Bypass VFD for soft start is a common method to reduce the inrush current and mechanical stress of motors during startup. However, it also has some safety and environmental implications that you should be aware of. In this article, we will explain what bypass VFD is, how it works, and what are the main advantages and disadvantages of using it.
Bypass VFD is a configuration of variable frequency drive (VFD) that allows the motor to be switched from the VFD control to the line power supply after reaching a certain speed. This is done by using a bypass contactor that disconnects the VFD from the motor and connects the motor directly to the mains. The VFD remains in standby mode until the next startup or a fault condition.
In the event of a drive failure VFD failure owners still have the ability to manually control the device without the use of the drive. VFD’s can be utilized to filter out bad harmonics, or noise associated with the operation of an electric motor. They can Mitigate conditions of surge associated with Large chilled water applications as well.
Bypass VFD works by using the VFD as a soft starter for the motor. The VFD ramps up the frequency and voltage of the output to gradually increase the speed and torque of the motor. This reduces the inrush current and the mechanical stress on the motor and the load. Once the motor reaches a predefined speed, usually close to the rated speed, the bypass contactor is activated and the motor is switched to the line power supply. The VFD is then isolated from the motor and the load.
Bypass VFD has several advantages over using only a VFD or only a soft starter, such as reducing energy consumption and heat dissipation of the VFD, improving the power factor and harmonics of the system, extending the life and reliability of the VFD, as well as providing redundancy and flexibility in case of a VFD failure or maintenance. When running at full speed, bypassing the VFD output eliminates the need for its use, allowing for motor operation on the line power supply using the bypass contactor.
Bypass VFD can reduce the control and efficiency of the motor and load by switching to a fixed frequency and voltage when running at full speed, which may affect performance and quality. Furthermore, it increases wear and tear by switching between the VFD and line power supply, leading to voltage spikes, transients, and mechanical shocks. Additionally, it requires additional components and wiring such as a bypass contactor, isolation switch, and interlocks, which can increase cost, complexity, and space. Lastly, it exposes the motor and load to risks and faults of the line power supply like overvoltage, undervoltage, overcurrent, and short circuit.
Bypass VFD involves several safety and environmental considerations that must be taken into account to ensure a safe and sustainable operation. These include using the right protection devices and settings for the motor and the load when running on the line power supply, such as fuses, circuit breakers, overload relays, and thermal sensors. Additionally, proper grounding and shielding techniques should be used to prevent electrical noise and interference from affecting the VFD and the motor, such as grounding rods, filters, reactors, and shielded cables. Furthermore, it is important to monitor and maintain the condition and performance of the VFD, the motor, and the load regularly. This includes checking temperature, current, voltage, frequency, torque, speed, vibration, and noise levels. Lastly, you should follow the manufacturer's recommendations and guidelines for installing, operating, and troubleshooting the bypass VFD system. This may involve reading manuals, following wiring diagrams, or contacting technical support.
Well, cost would be a consideration too. A vfd is typically more expensive than a traditional scr based soft start unit. If you don't need the vfd for it's speed control capabilities (or torque control) then maybe using it as a glorified starter won't make sense due to cost.