Bypass in VFD applications

Have you also come across that question: ‘What is actually meant with VFD bypass?’ This article presents 3 different meanings of bypass in VFD applications.

Confusion about meaning of bypass

I have noticed a common misunderstanding when talking about bypass in VFD applications that inspired me to write this short post.

A simple question such as Does the VFD have bypass option? may easily get a bit tricky.
Huh, ehh, well… What do you mean with bypass? How many times have you got confused by the subject of VFD bypass? Let’s try to shed some light to the topic and hopefully get more clarity.

Different types of bypass

There are at least three different things in VFD systems that may be called “bypass”. No wonder that people may get confused. Each “bypass” has somewhat different purpose and technical realization. We will briefly describe each of them.

1. Mechanical (manual) bypass of entire VFD

The first type is a mechanical bypass of the whole VFD. It is used e.g. in case of VFD failure or during maintenance works. In this case the entire VFD is bypassed and the motor can (resp. must) be operated as direct on-line. This bypass is normally operated at no-load condition. It does not provide uninterrupted operation. After the VFD is taken out of service and isolated the bypass is applied while the motor is at standstill. Therefore, motor must be suitable for direct on-line (DOL) start and the grid must be able to tolerate the starting current in such case.

Aim of this bypass is to keep the process alive even though the VFD is shut down. Machine will be operated at fix speed (i.e. speed control must be sacrificed) and both motor and grid must tolerate the harsh DOL start.

2. Synchronous bypass /Automatic bypass

Second case is a bypass used to synchronize the motor to the grid at the end of a VFD soft start. This bypass allows the transfer VFD operation ⇔ DOL operation. Every VFD soft starter has such a bypass. It is also used in combined soft starters/full drives where some loads are soft started and transferred to this grid while other loads continuously use the speed control.

The bypass always operates when the machine is synchronized with the grid. Therefore the name synchronous bypass. The switches are controlled by a synchronization equipment that compares grid and machine voltages, frequencies and phase angles. Once the conditions are fulfilled, the switches are automatically operated. So it may also be called automatic bypass.

3. Cell bypass

Last type of bypass mentioned in our article is used in cell based VFDs. Purpose of this bypass is to increase the availability of the VFD, especially when it consists of lot of power components (particularly medium voltage drives based on low voltage semiconductors).

A failed power cell can be bypassed so that the VFD remains operational. This feature is therefore used to realize N+1 or N-1 redundancy in hardware design. Cell bypass may be realized as fast mechanical switch or as an additional semiconductor that is triggered when a cell failure is detected.

More types of bypass in VFD applications?

Do you know any other type of bypass used in VFD applications that we do not cover here? Feel free to share it with us per email or in the comment section at the bottom.

Summary

As you can see, there are multiple bypass variants in drive systems utilizing VFDs. As the purpose and functionality of each bypass solution is quite different we shall be more precise when talking about bypass.

Mechanical bypass of entire VFD is typically the last option to keep the process running after a VFD failure. This bypass is operated at no-load with motor at standstill. Speed control must be sacrificed and the motor must start direct on-line with all negative consequences (starting current, voltage drop, torque pulsations etc).

Synchronous bypass is inherently used in all VFD soft starters to transfer the machine from inverter operation to DOL operation (and vice versa in case of bi-directional synchronous bypass). Full size VFDs might have synchronous bypass as well – especially if the VFD shall soft start and synchronize one load and afterwards continuously control speed or torque of another load. Synchronous bypass may also be used to keep the motor running while the VFD needs some maintenance work. However, that is certainly not the main purpose of such bypass.

Finally cell bypass is used to bridge a failed component inside the VFD (typically a power cell) so that the VFD can remain in service. Such bypass is frequently used in multi-cell drives with large quantity of hardware components that would otherwise feature too low reliability figures. The cell configuration is either N+1 or N-1. Realization of cell bypass is either mechanical and electronic (depending on manufacturer’s philosophy).

There are lot of topics related to VFDs and drive systems. Above explanation on bypass feature is just one small example. Join one of our programs, grow your VFD knowledge and become a drive expert!

References

[1] Motor starting methods: Direct on-line start, https://mb-drive-services.com/direct-on-line-start/

[2] VFD as soft starter, https://mb-drive-services.com/vfd-as-soft-starter/

[3] Cell bypass – Things you should know, https://mb-drive-services.com/facts-about-cell-bypass/

Bypass in VFD applications – What does it actually mean?

Published by admin on July 31, 2021July 31, 2021