How do you configure and control a VFD bypass using a PLC or HMI?

1 PLC-based bypass

A PLC-based bypass uses a PLC to monitor the status of the VFD and the motor, and to switch between the VFD and the bypass mode. The PLC can also provide logic and interlocks to ensure safe and smooth transitions. A PLC-based bypass requires a separate contactor or switch to isolate the VFD from the line power, and another contactor or switch to connect the motor to the line power. The PLC can control these contactors or switches using output relays or digital outputs. A PLC-based bypass can be more flexible and customizable, but also more complex and costly.

2 HMI-based bypass

An HMI-based bypass uses an HMI to display the status of the VFD and the motor, and to enable the operator to manually select the VFD or the bypass mode. An HMI-based bypass does not require a separate PLC, but relies on the built-in logic and protection of the VFD. An HMI-based bypass requires a bypass contactor or switch that is controlled by the VFD, and a feedback signal from the contactor or switch to the VFD. An HMI-based bypass can be simpler and cheaper, but also less reliable and secure.

3 Bypass advantages and disadvantages

A VFD bypass can provide some advantages, such as redundancy, reliability, and maintenance convenience. A VFD bypass can allow the motor to keep running in case of a VFD failure or fault, or during maintenance or repair of the VFD. A VFD bypass can also reduce the stress and heat on the VFD components, extending their lifespan. However, a VFD bypass can also have some disadvantages, such as loss of control, efficiency, and protection. A VFD bypass can eliminate the benefits of speed and torque control, energy savings, and soft start and stop of the VFD. A VFD bypass can also expose the motor to higher currents, voltages, and harmonics, reducing its performance and durability.

4 Bypass selection criteria

The decision of whether to use a VFD bypass and which type of bypass to use (PLC-based or HMI-based) depends on various factors, such as the application, the environment, the budget, and the preference. When considering which bypass to use, take into account the criticality of the motor and process, the frequency and duration of the bypass mode, and the availability and accessibility of the operator and maintenance staff. For instance, if the motor is essential for operation or safety of the process, a VFD bypass can provide backup and continuity. On the other hand, if the bypass mode is used rarely or for short periods, an HMI-based bypass may be sufficient. Furthermore, if operator and maintenance staff are readily available and accessible, an HMI-based bypass can be convenient and user-friendly; however, if they are not easily available or accessible, a PLC-based bypass can be more automated and remote.

5 Bypass configuration steps

The configuration steps for a VFD bypass may differ depending on the type of bypass, the type of VFD, and the type of PLC or HMI. However, some of the common steps include wiring the VFD, motor, contactors/switches, and PLC/HMI in accordance with the manufacturer's instructions and electrical codes. Additionally, programming the PLC/HMI to control and monitor the VFD and bypass mode is necessary, using the appropriate software and communication protocols. It is also important to set up the parameters and functions of the VFD to enable/disable bypass mode using either keypad, HMI, or software. Finally, testing and troubleshooting the VFD and bypass mode using manual/automatic modes while checking signals/feedbacks is essential.

6 Bypass control tips

For safe and smooth operation, it is important to pay attention when controlling a VFD bypass. Ensure that you follow the manufacturer's recommendations and instructions for the VFD, motor, contactors or switches, and PLC or HMI. Also, verify that the line power and motor are compatible and within the specifications of the VFD and bypass mode. When switching to bypass mode, wait for the VFD to stop completely. Additionally, use proper isolation and protection devices such as fuses, circuit breakers, disconnects, and overload relays to prevent short circuits, overloads, or faults. Lastly, monitor and maintain the VFD, motor, contactors or switches, and PLC or HMI regularly to ensure optimal performance and reliability.

7 Here’s what else to consider

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