Bypassing a Variable Frequency Drive VFD for Star-Delta Starter Motor: A Comprehensive Guide

When a motor is normally started with a star-delta starter and you need to bypass the Variable Frequency Drive (VFD), there are specific steps and considerations to ensure a safe and efficient transition. This article provides a detailed guide on how to bypass a VFD for a star-delta starter motor, along with important safety and compatibility considerations.

Steps to Bypass the VFD

Understand the System

Review Motor Specifications and VFD Settings: Begin by thoroughly reviewing the motor specifications and VFD settings to ensure that the motor can operate directly from the power supply without a VFD. Verify the power rating, voltage, and any requirements for starting and running the motor.

Isolate Power

Turn Off the Power Supply: Turn off the power supply to both the motor and the VFD. To ensure absolute safety, lockout/tagout (LOTO) the system.

Wiring Configuration

Identify the Wiring Connections: Identify the wiring connections from the VFD to the motor. Disconnect the output terminals of the VFD from the motor and connect the motor directly to the power supply using the star-delta starter circuit.

Star Connection: Initially connect the motor in a star configuration to reduce the voltage during starting. This helps to limit the inrush current and protect the motor.

Delta Connection: After a predetermined time or once the motor reaches a certain speed, switch the motor to a delta connection for full voltage operation. This ensures smooth operation under full load conditions.

Control Circuit Adjustment

Modify the Control Circuit: Adjust the control circuit to operate the star-delta starter directly instead of the VFD. Ensure that any control signals or logic previously used to control the VFD are redirected to control the star-delta starter.

Testing

Pre-Startup Check: Before restoring power, double-check all connections to ensure they are correct and secure. Power up the system and monitor the motor during startup to ensure it operates as expected without any issues.

Documentation

Update Schematics: Update any schematics or documentation to reflect the new setup without the VFD. Note any changes made for future reference or maintenance.

Important Considerations

Motor Protection

Ensure that the motor has appropriate overload protection as the VFD may have provided some protective features. Direct starting without a VFD can result in higher inrush currents, which could affect the motor's lifespan.

Starting Conditions

Be aware that the direct starting method may result in higher inrush currents compared to VFD operation. This could affect the power supply and connected equipment, potentially leading to voltage drops or equipment damage.

Operational Changes

Understand that bypassing the VFD means losing speed control and other benefits such as soft starting, energy savings, and variable speed operation. Direct starting is typically less efficient in terms of energy usage and motor protection.

Conclusion

Bypassing a VFD requires careful planning and execution to ensure safety and functionality. If you are not familiar with electrical systems, it is advisable to consult with a qualified electrician or engineer to assist with the bypass process.