be shutting down fast enough to prevent the breaker from tripping on the input side. The internal protection should be protecting the drive, but breakers have trip curves.

Possible reason for breaker tripping

If the inrush current is very high, the breaker might trip before the VFD's internal short-circuit protection can react. Breakers are generally designed to trip faster with higher currents.2.

Motor Issues



Winding Fault

Although unlikely since the error is an SCF, the larger motor could have a developing short in its windings. Even a partial short could draw excessive current at startup.

Locked Rotor

The motor might be seized or have excessive friction, preventing it from starting normally. This would cause a very high current draw.

Incorrect Motor Parameters

The VFD might be configured with parameters suitable for the 1.5HP motor, but not for the 2.5HP motor. Although, since its an SCF, it would have to be very bad.
3.

VFD Issues



Component Failure

While it's working on the smaller motor, there could be a component within the VFD that's borderline. The larger motor could be pushing it over the edge, causing the short circuit.

Internal Short

There could be a very slight internal short that's only triggered by the higher current demand of the larger motor.

VFD Settings

Double-check VFD settings. Some VFDs have adjustable current limits, acceleration times, and motor parameters.4.

Wiring and Connections



Loose Connections

A loose connection in the wiring to the larger motor could be causing arcing and high resistance, leading to excessive current draw and potentially triggering the short circuit fault.

Damaged Wiring

Inspect the wiring to the larger motor for any signs of damage, kinks, or wear that could be causing a short.

Incorrect Wiring

Confirm the wiring to the motor is correct (U, V, W to the correct motor terminals) and that the motor is wired for the correct voltage (usually 230V in the US).5.

Phase Converter Issues



Phase Imbalance

While this should be handled by the VFD, a severe phase imbalance caused by the larger motor might be contributing to the problem.

Input Voltage Sag

The increased load from the larger motor could be causing a significant voltage sag on the input side of the VFD, which could affect its operation.

Troubleshooting Steps1. Motor Inspection



Megger Test

Perform a megger test on the windings of the 2.5HP motor to check for insulation breakdown (shorts to ground).

Visual Inspection

Check the motor for signs of overheating, damage, or loose connections.

Rotation

Ensure the motor shaft turns freely.2.

VFD Configuration



Motor Parameters

Double-check the VFD's motor parameters (voltage, current, frequency, HP, speed) and ensure they are correctly set for the 2.5HP motor. Especially the rated motor current.

Current Limit

Check the VFD's current limit setting. It might be set too low.

Acceleration/Deceleration

Increase the acceleration and deceleration times slightly. A gentler ramp-up can reduce inrush current.

Factory Reset

Consider a factory reset of the VFD and then reconfigure it for the 2.5HP motor.
3.

Wiring Inspection



Check all connections

Tighten all connections at the VFD, the motor, and the breaker panel.

Inspect Wiring

Check for damaged or frayed wires.4.

Input Voltage



Measure Voltage

Use a multimeter to measure the input voltage to the VFD under load (while trying to start the 2.5HP motor). A significant voltage drop could indicate a problem with the incoming power supply.
5.

Test the VFD Independently



No Load Test

Disconnect the motor and run the VFD with no load. See if it still throws the SCF fault. If it does, the VFD itself is likely the problem.

Smaller Load Test

If possible, try a different 1.5HP or similar motor to see if the problem persists.
6.

Breaker



Breaker Integrity

Although the 30amp breaker is tripping, you could have a defective one. Verify with a clamp meter, the AMPs the drive is pulling before it blows the breaker.

Breaker Rating

It is a 30amp breaker for the load or is it the correct breaker for the drive size?

Addressing the "SCF" Fault

The "SCF" fault indicates a short circuit. This means the VFD's protection circuitry is detecting a very high current flow, suggesting a direct short (or near short) somewhere in the circuit. This needs to be investigated carefully as it could indicate a serious problem.

Important Considerations Single-Phase to Three-Phase Derating

Keep in mind that converting single-phase to three-phase power derates the VFD's capacity. A 3HP rated VFD used on a single-phase input might not be able to handle a full 3HP three-phase load. In other words, you need a larger VFD for single-phase to 3-phase conversion compared to a native 3-phase input. The 2.5HP motor may be pushing the VFD beyond its capabilities in this configuration.

Service Factor

Check the service factor of both motors. A motor with a higher service factor can handle momentary overloads better.

Contactors Wiring of contactors. You mentioned that they are using ATL contactors. It is imperative that they are wired correctly.

Recommendations1. Start with the motor inspection (Megger test). This is the most likely culprit in my opinion, followed by VFD configuration.2. Carefully review the VFD's manual for proper setup and configuration for the specific motor.3. If the problem persists after checking the motor and configuration, the VFD itself may be failing and need to be replaced. Consider a larger VFD, given the single-phase input and the load.4. Ensure safe electrical practices. Work with a qualified electrician to perform these tests and repairs.Good luck troubleshooting! Let me know if you have any more information after performing some of these tests. Flag for review